LR11008
The WIMPless Miracle
We have recently shown that the standard paradigm for dark matter can be
generalized, yielding many new avenues for experimental searches. It
is now well-established that 25% of the energy in the universe is in
dark matter, a new form of matter that does not interact with light. One
of the leading theoretical candidates for dark matter is weakly interacting
massive particles (WIMPs). This is largely due to the "WIMP Miracle," the
remarkable coincidence that a particle with mass and interactions characteristic
of the weak force has approximately the correct energy density to match our
observations. This coincidence has guided most theoretical models and
experimental searches for dark matter to date. In this paper, we showed
that the WIMP miracle is actually just a specific example of a much
more general "WIMPless Miracle." In the more general framework,
particles naturally have the correct density and properties to
be dark matter, regardless of whether or not they are related to the weak
force. This "opens up the playbook," allowing realistic dark matter
candidates with masses very different from those typically searched for at
experiments and possibly explaining anomalous experimental results that
have thus far defied explanation.
***
LV11333
Electric circuit forms an artificial molecule
Besides the motion of electrons, a diatomic molecule has two nuclei
that that can vibrate around their equilibrium separation. We have
fabricated an electric circuit that has an analog of both these
motions. Measurements on the circuit reveal transitions between
quantum states that are similar as observed in diatomic molecules. In
particular, we see vibronic transitions where both the electronic and
vibrational states change simultaneously. These transitions obey the
principles that were established for molecules in the early days of
quantum mechanics. Our circuit uses superconducting components at
millikelvin temperatures. The electronic states of the molecule
correspond to a metallic island between two nanoscale Josephson
junctions. The vibrations of the nuclei correspond to an electric LC
oscillator. Besides adjusting the parameters of the artificial
molecule in fabrication, there are two parameters that can tuned
during the measurement and allow clear mapping of the transitions.
Our measurement also shows a cooling effect of the vibronic transitions.
Friday, November 21, 2008
Thursday, November 20, 2008
November 20, 2008
LU11804
Signatures of quark matter in compact stars
A new study carried out by Mannarelli, Manuel and Sa'd reveals that
a star formed by highly compressed quark matter cannot rotate
faster that one revolution per second.
Pulsars are rapidly rotating compact objects formed during the supernova
explosions of massive stars. While they are believed to be formed mainly
by neutrons, some physicists think that they could be made up of quark
matter. Different properties of rotating neutron and quark matter
may allow scientists to distinguish among these possibilities.
In all rotating stars there are fluid motions, the so called r-modes,
which are similar to current streams in the Earth's oceans. These fluid
motions radiate gravity waves, losing energy and angular momentum and
forcing the star to spin-down while they are growing. This growth is
counteracted by fluid dissipative forces. After establishing a balance
between all these effects one gets the maximal rotational frequency that a
star of a given kind can stand.
The authors' results for a model of a quark star imply that 75 % of
the observed pulsars cannot be made up of this exotic material.
As for the remaining 25%, more properties need to be studied
to find evidence of quark matter in the heavens.
***
LR11074
Noble metal film as spin filter
Imagine an electrical power supply where the output sockets are not
marked by plus and minus but by arrows meaning spin up and spin down.
Such device is fancied in spintronics where signals shall be transmitted
by the spin of the electron instead of its charge.
The authors of the present paper have made a significant step towards
realizing such a unit. They produced a quantum cavity which is able to
filter electrons according to their spin. The physical principle which
achieves this is the spin-orbit interaction. It leads to a small
energy shift between a spin up electron orbiting the atomic core clockwise
and anticlockwise. This interaction plays a fundamental role in atoms and
in recent years researchers have been tracing its effect on electrons at
heavy metal surfaces.
What is new with the present work is the combination
of this effect with that of a quantum cavity inside which electrons are
trapped. This does not only enhance the spin effect by subjecting all the
electrons in the entire gold film to it, but it also lets a former surface
property, existant only in ultrahigh vacuum, slip into the protected inside
of a metal film - a much better perspective for a practical device.
***
LQ11147BR
Breakdown of magnetic order by orbital disorder in Mott insulators
Interatomic interactions between localized-electron spins are
responsible for long-range magnetic order in Mott insulators. Where the
localized electrons partially occupy atomic orbitals of the same energy,
orbital order above or at the magnetic ordering temperature determines how
spins are ordered. Therefore, the question of how or whether long-range
magnetic order occurs where the orbitals remain disordered has remained an
open question. High-pressure experiments that create a lattice symmetry that
does not support static orbital order have revealed that disordering the
electronic orbitals frustrates the spin-spin interactions to suppress
long-range magnetic order abruptly at the structural transition. By
measuring the magnetic susceptibility, transport property, and the crystal
structure of both antiferromagnetic LaMnO3 and ferromagnetic
LaMn0.5Ga0.5O3under pressure across a perovskite orthorhombic to
rhombohedral transition,
we have shown that the long-range magnetic ordering disappears at the
critical pressure for stabilization of the rhombohedral phase where
long-range order of the orbitals is not allowed by symmetry.
***
LT11635
“No-Loss” Metamaterials Open Door to Major Advances in Optics
Researchers have removed one of the major roadblocks that prevent more
practical applications of plasmonic metamaterials, metal-based nanocomposites
that allow for novel optical phenomena, ranging from biosensing to
subwavelength light guiding, the superlens, or even optical invisibility
cloaks. One obstacle to greater advances of metamaterials has been
metal-induced optical absorption. The absorption losses impose fundamental
limits on the sensitivity of plasmonic biosensors, on the resolution of the
superlens, and on the size and quality of optical invisibility cloaks. It has
been suggested that optical gain could be incorporated into metamaterials to
address this problem – although the possibility of complete loss compensation
has been the topic of considerable debate. In this work we have achieved the
level of gain necessary to completely compensate for optical plasmonic
losses, and clearly demonstrate this by the distinct threshold in emission
from the plasmonic modes (see figure). Apart from enabling a number of
dreamed-about applications of metamaterials, our research opens the door to
important new areas of metamaterial optics.
Figure caption: Stimulated emission of externally pumped molecules of
Rhodamine 6G (dots) into surface plasmon polariton (colored profile) that
exponentially grows in magnitude as it propagates along the boundary between
gain region (red) and silver film (gray strip), simultaneously decoupling
into the prism (bottom of the figure).
Signatures of quark matter in compact stars
A new study carried out by Mannarelli, Manuel and Sa'd reveals that
a star formed by highly compressed quark matter cannot rotate
faster that one revolution per second.
Pulsars are rapidly rotating compact objects formed during the supernova
explosions of massive stars. While they are believed to be formed mainly
by neutrons, some physicists think that they could be made up of quark
matter. Different properties of rotating neutron and quark matter
may allow scientists to distinguish among these possibilities.
In all rotating stars there are fluid motions, the so called r-modes,
which are similar to current streams in the Earth's oceans. These fluid
motions radiate gravity waves, losing energy and angular momentum and
forcing the star to spin-down while they are growing. This growth is
counteracted by fluid dissipative forces. After establishing a balance
between all these effects one gets the maximal rotational frequency that a
star of a given kind can stand.
The authors' results for a model of a quark star imply that 75 % of
the observed pulsars cannot be made up of this exotic material.
As for the remaining 25%, more properties need to be studied
to find evidence of quark matter in the heavens.
***
LR11074
Noble metal film as spin filter
Imagine an electrical power supply where the output sockets are not
marked by plus and minus but by arrows meaning spin up and spin down.
Such device is fancied in spintronics where signals shall be transmitted
by the spin of the electron instead of its charge.
The authors of the present paper have made a significant step towards
realizing such a unit. They produced a quantum cavity which is able to
filter electrons according to their spin. The physical principle which
achieves this is the spin-orbit interaction. It leads to a small
energy shift between a spin up electron orbiting the atomic core clockwise
and anticlockwise. This interaction plays a fundamental role in atoms and
in recent years researchers have been tracing its effect on electrons at
heavy metal surfaces.
What is new with the present work is the combination
of this effect with that of a quantum cavity inside which electrons are
trapped. This does not only enhance the spin effect by subjecting all the
electrons in the entire gold film to it, but it also lets a former surface
property, existant only in ultrahigh vacuum, slip into the protected inside
of a metal film - a much better perspective for a practical device.
***
LQ11147BR
Breakdown of magnetic order by orbital disorder in Mott insulators
Interatomic interactions between localized-electron spins are
responsible for long-range magnetic order in Mott insulators. Where the
localized electrons partially occupy atomic orbitals of the same energy,
orbital order above or at the magnetic ordering temperature determines how
spins are ordered. Therefore, the question of how or whether long-range
magnetic order occurs where the orbitals remain disordered has remained an
open question. High-pressure experiments that create a lattice symmetry that
does not support static orbital order have revealed that disordering the
electronic orbitals frustrates the spin-spin interactions to suppress
long-range magnetic order abruptly at the structural transition. By
measuring the magnetic susceptibility, transport property, and the crystal
structure of both antiferromagnetic LaMnO3 and ferromagnetic
LaMn0.5Ga0.5O3under pressure across a perovskite orthorhombic to
rhombohedral transition,
we have shown that the long-range magnetic ordering disappears at the
critical pressure for stabilization of the rhombohedral phase where
long-range order of the orbitals is not allowed by symmetry.
***
LT11635
“No-Loss” Metamaterials Open Door to Major Advances in Optics
Researchers have removed one of the major roadblocks that prevent more
practical applications of plasmonic metamaterials, metal-based nanocomposites
that allow for novel optical phenomena, ranging from biosensing to
subwavelength light guiding, the superlens, or even optical invisibility
cloaks. One obstacle to greater advances of metamaterials has been
metal-induced optical absorption. The absorption losses impose fundamental
limits on the sensitivity of plasmonic biosensors, on the resolution of the
superlens, and on the size and quality of optical invisibility cloaks. It has
been suggested that optical gain could be incorporated into metamaterials to
address this problem – although the possibility of complete loss compensation
has been the topic of considerable debate. In this work we have achieved the
level of gain necessary to completely compensate for optical plasmonic
losses, and clearly demonstrate this by the distinct threshold in emission
from the plasmonic modes (see figure). Apart from enabling a number of
dreamed-about applications of metamaterials, our research opens the door to
important new areas of metamaterial optics.
Figure caption: Stimulated emission of externally pumped molecules of
Rhodamine 6G (dots) into surface plasmon polariton (colored profile) that
exponentially grows in magnitude as it propagates along the boundary between
gain region (red) and silver film (gray strip), simultaneously decoupling
into the prism (bottom of the figure).
Wednesday, November 19, 2008
November 19, 2008
AX10219
Quantum “split personality syndrome” leads to survival collapse!
One bucket full of water with a small hole in the
bottom empties with a flux proportional to the
volume of liquid left. This form of decay,
called exponential decay, is a ubiquitous
phenomenon in Nature, as a phosphorescent toy or
a chunk of radioactive atoms tend to decay or
transmute by emitting visible light or radiation
respectively. Quantum mechanics describes these,
and innumerable other processes, through the
Fermi Golden Rule, named after Enrico Fermi, the
inventor of the atomic pile. However, quantum
mechanics has surprises stored for us. Chances
are that, instead of a monotonous exponential
decay, a system collapses and reassembles itself
all of a sudden! This paper shows that this
surprising revival is inherent to quantum
mechanics. It shows up even in system with some
internal dynamics, such as a Rabi oscillator.
Experimental implementations are suggested that
exploit this and related phenomena for
information processing and spectroscopy. The
conceptual key lies in the too frequently
neglected “environment”, where the resulting
system’s fragments retain some memory of their
previous state. Indeed, they have a
non-negligible chance to be simultaneously
reflected back into the original state. This
enables a sort of “split-personality syndrome”,
inherent to quantum physics: its surviving
personality interferes with its returning one
destroying each other. Indeed, this is a simple,
but counterintuitive, interference among
probability waves that was previously
inaccessible. Metamaterials and nanoscience are
now ready for the exploration of the wealth of
phenomena in the realm of time domain.
***
BK10751
Picturing the Propagation of Localized Electrons
In some branches of theoretical physics it often turns out that a
picture can
be worth a thousand quite technical words (or nowadays 1024). These
pictures,
or 'diagrams', provide a compact, intuitive, and often highly physical
representation of objects that arise in theories developed to describe
systems
of many, many particles, usually interacting. In the ensuing quantum
field
theory and in perturbation many-body theory, the scattering of (quasi)
particles is represented as a convergent series of so-called Feynman
diagrams*.
Here the probability amplitude for a particle to travel between two
scattering events, or space-time points, is given by the so-called
Feynman
propagator.
The traditional domain of many-body perturbation theory has been
spatially
uniform systems. What is now presented is a diagrammatic quantum field
formalism for calculating quantum mechanical quantities in just the
opposite
limit. This is a limit where the physics of the system deems the
electrons to
be strongly localized but where the wave functions still overlap, just
a bit.
Despite these conceptual differences, a diagrammatic language can be
introduced for this important case by direct analogy with that of
standard
field theory. It is based, in particular, on the idea that overlaps
between
localized single-electron wave functions can themselves be viewed as
quantum
perturbations that introduce scattering of the single-electron
amplitudes. A key quantity in the emerging diagrammatic language is
indeed
the overlap between single-electron wave functions. It is measured by
a quantity referred to as the overlap integral, which in our formalism
becomes
the equivalent of the Feynman propagator in the traditional
diagrammatic field
theory. The resulting framework then turns out to be exceedingly
general and
flexible, and the corresponding 'rules' for constructing the diagrams
for
physical objects of interest are quite straightforward. The new
diagrammatic
method should be of some utility both in theoretical condensed matter
physics
and in quantum chemistry.
***
LS11546B
How are ferroelectrics getting older?
The properties of ferroelectric materials can be tuned for specific
applications through doping with various metal ions. However, for unknown
reasons the doped materials exhibit aging: an undesirable gradual
degradation of material parameters, in particular the stabilization of the
polarisation domain pattern which makes the material hardly polarisable.
Previously, it has been assumed that the deterioration of materials during a
period from hours to months is caused by charged defects, although the
mechanism of this phenomenon is still highly disputable. The theory of
collective charge defect migration advanced in this paper seems to shed
light on some still inexplicable features of aging and its hidden mechanism.
Namely, it predicts, in agreement with experiments, a characteristic aging
time distinctly dependent on the doping concentration and a saturation of
the domain pinning strength already at medium defect concentrations. This is
in contrast to the most popular explanation during the last two decades of
the mechanism of aging due to individual gradual orientation of defect
dipoles formed by the acceptor defects and oxygen vacancies that predicts
independence of the aging time on doping and a linear increase of the domain
pinning strength with defect concentration.
***
BU11077
Artificial Metamaterial Twists Light Polarization a Million Times More
Strongly than Natural Media, Forcing Light to Go Backwards
Our experimental study of a novel twisted (chiral) metamaterial reveals
a whole range of useful properties including negative refraction and the
ability to act as a polarization rotator and circular polarizer.
Materials with a negative index of refraction allow focusing of light to
sub-wavelength spots and are therefore key to next generation
ultra-high-capacity data storage devices and super-resolution
microscopes. In this paper we prove that a negative index of refraction
can arise from the chirality of media. Our metamaterial consists of
pairs of flat, mutually-twisted metal patterns in parallel planes. In a
manner similar to natural chiral media, such as quartz crystal or sugar
solution, our artificial structure can rotate the polarization state of
light and is more transparent for one circular polarization than the
other. However, in the artificial structure, the polarization rotation
occurring between the two layers of meta-molecules is a million times
stronger than in quartz. This allows the realization of polarization
rotators and circular polarizers of sub-wavelength thickness, and our
novel metamaterial therefore constitutes an important step towards the
miniaturization of polarization control elements for microwave and
optoelectronic applications, which are conventionally many wavelengths
thick.
***
LW11416
Emerging regularity in long-range dynamics
The vast majority of phenomena observed in nature result from the
complex interaction between elementary constituents. A widespread
observation is the emergence of regular trajectories despite the
extended network of simultaneously active couplings. These
trajectories are for instance found within the Solar system, and
their stability allows for an accurate long-term predictability.
What determines their ubiquity? Is there a general theory to describe
their dynamical behaviour?
This paper reports on a novel interpretative framework for explaining
the emergence of regular trajectories in systems subject to long
range couplings. With reference to a paradigmatic case study, and at
variance with the customarily invoked Kolmogorov-Arnold-Moser
picture, the paper demonstrate that invariant tori (enclosed pictures)
do appear in phase space as the number of degrees of freedom
is increased, due to the self-consistent nature of the interaction.
Surprisingly, regularity is an emergent property, which manifests at
the thermodynamic limit. This is at odd with intuition that would
favor chaotic orbits and, consequently, well mixed dynamics, as a
result of the simultaneous action of an increasingly large ensemble
of microscopic constituents. This observation is general and opens
up interesting scenarios for those fields of applications (e.g.
celestial mechanics) where long range forces are in play. Galaxies
potentially represent the most spectacular example where such far-
from-equilibrium processes do occur.
---
***
BW10640
Light Separates the Cis- and Trans-Distorted Isomers of Polyacene
Optical measurements on polyacene should reveal a striking constrast
between its structural instability of two types: double bonds in a cis
pattern and those in a trans pattern, which have hardly been identified
distinguishably in the past decades. The two Peierls-distorted states
were argued again and again in the literature to be highly degenerate
in their energetics, but in this paper, they have turned out quite
distinct in their optics. For light polarized in the chain direction,
two well-separate absorption bands will be expected against the
trans-distorted background, while a single absorption band with the
cis-distorted background is to appear in between, provided that the
electron-lattice coupling is strong enough.
***
LU11959
Are the newly discovered Iron-based high temperature superconductor
different or similar to the cuprate high temperature superconductors?
The recent discovery of high-temperature superconductivity in ironoxypnictides and related materials has generated enormous excitement in the community. Here we demonstrate that the basic electronic structure in the normal state of the iron-oxypnictide CeFeAsO0.89F0.11 appears to be quite different from that of copper-based high-temperature superconductors (cuprates). Both x-ray absorption and photoemission spectra exhibit signatures typical of delocalized, itinerant electrons, while exchange multiplets appearing in the iron photoemission spectra indicate itinerant spin fluctuations. This is particularly important in light of the competition between a magnetically ordered state and superconductivity displayed in the general phase diagram of oxypnictide materials. The detection of magnetic fluctuations by means of magnetic
probes has so far remained elusive because of the extremely fast time scales involved. Our experiment provides a strong and unique test case for the occurrence of itinerant magnetic fluctuations, whose detection is made possible by the rapid time scales in the photoemission process. By finding direct signatures of magnetic fluctuations in a prototypical oxypnictide material, our work clarifies some modalities of the interactions between magnetism and superconductivity.
These findings suggest that the underlying physics and the origin of
superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors, and also from those of MgB2 and BCS-like electron-phonon superconductors.
Quantum “split personality syndrome” leads to survival collapse!
One bucket full of water with a small hole in the
bottom empties with a flux proportional to the
volume of liquid left. This form of decay,
called exponential decay, is a ubiquitous
phenomenon in Nature, as a phosphorescent toy or
a chunk of radioactive atoms tend to decay or
transmute by emitting visible light or radiation
respectively. Quantum mechanics describes these,
and innumerable other processes, through the
Fermi Golden Rule, named after Enrico Fermi, the
inventor of the atomic pile. However, quantum
mechanics has surprises stored for us. Chances
are that, instead of a monotonous exponential
decay, a system collapses and reassembles itself
all of a sudden! This paper shows that this
surprising revival is inherent to quantum
mechanics. It shows up even in system with some
internal dynamics, such as a Rabi oscillator.
Experimental implementations are suggested that
exploit this and related phenomena for
information processing and spectroscopy. The
conceptual key lies in the too frequently
neglected “environment”, where the resulting
system’s fragments retain some memory of their
previous state. Indeed, they have a
non-negligible chance to be simultaneously
reflected back into the original state. This
enables a sort of “split-personality syndrome”,
inherent to quantum physics: its surviving
personality interferes with its returning one
destroying each other. Indeed, this is a simple,
but counterintuitive, interference among
probability waves that was previously
inaccessible. Metamaterials and nanoscience are
now ready for the exploration of the wealth of
phenomena in the realm of time domain.
***
BK10751
Picturing the Propagation of Localized Electrons
In some branches of theoretical physics it often turns out that a
picture can
be worth a thousand quite technical words (or nowadays 1024). These
pictures,
or 'diagrams', provide a compact, intuitive, and often highly physical
representation of objects that arise in theories developed to describe
systems
of many, many particles, usually interacting. In the ensuing quantum
field
theory and in perturbation many-body theory, the scattering of (quasi)
particles is represented as a convergent series of so-called Feynman
diagrams*.
Here the probability amplitude for a particle to travel between two
scattering events, or space-time points, is given by the so-called
Feynman
propagator.
The traditional domain of many-body perturbation theory has been
spatially
uniform systems. What is now presented is a diagrammatic quantum field
formalism for calculating quantum mechanical quantities in just the
opposite
limit. This is a limit where the physics of the system deems the
electrons to
be strongly localized but where the wave functions still overlap, just
a bit.
Despite these conceptual differences, a diagrammatic language can be
introduced for this important case by direct analogy with that of
standard
field theory. It is based, in particular, on the idea that overlaps
between
localized single-electron wave functions can themselves be viewed as
quantum
perturbations that introduce scattering of the single-electron
amplitudes. A key quantity in the emerging diagrammatic language is
indeed
the overlap between single-electron wave functions. It is measured by
a quantity referred to as the overlap integral, which in our formalism
becomes
the equivalent of the Feynman propagator in the traditional
diagrammatic field
theory. The resulting framework then turns out to be exceedingly
general and
flexible, and the corresponding 'rules' for constructing the diagrams
for
physical objects of interest are quite straightforward. The new
diagrammatic
method should be of some utility both in theoretical condensed matter
physics
and in quantum chemistry.
***
LS11546B
How are ferroelectrics getting older?
The properties of ferroelectric materials can be tuned for specific
applications through doping with various metal ions. However, for unknown
reasons the doped materials exhibit aging: an undesirable gradual
degradation of material parameters, in particular the stabilization of the
polarisation domain pattern which makes the material hardly polarisable.
Previously, it has been assumed that the deterioration of materials during a
period from hours to months is caused by charged defects, although the
mechanism of this phenomenon is still highly disputable. The theory of
collective charge defect migration advanced in this paper seems to shed
light on some still inexplicable features of aging and its hidden mechanism.
Namely, it predicts, in agreement with experiments, a characteristic aging
time distinctly dependent on the doping concentration and a saturation of
the domain pinning strength already at medium defect concentrations. This is
in contrast to the most popular explanation during the last two decades of
the mechanism of aging due to individual gradual orientation of defect
dipoles formed by the acceptor defects and oxygen vacancies that predicts
independence of the aging time on doping and a linear increase of the domain
pinning strength with defect concentration.
***
BU11077
Artificial Metamaterial Twists Light Polarization a Million Times More
Strongly than Natural Media, Forcing Light to Go Backwards
Our experimental study of a novel twisted (chiral) metamaterial reveals
a whole range of useful properties including negative refraction and the
ability to act as a polarization rotator and circular polarizer.
Materials with a negative index of refraction allow focusing of light to
sub-wavelength spots and are therefore key to next generation
ultra-high-capacity data storage devices and super-resolution
microscopes. In this paper we prove that a negative index of refraction
can arise from the chirality of media. Our metamaterial consists of
pairs of flat, mutually-twisted metal patterns in parallel planes. In a
manner similar to natural chiral media, such as quartz crystal or sugar
solution, our artificial structure can rotate the polarization state of
light and is more transparent for one circular polarization than the
other. However, in the artificial structure, the polarization rotation
occurring between the two layers of meta-molecules is a million times
stronger than in quartz. This allows the realization of polarization
rotators and circular polarizers of sub-wavelength thickness, and our
novel metamaterial therefore constitutes an important step towards the
miniaturization of polarization control elements for microwave and
optoelectronic applications, which are conventionally many wavelengths
thick.
***
LW11416
Emerging regularity in long-range dynamics
The vast majority of phenomena observed in nature result from the
complex interaction between elementary constituents. A widespread
observation is the emergence of regular trajectories despite the
extended network of simultaneously active couplings. These
trajectories are for instance found within the Solar system, and
their stability allows for an accurate long-term predictability.
What determines their ubiquity? Is there a general theory to describe
their dynamical behaviour?
This paper reports on a novel interpretative framework for explaining
the emergence of regular trajectories in systems subject to long
range couplings. With reference to a paradigmatic case study, and at
variance with the customarily invoked Kolmogorov-Arnold-Moser
picture, the paper demonstrate that invariant tori (enclosed pictures)
do appear in phase space as the number of degrees of freedom
is increased, due to the self-consistent nature of the interaction.
Surprisingly, regularity is an emergent property, which manifests at
the thermodynamic limit. This is at odd with intuition that would
favor chaotic orbits and, consequently, well mixed dynamics, as a
result of the simultaneous action of an increasingly large ensemble
of microscopic constituents. This observation is general and opens
up interesting scenarios for those fields of applications (e.g.
celestial mechanics) where long range forces are in play. Galaxies
potentially represent the most spectacular example where such far-
from-equilibrium processes do occur.
---
***
BW10640
Light Separates the Cis- and Trans-Distorted Isomers of Polyacene
Optical measurements on polyacene should reveal a striking constrast
between its structural instability of two types: double bonds in a cis
pattern and those in a trans pattern, which have hardly been identified
distinguishably in the past decades. The two Peierls-distorted states
were argued again and again in the literature to be highly degenerate
in their energetics, but in this paper, they have turned out quite
distinct in their optics. For light polarized in the chain direction,
two well-separate absorption bands will be expected against the
trans-distorted background, while a single absorption band with the
cis-distorted background is to appear in between, provided that the
electron-lattice coupling is strong enough.
***
LU11959
Are the newly discovered Iron-based high temperature superconductor
different or similar to the cuprate high temperature superconductors?
The recent discovery of high-temperature superconductivity in ironoxypnictides and related materials has generated enormous excitement in the community. Here we demonstrate that the basic electronic structure in the normal state of the iron-oxypnictide CeFeAsO0.89F0.11 appears to be quite different from that of copper-based high-temperature superconductors (cuprates). Both x-ray absorption and photoemission spectra exhibit signatures typical of delocalized, itinerant electrons, while exchange multiplets appearing in the iron photoemission spectra indicate itinerant spin fluctuations. This is particularly important in light of the competition between a magnetically ordered state and superconductivity displayed in the general phase diagram of oxypnictide materials. The detection of magnetic fluctuations by means of magnetic
probes has so far remained elusive because of the extremely fast time scales involved. Our experiment provides a strong and unique test case for the occurrence of itinerant magnetic fluctuations, whose detection is made possible by the rapid time scales in the photoemission process. By finding direct signatures of magnetic fluctuations in a prototypical oxypnictide material, our work clarifies some modalities of the interactions between magnetism and superconductivity.
These findings suggest that the underlying physics and the origin of
superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors, and also from those of MgB2 and BCS-like electron-phonon superconductors.
Tuesday, November 18, 2008
November 18, 2008
LW11529
New study finds we're not at centre of Universe, reaffirms mysterious dark energy
A new study to be published in Physical Review Letters has found that
models of the Universe which place us near the centre of a large, sparse
region or "void" are very poor fits to astronomical observations, and
reaffirms the presence of a perplexing dark energy. In recent years
many studies have indicated that the expansion of the Universe is
accelerating, which may be due to a mysterious form of "dark energy".
Under an alternative interpretation, the Universe would only appear to
accelerate if we happened to live near the centre of an enormous cosmic
void, empty of most matter. Now Jim Zibin, Adam Moss, and Douglas Scott,
cosmologists at the University of British Columbia, in Vancouver, Canada,
have examined the latest data, in particular subtle features in the cosmic
microwave background radiation (the "afterglow" of the Big Bang) and
ripples in the large-scale distribution of matter. They found that void
models, unlike standard dark energy models, do a very poor job of
explaining all of the latest data, taken together. This new study helps
to solidify our place in the Universe as a completely typical and
unremarkable one. But it also reaffirms that most of the stuff in the
Universe is far from unremarkable: dark energy remains as enigmatic as
ever.
***
LU11257
Local transport reveals the spin-triplet superconductivity
By using micro fabrication technique, we have measured local transports and determined
the pairing symmetry of Sr2RuO4-Ru eutectic system which is called the 3-K phase superconductivity
(Tc ~ 3 K). Up to now, pure Sr2RuO4 (Tc = 1.5 K) is widely recognized to be a rare example of spin-triplet
superconductor. However, the enhancement mechanism of Tc up to 3 K and the pairing symmetry for
the 3-K phase have not been understood clearly because the 3-K phase is the interface superconductivity
with low-volume fraction between Sr2RuO4 and Ru inclusion which is randomly distributed in a crystal.
In this paper, we have controlled the number of Ru inclusions with the micro fabrication and
succeeded to extract individual superconducting channels for the 3-K phase. We confirmed that
the 3-K phase is an odd-parity superconductor similar to pure Sr2RuO4 from the monotonous temperature
dependence of the critical currents. In addition, we observed quite unusual behavior in current-voltage
characteristics below 2 K, which indicates the internal degrees of freedom of the superconducting state,
most probably the chiral p-wave state. This experiment revealed nanoscale physics in inhomogeneous
spin-triplet superconductivity by means of micro fabrication technique.
***
LT11016
Is it possible for two parties to perfectly communicate without having a
clue about the medium they have been using for such a task?
The answer is a resounding "Yes" if you are doing it quantum mechanically, as it is
proved in this work!
Many-body interactions in spin systems have recently emerged as fruitful
mechanisms for the achievement of information-transfer among distant
locations of a network. So far, protocols for "perfect" transfer of
information across interacting quantum spin systems have been designed,
taking in careful consideration both the arrangement of proper coupling
strengths and the correct and most suitable preparation of the state of
the transportation medium. However, this latter requirement is not exempt
from difficulties, especially in many-body systems with a large number of
degrees of freedom. Even small deviations from the desired initial state
may result in a spoiled transfer-efficiency.
In our manuscript, we report a significant step forward in this direction:
we present a general protocol for perfect quantum state transfer in an
interacting-spin system which, with only a limited amount of local
resources, bypasses the necessity of challenging state-initialization. Our
protocol does not require fine control of the participants' dynamics, nor
demanding properties of the interacting systems.
***
LV11034
Scientists Ignite Optical Rogue Waves
One of the most surprising and technologically important examples of
nonlinear action in physics is the generation of supercontinuum radiation,
pulses of light containing a broad spectrum of wavelengths. Recently, we
discovered a new phenomenon arising in supercontinuum generation known as
optical rogue waves-rare, bright flashes of white light analogous to the
infamous monster waves that roam the high seas [Solli et al., Nature 450
(2007)]. Rogue waves testify to the extreme influence of noise in many
nonlinear systems. A process capable of controlling these rogue events would
have great scientific and practical importance. In the manuscript entitled
"Active Control of Rogue Waves for Stimulated Supercontinuum Generation,"
the authors demonstrate active control over rogue waves for the first time.
Active control over rogue waves has never been accomplished before in any
system, and rogue waves have never been harnessed for applications. As a
striking example of the importance of this phenomenon in optics, the authors
show that it can be exploited to stimulate supercontinuum generation, a new
concept that results in a greatly enhanced, controllable source of broadband
light.
Image: Optical rogue waves appear as rare flashes of broadband light in
optical fiber. Controlling these extreme events presents a new tool with
numerous potential applications.
***
LW11327
New type of precise atomic clocks.
The present definition of the unit of time, the second, is based on the
microwave transition in cesium atom. The most precise cesium clock
is a big apparatus called the fountain clock. The size of the clock may
be substantially reduced if the cesium atoms will be placed inside
an optical lattice produced by laser light. A relative compactness of the
proposed cesium clock could benefit numerous scientific and general
applications such as navigation systems.
New study finds we're not at centre of Universe, reaffirms mysterious dark energy
A new study to be published in Physical Review Letters has found that
models of the Universe which place us near the centre of a large, sparse
region or "void" are very poor fits to astronomical observations, and
reaffirms the presence of a perplexing dark energy. In recent years
many studies have indicated that the expansion of the Universe is
accelerating, which may be due to a mysterious form of "dark energy".
Under an alternative interpretation, the Universe would only appear to
accelerate if we happened to live near the centre of an enormous cosmic
void, empty of most matter. Now Jim Zibin, Adam Moss, and Douglas Scott,
cosmologists at the University of British Columbia, in Vancouver, Canada,
have examined the latest data, in particular subtle features in the cosmic
microwave background radiation (the "afterglow" of the Big Bang) and
ripples in the large-scale distribution of matter. They found that void
models, unlike standard dark energy models, do a very poor job of
explaining all of the latest data, taken together. This new study helps
to solidify our place in the Universe as a completely typical and
unremarkable one. But it also reaffirms that most of the stuff in the
Universe is far from unremarkable: dark energy remains as enigmatic as
ever.
***
LU11257
Local transport reveals the spin-triplet superconductivity
By using micro fabrication technique, we have measured local transports and determined
the pairing symmetry of Sr2RuO4-Ru eutectic system which is called the 3-K phase superconductivity
(Tc ~ 3 K). Up to now, pure Sr2RuO4 (Tc = 1.5 K) is widely recognized to be a rare example of spin-triplet
superconductor. However, the enhancement mechanism of Tc up to 3 K and the pairing symmetry for
the 3-K phase have not been understood clearly because the 3-K phase is the interface superconductivity
with low-volume fraction between Sr2RuO4 and Ru inclusion which is randomly distributed in a crystal.
In this paper, we have controlled the number of Ru inclusions with the micro fabrication and
succeeded to extract individual superconducting channels for the 3-K phase. We confirmed that
the 3-K phase is an odd-parity superconductor similar to pure Sr2RuO4 from the monotonous temperature
dependence of the critical currents. In addition, we observed quite unusual behavior in current-voltage
characteristics below 2 K, which indicates the internal degrees of freedom of the superconducting state,
most probably the chiral p-wave state. This experiment revealed nanoscale physics in inhomogeneous
spin-triplet superconductivity by means of micro fabrication technique.
***
LT11016
Is it possible for two parties to perfectly communicate without having a
clue about the medium they have been using for such a task?
The answer is a resounding "Yes" if you are doing it quantum mechanically, as it is
proved in this work!
Many-body interactions in spin systems have recently emerged as fruitful
mechanisms for the achievement of information-transfer among distant
locations of a network. So far, protocols for "perfect" transfer of
information across interacting quantum spin systems have been designed,
taking in careful consideration both the arrangement of proper coupling
strengths and the correct and most suitable preparation of the state of
the transportation medium. However, this latter requirement is not exempt
from difficulties, especially in many-body systems with a large number of
degrees of freedom. Even small deviations from the desired initial state
may result in a spoiled transfer-efficiency.
In our manuscript, we report a significant step forward in this direction:
we present a general protocol for perfect quantum state transfer in an
interacting-spin system which, with only a limited amount of local
resources, bypasses the necessity of challenging state-initialization. Our
protocol does not require fine control of the participants' dynamics, nor
demanding properties of the interacting systems.
***
LV11034
Scientists Ignite Optical Rogue Waves
One of the most surprising and technologically important examples of
nonlinear action in physics is the generation of supercontinuum radiation,
pulses of light containing a broad spectrum of wavelengths. Recently, we
discovered a new phenomenon arising in supercontinuum generation known as
optical rogue waves-rare, bright flashes of white light analogous to the
infamous monster waves that roam the high seas [Solli et al., Nature 450
(2007)]. Rogue waves testify to the extreme influence of noise in many
nonlinear systems. A process capable of controlling these rogue events would
have great scientific and practical importance. In the manuscript entitled
"Active Control of Rogue Waves for Stimulated Supercontinuum Generation,"
the authors demonstrate active control over rogue waves for the first time.
Active control over rogue waves has never been accomplished before in any
system, and rogue waves have never been harnessed for applications. As a
striking example of the importance of this phenomenon in optics, the authors
show that it can be exploited to stimulate supercontinuum generation, a new
concept that results in a greatly enhanced, controllable source of broadband
light.
Image: Optical rogue waves appear as rare flashes of broadband light in
optical fiber. Controlling these extreme events presents a new tool with
numerous potential applications.
***
LW11327
New type of precise atomic clocks.
The present definition of the unit of time, the second, is based on the
microwave transition in cesium atom. The most precise cesium clock
is a big apparatus called the fountain clock. The size of the clock may
be substantially reduced if the cesium atoms will be placed inside
an optical lattice produced by laser light. A relative compactness of the
proposed cesium clock could benefit numerous scientific and general
applications such as navigation systems.
Monday, November 17, 2008
November 17, 2008
LW11101
Environment Variation and Extinction Risk
A small isolated population of living organisms ultimately goes
extinct because of a random chain of rare events of predominance
of deaths over births. Average time to extinction, however, can be quite
large for not too small populations. Irregular variations of environmental
conditions affect the birth and death rates, once in a while decreasing
the population size and accelerating its
extinction. Earlier theoretical work assumed that the variations
of the environmental parameters, for example of the daily
temperature, look as ``white noise": they are uncorrelated, that
is completely independent from each other. The real environmental
variations, however, are correlated. One can say that they are
colored rather than white. For example, the daily temperature
varies much more gradually than what the white noise model would
predict. The complicated interplay between the color and magnitude of the
environmental variations on the one side, and the
population birth and death rates on the other side has eluded
understanding for
many years. Our work shows that the color of environmental
variations dramatically enhances the extinction risk. Our results will
help ecologists to correctly assess the viability of isolated populations.
***
LS11243
The signature of QCD in baryon magnetic moments
Baryon magnetic moments are approximately described by the
celebrated Coleman-Glashow relations that assume an
exact flavor symmetry between u, d and s quarks.
The present study improves these relations by including
the SU(3)-breaking corrections given by the low-energy realization of
Quantum Chromodynamics (QCD) known as Chiral Perturbation Theory (ChPT).
Baryons are composite objects made of interacting quarks and gluons.
For this reason, their magnetic moments are different from those of
elementary fermions with the same charges and masses. Unfortunately,
a full calculation of such a simple and fundamental quantity using QCD,
the theory of the strong interaction, is not yet feasible due to its
non-perturbative nature. Nonetheless, the magnetic moments of the lightest
spin 1/2 baryons are related among themselves providing a clear manifestation
of the underlying approximate SU(3) flavor symmetry. Previous attempts to
describe the breaking of this symmetry using ChPT have encountered problems
pointing at a poor convergence of the perturbative results.
It is found that in order to obtain a good description of these
magnetic moments it is essential that the low energy
realization of the fundamental strong interaction theory fulfills the
fundamental principles of analyticity and relativistic covariance. The results
of this work provide a solution for a long standing puzzle and neatly reveal
the manifestation of QCD on a basic observable such as baryon magnetic moments.
***
LR11242E
Shear induced mesostructures in biaxial liquid crystals
In 2004 a new "biaxial phase" of thermotropic (or temperature
sensitive) biaxial liquid crystalline polymers (LCPs), was discovered
experimentally by two seperate groups.
Biaxial LCPs are the brick-shaped or the ellipsoidal molecules that
have 2 length scales: a longer one along the long molecular axis and
the shorter one along the shorter axis.
It is widely hypothesized that the properties of the liquid crystal
polymers, which is seen along the long molecular axis; can also be
duplicated along the shorter axis; with a smaller time-scale. Hence
the presence of these 2 length-scales phenomena in these "smart"
materials can significantly improve the efficiency of certain
industries using semiconductor devices.
This biaxial phase, however, is very elusive at the molecular level
and extremely difficult to visualize and predict experimentally; in
the sense that they require an extreme conditions of temperature,
pressure and high magnetic field to be predicted experimentally.
To our knowledge, for the first time, we predict and present the
various phases of biaxial LCPs in the presence of an external
homogeneous shear flow, using mathematical modeling and large-scale
computation.
We discuss about the sequence of the orientational phases observed in
the selected regions of material parameter space. The underlying
hydrodynamic theory is also briefly discussed.
Environment Variation and Extinction Risk
A small isolated population of living organisms ultimately goes
extinct because of a random chain of rare events of predominance
of deaths over births. Average time to extinction, however, can be quite
large for not too small populations. Irregular variations of environmental
conditions affect the birth and death rates, once in a while decreasing
the population size and accelerating its
extinction. Earlier theoretical work assumed that the variations
of the environmental parameters, for example of the daily
temperature, look as ``white noise": they are uncorrelated, that
is completely independent from each other. The real environmental
variations, however, are correlated. One can say that they are
colored rather than white. For example, the daily temperature
varies much more gradually than what the white noise model would
predict. The complicated interplay between the color and magnitude of the
environmental variations on the one side, and the
population birth and death rates on the other side has eluded
understanding for
many years. Our work shows that the color of environmental
variations dramatically enhances the extinction risk. Our results will
help ecologists to correctly assess the viability of isolated populations.
***
LS11243
The signature of QCD in baryon magnetic moments
Baryon magnetic moments are approximately described by the
celebrated Coleman-Glashow relations that assume an
exact flavor symmetry between u, d and s quarks.
The present study improves these relations by including
the SU(3)-breaking corrections given by the low-energy realization of
Quantum Chromodynamics (QCD) known as Chiral Perturbation Theory (ChPT).
Baryons are composite objects made of interacting quarks and gluons.
For this reason, their magnetic moments are different from those of
elementary fermions with the same charges and masses. Unfortunately,
a full calculation of such a simple and fundamental quantity using QCD,
the theory of the strong interaction, is not yet feasible due to its
non-perturbative nature. Nonetheless, the magnetic moments of the lightest
spin 1/2 baryons are related among themselves providing a clear manifestation
of the underlying approximate SU(3) flavor symmetry. Previous attempts to
describe the breaking of this symmetry using ChPT have encountered problems
pointing at a poor convergence of the perturbative results.
It is found that in order to obtain a good description of these
magnetic moments it is essential that the low energy
realization of the fundamental strong interaction theory fulfills the
fundamental principles of analyticity and relativistic covariance. The results
of this work provide a solution for a long standing puzzle and neatly reveal
the manifestation of QCD on a basic observable such as baryon magnetic moments.
***
LR11242E
Shear induced mesostructures in biaxial liquid crystals
In 2004 a new "biaxial phase" of thermotropic (or temperature
sensitive) biaxial liquid crystalline polymers (LCPs), was discovered
experimentally by two seperate groups.
Biaxial LCPs are the brick-shaped or the ellipsoidal molecules that
have 2 length scales: a longer one along the long molecular axis and
the shorter one along the shorter axis.
It is widely hypothesized that the properties of the liquid crystal
polymers, which is seen along the long molecular axis; can also be
duplicated along the shorter axis; with a smaller time-scale. Hence
the presence of these 2 length-scales phenomena in these "smart"
materials can significantly improve the efficiency of certain
industries using semiconductor devices.
This biaxial phase, however, is very elusive at the molecular level
and extremely difficult to visualize and predict experimentally; in
the sense that they require an extreme conditions of temperature,
pressure and high magnetic field to be predicted experimentally.
To our knowledge, for the first time, we predict and present the
various phases of biaxial LCPs in the presence of an external
homogeneous shear flow, using mathematical modeling and large-scale
computation.
We discuss about the sequence of the orientational phases observed in
the selected regions of material parameter space. The underlying
hydrodynamic theory is also briefly discussed.
Friday, November 14, 2008
November 14, 2008
LR10964
Exotic braids for cold atoms: simulating anyons in the lab
We present a method to simulate quantum computations by the creation,
transport, and fusion of anyonic particles in a planar system of cold
atoms in an optical lattice. Particles in our three-dimensional world
come in two fundamentally different types depending on their
statistical properties, determined by their behaviour under exchange,
or braiding, of a pair of particles: Fermions, building blocks of
matter, and bosons, carriers of interactions (or forces.) In planar
materials, however, quantum mechanics allows for anyons, particles
with exotic braiding and statistical properties, which in some cases
may be used to perform universal quantum computations. Accordingly,
some planar systems with anyons have been proposed as architectures
for a fault-tolerant quantum computer, where operations are carried
out by creating, moving, and fusing these particles. In this paper we
show how these operations can be performed on a suitable planar system
simulated in a gas of cold atoms trapped in an optical lattice, using
atoms of a different species as "moving heads" to control and drive
the computations. Our method requires putting together already
demonstrated experimental techniques, which makes a simulation of
anyonic properties feasible in the lab.
***
LX10901
MAGNETIC TURBULENT WAVES
Wave turbulence describes the statistical behavior of a set of
randomly interacting waves and therefore has been applied to a great
variety of systems (ocean surface waves, plasma waves in solar wind,
spin waves in solids…), but few laboratory experiments have been
performed so far. Now, in a paper appearing in Physical Review
Letters, François Boyer and Eric Falcon of the University Paris
Diderot, France, report the first observation of magnetic wave
turbulence on the surface of a ferrofluid submitted to a magnetic
field, a regime that has not yet been envisaged in theoretical studies.
When wave amplitudes are high enough, the wave turbulence theory
predicts a nonlinear resonant process between waves which generates
smaller wavelengths. In a ferrofluid (a fluid with a suspension of
nanometric magnetic particles), the dispersion relation of surface
waves was known to be tuned by a magnetic field. This leads the
authors to the first observation of a magnetic wave turbulence
regime. The existence domains of gravity and capillary wave
turbulence are also documented as well as a triple point of
coexistence of these three regimes: these new results are understood
using dimensional analysis. Such an experimental system where the
dispersion relation is tuned by the operator from a non dispersive to
a dispersive system is thus of primary interest to test the wave
turbulence theory.
***
BUR1096B
Surface Superconductivity and Paramagnetic Meissner Effect in a
conventional type-II Superconductor
New results pertaining to the simultaneous observation of surface
superconductivity and paramagnetic Meissner effect in a high purity single
crystal of elemental Nb have revealed the sequence of steps leading to an
eventual stabilization of ordered flux line lattice in a type-II
superconductor. Superconductors are materials with zero electrical
resistance and are classified into two categories (type-I and type-II) on
the basis of their magnetic characteristics. Using a mean field
description, Abrikosov had predicted the nucleation of quantized flux
lines below the upper critical field (Hc2) in type-II superconductors. The
temperature dependence of the shielding response of a small superimposed
ac field has elucidated that superconductivity nucleates first near the
surface of a Nb sphere at a threshold field value (Hc3), much larger than
Hc2. As the super currents flow, the magnetic flux gets compressed within
the sphere, resulting in a positive dc magnetization signal (often termed
as Paramagnetic Meissner Effect) for field values lying in between Hc3 and
Hc2. Diamagnetic signal appears when the quantized flux lines get formed
below Hc2. Initially this vortex array is disordered (amorphous), however,
the spatial order among these vortices emerges (Abrikosov vortex lattice)
below the onset temperature of peak effect phenomenon in the critical
current density of weakly (collective) pinned array of vortices.
Accordingly we can define a new (H,T) phase diagram in a realistic
specimen of a high purity type-II superconductor, as given in the figure.
***
EV10436
Making (One-way) Waves
We have constructed a unique mechanical arrangement of one-way
coupled oscillators that enables the endless propagation of solitary
waves, but only in one direction, and only if there is an odd number
of elements. Typical oscillators, such as the coiled springs in a
mattress, are coupled so that vibrating one vibrates the others, and
vice versa, a consequence of Newton's famous third law of action and
reaction. However, in our array, disturbances propagate in only one
direction. Each oscillator is an inverted pendulum restrained by
springs, whose left-right oscillation directs jets of falling water
onto the next oscillator. An even number of such oscillators reaches
a quiescent equilibrium where the pendulums alternate left-right in
pairs. However, for odd arrays, there is always one unpaired
oscillator that frustrates this quiescent equilibrium and propagates
indefinitely as a single left-right pendulum oscillation. Our first
prototype array was circular and made from K’NEX construction
pieces, while our final apparatus is linear with interconnected ends
and constructed from machined aluminum, PVC pipe, rubber hoses, and
other inexpensive materials. Our array realizes an extreme case of
wave propagation in anisotropic media and is a dramatic mechanical
example of one-way coupling.
***
LM11138E
A New View of Cavitation
Cavitation is a fascinating phenomenon in which a liquid forms bubbles
of vapour and gas as a result of a variation of pressure. The bubbles
themselves usually attract most attention: their collapse leads to huge
temperatures and pressures, making them suitable for use as tiny
chemical reactors. We have examined cavitation from a different point of
view: not as an evolution of the bubbles, but as an evolution of the
molecules of gas that actually comprise the bubbles.
Imagine that as soon as a gas molecule enters a cavitating bubble, we
colour it black. How long is it before all gas molecules in the vessel
are blackened? For how long do the gas molecules reside in cavitating
bubbles? To address such questions, we have measured, for the first
time, the dynamics of both the dissolved gas and the liquid in a fluid,
in which cavitation is caused by a strong acoustic field. We dissolved
an NMR-sensitive gas in water and used Magnetic Resonance Imaging to
trace its motion and, separately, the motion of the water. The motion of
the water and the dissolved gas can be very different, because the gas
jumps from its dissolved state into and out of the cavitating bubbles
and travels with them. The residence time for gas molecules inside the
bubble can be very short, on the order of two oscillation cycles.
Depending on the cavitation intensity, all molecules of dissolved gas in
the vessel can go through the "bubbly" state within a second.
***
YU10004
Link Maps – Helping first year physics students stay on course
A new type of concept map helps first year students without a physics
background achieve a whole letter grade better than their peers! Concept
maps are a well known study aid, but they have not been used extensively
in physics – until now. At the University of Sydney a new type of concept
map, dubbed Link Maps, focus on the central concepts covered in the first
year physics course, and the equations which show how they are related. In
the pilot study, first year students without background in physics were
invited to attend one extra tutorial per week, called Map Meetings. In
these, Link Maps were the central feature around which the relevant weekly
topic was covered and problems were solved. The Map Meetings were very
popular; weekly attracting around 20% of the population comprising 351
students. However, in addition to their popularity, students who
consistently attended Map Meetings achieved 9 marks out of 90 (translating
to a whole letter grade) better in the final exam than students with
comparable academic background who were unable to attend the Map Meetings.
***
LN11136
Evidence for magnetic proximity effect up to room
temperature in Fe(Ga,Mn)As interfaces
The control of the mutual interaction between magnetically
''active'' layers at the interface of heterostructures is
a major challenge in solid state physics and one of the
key points of spintronics research. Diluted magnetic
semiconductors (DMS), in which magnetic impurities are
artificially imbedded into a semiconductor host crystal
lattice may allow to integrate the spin degree of freedom
and semiconducting properties in a single material.
Although the correlation between magnetic and transport
properties in DMS is likely to be a crucial ingredient in
possible applications, the physical mechanisms underlying
the magnetic properties are still a matter of an intense
debate. We report experimental evidence of magnetic
coupling at the interface between a thin ferromagnetic Fe
film and a DMS (in the present case (GaMn)As). Our
results reveal the presence of a long range ferromagnetic
order, up to room temperature, in a (Ga,Mn)As region as
thick as 2 nm induced by the ferromagnetic order of the Fe
film. The Mn magnetization at the interface is aligned
antiparallel with respect to the magnetization direction
of Fe, and increases with the thickness of the Fe
overlayer
Exotic braids for cold atoms: simulating anyons in the lab
We present a method to simulate quantum computations by the creation,
transport, and fusion of anyonic particles in a planar system of cold
atoms in an optical lattice. Particles in our three-dimensional world
come in two fundamentally different types depending on their
statistical properties, determined by their behaviour under exchange,
or braiding, of a pair of particles: Fermions, building blocks of
matter, and bosons, carriers of interactions (or forces.) In planar
materials, however, quantum mechanics allows for anyons, particles
with exotic braiding and statistical properties, which in some cases
may be used to perform universal quantum computations. Accordingly,
some planar systems with anyons have been proposed as architectures
for a fault-tolerant quantum computer, where operations are carried
out by creating, moving, and fusing these particles. In this paper we
show how these operations can be performed on a suitable planar system
simulated in a gas of cold atoms trapped in an optical lattice, using
atoms of a different species as "moving heads" to control and drive
the computations. Our method requires putting together already
demonstrated experimental techniques, which makes a simulation of
anyonic properties feasible in the lab.
***
LX10901
MAGNETIC TURBULENT WAVES
Wave turbulence describes the statistical behavior of a set of
randomly interacting waves and therefore has been applied to a great
variety of systems (ocean surface waves, plasma waves in solar wind,
spin waves in solids…), but few laboratory experiments have been
performed so far. Now, in a paper appearing in Physical Review
Letters, François Boyer and Eric Falcon of the University Paris
Diderot, France, report the first observation of magnetic wave
turbulence on the surface of a ferrofluid submitted to a magnetic
field, a regime that has not yet been envisaged in theoretical studies.
When wave amplitudes are high enough, the wave turbulence theory
predicts a nonlinear resonant process between waves which generates
smaller wavelengths. In a ferrofluid (a fluid with a suspension of
nanometric magnetic particles), the dispersion relation of surface
waves was known to be tuned by a magnetic field. This leads the
authors to the first observation of a magnetic wave turbulence
regime. The existence domains of gravity and capillary wave
turbulence are also documented as well as a triple point of
coexistence of these three regimes: these new results are understood
using dimensional analysis. Such an experimental system where the
dispersion relation is tuned by the operator from a non dispersive to
a dispersive system is thus of primary interest to test the wave
turbulence theory.
***
BUR1096B
Surface Superconductivity and Paramagnetic Meissner Effect in a
conventional type-II Superconductor
New results pertaining to the simultaneous observation of surface
superconductivity and paramagnetic Meissner effect in a high purity single
crystal of elemental Nb have revealed the sequence of steps leading to an
eventual stabilization of ordered flux line lattice in a type-II
superconductor. Superconductors are materials with zero electrical
resistance and are classified into two categories (type-I and type-II) on
the basis of their magnetic characteristics. Using a mean field
description, Abrikosov had predicted the nucleation of quantized flux
lines below the upper critical field (Hc2) in type-II superconductors. The
temperature dependence of the shielding response of a small superimposed
ac field has elucidated that superconductivity nucleates first near the
surface of a Nb sphere at a threshold field value (Hc3), much larger than
Hc2. As the super currents flow, the magnetic flux gets compressed within
the sphere, resulting in a positive dc magnetization signal (often termed
as Paramagnetic Meissner Effect) for field values lying in between Hc3 and
Hc2. Diamagnetic signal appears when the quantized flux lines get formed
below Hc2. Initially this vortex array is disordered (amorphous), however,
the spatial order among these vortices emerges (Abrikosov vortex lattice)
below the onset temperature of peak effect phenomenon in the critical
current density of weakly (collective) pinned array of vortices.
Accordingly we can define a new (H,T) phase diagram in a realistic
specimen of a high purity type-II superconductor, as given in the figure.
***
EV10436
Making (One-way) Waves
We have constructed a unique mechanical arrangement of one-way
coupled oscillators that enables the endless propagation of solitary
waves, but only in one direction, and only if there is an odd number
of elements. Typical oscillators, such as the coiled springs in a
mattress, are coupled so that vibrating one vibrates the others, and
vice versa, a consequence of Newton's famous third law of action and
reaction. However, in our array, disturbances propagate in only one
direction. Each oscillator is an inverted pendulum restrained by
springs, whose left-right oscillation directs jets of falling water
onto the next oscillator. An even number of such oscillators reaches
a quiescent equilibrium where the pendulums alternate left-right in
pairs. However, for odd arrays, there is always one unpaired
oscillator that frustrates this quiescent equilibrium and propagates
indefinitely as a single left-right pendulum oscillation. Our first
prototype array was circular and made from K’NEX construction
pieces, while our final apparatus is linear with interconnected ends
and constructed from machined aluminum, PVC pipe, rubber hoses, and
other inexpensive materials. Our array realizes an extreme case of
wave propagation in anisotropic media and is a dramatic mechanical
example of one-way coupling.
***
LM11138E
A New View of Cavitation
Cavitation is a fascinating phenomenon in which a liquid forms bubbles
of vapour and gas as a result of a variation of pressure. The bubbles
themselves usually attract most attention: their collapse leads to huge
temperatures and pressures, making them suitable for use as tiny
chemical reactors. We have examined cavitation from a different point of
view: not as an evolution of the bubbles, but as an evolution of the
molecules of gas that actually comprise the bubbles.
Imagine that as soon as a gas molecule enters a cavitating bubble, we
colour it black. How long is it before all gas molecules in the vessel
are blackened? For how long do the gas molecules reside in cavitating
bubbles? To address such questions, we have measured, for the first
time, the dynamics of both the dissolved gas and the liquid in a fluid,
in which cavitation is caused by a strong acoustic field. We dissolved
an NMR-sensitive gas in water and used Magnetic Resonance Imaging to
trace its motion and, separately, the motion of the water. The motion of
the water and the dissolved gas can be very different, because the gas
jumps from its dissolved state into and out of the cavitating bubbles
and travels with them. The residence time for gas molecules inside the
bubble can be very short, on the order of two oscillation cycles.
Depending on the cavitation intensity, all molecules of dissolved gas in
the vessel can go through the "bubbly" state within a second.
***
YU10004
Link Maps – Helping first year physics students stay on course
A new type of concept map helps first year students without a physics
background achieve a whole letter grade better than their peers! Concept
maps are a well known study aid, but they have not been used extensively
in physics – until now. At the University of Sydney a new type of concept
map, dubbed Link Maps, focus on the central concepts covered in the first
year physics course, and the equations which show how they are related. In
the pilot study, first year students without background in physics were
invited to attend one extra tutorial per week, called Map Meetings. In
these, Link Maps were the central feature around which the relevant weekly
topic was covered and problems were solved. The Map Meetings were very
popular; weekly attracting around 20% of the population comprising 351
students. However, in addition to their popularity, students who
consistently attended Map Meetings achieved 9 marks out of 90 (translating
to a whole letter grade) better in the final exam than students with
comparable academic background who were unable to attend the Map Meetings.
***
LN11136
Evidence for magnetic proximity effect up to room
temperature in Fe(Ga,Mn)As interfaces
The control of the mutual interaction between magnetically
''active'' layers at the interface of heterostructures is
a major challenge in solid state physics and one of the
key points of spintronics research. Diluted magnetic
semiconductors (DMS), in which magnetic impurities are
artificially imbedded into a semiconductor host crystal
lattice may allow to integrate the spin degree of freedom
and semiconducting properties in a single material.
Although the correlation between magnetic and transport
properties in DMS is likely to be a crucial ingredient in
possible applications, the physical mechanisms underlying
the magnetic properties are still a matter of an intense
debate. We report experimental evidence of magnetic
coupling at the interface between a thin ferromagnetic Fe
film and a DMS (in the present case (GaMn)As). Our
results reveal the presence of a long range ferromagnetic
order, up to room temperature, in a (Ga,Mn)As region as
thick as 2 nm induced by the ferromagnetic order of the Fe
film. The Mn magnetization at the interface is aligned
antiparallel with respect to the magnetization direction
of Fe, and increases with the thickness of the Fe
overlayer
Wednesday, November 12, 2008
November 12, 2008
LT11818
Putting an end to turbulence
When reaching sufficiently large velocities all fluid flows become turbulent, whether it
is water travelling through a garden hose or blood being pumped through veins. Scientists
from the Max-Planck Institute for Dynamics and Self-Organization in Germany and the Delft
University of Technology in the Netherlands, however, have now shown that in pipe flows
turbulence is not stable. After a finite lifetime it disappears and the flow turns laminar -
even if these lifetimes may be extremely long. The researchers studied water flowing through
narrow pipes of up to 14 meters length and monitored the breakdown of turbulence with an
accuracy never before achieved in experiment or numerical simulation. Their results therefore
allow new insights into the elusive nature of turbulence and confirm a 20 year old conjecture
speculating that turbulence may fall into the category of so-called super transient states.
Since turbulent flows consume much more energy than laminar ones, these findings may be of
great importance for many applications like oil pipelines or gas and water supplies. It may
be possible to speed up the extremely slow decay and deliberately change the flow from turbulent
to laminar.
***
LT11941
First observation of doubly magic tin-100 isotope in North
America sheds light on heavy element synthesis in cosmos
Summary: Researchers at Michigan State University's National
Superconducting Cyclotron Laboratory have measured the half-lives of
tin-100 and cadmium-96, two nuclei near the proton-rich limit of
stability with equal numbers of protons and neutrons. The tin-100
observation, the first ever creation of this nucleus in a North
American laboratory, narrows the error range of previous half-life
measurements of this doubly magic isotope, so called because both its
neutrons and protons form a closed configuration which gives extra
stability to the nucleus.
The cadmium-96 measurement, the first of its kind in the world,
undercuts predictions about the role of the isotope as a waiting point
in the rp-process – a key part of heavy element synthesis in the
cosmos. And the finding implies that a new, as-yet-unknown mechanism
is responsible for ruthenium-96, the abundance of which in the solar
system has long been unexplained in nuclear astrophysics.
***
AW10339
Quantum interferometry for absolute spectroscopy
Optical spectra are often regarded as fingerprints of atoms, clusters
and molecules.
We here propose a new scheme to measure the absolute optical absorption
cross section of such nanoparticles. This scheme is based on
near-field matter wave experiments which have already convincingly
demonstrated the quantum wave nature of large
molecules by generating a periodic molecular density pattern, the
interferogramm.
In the present work we exploit the fact that the recoil caused by a
single (!)
photon already suffices to change this pattern significantly.
This is predicted to permit precise and absolute measurements of the
optical absorption cross section and also of fluorescence yields for
isolated
particles in molecular beams.
Compared to other techniques the new method does not require
any knowledge of the particle beam density, nor does it invoke
photo-induced structural changes of the irradiated particles.
Its sensitivity can be high even for very dilute molecular beams.
***
LU11591
How well do electron microscopes resolve the nanoworld?
With this paper we provide for the first time an accurate answer to this fundamental
question. While several multi-million dollar projects worldwide aim for a further
resolution improvement of transmission electron microscopes towards the deep sub-Angstrom
range (see e.g. http://ncem.lbl.gov/TEAM-project), the quantitative assessment of the
actually achieved optical resolution is still a challenge. In this paper we demonstrate
that the commonly accepted traditional resolution assessment procedure, which dates back
to the mid 1970s, is not adequate for a quantitative resolution assessment on the atomic
scale. Motivated by this finding, we succeeded to develop a new measurement principle,
which allows one for the first time to determine the resolution of a transmission electron
microscope on a reliable and quantitative basis. The comparison between the traditional
method and our new method reveals in some cases drastic discrepancies, which are due to
the failure of the commonly used traditional method. Therefore, the new method and its
outcome are highly relevant for the microscope manufacturers as well as for the scientific
user community along their path to obtain an even closer look into the nanoworld.
***
LS11387
JAMMING CAN OCCUR EVEN IN THIN GRANULAR LAYERS
Granular materials play a key role in a broad spectrum of problems
ranging from industrial handling to shear in tectonic fault zones.
Granular jamming has received considerable attention as a possible
mechanism for frictional stick-slip such as occurs in earthquake
faulting.
We study the jamming transition for layers of spherical beads ranging
in thickness from 1 to 3 times the grain diameter d. Our work
represents the first systematic study of thin granular layers and
reveals a previously unobserved property of granular matter: the
jamming transition in thin layers is discontinuous and, in contrast to
bulk granular matter, associated with a decrease in packing fraction.
As layer thickness increases slightly above d, friction jumps
discontinuously from 0.02 to > 0.1. We interpret the jump as the
transition from rolling to jamming. Effective granular pressure P
increases as a power law of the excess layer thickness above a
critical value for jamming. Layers below the critical thickness thin
to a monolayer and weaken. For thin layers, friction and P increase
as packing fraction decreases near the jamming transition.
Putting an end to turbulence
When reaching sufficiently large velocities all fluid flows become turbulent, whether it
is water travelling through a garden hose or blood being pumped through veins. Scientists
from the Max-Planck Institute for Dynamics and Self-Organization in Germany and the Delft
University of Technology in the Netherlands, however, have now shown that in pipe flows
turbulence is not stable. After a finite lifetime it disappears and the flow turns laminar -
even if these lifetimes may be extremely long. The researchers studied water flowing through
narrow pipes of up to 14 meters length and monitored the breakdown of turbulence with an
accuracy never before achieved in experiment or numerical simulation. Their results therefore
allow new insights into the elusive nature of turbulence and confirm a 20 year old conjecture
speculating that turbulence may fall into the category of so-called super transient states.
Since turbulent flows consume much more energy than laminar ones, these findings may be of
great importance for many applications like oil pipelines or gas and water supplies. It may
be possible to speed up the extremely slow decay and deliberately change the flow from turbulent
to laminar.
***
LT11941
First observation of doubly magic tin-100 isotope in North
America sheds light on heavy element synthesis in cosmos
Summary: Researchers at Michigan State University's National
Superconducting Cyclotron Laboratory have measured the half-lives of
tin-100 and cadmium-96, two nuclei near the proton-rich limit of
stability with equal numbers of protons and neutrons. The tin-100
observation, the first ever creation of this nucleus in a North
American laboratory, narrows the error range of previous half-life
measurements of this doubly magic isotope, so called because both its
neutrons and protons form a closed configuration which gives extra
stability to the nucleus.
The cadmium-96 measurement, the first of its kind in the world,
undercuts predictions about the role of the isotope as a waiting point
in the rp-process – a key part of heavy element synthesis in the
cosmos. And the finding implies that a new, as-yet-unknown mechanism
is responsible for ruthenium-96, the abundance of which in the solar
system has long been unexplained in nuclear astrophysics.
***
AW10339
Quantum interferometry for absolute spectroscopy
Optical spectra are often regarded as fingerprints of atoms, clusters
and molecules.
We here propose a new scheme to measure the absolute optical absorption
cross section of such nanoparticles. This scheme is based on
near-field matter wave experiments which have already convincingly
demonstrated the quantum wave nature of large
molecules by generating a periodic molecular density pattern, the
interferogramm.
In the present work we exploit the fact that the recoil caused by a
single (!)
photon already suffices to change this pattern significantly.
This is predicted to permit precise and absolute measurements of the
optical absorption cross section and also of fluorescence yields for
isolated
particles in molecular beams.
Compared to other techniques the new method does not require
any knowledge of the particle beam density, nor does it invoke
photo-induced structural changes of the irradiated particles.
Its sensitivity can be high even for very dilute molecular beams.
***
LU11591
How well do electron microscopes resolve the nanoworld?
With this paper we provide for the first time an accurate answer to this fundamental
question. While several multi-million dollar projects worldwide aim for a further
resolution improvement of transmission electron microscopes towards the deep sub-Angstrom
range (see e.g. http://ncem.lbl.gov/TEAM-project), the quantitative assessment of the
actually achieved optical resolution is still a challenge. In this paper we demonstrate
that the commonly accepted traditional resolution assessment procedure, which dates back
to the mid 1970s, is not adequate for a quantitative resolution assessment on the atomic
scale. Motivated by this finding, we succeeded to develop a new measurement principle,
which allows one for the first time to determine the resolution of a transmission electron
microscope on a reliable and quantitative basis. The comparison between the traditional
method and our new method reveals in some cases drastic discrepancies, which are due to
the failure of the commonly used traditional method. Therefore, the new method and its
outcome are highly relevant for the microscope manufacturers as well as for the scientific
user community along their path to obtain an even closer look into the nanoworld.
***
LS11387
JAMMING CAN OCCUR EVEN IN THIN GRANULAR LAYERS
Granular materials play a key role in a broad spectrum of problems
ranging from industrial handling to shear in tectonic fault zones.
Granular jamming has received considerable attention as a possible
mechanism for frictional stick-slip such as occurs in earthquake
faulting.
We study the jamming transition for layers of spherical beads ranging
in thickness from 1 to 3 times the grain diameter d. Our work
represents the first systematic study of thin granular layers and
reveals a previously unobserved property of granular matter: the
jamming transition in thin layers is discontinuous and, in contrast to
bulk granular matter, associated with a decrease in packing fraction.
As layer thickness increases slightly above d, friction jumps
discontinuously from 0.02 to > 0.1. We interpret the jump as the
transition from rolling to jamming. Effective granular pressure P
increases as a power law of the excess layer thickness above a
critical value for jamming. Layers below the critical thickness thin
to a monolayer and weaken. For thin layers, friction and P increase
as packing fraction decreases near the jamming transition.
Tuesday, November 11, 2008
November 11, 2008
LU11087B
Magnetism induces a shape change
Magnetic field affects shapes of magnetic crystals, yet commonly these
changes amount to a few parts per million (ppm). Only a handful of
materials exhibit ?magnetostriction? reaching thousands of ppm.
Overwhelmingly, such crystals have anisotropic charge densities from
atomic orbitals coupled with magnetic moments and as the latter align
with the field, they pull the associated charge, thereby altering
external dimensions of the crystal. This has led to devices such as
field and stress sensors, and transducers. This paper shows that the
applied field does indeed line up the Gd magnetic moments in a
compound with Ni, but the giant anisotropic linear magnetostriction
with negligible volume change is unexpected because the charge density
of Gd is spherical (isotropic). Magnetostriction arises because the
electrons that are responsible for electrical conductivity interact
with the moment-carrying electrons of Gd and gain an additional
magnetic moment as the field is increased. This also affects the
chemical bonding. The same happens on cooling, when the Gd moments
order spontaneously. Calculations and experiments show a smooth
transition between the magnetically ordered and non-magnetic states.
The smooth magnetostriction may have applications in advanced sensors
and energy conversion devices, but currently cooling by liquid
nitrogen is required.
***
LP11420BR
Entangled states in graphene- detection and use
Graphene has captured the interest of the physics community
because of its many versatile applications to microelectronics
and its connection to relativistic quantum field theory.
Here, we create and detect entangled states in graphene sheets where
superconductivity is induced via the proximity effect. The
idea involves extracting the Cooper pair, the most entangled
state found in matter, into different sheets such that the
electrons involved are not paired but remember their original correlations.
In normal metals coupled to a superconductor it was found that such processes
detected via the current-current correlations across the two
normal metals could be positive. This is in contrast to
the expectation that since there are only fermionic excitations
in normal metals these correlations should be negative. The
possibility of obtaining positive value is a good indicator of
spin singlet entanglement. Unfortunately, experiments in such a normal metal
Y-junction coupled to a superconductor do not give the desired
results. One reason being the difficulty in manipulating the
Fermi energy of metals with the application of a gate voltage.
This difficulty does not arise in graphene, where its Fermi
energy can be very easily tuned. These entangled
states could be used to teleport and swap the entanglement between pairs of electrons. In theory, it is possible to
teleport the information encoded in a quantum object to
another place arbitrarily far via entangled states. In
practice, though, only information about a photon has been
teleported. It is advantageous to perform quantum
teleportation via electrons. The aim being to speed up
processing since teleportation would obviate any need for the
physical transport of electrons.
***
LU11548
Universe created by extra dimensional bubble collision.
In this paper, I suggest the possibility that the entire visible
universe may have been produced from a single bubble that expanded and
collided with *itself* after winding round a compact extra dimension.
The extra dimension is "compact" because, like the surface of the
earth, if you go far enough in one direction you end up where you
started. More to the point, the bubble goes "far enough" and ends up
colliding with itself, and that collision produces all the matter and
radiation and everything we see around us.
The extra dimension would have to be really small for us not to have
noticed it yet. The Large Hadron Collider, inter alia, is searching
for small extra dimensions.
***
LU11958ER
Dynamics of molecular clocks reveal hidden structure of cell populations
Telomeres are DNA stretches at the ends of chromosomes that shorten with each cell division, essentially acting as molecular clocks, which eventually signal cells to stop dividing. Although telomeres of cells grown in a plate shorten linearly with each cell division, the average telomere length of white blood cells in our body decreases non-linearly - a rapid decrease in the first years of life is followed by a successively slower decrease. In this paper we show that this dynamics can be explained if cell populations in our body contain a small pool of 'repopulating' cells with long telomeres, which provide an influx into a larger pool of derived cells, telomeres of which shorten linearly. The model gives rise to an exponential decrease in average telomere length providing an excellent fit to available data, and allows estimating the biologically relevant parameters such as the repopulation rate from the telomere dynamics. The model also accounts for the previously unexplained phenomena of telomeres elongation after cells are exposed to stress, e.g. in AIDS patients and after bone marrow transplantation. Such elongation can be explained by an increase in the influx of repopulating cells to compensate for the loss of derived cells. In summary the model provides a tool for inferring the hidden structure of cell populations from the dynamics of molecular clocks.
***
ET10480
Re-entrant phase transition found with new mesoscopic model
Small solid particles - colloids - that attract each other at short
distances are known to form either a crystalline structure or a
disordered (liquid or vapor) phase. Since about a decade we know that if
the range of attraction is less than 1/6 of the particle diameter, such
a colloidal crystal does not melt on heating - as for longer force range
- but it sublimes into its gas phase. We have now found from computer
simulation that if the range of attraction is decreased further and
further, something quite unexpected happens. For a force range shorter
than 1/1000th of the particle diameter a stable liquid phase reappears
again. On heating such a crystal, it first melts and then vaporizes.
Why? For such a small attraction range the particles have no room to
move in a crystalline structure, whereas some freedom is left in the
liquid phase. The same mesoscopic model (so called because the objects
in the simulation are small, but much larger than atoms) can be applied
to powders and granular solids. Then it correctly predicts the fracture
behavior seen in experiments.
Magnetism induces a shape change
Magnetic field affects shapes of magnetic crystals, yet commonly these
changes amount to a few parts per million (ppm). Only a handful of
materials exhibit ?magnetostriction? reaching thousands of ppm.
Overwhelmingly, such crystals have anisotropic charge densities from
atomic orbitals coupled with magnetic moments and as the latter align
with the field, they pull the associated charge, thereby altering
external dimensions of the crystal. This has led to devices such as
field and stress sensors, and transducers. This paper shows that the
applied field does indeed line up the Gd magnetic moments in a
compound with Ni, but the giant anisotropic linear magnetostriction
with negligible volume change is unexpected because the charge density
of Gd is spherical (isotropic). Magnetostriction arises because the
electrons that are responsible for electrical conductivity interact
with the moment-carrying electrons of Gd and gain an additional
magnetic moment as the field is increased. This also affects the
chemical bonding. The same happens on cooling, when the Gd moments
order spontaneously. Calculations and experiments show a smooth
transition between the magnetically ordered and non-magnetic states.
The smooth magnetostriction may have applications in advanced sensors
and energy conversion devices, but currently cooling by liquid
nitrogen is required.
***
LP11420BR
Entangled states in graphene- detection and use
Graphene has captured the interest of the physics community
because of its many versatile applications to microelectronics
and its connection to relativistic quantum field theory.
Here, we create and detect entangled states in graphene sheets where
superconductivity is induced via the proximity effect. The
idea involves extracting the Cooper pair, the most entangled
state found in matter, into different sheets such that the
electrons involved are not paired but remember their original correlations.
In normal metals coupled to a superconductor it was found that such processes
detected via the current-current correlations across the two
normal metals could be positive. This is in contrast to
the expectation that since there are only fermionic excitations
in normal metals these correlations should be negative. The
possibility of obtaining positive value is a good indicator of
spin singlet entanglement. Unfortunately, experiments in such a normal metal
Y-junction coupled to a superconductor do not give the desired
results. One reason being the difficulty in manipulating the
Fermi energy of metals with the application of a gate voltage.
This difficulty does not arise in graphene, where its Fermi
energy can be very easily tuned. These entangled
states could be used to teleport and swap the entanglement between pairs of electrons. In theory, it is possible to
teleport the information encoded in a quantum object to
another place arbitrarily far via entangled states. In
practice, though, only information about a photon has been
teleported. It is advantageous to perform quantum
teleportation via electrons. The aim being to speed up
processing since teleportation would obviate any need for the
physical transport of electrons.
***
LU11548
Universe created by extra dimensional bubble collision.
In this paper, I suggest the possibility that the entire visible
universe may have been produced from a single bubble that expanded and
collided with *itself* after winding round a compact extra dimension.
The extra dimension is "compact" because, like the surface of the
earth, if you go far enough in one direction you end up where you
started. More to the point, the bubble goes "far enough" and ends up
colliding with itself, and that collision produces all the matter and
radiation and everything we see around us.
The extra dimension would have to be really small for us not to have
noticed it yet. The Large Hadron Collider, inter alia, is searching
for small extra dimensions.
***
LU11958ER
Dynamics of molecular clocks reveal hidden structure of cell populations
Telomeres are DNA stretches at the ends of chromosomes that shorten with each cell division, essentially acting as molecular clocks, which eventually signal cells to stop dividing. Although telomeres of cells grown in a plate shorten linearly with each cell division, the average telomere length of white blood cells in our body decreases non-linearly - a rapid decrease in the first years of life is followed by a successively slower decrease. In this paper we show that this dynamics can be explained if cell populations in our body contain a small pool of 'repopulating' cells with long telomeres, which provide an influx into a larger pool of derived cells, telomeres of which shorten linearly. The model gives rise to an exponential decrease in average telomere length providing an excellent fit to available data, and allows estimating the biologically relevant parameters such as the repopulation rate from the telomere dynamics. The model also accounts for the previously unexplained phenomena of telomeres elongation after cells are exposed to stress, e.g. in AIDS patients and after bone marrow transplantation. Such elongation can be explained by an increase in the influx of repopulating cells to compensate for the loss of derived cells. In summary the model provides a tool for inferring the hidden structure of cell populations from the dynamics of molecular clocks.
***
ET10480
Re-entrant phase transition found with new mesoscopic model
Small solid particles - colloids - that attract each other at short
distances are known to form either a crystalline structure or a
disordered (liquid or vapor) phase. Since about a decade we know that if
the range of attraction is less than 1/6 of the particle diameter, such
a colloidal crystal does not melt on heating - as for longer force range
- but it sublimes into its gas phase. We have now found from computer
simulation that if the range of attraction is decreased further and
further, something quite unexpected happens. For a force range shorter
than 1/1000th of the particle diameter a stable liquid phase reappears
again. On heating such a crystal, it first melts and then vaporizes.
Why? For such a small attraction range the particles have no room to
move in a crystalline structure, whereas some freedom is left in the
liquid phase. The same mesoscopic model (so called because the objects
in the simulation are small, but much larger than atoms) can be applied
to powders and granular solids. Then it correctly predicts the fracture
behavior seen in experiments.
Thursday, November 6, 2008
November 6, 2008
LR11814
A First for Spintronics: Measuring the Strength of a Spin-Polarized Current
Traditional electronic devices depend on manipulating electrical charge, the
negative charge of electrons or the positive charge of 'holes,' but future
information-technology devices will depend not on charge electronics but on
spin electronics, or 'spintronics,' which manipulates the orientation of the
(quantum-mechanical) spin of the electrons. One way to achieve this is to
inject a spin-polarized current into a magnetic element and use the torque
the current exerts on the magnetic moment to reverse its direction. For the
first time, Kasai and colleagues have determined the strength of such spin
currents directly and unambiguously, using time resolved magnetic soft x-ray
microscopy to image the effects of spin-polarized electron currents on
magnetic vortex cores confined to a disk of ferromagnetic Permalloy. Working
at the U.S. Department of Energy's Advanced Light Source, the researchers
used specialized x-ray optics (Fresnel zone-plate lenses) to achieve a
spatial resolution of less than 25 nanometers, with a temporal resolution of
70 picoseconds, as determined by the duration of the x-ray flashes. This
novel technique offers a window to understanding fundamental magnetic
processes, with tremendous potential applications in such areas of
nanoscience as the development of novel magnetic materials,
ultrahigh-density magnetic sensors, and new storage technologies.
***
LT11222
A FEW ELECTRONS CONTROL THE HEAT
Much current research with an eye toward technologies involves
materials with physical properties that can be manipulated with
weak electric or magnetic fields. Recently, researchers led by
Prof. J. Cohn (University of Miami), found that weak electric
fields have a surprisingly large effect on how well heat is
conducted in the magnetic oxide compound CaMnO(3) (calcium
manganite).
In magnetic solids -- those composed of ions which have a magnetic
"moment" -- the separation between atoms and the mobility of
electrons can be sensitive to the arrangement of the moments and
vice versa. This is particularly evident at the magnetic
transition or ordering temperature, below which neighboring
moments become aligned (a ferromagnet) or anti-aligned (an
antiferromagnet). Well above the antiferromagnet transition
temperature of CaMnO(3) (125 degrees Kelvin), fluctuating
nanoscale magnetically ordered regions form as a precursor to the
transition. This short-ranged order causes distortions in the
structure that impede the flow of heat. Turning on a modest
electric field mobilizes a small density of loosely bound
electrons and improves the heat conduction by a factor of two.
Because the electrons themselves are too few to account for the
improved heat conduction, the researchers suggest the novel
possibility that the liberated electrons suppress the magnetic
fluctuations.
***
LQ11199E
Shake it stronger and it will get denser.
Until now, experimental evidences showed that the stronger you tap a
collection of beads poured into a box, the fluffier it gets. This new study
shows that, if you go beyond a certain tapping intensity, the material starts
compacting back again. It is known that tapping a granular sample promotes
compaction. However, if the intensity of the taps is very low, the system
compacts better, although one needs to tap for longer. This remarkable fact
is supported by a number of experiments. However, none of these previous
studies probed really strong taps. Testing a number of computational models,
this paper shows that, eventually, high intensity taps will induce denser
arrangements. Therefore, there is an optimum tapping intensity at which the
looser state can be obtained. The paper propose an explanation based on the
formation and breakdown of arches. The results suggest that in a sample
confined to two dimensions the effect is much more evident. Preliminary
experimental research seems to confirm the prediction.
***
LV11004
A reversible breaking of inter-molecular chemical bonds with decreasing temperature and the coexistence of multiple molecular charge states are observed in a novel class of molecular solids known as fullerides. These materials, discovered only fifteen years ago, are composed of hollow spherically-shaped molecules (C60) with a diameter of one nanometer and made entirely of carbon which exhibit features that are rare in solids, such as the possibility of almost free rotation, distortion into elongated shapes, inter-molecular chemical binding, magnetism without transition elements, and charge conduction and superconductivity whereas an insulating electrical behavior is expected. We report on yet more surprises in our study: the strong (covalent) chemical bond linking two C60 molecules is observed to break as the temperature is lowered; while this cannot occur for an isolated pair of molecules, it is allowed for a system of many molecules such as a solid. The breaking/forming of the bond is perfectly reversible. When the bonds break a metallic phase results in which we could distinguish molecules carrying distinct electrical charges. This is quite unusual for such materials and indicates the occurrence of local charge fluctuations, which may provide a clue to solving the puzzle of electrical conductivity in fullerides.
***
LW10985
Dressed matter waves - new frontiers for quantum state engineering
When an atom interacts with an electromagnetic field, it becomes
"dressed" by the photons and changes its properties. Similarly, one
can "dress" a quantum mechanical matter wave, consisting of a large
number of ultracold atoms stored in an optical lattice, simply by
shaking that lattice with kilohertz frequencies. This means that
the matter wave responds to the shaking not by being disturbed in an
uncontrollable manner, as one might have expected on naive grounds,
but rather by acquiring new properties which the "bare" (i.e. unshaken)
system did not have. A striking demonstration of this concept had
recently been given by an experimental team from Pisa, Italy: These
authors succceeded to turn a superfluid matter wave into a Mott insulator
by varying the shaking strength [arXiv:0809.0768]. In their paper entitled
"Avoided level crossing spectroscopy with dressed matter waves", which
has just been accepted for publication in the Physical Review Letters,
two theoreticians from the Instituto de Ciencias Fotonicas (Spain) and
Oldenburg University (Germany) now have shown that the analogy between
"dressed atoms" and "dressed mesoscopic matter waves" has further high
potential for systematic quantum state engineering with ultracold atoms.
***
LW10842
Super-diffusive mass transport of Pb on Si(111) at low temperatures
An exceptionally fast and unusual mass transport behavior has been
discovered in the dense Pb wetting layer on the Si(111) surface at
temperatures as low as 150 K. The experiments were carried out using a
single laser pulse to desorb Pb from the wetting layer by heating the
surface locally. Low energy electron microscopy is used to observe the
resulting non-equilibrium coverage profile relax to equilibrium uniform
distribution in real-time. According to the classical textbook
description of surface mass transport, the initial coverage profile is
expected to broaden as it equilibrates progressively slower with time.
Surprisingly, profile evolution in the Pb wetting layer lacks these
classical features and is dominated instead by a much more efficient
convection-like mass transport. This behavior is unprecedented at
crystal surfaces and more characteristic of fluid motion. The authors
demonstrate that the transition to this novel super-diffusive state
occurs above a critical wetting layer coverage. Super-diffusive mass
transport can account for the highly efficient self-organization of
uniform height Pb nanostructures that was reported on widely in the
past. Its discovery also signals the possible existence of similar
intriguing mass transport mechanisms at low temperatures in other
systems that are yet to be discovered.
***
LT11935
To follow or not to follow: Laser-controlled electron motion in molecule
defies Newton's law
We show that ultrashort and intense laser pulses can be used to direct
and control electrons inside molecules. For the specific example of the
most simple molecule, the hydrogen molecular ion, we find that the
electronic probability current either follows or opposes the external
laser electric force, depending on the laser electric field strength.
According to Newton's second law of classical mechanics, an object
responds to an external force by changing its momentum. This classical
concept often applies for quantum systems too, where the correspondence
between classical variables and expectation values of quantum mechanical
operators is guaranteed by the Ehrenfest'S theorem. This correspondence
is frequently taken advantage of in order to explain complex quantum
mechanical phenomena, such as the interaction of intense laser light
with matter. For example, the generation of high harmonics radiation and
attosecond pulses can be understood in terms of the classical motion
of an electron in the laser field and its forced recollison with the parent
ion. The situation changes, if the electron is located inside a
molecule, as our numerical simulations for a dissociating hydrogen
molecular ion in an ultrashort intense laser pulse show. Here, the
momentum distribution of the electron is not only shifted by the
external laser field, but also modulated by an interference effect due
to the two nuclei. Due to this modulation the net electronic probability
current inside the molecular ion, that corresponds to the classical motion
of the electron, can follow or oppose to the laser electric force,
depending on the laser intensity. As ultrashort laser pulses are
acting on the time scale of the electronic motion in atoms and
molecules, our results are relevant for the control of chemical
reaction dynamics with light.
***
LN11497
Tuning nucleation on top of Pb islands by electron confinement.
On the nanoscale novel phenomena are present as a manifestation of
Quantum Mechanics. For nanotechnology applications it is important
to prepare nanostructures of uniform dimensions which requires
understanding nucleation on the atomistic level. Surprisingly a
dramatic variation of the nucleated island density is found for the
same orientation and the same substrate (i.e. Pb islands of (111)
orientation) as a function of island height. Normally nucleation is
varied only by changing the surface orientation and/or the substrate.
The new discovery is a result of how free electrons within the Pb
islands are confined according to the laws of Quantum Mechanics.
The electrons form standing waves:how well the standing waves fit
the island affects how easy Pb atoms nucleate the second layer
islands. This coupling of how electrons are confined to adatom
diffusion is unexpected and intriguing. The island density is 60
times larger on islands where the standing waves do not fit perfectly
the island. These unusual results suggest that it is possible to
affect atomistic processes not by varying the surface orientation or
substrate, but by simply changing island geometry.
***
BW10457
High-Temperature Superconductivity: Learning from Failure
In attempting to understand a particular phenomenon, the obvious
choice would be to study particularly successful examples; however,
sometimes one can learn as much or more from nominal failures. In
studying high temperature superconductors, we have discovered
potentially important clues to the mechanism of superconductivity by
studying a particular layered copper-oxide compound with anomalously
depressed superconductivity. This particular material is known to
exhibit unusual patterns of charge and magnetism known as "stripes".
It has been a common belief that this stripe order competes with
superconductivity. Thus, it comes as a considerable surprise that our
collective measurements of electronic transport and magnetic
properties, on one hand, and charge and spin order, on the other,
indicate that a fragile sort of short-range superconductivity develops
simultaneously with magnetic stripe order at a relatively high
temperature. Theorists have proposed that this coexistence may involve
an interweaving of the superconductivity and magnetism, much like warp
and weft. In any case, our results provide support for the
theoretical idea that spatial modulations of charge and spin densities
can enhance the strength of electron pairing, a prerequisite for high-
temperature superconductivity.
***
LR11507
A Quantum Look at Surface Plasmons
Recent developments in nanoplasmonics have led to the rapid development of
plasmonic circuitry for guiding and manipulation of light on subwavelength
scales as well as for engineering various devices and applications utilizing
the interaction of plasmonic modes with molecular excitations. The
development of plasmonic-based nanophotonic devices has also attracted a
keen interest from the quantum optics community for their use in quantum
information processing, due to a superior enhancement of nonlinear optical
effects and faster interaction times.
Most research in plasmonics is restricted to a completely classical
description of the underlying physics. This becomes inadequate once quantum
mechanical effects are prevalent in the investigation of plasmon-molecule
interactions or quantum plasmonic devices such as spasers (plasmon-based
lasers). Further development of molecular plasmonics and the next-generation
of photonic and plasmonic-based devices operating at a faithful
single-photon excitation level in nanoscale structures (desirable for the
purposes of integratable quantum information processing) will require a
genuine ab initio quantum approach.
In our article we provide key insights into the physics of photon-surface
plasmon coupling at the quantum level and the statistics of plasmonic
quantum states. We have established that excited surface plasmons can
completely preserve important quantum mechanical features of the original
photons as they travel along metal surfaces under realistic experimental
conditions. Efficient single-photon excitation in addition to few-photon
excitation of surface plasmons is considered. We also derive the
second-order quantum coherence function, which is an accurate measure of the
quantum nature of a surface plasmon.
Our results open up a new route toward the efficient manipulation of surface
plasmons at the quantum level in nanoplasmonic-based quantum information
processing experiments.
***
LS11197
Electric manipulation of molecular spins
Magnetism of single molecules has received much attention in
recent years. However, so far all the studies of molecular
magnetism have focused on the response of spins to magnetic
fields. Our results show that single-molecule magnets also
respond to electric fields by rearranging their spins. We have
identified a novel mechanism, based on broken inversion symmetry and
the concept of chirality, which enables access to the spin texture of
a special class of single-molecule magnets via static electric fields.
This provides a quantum degree of freedom that is amenable to electric
control, and can be exploited in a variety of ways. At the nanoscale,
strong controllable electric fields are readily available near the
tips of scanning tunneling microscopes, while the precise control over
magnetic fields remains a challenge. Spin-electric coupling therefore
enables a new generation of experimental studies as well as new
technologies based on electric manipulation of molecular spins.
A First for Spintronics: Measuring the Strength of a Spin-Polarized Current
Traditional electronic devices depend on manipulating electrical charge, the
negative charge of electrons or the positive charge of 'holes,' but future
information-technology devices will depend not on charge electronics but on
spin electronics, or 'spintronics,' which manipulates the orientation of the
(quantum-mechanical) spin of the electrons. One way to achieve this is to
inject a spin-polarized current into a magnetic element and use the torque
the current exerts on the magnetic moment to reverse its direction. For the
first time, Kasai and colleagues have determined the strength of such spin
currents directly and unambiguously, using time resolved magnetic soft x-ray
microscopy to image the effects of spin-polarized electron currents on
magnetic vortex cores confined to a disk of ferromagnetic Permalloy. Working
at the U.S. Department of Energy's Advanced Light Source, the researchers
used specialized x-ray optics (Fresnel zone-plate lenses) to achieve a
spatial resolution of less than 25 nanometers, with a temporal resolution of
70 picoseconds, as determined by the duration of the x-ray flashes. This
novel technique offers a window to understanding fundamental magnetic
processes, with tremendous potential applications in such areas of
nanoscience as the development of novel magnetic materials,
ultrahigh-density magnetic sensors, and new storage technologies.
***
LT11222
A FEW ELECTRONS CONTROL THE HEAT
Much current research with an eye toward technologies involves
materials with physical properties that can be manipulated with
weak electric or magnetic fields. Recently, researchers led by
Prof. J. Cohn (University of Miami), found that weak electric
fields have a surprisingly large effect on how well heat is
conducted in the magnetic oxide compound CaMnO(3) (calcium
manganite).
In magnetic solids -- those composed of ions which have a magnetic
"moment" -- the separation between atoms and the mobility of
electrons can be sensitive to the arrangement of the moments and
vice versa. This is particularly evident at the magnetic
transition or ordering temperature, below which neighboring
moments become aligned (a ferromagnet) or anti-aligned (an
antiferromagnet). Well above the antiferromagnet transition
temperature of CaMnO(3) (125 degrees Kelvin), fluctuating
nanoscale magnetically ordered regions form as a precursor to the
transition. This short-ranged order causes distortions in the
structure that impede the flow of heat. Turning on a modest
electric field mobilizes a small density of loosely bound
electrons and improves the heat conduction by a factor of two.
Because the electrons themselves are too few to account for the
improved heat conduction, the researchers suggest the novel
possibility that the liberated electrons suppress the magnetic
fluctuations.
***
LQ11199E
Shake it stronger and it will get denser.
Until now, experimental evidences showed that the stronger you tap a
collection of beads poured into a box, the fluffier it gets. This new study
shows that, if you go beyond a certain tapping intensity, the material starts
compacting back again. It is known that tapping a granular sample promotes
compaction. However, if the intensity of the taps is very low, the system
compacts better, although one needs to tap for longer. This remarkable fact
is supported by a number of experiments. However, none of these previous
studies probed really strong taps. Testing a number of computational models,
this paper shows that, eventually, high intensity taps will induce denser
arrangements. Therefore, there is an optimum tapping intensity at which the
looser state can be obtained. The paper propose an explanation based on the
formation and breakdown of arches. The results suggest that in a sample
confined to two dimensions the effect is much more evident. Preliminary
experimental research seems to confirm the prediction.
***
LV11004
A reversible breaking of inter-molecular chemical bonds with decreasing temperature and the coexistence of multiple molecular charge states are observed in a novel class of molecular solids known as fullerides. These materials, discovered only fifteen years ago, are composed of hollow spherically-shaped molecules (C60) with a diameter of one nanometer and made entirely of carbon which exhibit features that are rare in solids, such as the possibility of almost free rotation, distortion into elongated shapes, inter-molecular chemical binding, magnetism without transition elements, and charge conduction and superconductivity whereas an insulating electrical behavior is expected. We report on yet more surprises in our study: the strong (covalent) chemical bond linking two C60 molecules is observed to break as the temperature is lowered; while this cannot occur for an isolated pair of molecules, it is allowed for a system of many molecules such as a solid. The breaking/forming of the bond is perfectly reversible. When the bonds break a metallic phase results in which we could distinguish molecules carrying distinct electrical charges. This is quite unusual for such materials and indicates the occurrence of local charge fluctuations, which may provide a clue to solving the puzzle of electrical conductivity in fullerides.
***
LW10985
Dressed matter waves - new frontiers for quantum state engineering
When an atom interacts with an electromagnetic field, it becomes
"dressed" by the photons and changes its properties. Similarly, one
can "dress" a quantum mechanical matter wave, consisting of a large
number of ultracold atoms stored in an optical lattice, simply by
shaking that lattice with kilohertz frequencies. This means that
the matter wave responds to the shaking not by being disturbed in an
uncontrollable manner, as one might have expected on naive grounds,
but rather by acquiring new properties which the "bare" (i.e. unshaken)
system did not have. A striking demonstration of this concept had
recently been given by an experimental team from Pisa, Italy: These
authors succceeded to turn a superfluid matter wave into a Mott insulator
by varying the shaking strength [arXiv:0809.0768]. In their paper entitled
"Avoided level crossing spectroscopy with dressed matter waves", which
has just been accepted for publication in the Physical Review Letters,
two theoreticians from the Instituto de Ciencias Fotonicas (Spain) and
Oldenburg University (Germany) now have shown that the analogy between
"dressed atoms" and "dressed mesoscopic matter waves" has further high
potential for systematic quantum state engineering with ultracold atoms.
***
LW10842
Super-diffusive mass transport of Pb on Si(111) at low temperatures
An exceptionally fast and unusual mass transport behavior has been
discovered in the dense Pb wetting layer on the Si(111) surface at
temperatures as low as 150 K. The experiments were carried out using a
single laser pulse to desorb Pb from the wetting layer by heating the
surface locally. Low energy electron microscopy is used to observe the
resulting non-equilibrium coverage profile relax to equilibrium uniform
distribution in real-time. According to the classical textbook
description of surface mass transport, the initial coverage profile is
expected to broaden as it equilibrates progressively slower with time.
Surprisingly, profile evolution in the Pb wetting layer lacks these
classical features and is dominated instead by a much more efficient
convection-like mass transport. This behavior is unprecedented at
crystal surfaces and more characteristic of fluid motion. The authors
demonstrate that the transition to this novel super-diffusive state
occurs above a critical wetting layer coverage. Super-diffusive mass
transport can account for the highly efficient self-organization of
uniform height Pb nanostructures that was reported on widely in the
past. Its discovery also signals the possible existence of similar
intriguing mass transport mechanisms at low temperatures in other
systems that are yet to be discovered.
***
LT11935
To follow or not to follow: Laser-controlled electron motion in molecule
defies Newton's law
We show that ultrashort and intense laser pulses can be used to direct
and control electrons inside molecules. For the specific example of the
most simple molecule, the hydrogen molecular ion, we find that the
electronic probability current either follows or opposes the external
laser electric force, depending on the laser electric field strength.
According to Newton's second law of classical mechanics, an object
responds to an external force by changing its momentum. This classical
concept often applies for quantum systems too, where the correspondence
between classical variables and expectation values of quantum mechanical
operators is guaranteed by the Ehrenfest'S theorem. This correspondence
is frequently taken advantage of in order to explain complex quantum
mechanical phenomena, such as the interaction of intense laser light
with matter. For example, the generation of high harmonics radiation and
attosecond pulses can be understood in terms of the classical motion
of an electron in the laser field and its forced recollison with the parent
ion. The situation changes, if the electron is located inside a
molecule, as our numerical simulations for a dissociating hydrogen
molecular ion in an ultrashort intense laser pulse show. Here, the
momentum distribution of the electron is not only shifted by the
external laser field, but also modulated by an interference effect due
to the two nuclei. Due to this modulation the net electronic probability
current inside the molecular ion, that corresponds to the classical motion
of the electron, can follow or oppose to the laser electric force,
depending on the laser intensity. As ultrashort laser pulses are
acting on the time scale of the electronic motion in atoms and
molecules, our results are relevant for the control of chemical
reaction dynamics with light.
***
LN11497
Tuning nucleation on top of Pb islands by electron confinement.
On the nanoscale novel phenomena are present as a manifestation of
Quantum Mechanics. For nanotechnology applications it is important
to prepare nanostructures of uniform dimensions which requires
understanding nucleation on the atomistic level. Surprisingly a
dramatic variation of the nucleated island density is found for the
same orientation and the same substrate (i.e. Pb islands of (111)
orientation) as a function of island height. Normally nucleation is
varied only by changing the surface orientation and/or the substrate.
The new discovery is a result of how free electrons within the Pb
islands are confined according to the laws of Quantum Mechanics.
The electrons form standing waves:how well the standing waves fit
the island affects how easy Pb atoms nucleate the second layer
islands. This coupling of how electrons are confined to adatom
diffusion is unexpected and intriguing. The island density is 60
times larger on islands where the standing waves do not fit perfectly
the island. These unusual results suggest that it is possible to
affect atomistic processes not by varying the surface orientation or
substrate, but by simply changing island geometry.
***
BW10457
High-Temperature Superconductivity: Learning from Failure
In attempting to understand a particular phenomenon, the obvious
choice would be to study particularly successful examples; however,
sometimes one can learn as much or more from nominal failures. In
studying high temperature superconductors, we have discovered
potentially important clues to the mechanism of superconductivity by
studying a particular layered copper-oxide compound with anomalously
depressed superconductivity. This particular material is known to
exhibit unusual patterns of charge and magnetism known as "stripes".
It has been a common belief that this stripe order competes with
superconductivity. Thus, it comes as a considerable surprise that our
collective measurements of electronic transport and magnetic
properties, on one hand, and charge and spin order, on the other,
indicate that a fragile sort of short-range superconductivity develops
simultaneously with magnetic stripe order at a relatively high
temperature. Theorists have proposed that this coexistence may involve
an interweaving of the superconductivity and magnetism, much like warp
and weft. In any case, our results provide support for the
theoretical idea that spatial modulations of charge and spin densities
can enhance the strength of electron pairing, a prerequisite for high-
temperature superconductivity.
***
LR11507
A Quantum Look at Surface Plasmons
Recent developments in nanoplasmonics have led to the rapid development of
plasmonic circuitry for guiding and manipulation of light on subwavelength
scales as well as for engineering various devices and applications utilizing
the interaction of plasmonic modes with molecular excitations. The
development of plasmonic-based nanophotonic devices has also attracted a
keen interest from the quantum optics community for their use in quantum
information processing, due to a superior enhancement of nonlinear optical
effects and faster interaction times.
Most research in plasmonics is restricted to a completely classical
description of the underlying physics. This becomes inadequate once quantum
mechanical effects are prevalent in the investigation of plasmon-molecule
interactions or quantum plasmonic devices such as spasers (plasmon-based
lasers). Further development of molecular plasmonics and the next-generation
of photonic and plasmonic-based devices operating at a faithful
single-photon excitation level in nanoscale structures (desirable for the
purposes of integratable quantum information processing) will require a
genuine ab initio quantum approach.
In our article we provide key insights into the physics of photon-surface
plasmon coupling at the quantum level and the statistics of plasmonic
quantum states. We have established that excited surface plasmons can
completely preserve important quantum mechanical features of the original
photons as they travel along metal surfaces under realistic experimental
conditions. Efficient single-photon excitation in addition to few-photon
excitation of surface plasmons is considered. We also derive the
second-order quantum coherence function, which is an accurate measure of the
quantum nature of a surface plasmon.
Our results open up a new route toward the efficient manipulation of surface
plasmons at the quantum level in nanoplasmonic-based quantum information
processing experiments.
***
LS11197
Electric manipulation of molecular spins
Magnetism of single molecules has received much attention in
recent years. However, so far all the studies of molecular
magnetism have focused on the response of spins to magnetic
fields. Our results show that single-molecule magnets also
respond to electric fields by rearranging their spins. We have
identified a novel mechanism, based on broken inversion symmetry and
the concept of chirality, which enables access to the spin texture of
a special class of single-molecule magnets via static electric fields.
This provides a quantum degree of freedom that is amenable to electric
control, and can be exploited in a variety of ways. At the nanoscale,
strong controllable electric fields are readily available near the
tips of scanning tunneling microscopes, while the precise control over
magnetic fields remains a challenge. Spin-electric coupling therefore
enables a new generation of experimental studies as well as new
technologies based on electric manipulation of molecular spins.
Monday, November 3, 2008
November 3, 2008
LS11863
Transforming the Biphotons
How to shape the biphotons according to need is a hot topic. In an upcoming
issue of Physical Review Letters, researchers transform the spatial profile
of the biphotons by manipulating the domain structure of the nonlinear
crystal. The experiment results reveal that the modulation of the domain
pattern is transferred into the spatial mode of the generated biphotons.
This technique is very useful for integration of the quantum optics device
and generation of the novel entanglement state.
***
LQ11802
When neutron constituents behave like a single constituent
In the first precision test of the neutron¹s so-called ³quark-hadron
duality,² physicists have found that spinning neutrons exibit quark-hadron
duality, similar to the observations on protons before. This observation may
provide insight into the particles and the force that builds nearly all of
the visible matter in the universe.
From large distances, a proton or a neutron appears as a strongly
interacting hadron, a tight cluster of quarks and gluons, which can be
excited into various quark configurations called resonances. When probed
from short distances, protons and neutrons appear as an incoherent state of
quasi-free quarks. Though seemingly far different, measurements in these two
regions show, on average, a remarkably similar behavior, a phenomenon called
³quark-hadron duality.²
In the last decade, there has been significant improvement in high-precision
duality data on the spin-averaged structure of the proton and of nuclei. It
was also shown for the first time that quark-hadron duality holds for the
proton spin-polarized structure function. But one essential experimental
result that has been missing is a test on the neutron structure.
During the early months of 2003, a group of physicists at Jefferson Lab
conducted an experiment aimed at measuring the neutron spin structure in the
resonance region at moderate energies. These resonance measurements were
then compared to the expected curve derived from the data taken on the
quasi-free quarks. It was found that, within experimental errors,
quark-hadron duality starts to work for the neutron spin structure at about
the same energy as for the proton.
***
LD11686
Spindles, cusps and bifurcation for capsules in strong flows
Capsules are commonly used in a variety of industrial and biomedical
applications that require controlled release of medical agents (as in
drug delivery), aromas and flavors. However, little is currently known
at the deformation and dynamics of these membrane-enclosed fluid volumes
at strong flows owing to the complicated coupling of the fluid dynamics
with the membrane properties.
Based on computational investigation, our study shows that
strain-hardening capsules in strong extensional flows develop steady-state
shapes whose edges from spindled (or concave) become cusped with
increasing flow rate owing to a transition of the edge tensions from
tensile to compressive. A bifurcation in the steady-state shapes
is also found (i.e. existence of both spindled and cusped edges for
a range of high flow rates) by implementing different experiments,
owing to the different evolution of the membrane tensions. Thus our
present work complements the similar evolution for low-viscosity drops
which was first identified by the famous experiments of G. I. Taylor
in 1934, further explained in the 1970's and 1980's and still finds
useful applications nowadays. In addition, our study elucidates the
importance of compressive tensions in capsule mechanical deformation
that is applicable to both industrial and physiological processes.
***
BU10860
Can a glass be superfluid?
Glasses are often described as just liquids
that have stopped to flow. Superfluids instead flow without any
resistance. The existence of a phase of matter characterized by both
properties at the same time seems therefore utterly impossible. In
this paper we show instead that interacting quantum particles can form
a "super-glass" phase at very low temperature and high density; this
theory of the super-glass phase confirms previous numerical
results. This purely quantum phase is indeed characterized by an
amorphous density profile, as e.g. window glasses, but at the same
time by a finite fraction of the atoms that flow without any
resistance. The properties of this new phase of matter are
investigated for a class of models that turn out to be particularly
easy to analyze. The super-glass phase is not only a dream of
theoreticians but it is very likely to be observed in
experiments. Many recent experimental results on solid Helium 4 indeed
show many evidences of a super-solid phase of matter. Super-glass
phases are also likely to appear in another very different context:
mixture of cold atoms trapped by lasers in frustrated lattices.
***
BUR1079
The magnetic response of a bilayer of ferromagnetic (FM) and
antiferromagnetic (AF) thin films can be shifted along the field axis by
the exchange coupling of pinned interfacial spins with the FM. This
behavior is called exchange bias, and is widely used in the information
storage industry. We have used element-specific, soft
x-ray reflection to examine the interfacial spin configurations that
produce exchange bias in the prototypical (FM = Permalloy)/(AF = CoO)
system. In a ~1nm interfacial region, Co atoms showing a ferromagnetic
responses are present in two forms. Most of the FM Co responds to
applied fields exactly as does the Permalloy. However, about 10% of the
interfacial FM Co is pinned in a direction antiparallel to the applied
cooling field, and therefore is exchange-coupled antiferromagnetically
to the Permalloy, and is likely to be responsible for the exchange bias.
There are no measurable interfacial pinned spins in the Permalloy. The
interfacial region exhibits a temperature dependent magnetic behavior
that strongly differs from that of CoO and Permalloy.
***
LU12019
New test for mystery dark energy
How do we detect something we don't understand?
Over the last half century our understanding of the large scale
universe has gone from basically zero to a refined science driven by
the interplay between precise observations and sophisticated model
building. Of course, major questions remain such as the nature of the
big bang, and why the universe is here at all. Arguably the most
important at the moment is the most bizarre: the expansion of our
universe is speeding up, not slowing down. The 'dark energy'
apparently responsible is categorically unexpected in fundamental
physics, and is now one of the most important and far-reaching
mysteries in science today.
The critical issue facing cosmologists lies in determining if
Einstein's 'cosmological constant' is the underlying ingredient of
dark energy. Until a physically-motivated alternative is discovered,
it is imperative that we establish whether this simplest and currently
observationally favoured model is in fact correct or not. But our
total lack of understanding of the physics of dark energy - including
this
cosmological constant - places
severe constrains on our ability to say anything about its possible
dynamical nature: is this a field which changes with the cosmic
expansion? In this Letter, we present a new observational test which can
signal if the cosmological constant is the wrong answer, but,
critically, without having to invent ad hoc phenomenological
alternatives to it.
***
AP10366
NEW POSSIBILITIES FOR ELECTROMAGNETIC PULSES SLOWING IN BEC
As known, the propagation velocity of an electromagnetic pulse in medium
can be lower than in vacuum. For example, one of the most common and dense
materials in this meaning is diamond, where the light propagates slower in
2.5 times. But, the fact that the signal velocity in medium can differ
more than in million times in comparison with its value in free space is
rather unusual. The mentioned optical properties, in particular, are
typical of gases in the Bose-Einstein condensation (BEC) phase. The
authors theoretically investigate the ultra-slow light phenomenon in such
systems in the framework of the developed microscopic approach. It is
shown that for BEC of sodium atoms without a stimulated transparency the
minimum velocity for light pulses reaches the limit of 300 m/s. We find
that microwave signals can be more intensively slowed to 0.001 m/s. It is
also shown that the propagation velocity can strongly depend on the
intensity of the external magnetic field. Moreover, in some cases there
can propagate electromagnetic pulses with a negative group velocity. Thus
the investigation emphasizes one more time the uniqueness of the
properties of a new phase, which was reached at first time in 1995.
Transforming the Biphotons
How to shape the biphotons according to need is a hot topic. In an upcoming
issue of Physical Review Letters, researchers transform the spatial profile
of the biphotons by manipulating the domain structure of the nonlinear
crystal. The experiment results reveal that the modulation of the domain
pattern is transferred into the spatial mode of the generated biphotons.
This technique is very useful for integration of the quantum optics device
and generation of the novel entanglement state.
***
LQ11802
When neutron constituents behave like a single constituent
In the first precision test of the neutron¹s so-called ³quark-hadron
duality,² physicists have found that spinning neutrons exibit quark-hadron
duality, similar to the observations on protons before. This observation may
provide insight into the particles and the force that builds nearly all of
the visible matter in the universe.
From large distances, a proton or a neutron appears as a strongly
interacting hadron, a tight cluster of quarks and gluons, which can be
excited into various quark configurations called resonances. When probed
from short distances, protons and neutrons appear as an incoherent state of
quasi-free quarks. Though seemingly far different, measurements in these two
regions show, on average, a remarkably similar behavior, a phenomenon called
³quark-hadron duality.²
In the last decade, there has been significant improvement in high-precision
duality data on the spin-averaged structure of the proton and of nuclei. It
was also shown for the first time that quark-hadron duality holds for the
proton spin-polarized structure function. But one essential experimental
result that has been missing is a test on the neutron structure.
During the early months of 2003, a group of physicists at Jefferson Lab
conducted an experiment aimed at measuring the neutron spin structure in the
resonance region at moderate energies. These resonance measurements were
then compared to the expected curve derived from the data taken on the
quasi-free quarks. It was found that, within experimental errors,
quark-hadron duality starts to work for the neutron spin structure at about
the same energy as for the proton.
***
LD11686
Spindles, cusps and bifurcation for capsules in strong flows
Capsules are commonly used in a variety of industrial and biomedical
applications that require controlled release of medical agents (as in
drug delivery), aromas and flavors. However, little is currently known
at the deformation and dynamics of these membrane-enclosed fluid volumes
at strong flows owing to the complicated coupling of the fluid dynamics
with the membrane properties.
Based on computational investigation, our study shows that
strain-hardening capsules in strong extensional flows develop steady-state
shapes whose edges from spindled (or concave) become cusped with
increasing flow rate owing to a transition of the edge tensions from
tensile to compressive. A bifurcation in the steady-state shapes
is also found (i.e. existence of both spindled and cusped edges for
a range of high flow rates) by implementing different experiments,
owing to the different evolution of the membrane tensions. Thus our
present work complements the similar evolution for low-viscosity drops
which was first identified by the famous experiments of G. I. Taylor
in 1934, further explained in the 1970's and 1980's and still finds
useful applications nowadays. In addition, our study elucidates the
importance of compressive tensions in capsule mechanical deformation
that is applicable to both industrial and physiological processes.
***
BU10860
Can a glass be superfluid?
Glasses are often described as just liquids
that have stopped to flow. Superfluids instead flow without any
resistance. The existence of a phase of matter characterized by both
properties at the same time seems therefore utterly impossible. In
this paper we show instead that interacting quantum particles can form
a "super-glass" phase at very low temperature and high density; this
theory of the super-glass phase confirms previous numerical
results. This purely quantum phase is indeed characterized by an
amorphous density profile, as e.g. window glasses, but at the same
time by a finite fraction of the atoms that flow without any
resistance. The properties of this new phase of matter are
investigated for a class of models that turn out to be particularly
easy to analyze. The super-glass phase is not only a dream of
theoreticians but it is very likely to be observed in
experiments. Many recent experimental results on solid Helium 4 indeed
show many evidences of a super-solid phase of matter. Super-glass
phases are also likely to appear in another very different context:
mixture of cold atoms trapped by lasers in frustrated lattices.
***
BUR1079
The magnetic response of a bilayer of ferromagnetic (FM) and
antiferromagnetic (AF) thin films can be shifted along the field axis by
the exchange coupling of pinned interfacial spins with the FM. This
behavior is called exchange bias, and is widely used in the information
storage industry. We have used element-specific, soft
x-ray reflection to examine the interfacial spin configurations that
produce exchange bias in the prototypical (FM = Permalloy)/(AF = CoO)
system. In a ~1nm interfacial region, Co atoms showing a ferromagnetic
responses are present in two forms. Most of the FM Co responds to
applied fields exactly as does the Permalloy. However, about 10% of the
interfacial FM Co is pinned in a direction antiparallel to the applied
cooling field, and therefore is exchange-coupled antiferromagnetically
to the Permalloy, and is likely to be responsible for the exchange bias.
There are no measurable interfacial pinned spins in the Permalloy. The
interfacial region exhibits a temperature dependent magnetic behavior
that strongly differs from that of CoO and Permalloy.
***
LU12019
New test for mystery dark energy
How do we detect something we don't understand?
Over the last half century our understanding of the large scale
universe has gone from basically zero to a refined science driven by
the interplay between precise observations and sophisticated model
building. Of course, major questions remain such as the nature of the
big bang, and why the universe is here at all. Arguably the most
important at the moment is the most bizarre: the expansion of our
universe is speeding up, not slowing down. The 'dark energy'
apparently responsible is categorically unexpected in fundamental
physics, and is now one of the most important and far-reaching
mysteries in science today.
The critical issue facing cosmologists lies in determining if
Einstein's 'cosmological constant' is the underlying ingredient of
dark energy. Until a physically-motivated alternative is discovered,
it is imperative that we establish whether this simplest and currently
observationally favoured model is in fact correct or not. But our
total lack of understanding of the physics of dark energy - including
this
cosmological constant - places
severe constrains on our ability to say anything about its possible
dynamical nature: is this a field which changes with the cosmic
expansion? In this Letter, we present a new observational test which can
signal if the cosmological constant is the wrong answer, but,
critically, without having to invent ad hoc phenomenological
alternatives to it.
***
AP10366
NEW POSSIBILITIES FOR ELECTROMAGNETIC PULSES SLOWING IN BEC
As known, the propagation velocity of an electromagnetic pulse in medium
can be lower than in vacuum. For example, one of the most common and dense
materials in this meaning is diamond, where the light propagates slower in
2.5 times. But, the fact that the signal velocity in medium can differ
more than in million times in comparison with its value in free space is
rather unusual. The mentioned optical properties, in particular, are
typical of gases in the Bose-Einstein condensation (BEC) phase. The
authors theoretically investigate the ultra-slow light phenomenon in such
systems in the framework of the developed microscopic approach. It is
shown that for BEC of sodium atoms without a stimulated transparency the
minimum velocity for light pulses reaches the limit of 300 m/s. We find
that microwave signals can be more intensively slowed to 0.001 m/s. It is
also shown that the propagation velocity can strongly depend on the
intensity of the external magnetic field. Moreover, in some cases there
can propagate electromagnetic pulses with a negative group velocity. Thus
the investigation emphasizes one more time the uniqueness of the
properties of a new phase, which was reached at first time in 1995.
Wednesday, October 29, 2008
October 29, 2008
BT10895E
Discovery of Electric Conduction in the Nuclei of Living Cells
Recently, with the helps of X-ray investigation the relative positions of DNA
and proteins in the cell have been determined. It has turned out that there
are a large number of places where electric charge can flow from DNA to
proteins. This causes a current of positive charges in DNA and an electron
flow in proteins. If the DNA-protein structures are disturbed in the cell
nucleus by chemicals or radiations, a part of DNA moves away from the
proteins. This interrupts the currents and implicitly the attractive
interaction between them. Therefore parts of the long DNA chain in the cell
become readable which final end leads to the disturbance of the cell's
regulation. This may lead to the onset of cancer. To understand the details of
these phenomena Dr.-s A. Bende, F. Bogar, F. Beleznay and J. Ladik have
started quantum theoretical calculations of the energy level distributions of
DNA and proteins. In the present paper they show, using methods of
semiconductor physics, that really a non-negligible electric current of
positive changes flows in DNA.
***
LT11084
A SEQUENTIAL FACTORY OF QUANTUM OPERATIONS IS IMPOSSIBLE
This work proves the impossibility of building a sequential factory
of quantum operations, contrary to the existence of the massively used
sequential factory of operations in the "classical" industry.
Typically, in classical sequential factories, an operating head goes
down and up succesively through raw material in a row aiming at
producing the desired product at the end of the rolling band. If the
final product is a nontrivial quantum operation acting on the
circulating material, there is no way of implementing this task in a
sequential quantum factory. It is forbidden by the laws of quantum
physics and by the striking properties of entanglement among quantum
objects, an extension of the weirdness appearing already in the
"spooky action at distance" (as termed by Albert Einstein) of two
entangled objects. For simplicity, we may consider our raw material at
each step as formed by two-level quantum systems, also called quantum
bits or qubits, key elements in the description of many quantum
information protocols, including the possibility of building an
ultrafast quantum computer. This impressive negative result may have
deep implications in the design of future quantum technology, be for
sequential quantum Turing machines or the implementation of sequential
quantum information devices.
***
BV10878
Novel Quantum Effects Discovered in Graphene
Since its fabrication in 2005 monolayer graphene has been found to have
remarkable properties. The low energy behaviour of the system, remarkably,
can in certain regions, be modeled by a mass zero Dirac equation which
usually represents a highly relativistic system. The presence of charge
impurities in the system can introduce a mass (gapped graphene) and, as we
show, an additional real parameter which cannot be theoretically
determined. The parameter changes the physical properties of graphene such
as its energy levels and thus can be fixed experimentally. The reason for
this parameter lies in subtle feature of quantum mechanics connected with
the property of self-adjointness. The experimental determination of this
parameter would establish its usefulness in describing the properties of
gapped graphene and would also provide an example of presence of a subtle
quantum effect.
***
ET10355
Prime numbers, fractals and quantum mechanics.
The seemingly random distribution of the prime numbers has been
a source of fascination for mathematicians since before Zeno took
up archery. This distribution is related to Riemann's famous
hypothesis, regarding the nature of the zeros of his zeta-function,
for which the Clay Mathematics Institute offers a million dollar
Millennium Prize for a proof (or disproof). What can physicists
say about this problem? One method of investigating the distribution
of the primes or Riemann zeros is to assume they correspond to the
energy levels of some fictional physical system and then investigate
the properties of that system. This is the approach taken by Kiwi
and Canadian authors in this month's Physical Review E. Using
techniques from quantum scattering theory, they reconstruct potentials
with bound states corresponding to the primes and Riemann zeros,
showing that these potentials must have a fractal nature to reproduce
the seemingly chaotic distributions. Indeed, confusion in the
literature regarding the fractal dimension of these potentials is
resolved by showing that the potentials actually have a multi-fractal
nature. Analytic results also show possible connections to quantum
chaotic systems and perhaps offer hints for further such analytic
progress.
***
EU10352
What can mechanical systems tell us about the firing of brain cells?
Hamilton's equations are a mathematical description that is usually
applied to mechanical movement, such as the swinging of a pendulum.
However, we have used them to describe an unusual non-mechanical
situation, namely a cortical neuron (brain cell). In particular, we have
looked at the processes involved in the formation of an 'action
potential', or 'firing' event in the neuron. The equations, related to
Newton's Laws of Motion, analyze a system in terms of the flows of
momentum and energy. In this work, we have identified pseudo-momentum
and pseudo-energy terms for the neuron. There are a number of
similarities with classical mechanics, for example the total
pseudo-energy is conserved. The pseudo-momenta are directly related to
the electrical noise that the neuron experiences from its environment.
This fact implies that when a neuron fires, it is as a result of an
initial exponentially-increasing bias in the random noise that buffets
it, pulling the neuron out of a stable non-firing state, into a firing
regime.
***
BW10636
Looking for the mysterious binding force
High-Temperature Superconductivity (HTS) holds promise of a new
technological revolution, with the potential for changing our everyday
lives. The first family of HTS compounds, discovered twenty years ago and
known as copper-oxides or cuprates, has already found many important
practical applications, from medical imaging to cell telephony to powerful
electromagnetic motors. The recent discovery of the second such family,
dubbed iron pnictides, may prove to be even more important, since they
appear to have significant practical advantages over the cuprates. In our
paper we undertake a theoretical analysis of these new materials and focus
on the key mystery in iron pnictides: what is the nature of the
superconducting state and what is the microscopic mechanism that produces
it. In this regard, the crucial physical quantity is the superconducting
"order parameter," loosely the quantum state of the electron pairs that are
formed when a material goes superconducting. This order parameter encodes the
information about the nature of the force that binds electrons into pairs
and thus leads to superconductivity. A flurry of experiments indicates that
the superconducting order parameter of iron pnictides is different from the
one in cuprates. This is extremely exciting to physicists since it suggests
that the two families might represent two distinct paths to the ever
increasing HTS. In our paper, we suggest that the essential difference
between the two families can be established by probing the nature of the
order parameter in the momentum rather than in the real space, as was the
traditional approach for cuprate HTS. The momentum space is a mathematical
dual of the real space, allowing for easy visualization of the intrinsic
structure of an electron pair. The secret to "reaching
into" this momentum space is to use an experimental probe which carries a
physical momentum - a microworld version of an oncoming train - which
couples with the spins of the superconducting electrons. The results of this
quantum "demolition derby" depend sensitively on the specific way in which
electrons are bound into pairs thus giving us a powerful diagnostic tool
which can probe the very heart of the superconducting state.
***
LT11024
Simulations Show Polymers Can Be Good Heat Conductors
For more than 50 years scientists have pondered the "remarkable little
discovery" of Fermi, Pasta and Ulam (FPU) whose simulations showed that a
chain of atoms can have unlimited ability to transfer heat. Most of the work
that followed this discovery used highly simplified models of fictitious
molecular chains to study the circumstances where this unexpected phenomenon
arises. In this letter, a realistic model of a polymer chain is considered
and the simulations show that this phenomenon can still persist. Although
bulk polymers are generally considered poor thermal conductors, the
intrinsic properties of individual polymer molecules indicate that they can
be engineered as good thermal conductors. The simulation results suggest
that one day, polymers may be able to compete with the more expensive heat
transfer materials we use today, most of which are based on metals.
Discovery of Electric Conduction in the Nuclei of Living Cells
Recently, with the helps of X-ray investigation the relative positions of DNA
and proteins in the cell have been determined. It has turned out that there
are a large number of places where electric charge can flow from DNA to
proteins. This causes a current of positive charges in DNA and an electron
flow in proteins. If the DNA-protein structures are disturbed in the cell
nucleus by chemicals or radiations, a part of DNA moves away from the
proteins. This interrupts the currents and implicitly the attractive
interaction between them. Therefore parts of the long DNA chain in the cell
become readable which final end leads to the disturbance of the cell's
regulation. This may lead to the onset of cancer. To understand the details of
these phenomena Dr.-s A. Bende, F. Bogar, F. Beleznay and J. Ladik have
started quantum theoretical calculations of the energy level distributions of
DNA and proteins. In the present paper they show, using methods of
semiconductor physics, that really a non-negligible electric current of
positive changes flows in DNA.
***
LT11084
A SEQUENTIAL FACTORY OF QUANTUM OPERATIONS IS IMPOSSIBLE
This work proves the impossibility of building a sequential factory
of quantum operations, contrary to the existence of the massively used
sequential factory of operations in the "classical" industry.
Typically, in classical sequential factories, an operating head goes
down and up succesively through raw material in a row aiming at
producing the desired product at the end of the rolling band. If the
final product is a nontrivial quantum operation acting on the
circulating material, there is no way of implementing this task in a
sequential quantum factory. It is forbidden by the laws of quantum
physics and by the striking properties of entanglement among quantum
objects, an extension of the weirdness appearing already in the
"spooky action at distance" (as termed by Albert Einstein) of two
entangled objects. For simplicity, we may consider our raw material at
each step as formed by two-level quantum systems, also called quantum
bits or qubits, key elements in the description of many quantum
information protocols, including the possibility of building an
ultrafast quantum computer. This impressive negative result may have
deep implications in the design of future quantum technology, be for
sequential quantum Turing machines or the implementation of sequential
quantum information devices.
***
BV10878
Novel Quantum Effects Discovered in Graphene
Since its fabrication in 2005 monolayer graphene has been found to have
remarkable properties. The low energy behaviour of the system, remarkably,
can in certain regions, be modeled by a mass zero Dirac equation which
usually represents a highly relativistic system. The presence of charge
impurities in the system can introduce a mass (gapped graphene) and, as we
show, an additional real parameter which cannot be theoretically
determined. The parameter changes the physical properties of graphene such
as its energy levels and thus can be fixed experimentally. The reason for
this parameter lies in subtle feature of quantum mechanics connected with
the property of self-adjointness. The experimental determination of this
parameter would establish its usefulness in describing the properties of
gapped graphene and would also provide an example of presence of a subtle
quantum effect.
***
ET10355
Prime numbers, fractals and quantum mechanics.
The seemingly random distribution of the prime numbers has been
a source of fascination for mathematicians since before Zeno took
up archery. This distribution is related to Riemann's famous
hypothesis, regarding the nature of the zeros of his zeta-function,
for which the Clay Mathematics Institute offers a million dollar
Millennium Prize for a proof (or disproof). What can physicists
say about this problem? One method of investigating the distribution
of the primes or Riemann zeros is to assume they correspond to the
energy levels of some fictional physical system and then investigate
the properties of that system. This is the approach taken by Kiwi
and Canadian authors in this month's Physical Review E. Using
techniques from quantum scattering theory, they reconstruct potentials
with bound states corresponding to the primes and Riemann zeros,
showing that these potentials must have a fractal nature to reproduce
the seemingly chaotic distributions. Indeed, confusion in the
literature regarding the fractal dimension of these potentials is
resolved by showing that the potentials actually have a multi-fractal
nature. Analytic results also show possible connections to quantum
chaotic systems and perhaps offer hints for further such analytic
progress.
***
EU10352
What can mechanical systems tell us about the firing of brain cells?
Hamilton's equations are a mathematical description that is usually
applied to mechanical movement, such as the swinging of a pendulum.
However, we have used them to describe an unusual non-mechanical
situation, namely a cortical neuron (brain cell). In particular, we have
looked at the processes involved in the formation of an 'action
potential', or 'firing' event in the neuron. The equations, related to
Newton's Laws of Motion, analyze a system in terms of the flows of
momentum and energy. In this work, we have identified pseudo-momentum
and pseudo-energy terms for the neuron. There are a number of
similarities with classical mechanics, for example the total
pseudo-energy is conserved. The pseudo-momenta are directly related to
the electrical noise that the neuron experiences from its environment.
This fact implies that when a neuron fires, it is as a result of an
initial exponentially-increasing bias in the random noise that buffets
it, pulling the neuron out of a stable non-firing state, into a firing
regime.
***
BW10636
Looking for the mysterious binding force
High-Temperature Superconductivity (HTS) holds promise of a new
technological revolution, with the potential for changing our everyday
lives. The first family of HTS compounds, discovered twenty years ago and
known as copper-oxides or cuprates, has already found many important
practical applications, from medical imaging to cell telephony to powerful
electromagnetic motors. The recent discovery of the second such family,
dubbed iron pnictides, may prove to be even more important, since they
appear to have significant practical advantages over the cuprates. In our
paper we undertake a theoretical analysis of these new materials and focus
on the key mystery in iron pnictides: what is the nature of the
superconducting state and what is the microscopic mechanism that produces
it. In this regard, the crucial physical quantity is the superconducting
"order parameter," loosely the quantum state of the electron pairs that are
formed when a material goes superconducting. This order parameter encodes the
information about the nature of the force that binds electrons into pairs
and thus leads to superconductivity. A flurry of experiments indicates that
the superconducting order parameter of iron pnictides is different from the
one in cuprates. This is extremely exciting to physicists since it suggests
that the two families might represent two distinct paths to the ever
increasing HTS. In our paper, we suggest that the essential difference
between the two families can be established by probing the nature of the
order parameter in the momentum rather than in the real space, as was the
traditional approach for cuprate HTS. The momentum space is a mathematical
dual of the real space, allowing for easy visualization of the intrinsic
structure of an electron pair. The secret to "reaching
into" this momentum space is to use an experimental probe which carries a
physical momentum - a microworld version of an oncoming train - which
couples with the spins of the superconducting electrons. The results of this
quantum "demolition derby" depend sensitively on the specific way in which
electrons are bound into pairs thus giving us a powerful diagnostic tool
which can probe the very heart of the superconducting state.
***
LT11024
Simulations Show Polymers Can Be Good Heat Conductors
For more than 50 years scientists have pondered the "remarkable little
discovery" of Fermi, Pasta and Ulam (FPU) whose simulations showed that a
chain of atoms can have unlimited ability to transfer heat. Most of the work
that followed this discovery used highly simplified models of fictitious
molecular chains to study the circumstances where this unexpected phenomenon
arises. In this letter, a realistic model of a polymer chain is considered
and the simulations show that this phenomenon can still persist. Although
bulk polymers are generally considered poor thermal conductors, the
intrinsic properties of individual polymer molecules indicate that they can
be engineered as good thermal conductors. The simulation results suggest
that one day, polymers may be able to compete with the more expensive heat
transfer materials we use today, most of which are based on metals.
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