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.
Subscribe to:
Posts (Atom)