Real graphene on a SiC substrate
Graphene is the name given to an isolated plane of carbon atoms arranged
on a honeycomb lattice. Because of pecular electronic properties, it
could play a major role in tomorrow's nanoelectronics. Graphene can be
grown on a SiC substrate opening a route to carbon based electronics. We
show in our letter that this system indeed has the properties of
isolated graphene thanks to the formation of an interface buffer layer.
Our calculations and X-Ray diffraction measurements evidence that
eventhough the cristallographic structure of this interfacial layer is
very close to the honeycomb lattice, the strong coupling to the
substrate prevents any graphene like behavior in this layer. Graphene
properties are recovered in the first carbon layer on top of the buffer
layer. In agreement with experiment, we find that the graphene layers
are doped and we show that the doping level depends on the interface
geometry. LX10753
***
Tuning of tunneling anisotropies by an electric field
The resistance measured in ferromagnetic metals usually depends on
the relative orientation of the magnetization and the current
direction, an effect known as anisotropic magnetoresistance (AMR).
Before the advent of giant magnetoresistance the effect was employed in
read-out heads of hard disks. In our manuscript we describe another
type of anisotropic resistance occurring if a ferromagnetic layer is
incorporated in a ferromagnet/insulator/non-magnetic metal sandwich, in
our case Fe/GaAs/Au with a single crystalline Fe/GaAs interface. There
the resistance which is due to quantum mechanical tunneling of
electrons between the ferromagnetic (Fe) and normal (Au) metal, depends
on the direction of magnetization in the iron film. While the
observation of such a tunneling anisotropic magnetoresistance (TAMR)
came as a surprise, more astonishing is the possibility to tune the
anisotropy. Indeed, the anisotropy can be reversed and even switched
off by means of the applied bias voltage. The experiments can be
perfectly fitted by taking the role of the spin-orbit interaction of
electrons, while tunneling, into account. Apart from potential
application, provided the effect can be increased, e.g. by other
material combinations, the experiment provides a striking experimental
manifestation of the role of spin-orbit interaction in the quantum
mechanical tunnelling process. LY10336
***
Phase Transition in Strongly Non-ideal Deutherium Plasma, Generated by Quasiisentropical Compression at Megabars.
High-explosive (HE) driven generators of cylindrical and plane shock waves in D2 and H2 were used for generation of warm and dense strongly non-ideal matter with intense inter-particle interaction and Fermi statistics. For the first time highly resolved flash X-ray diagnostics were used for measuring the adiabatic plasma compressibility. The thermodynamic measurements demonstrated the twenty percent increase of density at megabar pressure, just in the density range, where the electrical measurements indicated sharp - five order of magnitude - increase of electrical conductivity due to pressure ionization in strongly coupled plasmas. We think we have obtained the first direct experimental signature of the existence of new phase transition in strongly non-ideal hydrogen (deuterium) with intense Coulomb interaction other than the gas-liquid transition and melting of molecular crystal. LY10378
***
A chemical survey of the landscape beyond the nanoscale
A team of scientists from Australia and Spain have for the first time demonstrated simultaneous imaging and unambiguous chemical identification of columns of atoms in a solid. This was achieved by scanning the specimen with an atom-sized electron probe at the state-of-the-art SuperSTEM facility in Daresbury, England. Improved data processing techniques and detailed modeling of the imaging process facilitated this major step towards the goal articulated by Richard P. Feynman in his famous talk, Plenty of Room at the Bottom (1959): “It would be very easy to make an analysis of any complicated chemical structure; all one would have to do would be to look at it and see where the atoms are. The only trouble is that the electron microscope is one hundred times too poor... I put this out as a challenge: Is there no way to make the electron microscope more powerful?” Research in nanoscience and technology, design of new materials and drugs, protein structure and folding, to name just a few, are all areas where knowledge of structure and function at the atomic level is essential. LF11130
Friday, July 20, 2007
Thursday, July 19, 2007
7-19-07
Intense laser light, dark cavities and cool molecules
Laser cooling has been demonstrated to be a powerful technique
in experiments with atomic vapors.
It was instrumental in the achievement of Bose-Einstein
condensation, and is routinely applied in experiments dealing with
cold atoms for high-precision measurements and quantum information
processing. When applied to molecules, however,
direct implementations of laser cooling techniques
proved to be inefficient, because they in general heat the rotational and
vibrational motion of the molecules. Now, researchers in Barcelona, Munich
and Garching have devised a new method to cool molecules' external,
rotational and vibrational motion simultaneously by laser light in
combination with
high-quality optical cavities. The results are obtained with state of the art
quantum-chemical simulations for a prototype molecule, OH, and show
that the vibrations and rotations of OH molecules can be brought to
the ground state, while the motion is simultaneously cooled to
temperatures of the order of few millikelvin in a fraction of a
second. This proposal opens novel perspectives in the preparation
and control of ultracold matter of larger complexity. LC155455
***
A collection of atoms, cooled to a very low temperature, can form a
collective state with very interesting properties. Atoms in such a state
form a single coherent unit, called Bose-Einstein condensate (BEC). The
condensate is described by a macroscopic wave function- a matter wave,
which evolves in space and time according to the laws of quantum mechanics.
Initially a BEC is prepared in a trap. Then, at some instant , it is
released from the trap and let freely expand in space.
In this paper we considered the expansion of a BEC in the presence of a
random potential. This kind of a "random potential for atoms" can be
created by lasers. The potential causes scattering of the BEC and prevents
its free propagation. The BEC expansion is slowed down. It turns out
that, under the appropriate conditions, some part of the condensate will
"get stuck", whereas the other part will diffuse away. LA11400
***
New insight in ion specificity
Why is KCl twice more efficient for the crystallisation of lysozyme than
NaCl? Why does the surface tension of water decrease with addition of
HCl and increase with addition of NaCl? Numerous examples of such
questions which illustrate ionic /specificity/ where ions of the same
valency like Cl- and I- lead to dramatically different effects can be
found in biology, environmental and atmospheric science or
physical-chemistrry. They range from enzymatic activity and amyloidosis
to the stability of humics and halide heterogeneous chemistry in the
atmosphere and still continue to be the subject of intense research
since the seminal paper of Hofmeister in 1888.
Though the presence of ions at the surface has been recently evidenced,
we report in this letter the first quantitative determination of the air
aqueous solution interfacial composition, using grazing incidence x-ray
fluorescence.
All surface compositions for several salt mixtures could be explained
using a simple model including a short-range effective potential ~ kT in
addition to Coulombic and van der Waals forces, adding evidence to the
idea that the missing piece in order to understand ion specificity is
indeed very short-range couplings between solvent molecules and ions. LD11322
***
Do SuperCrystals Exist in Nature?
It is well known that the most stable phase of most chemical
compounds at low temperatures is a crystalline one. It is
characterized by periodically arranged atoms or molecules on a scale
of the inter-atomic distances. By analyzing experimental data of James
Brooks' group, we have theoretically come to the conclusion that the
experimentalists discovered a novel super-crystalline phase. In
super-crystalline phase, which is also called soliton wall
superlattice (SWS) one], some plane traps for electrons (the so-called
soliton walls) are periodically arranged on a scale, which is
typically many thousand times bigger than that in conventional
crystals. We have also shown that the period of super-crystals can be
changed and tuned by a magnetic field. To the best of
our knowledge, super-crystals had never been observed in nature before
and we argue that they were discovered by Brooks' experimental group. LB11636
Laser cooling has been demonstrated to be a powerful technique
in experiments with atomic vapors.
It was instrumental in the achievement of Bose-Einstein
condensation, and is routinely applied in experiments dealing with
cold atoms for high-precision measurements and quantum information
processing. When applied to molecules, however,
direct implementations of laser cooling techniques
proved to be inefficient, because they in general heat the rotational and
vibrational motion of the molecules. Now, researchers in Barcelona, Munich
and Garching have devised a new method to cool molecules' external,
rotational and vibrational motion simultaneously by laser light in
combination with
high-quality optical cavities. The results are obtained with state of the art
quantum-chemical simulations for a prototype molecule, OH, and show
that the vibrations and rotations of OH molecules can be brought to
the ground state, while the motion is simultaneously cooled to
temperatures of the order of few millikelvin in a fraction of a
second. This proposal opens novel perspectives in the preparation
and control of ultracold matter of larger complexity. LC155455
***
A collection of atoms, cooled to a very low temperature, can form a
collective state with very interesting properties. Atoms in such a state
form a single coherent unit, called Bose-Einstein condensate (BEC). The
condensate is described by a macroscopic wave function- a matter wave,
which evolves in space and time according to the laws of quantum mechanics.
Initially a BEC is prepared in a trap. Then, at some instant , it is
released from the trap and let freely expand in space.
In this paper we considered the expansion of a BEC in the presence of a
random potential. This kind of a "random potential for atoms" can be
created by lasers. The potential causes scattering of the BEC and prevents
its free propagation. The BEC expansion is slowed down. It turns out
that, under the appropriate conditions, some part of the condensate will
"get stuck", whereas the other part will diffuse away. LA11400
***
New insight in ion specificity
Why is KCl twice more efficient for the crystallisation of lysozyme than
NaCl? Why does the surface tension of water decrease with addition of
HCl and increase with addition of NaCl? Numerous examples of such
questions which illustrate ionic /specificity/ where ions of the same
valency like Cl- and I- lead to dramatically different effects can be
found in biology, environmental and atmospheric science or
physical-chemistrry. They range from enzymatic activity and amyloidosis
to the stability of humics and halide heterogeneous chemistry in the
atmosphere and still continue to be the subject of intense research
since the seminal paper of Hofmeister in 1888.
Though the presence of ions at the surface has been recently evidenced,
we report in this letter the first quantitative determination of the air
aqueous solution interfacial composition, using grazing incidence x-ray
fluorescence.
All surface compositions for several salt mixtures could be explained
using a simple model including a short-range effective potential ~ kT in
addition to Coulombic and van der Waals forces, adding evidence to the
idea that the missing piece in order to understand ion specificity is
indeed very short-range couplings between solvent molecules and ions. LD11322
***
Do SuperCrystals Exist in Nature?
It is well known that the most stable phase of most chemical
compounds at low temperatures is a crystalline one. It is
characterized by periodically arranged atoms or molecules on a scale
of the inter-atomic distances. By analyzing experimental data of James
Brooks' group, we have theoretically come to the conclusion that the
experimentalists discovered a novel super-crystalline phase. In
super-crystalline phase, which is also called soliton wall
superlattice (SWS) one], some plane traps for electrons (the so-called
soliton walls) are periodically arranged on a scale, which is
typically many thousand times bigger than that in conventional
crystals. We have also shown that the period of super-crystals can be
changed and tuned by a magnetic field. To the best of
our knowledge, super-crystals had never been observed in nature before
and we argue that they were discovered by Brooks' experimental group. LB11636
Tuesday, July 17, 2007
7-17-07
Transverse and longitudinal biopolymer response is coupled
Cellular biopolymers are rather stiff, and their response to forces is
highly anisotropic: they yield more easily in the transverse than in
the longitudinal direction. But they are also nearly inextensible, and
motion in the two directions is therefore coupled (just like in a
stiff rope). In this paper, we show that a nonlinear coupling between
transverse and longitudinal time-dependent response arises even in the
weakly-bending case of an almost straight contour, and significantly
weakens the transverse response compared to the widely used linear
response predictions. We analyze this coupling via scaling arguments
and by a systematic theory that also contains the experimentally and
biologically relevant case of prestretched filaments initially under
tension. Our results apply not only to the single filament response
(e.g., of DNA), but have implications also for the collective
dynamics, for instance in crosslinked actin networks and tensegrity
structures. LD11620
***
Ultrafast, controlled mode switching in a microlaser
We introduce the new concept of mode selection and wavelength switching
in a microlaser by injecting a short light pulse which has the same spatial
symmetry as the mode to be selected.
Operation of a microlaser at different light frequencies is highly desirable
for telecommunication, spectroscopic, and quantum optics applications.
Up to now, this has usually been achieved by mechanically or electrooptically
altering the properties of the laser resonator and, thus, tuning the frequency
of a single resonator mode. This is a relatively slow process. In our work we
demonstrate by analytic calculations and accurate numerical simulations that
stable lasing is possible in either of the modes of a bistable microlaser
resonator made of two coupled photonic crystal cavities. One can deliberately
switch between these modes (which can be 5-20 nm apart) by injecting light
pulses (seeding pulses) whose electric field distribution matches the
spatial symmetry of the mode. The switching time is controlled by the
resonator finesse and by the seeding pulse intensity. For realistic microlasers
switching times as short as 10 picoseconds can be achieved by a seeding pulse
intensity as low as 0.1 per cent of the saturated laser intensity.
Any system of microresonators or photonic molecules exhibiting a
similar mode multi-stability can be used in the same way to design
a switchable multi-wavelength laser source. LA11450
***
Nonlinear nano-resonators to test quantum deviations from classical
behavior of macroscopic mechanical objects
We propose a novel approach, taking advantage of the nonlinear nature
of nanomechanical resonators, for observing the transition of
macroscopic mechanical objects from classical to quantum behavior as
their masses and temperatures are decreased. More than 70 years after
Schroedinger described his famous cat paradox we still do not
understand why large mechanical objects fail to obey the laws of
quantum mechanics. Is it merely because of their large masses, as
Roger Penrose believes, or because they constantly interact with the
environment, as Tony Leggett suggests? The rapid development of
human-made nanometer-scale mechanical systems may soon resolve the
famous cat paradox, but it may take a while before we can actually
observe full-fledged quantum phenomena, such as superpositions or
quantization of energy levels, in these systems. It would be much
easier simply to follow the dynamics of a large object, and look for
deviation from classical behavior. Harmonic oscillators are useless
for such an approach, because their dynamics is essentially classical
even when they obey the laws of quantum mechanics. We demonstrate
here, by comparing classical and quantum-mechanical numerical
simulations, that with nonlinear nanomechanical resonators it should
be possible to observe clear quantum deviations from classical
behavior in the very near future. LB11074
***
Freezeout of electronic spins precession
Schnelzer and coworkers discovered in a molecular nanomagnet a spin configuration in which the spins seem to be frozen. These investigations were performed at extremely low temperatures on a ring-like nanoscopic magnet (CsFe8), a so called ferric wheel with magnetically coupled Fe(III) ions assembled to a ring-like structure. It is possible to detect the fingerprints of the electronic spins at each Fe site by measuring the magnetization via a spectroscopic technique, proton nuclear magnetic resonance (1H-NMR), where the resonance frequency of the protons is directly related to the local magnetic fields at the Fe sites.
The Fe spins couple antiferromagnetically, i. e., the ring itself is nonmagnetic at low temperatures, but becomes magnetic at higher magnetic fields or increasing temperature. Normally, only the very small component of a spin´s magnetic moment parallel to an external field is fixed, while the transverse component fluctuates with a high frequency corresponding to the coupling strength of neighboring spins. What has been observed by Schnelzer et al. is that below a critical temperature the electronic Fe spins in CsFe8 remain STAGGERED even in the direction perpendicular to the external field, due to a hitherto unobserved physical mechanism, the field-induced spin-Jahn-Teller effect. The additional transverse magnetic fields related to the static Fe spins is directly seen via the symmetry and width in the proton NMR spectrum, providing a clear fingerprint of this unique electronic spin configuration. LB11204
***
Two-Photon Exchange in Electron-Proton Scattering
Until a few years ago, our knowledge of the how the electric charge is
distributed within the proton was thought to be of textbook quality
and reliability. Surprisingly, a large difference has recently been
found between the results obtained from two different methods for
measuring the proton's electric form factor, which encodes this charge
distribution. It has been postulated that processes in which two
virtual photons are exchanged, rather than the usual single photon,
could explain this troubling discrepancy. However, two-photon exchange
effects are notoriously difficult to calculate, let alone to isolate
experimentally. In the present work, sizable two-photon exchange has
been directly observed in the elastic scattering of electrons from
protons.
The results also show, via comparison with model calculations, that a
significant portion of the two-photon exchange at these kinematics
involves the proton being knocked temporarily into an excited
state. This process therefore may provide a new probe of the structure
of the nucleon.
These results were obtained in a high-precision measurement of
the tiny single-spin asymmetry of the cross section for elastic
scattering of transversely polarized electrons from unpolarized
protons. LE11201
***
Microscopic magnetic ordering at interface explains small exchange-bias fields
There has been intense interest in the exchange-bias phenomenon in recent years due to its importance in technological applications, such as read heads in computers, sensors, and magnetic random access memory. Exchange-bias fields arise from the interaction at the interface between ferromagnetic (FM) and antiferromagnetic (AF) components. One of the problems that has baffled scientists over the last half century is the fact that the exchange bias fields are an order of magnitude smaller than expected using simple microscopic models. Until now it has proved impossible to study the AF component of the order at the interfaces using the most powerful technique, neutron diffraction, due to signal limitations. Using model FM/AF multilayers we have been able to determine the AF ordering at buried interfaces for the first time in the simplest “uncompensated” exchange-bias system. Our measurements of the microscopic magnetic order at the interfaces reveal completely unexpected orthogonal magnetic structures that readily explain why the bias fields are so small. The results show that the simple AF ordering assumed in a variety of exchange-biased systems may have to be revised. LA11498
***
Large scale motions in the Universe blur the picture
Type Ia Supernova are a special kind of exploding star. They act as
so-called standard candles, meaning that their absolute luminosity, i.e.
their brightness is always very nearly the same. Therefore, by measuring
the amount of energy received on earth, astronomers have been able to
determine how far away these objects are. By combining the distance
information with redshift information, astronomers are able to work out
how the expansion rate of the Universe changed with time. This is one of
the most direct proofs of dark energy, a mysterious component of the
Universe that drives its recent accelerated expansion. It has been known
for some time that expansion of the Universe is not the only physical
process that affects the apparent brightness of a supernovae. If the
galaxy hosting the supernova has a peculiar velocity in addition to the
overall expansion of the Universe, the supernova will appear brighter or
dimmer, depending on whether the peculiar velocity of the supernova is
approaching or receding from us. Now, three physicists from Oxford
University have measured this effect for the first time. Two galaxies that
are close together are likely to have similar peculiar velocities because
they follow the same large-scale flows of matter in the Universe and
therefore they are likely to appear dimmer or brighter in unison. By
statistically analysing all possible pairs of 130 nearby supernovae, the
three researchers were able to detect this effect with high statistical
significance. The signal shows an excellent agreement with theoretical
predictions. Moreover, they have shown that this effect will have to be
carefully taken into account in future large robotic supernovae surveys. LE11349
***
Do SuperCrystals Exist in Nature?
It is well known that the most stable phase of most chemical
compounds at low temperatures is a crystalline one. It is
characterized by periodically arranged atoms or molecules on a scale
of the inter-atomic distances. By analyzing experimental data of James
Brooks' group, we have theoretically come to the conclusion that the
experimentalists discovered a novel super-crystalline phase. In
super-crystalline phase [which is also called soliton wall
superlattice (SWS) one], some plane traps for electrons (the so-called
soliton walls) are periodically arranged on a scale, which is
typically many thousand times bigger than that in conventional
crystals. We have also shown that the period of super-crystals can be
changed and tuned by a magnetic field. To the best of our knowledge,
super-crystals had never been observed in nature before and we argue
that they were discovered by Brooks' experimental group. LB11636
***
Zonal flow spectrum of the tokamak plasmas
Zonal flow (ZF) is considered by theoreticians to play an important role in
the heat transport of the confined plasmas as the black sea current in the
Pacific Ocean is important in the heat transport from tropical area to the
north pole. In our experiment the spectrum of ZF and the turbulence changes
significantly as the low-density tokamak is additionally heated and density
increases, showing the effectiveness in the transport suppression of
frequency variations among zonal flows. LX10137
***
Ice from light
A strong pulse of focused laser light can trigger the freezing of
supercooled water. We describe this new phenomenon for the first time,
and we investigate details by high-speed videography. It turns out
that not the light directly is responsible for crystallization, but
probably intense pressure waves in the supercooled liquid, caused by
an optical breakdown and subsequent vapor bubble collapse. The
pressure values can reach many thousands of atmospheres, and they
presumably shift the liquid into a much more unstable state, thus
provoking the ice nucleation. The effect can be used to control
the location and initiation of solidification in supercooled
liquids. It also helps to learn more about the nucleation of solids by
a related effect which is induced by intense sound, the so-called
sonocrystallization. In both cases, collapsing bubbles in the
supercooled liquid play a crucial role. LV10261
Cellular biopolymers are rather stiff, and their response to forces is
highly anisotropic: they yield more easily in the transverse than in
the longitudinal direction. But they are also nearly inextensible, and
motion in the two directions is therefore coupled (just like in a
stiff rope). In this paper, we show that a nonlinear coupling between
transverse and longitudinal time-dependent response arises even in the
weakly-bending case of an almost straight contour, and significantly
weakens the transverse response compared to the widely used linear
response predictions. We analyze this coupling via scaling arguments
and by a systematic theory that also contains the experimentally and
biologically relevant case of prestretched filaments initially under
tension. Our results apply not only to the single filament response
(e.g., of DNA), but have implications also for the collective
dynamics, for instance in crosslinked actin networks and tensegrity
structures. LD11620
***
Ultrafast, controlled mode switching in a microlaser
We introduce the new concept of mode selection and wavelength switching
in a microlaser by injecting a short light pulse which has the same spatial
symmetry as the mode to be selected.
Operation of a microlaser at different light frequencies is highly desirable
for telecommunication, spectroscopic, and quantum optics applications.
Up to now, this has usually been achieved by mechanically or electrooptically
altering the properties of the laser resonator and, thus, tuning the frequency
of a single resonator mode. This is a relatively slow process. In our work we
demonstrate by analytic calculations and accurate numerical simulations that
stable lasing is possible in either of the modes of a bistable microlaser
resonator made of two coupled photonic crystal cavities. One can deliberately
switch between these modes (which can be 5-20 nm apart) by injecting light
pulses (seeding pulses) whose electric field distribution matches the
spatial symmetry of the mode. The switching time is controlled by the
resonator finesse and by the seeding pulse intensity. For realistic microlasers
switching times as short as 10 picoseconds can be achieved by a seeding pulse
intensity as low as 0.1 per cent of the saturated laser intensity.
Any system of microresonators or photonic molecules exhibiting a
similar mode multi-stability can be used in the same way to design
a switchable multi-wavelength laser source. LA11450
***
Nonlinear nano-resonators to test quantum deviations from classical
behavior of macroscopic mechanical objects
We propose a novel approach, taking advantage of the nonlinear nature
of nanomechanical resonators, for observing the transition of
macroscopic mechanical objects from classical to quantum behavior as
their masses and temperatures are decreased. More than 70 years after
Schroedinger described his famous cat paradox we still do not
understand why large mechanical objects fail to obey the laws of
quantum mechanics. Is it merely because of their large masses, as
Roger Penrose believes, or because they constantly interact with the
environment, as Tony Leggett suggests? The rapid development of
human-made nanometer-scale mechanical systems may soon resolve the
famous cat paradox, but it may take a while before we can actually
observe full-fledged quantum phenomena, such as superpositions or
quantization of energy levels, in these systems. It would be much
easier simply to follow the dynamics of a large object, and look for
deviation from classical behavior. Harmonic oscillators are useless
for such an approach, because their dynamics is essentially classical
even when they obey the laws of quantum mechanics. We demonstrate
here, by comparing classical and quantum-mechanical numerical
simulations, that with nonlinear nanomechanical resonators it should
be possible to observe clear quantum deviations from classical
behavior in the very near future. LB11074
***
Freezeout of electronic spins precession
Schnelzer and coworkers discovered in a molecular nanomagnet a spin configuration in which the spins seem to be frozen. These investigations were performed at extremely low temperatures on a ring-like nanoscopic magnet (CsFe8), a so called ferric wheel with magnetically coupled Fe(III) ions assembled to a ring-like structure. It is possible to detect the fingerprints of the electronic spins at each Fe site by measuring the magnetization via a spectroscopic technique, proton nuclear magnetic resonance (1H-NMR), where the resonance frequency of the protons is directly related to the local magnetic fields at the Fe sites.
The Fe spins couple antiferromagnetically, i. e., the ring itself is nonmagnetic at low temperatures, but becomes magnetic at higher magnetic fields or increasing temperature. Normally, only the very small component of a spin´s magnetic moment parallel to an external field is fixed, while the transverse component fluctuates with a high frequency corresponding to the coupling strength of neighboring spins. What has been observed by Schnelzer et al. is that below a critical temperature the electronic Fe spins in CsFe8 remain STAGGERED even in the direction perpendicular to the external field, due to a hitherto unobserved physical mechanism, the field-induced spin-Jahn-Teller effect. The additional transverse magnetic fields related to the static Fe spins is directly seen via the symmetry and width in the proton NMR spectrum, providing a clear fingerprint of this unique electronic spin configuration. LB11204
***
Two-Photon Exchange in Electron-Proton Scattering
Until a few years ago, our knowledge of the how the electric charge is
distributed within the proton was thought to be of textbook quality
and reliability. Surprisingly, a large difference has recently been
found between the results obtained from two different methods for
measuring the proton's electric form factor, which encodes this charge
distribution. It has been postulated that processes in which two
virtual photons are exchanged, rather than the usual single photon,
could explain this troubling discrepancy. However, two-photon exchange
effects are notoriously difficult to calculate, let alone to isolate
experimentally. In the present work, sizable two-photon exchange has
been directly observed in the elastic scattering of electrons from
protons.
The results also show, via comparison with model calculations, that a
significant portion of the two-photon exchange at these kinematics
involves the proton being knocked temporarily into an excited
state. This process therefore may provide a new probe of the structure
of the nucleon.
These results were obtained in a high-precision measurement of
the tiny single-spin asymmetry of the cross section for elastic
scattering of transversely polarized electrons from unpolarized
protons. LE11201
***
Microscopic magnetic ordering at interface explains small exchange-bias fields
There has been intense interest in the exchange-bias phenomenon in recent years due to its importance in technological applications, such as read heads in computers, sensors, and magnetic random access memory. Exchange-bias fields arise from the interaction at the interface between ferromagnetic (FM) and antiferromagnetic (AF) components. One of the problems that has baffled scientists over the last half century is the fact that the exchange bias fields are an order of magnitude smaller than expected using simple microscopic models. Until now it has proved impossible to study the AF component of the order at the interfaces using the most powerful technique, neutron diffraction, due to signal limitations. Using model FM/AF multilayers we have been able to determine the AF ordering at buried interfaces for the first time in the simplest “uncompensated” exchange-bias system. Our measurements of the microscopic magnetic order at the interfaces reveal completely unexpected orthogonal magnetic structures that readily explain why the bias fields are so small. The results show that the simple AF ordering assumed in a variety of exchange-biased systems may have to be revised. LA11498
***
Large scale motions in the Universe blur the picture
Type Ia Supernova are a special kind of exploding star. They act as
so-called standard candles, meaning that their absolute luminosity, i.e.
their brightness is always very nearly the same. Therefore, by measuring
the amount of energy received on earth, astronomers have been able to
determine how far away these objects are. By combining the distance
information with redshift information, astronomers are able to work out
how the expansion rate of the Universe changed with time. This is one of
the most direct proofs of dark energy, a mysterious component of the
Universe that drives its recent accelerated expansion. It has been known
for some time that expansion of the Universe is not the only physical
process that affects the apparent brightness of a supernovae. If the
galaxy hosting the supernova has a peculiar velocity in addition to the
overall expansion of the Universe, the supernova will appear brighter or
dimmer, depending on whether the peculiar velocity of the supernova is
approaching or receding from us. Now, three physicists from Oxford
University have measured this effect for the first time. Two galaxies that
are close together are likely to have similar peculiar velocities because
they follow the same large-scale flows of matter in the Universe and
therefore they are likely to appear dimmer or brighter in unison. By
statistically analysing all possible pairs of 130 nearby supernovae, the
three researchers were able to detect this effect with high statistical
significance. The signal shows an excellent agreement with theoretical
predictions. Moreover, they have shown that this effect will have to be
carefully taken into account in future large robotic supernovae surveys. LE11349
***
Do SuperCrystals Exist in Nature?
It is well known that the most stable phase of most chemical
compounds at low temperatures is a crystalline one. It is
characterized by periodically arranged atoms or molecules on a scale
of the inter-atomic distances. By analyzing experimental data of James
Brooks' group, we have theoretically come to the conclusion that the
experimentalists discovered a novel super-crystalline phase. In
super-crystalline phase [which is also called soliton wall
superlattice (SWS) one], some plane traps for electrons (the so-called
soliton walls) are periodically arranged on a scale, which is
typically many thousand times bigger than that in conventional
crystals. We have also shown that the period of super-crystals can be
changed and tuned by a magnetic field. To the best of our knowledge,
super-crystals had never been observed in nature before and we argue
that they were discovered by Brooks' experimental group. LB11636
***
Zonal flow spectrum of the tokamak plasmas
Zonal flow (ZF) is considered by theoreticians to play an important role in
the heat transport of the confined plasmas as the black sea current in the
Pacific Ocean is important in the heat transport from tropical area to the
north pole. In our experiment the spectrum of ZF and the turbulence changes
significantly as the low-density tokamak is additionally heated and density
increases, showing the effectiveness in the transport suppression of
frequency variations among zonal flows. LX10137
***
Ice from light
A strong pulse of focused laser light can trigger the freezing of
supercooled water. We describe this new phenomenon for the first time,
and we investigate details by high-speed videography. It turns out
that not the light directly is responsible for crystallization, but
probably intense pressure waves in the supercooled liquid, caused by
an optical breakdown and subsequent vapor bubble collapse. The
pressure values can reach many thousands of atmospheres, and they
presumably shift the liquid into a much more unstable state, thus
provoking the ice nucleation. The effect can be used to control
the location and initiation of solidification in supercooled
liquids. It also helps to learn more about the nucleation of solids by
a related effect which is induced by intense sound, the so-called
sonocrystallization. In both cases, collapsing bubbles in the
supercooled liquid play a crucial role. LV10261
Thursday, July 5, 2007
7-05-07
Researchers make tiny ratchet using quantum physics
Researchers at the University of Electro-Communications (UEC) in Tokyo,
led by Professor Ken'ichi Nakagawa, have used the weirdness of quantum
mechanics as a tool to build a new type of ratchet.
A ratchet is a device which converts back and forth movements into
forward motion. Scientists are interested in such behaviour on a
microscopic scale to explain movement in biological systems and also as
a novel means to transport atoms in a desired direction. However, for
atoms, so-called "quantum interference" becomes important, usually
leading to the ratchet effect stopping after a certain time. Now, a new
approach pioneered at UEC has created an atom ratchet which keeps on
going. The trick is to prepare atoms in a so-called "superposition"
where objects may be in two different states at once due to the strange
laws of quantum mechanics. Atoms are initially prepared to have zero and
non-zero speed simultaneously. If light pulses are then applied from
both sides to this state, movement occurs predominantly in one
direction. Unlike previous atom ratchets, in this case quantum effects
allow the motion to continue, rather than eventually stopping it. The
team plans to extend their technique to allow the storage and readout of
information using atoms. LC11385
***
An Elastic Pattern Switch
Nature makes extensive use of periodic and layered structures to achieve different properties and attributes. The brilliant coloring of many birds, butterflies, beetles and fish as well as the hydrophobic character of desert beetles and lotus leaves originates in their surface structures. Mimicking the complex periodic forms of the natural world in the manufacture of physical devices is a significant challenge at the required length-scales. The results of our investigation show that the application of a simple load to a periodic structure can trigger an unexpected global pattern switch above a critical point. This enables the possibility of creating prescribed complex patterns on currently available periodic lattices and also switching certain properties on and off with deformation. The effect is both reversible and repeatable. Its origin is an elastic instability so that it occurs over a narrow range of the applied load making precise switching practical.
Our results were obtained in samples at the millimeter-length scale relevant to phononic crystals but the effects should persist at the micro- and nano-scales. At this level, the new elastic pattern switch provides the exciting prospect controlling photonic crystals to manipulate light. LC10939
***
Capillary filling - a long-known effect with novel applications
Whether designing a modern ⿿lab-on-a-chip , DNA electrophoresis chips, or
nanowires of 1 nm diameter ⿿ one resorts nowadays to a long-known
phenomenon: if a capillary is inserted into a liquid, the liquid will rapidly
flow into it (fluid imbibition). Described and understood 90 years ago in terms
of well-defined macroscopic quantities such as surface tension, meniscus
curvature, and contact angle of wetting, this phenomenon needs reassessment in
the modern world of tiny (nano)-scale lengths where a nanotube diameter may
span several atom diameters only, and the discrete nature of matter cannot be
ignored. Now, checking the validity of classical laws within the realm of
nanoscales is most efficiently done by computer experiments, most notably by
Molecular Dynamics simulations of model systems.
To this end we simulate both a simple liquid, such as a liquefied noble gas,
and a model of polymer melt, like silicon oil, letting them fill a super narrow
capillary(with diameter of about ten times the atom size), immersed in the
liquid. In both cases, after a transient period (of a few nanoseconds), a
meniscus is observed to move up the tube on a distance which grows as a square
root of the time elapsed, exactly as predicted by Lucas and Washburn nearly a
century ago. As expected, the speed of filling increases as the square root of
the tube diameter. The flow velocity is maximal in the middle of the capillary
and goes gradually down to zero at the capillary wall.
For the polymer melt, however, it turns out that the melt slips with non-zero
velocity even immediately at the wall! The hypothetical distance from the wall,
where the extrapolated flow velocity would have reduced to zero, defines what
is called a slip length, and in our computer experiments this slip length is
comparable in size with the nanotube radius. Therefore, a straightforward
application to nanotubes of the classical Lucas-Washburn law for capillary
filling would not work. Our computer experiments show, however, that a
consistent description of the imbibition process in nanotubes is still possible
upon a simple modification of the Lucas-Washburn law whereby the capillary
radius is incremented by the observed slip length of the flow.
Thus a knowledge, accumulated over decades of scientific research, may now be
readily transferred to modern nano-applications and nano-technology.
Snapshot of a capillary being filled by a simple liquid upon immersion
into a reservoir. The magnifying glass is focused on the meniscus area
and shows the velocities of particle between tube axis and wall against
a density profile of the liquid. LC11834
***
Anderson localization goes oscillating
Have you ever thought how much "periodic" can be a random system?
Does natural disorder hide anything strictly periodic? A simple
model of a disordered optical system suggests that its mean
transmission should oscillate with sample length. The model,
supported by numerical experiments, shows for the first time that
the transmission periodically rises and drops as the system gets
thicker. Physicists know since half a century that Anderson
localization is the phenomenon ruling wave transport mechanism in
disordered systems. It is also widely accepted that the transmission
drops exponentially for thicker samples. Now the new results
demonstrate that in a generically produced random mixture of two
different units, e.g. special-sized "black" and "white balls", the
exponential law appears modulated by periodic oscillations.
Responsible for these unexpected oscillations are rarely forming
special states, known as "necklace states", which induce direct
transmission channels even through very thick samples. Such
necklaces periodically appear to be efficiently transmitting each
time the system reaches special thicknesses. The edifying feature of
the effect found adds a new insight into the physics of Anderson
localization. LC11146
***
.JPG)
Modified Thermal Emission from Self-Assembly: Opals Meet the Light Bulb
Han et al. predict that if a heated metal is first structured through simple self-assembly techniques, the color of its “glow” can be altered in useful ways. A heated object emits light according to its temperature and optical properties. This phenomenon, known as thermal emission, is utilized in conventional light bulb filaments. Unfortunately, light bulbs emit a large quantity of heat, resulting in inefficiency. Photonic crystals — materials that are structured on a micrometer length scale — are being explored for eliminating this heat, potentially providing more efficient thermal emission sources. However, the study of this effect has been limited due to difficulties in making the appropriate structures. Previously, photonic crystals have been obtained by emulating the natural process that leads to gemstone opals. Unfortunately, experiments on these self-assembled structures suggested that they were ill suited for modifying thermal emission. Han et al. considered the physics of this problem and found small, experimentally realizable changes to the structure that should lead to modified thermal emission. Thus, these new self-assembled photonic crystals, which are easy to prepare, have great potential for modifying thermal emission both for fundamental understanding and applications.
LC11750
***
Single-Molecule Force Spectroscopy Reveals Protein Folding Free
Energy Landscapes
Understanding biomolecular interaction is one of the most important
question of biological physics. Atomic force microscopes allow one
to unfold proteins or stretching molecules essentially by hand and,
with the help from modern statistical theory, to reconstruct the
free energy landscape of such interaction.
Kiang's group used a microscopic cantilever attached to the
atomic force microscope tips, to pull a protein while measuring
the protein's reaction force by the bending of the cantilever.
Using the modern nonequilibrium work theorem, the group
mapped the protein's free energy landscape along the entire unfolding
trajectory, which had not been accomplished to date by any other
methods. The new technique can be applied to most any biomolecule,
complex, or molecular interaction. LU10757
***
Magnetic Explosions
We studied the way microscopic spins in magnetic materials move upon
magnetization reversal, such as materials that are used in hard disk
drives. These systems can be designed to have avalanches where only
a few spins are needed to start a cascade, leading to many flipped
spins, for example, as you see with dominoes. However, we show that this
process is much more analogous to an explosion than had been previously
thought. Instead of spins just flipping once from up to down, they
continue to spin around, that is, the spins get hot, and in many materials
they stay hot for a substantial amount of time. This greatly alters the
microscopic physics of the system, which means that a change in one part
of a magnet can affect the behavior much further away. The hot region
forms a growth front and the heat from it recruits neighboring spins
into the explosive region before they eventually cool down. We analyzed
this behavior in detail using simulations and theoretical models and
were able to determine under what circumstances an explosion takes off.
The attached graphics shows a simulation of spins in a film during an
avalanche for: (a) high damping, and (b) low damping. The brightness
is a measure of the amount of spin motion. LR10732
***
From geometry class, we know that we can think about shapes that live
only in a two-dimensional world, but can real materials behave as if
they live in Flatland? New measurements of current flow in a special
type of copper-oxide high-temperature superconductor provide evidence
for two-dimensional superconductivity coexisting with one-dimensional
rivers of charge. The superconducting state, involving resistance-less
flow of electrical current, requires that conduction electrons pair up
and be able to flow in three-dimensional (3D) space. In copper-oxide
superconductors, the interactions that cause the pairing occur in 2D
planes; weak interactions between the planes allow the supercurrents to
flow in 3D. In the special copper-oxide compound considered here,
experiments indicate that the charge carriers tend to segregate into 1D
rivers known as "stripes". It had been common belief that the ordering
of charges into stripes within the Cu-O planes would prevent or compete
with superconductivity. The new measurements of current flow
contradict these expectations and are consistent with theoretical
predictions for superconductivity in Flatland. The observation of 2D
superconductivity within a 3D crystal over an extended temperature
range is extremely unusual, and the coexistence with 1D stripes
indicates the surprising nature of superconductivity in copper-oxide
compounds. LC11292
***
Face-to-face with correlated wave functions
The microscopic description of strongly interacting systems is
one of the latest frontiers in condensed matter theory.
Indeed, several materials which show spectacular - and still debated -
properties, like high-temperature superconductivity, superfluidity
and quantum Hall effect, go with a strong repulsion among particles.
The aforementioned phenomena generally manifest themselves at very low
temperatures, where the quantum-mechanical nature of particles comes
into play. As a consequence, all information related to their
collective quantum behavior are encoded into the ground-state
wave function of the system.
Nonetheless, the derivation of the ground-state wave function in presence of
strong correlation is a complicated issue, which so far has been solved in
few cases.
From the experimental point of view, the recent developments on cold atoms
trapped in optical lattices allow one to have a direct control on the
various couplings entering into the game, giving the unique opportunity to
make a close connection with theoretical models and to answer
fundamental questions.
In particular, by tuning the lattice potential depth on trapped-bosonic atoms,
it is possible to control the ratio between the kinetic energy (that tends to
delocalize the particles) and their local repulsion (that instead tends to
localize). It is therefore possible to drive the system from a superfluid to
a Mott insulating state. This behavior, that is also expected in electronic
systems, reflects the dramatic change in the electronic/bosonic arrangement
across a quantum phase transition.
In this work, we tackle the problem by introducing a wave function, which
turns out to be very effective to model a system of interacting bosons in
one, two, and three dimensions. We find that
the essential ingredient in order to capture
the physics of a strongly-interacting system is a two-body correlation term,
known as Jastrow factor.
In presence of this term, particles ''feel each other'' over all possible
distances, making it possible to correct the mean-field wave function and
describe a superfluid-insulator transition.
Remarkably, we find for the first time that the long-range
character of the Jastrow terms is essential in order to correctly reproduce
both the superfluid and the Mott insulating state.
Moreover, our accurate description of the correlated ground state allows us to
explain the peculiar excitation spectrum found in the
experiments of Stoferle and collaborators.
Indeed, the different behavior of the Jastrow correlation factor
across the transition naturally induces high-energy excitations in the
superfluid state in two and three dimensions, but not in one dimension.
These high-energy excitations signal that the superfluid state already contains
the main fingerprints of the correlated insulating state.
This is an interesting and also surprising result, that bears a
lot of similarities with the metal-insulator transition in electronic systems,
but is not accounted for by most accepted theories for bosonic systems.
Moreover, this general scheme indicates that this
class of wave function permit to have access to the Mott
transition in three-dimensional fermionic models, too,
all the more reason when realistic Coulomb interaction is taken
into account. LY10312
***
The addition of randomly scattered waves gives rise to a speckled intensity
pattern. The evolution of such patterns as the frequency scanned is observed
for microwave radiation, however speckle patterns arise for any type of
wave, including light, sound and electrons. Though the pattern is random, it
is not devoid of structure. A network of phase singularities appears about
points of vanishing intensity. We show that the motion of these points of
darkness provides a diagnostic of the nature of wave transport within the
scattering object. The probability distributions of the displacement of
phase singularities together with the variations of phase and intensity
throughout the speckle pattern are governed by a single parameter, which
provides a precise measure of the degree of spatial localization of the wave
within the sample. For exponentially peaked localized waves, the speckle
pattern seems to move in fits and starts as opposed to the smooth
development of speckle for extended waves. This reflects the degree to which
modes within the sample overlap. The paper provides sharp measures of
speckle evolution which can be exploded to monitor motion of hidden objects
and the development of structural defects. LY10404
***
Femtosecond field ion emission by surface optical rectification
Recent experimental evidence from separate fields of laser physics converges into indicating
that ultra short laser pulses may remove atoms from a metal surface within a time as short as
few hundreds of femtoseconds. This effect might lead to interesting applications such as nano
machining of materials with a control of the surface roughness at the atomic level. One
example is the recent advent of the atom probe tomography technique in which laser pulses
are used to remove a tip like shape sample material atom by atom allowing a full three-
dimensional image of the material to be obtained in the real space. Another example is the
possible strong anisotropic ablation of suspended gold nanoparticles induced by 100 fs laser
pulses. Finally a non-thermal ablation regime of planar metal surfaces has been reported. In
all these cases the underlying mechanism of this ultrafast emission is obscure and might be
ascribed to some yet unidentified nonlinear surface optical effect. In this article a possible
explanation for these observations is presented based on the optical rectification of the light
wave at the surface of materials. Moreover, we put forward the working hypothesis that the
same effect may be also at the root of the reported ultrafast ablation phenomena. LB11245
***
Unraveling a granular labyrinth
We describe a new pattern formation process where beautiful labyrinth
structures emerge as a mixture of grains and fluid slowly dries out. Key
to the pattern formation is an interesting interaction between granular
and fluidic influences. The granular-fluid system is confined between
two glass plates, and the receding fluid/air meniscus compiles a layer
of compacted grains and becomes unstable, whereby fingers of air invade
the system in a random fashion. We show that a uniform characteristic
length-scale in the pattern develops as a compromise between two
opposing forces: the surface tension of the fluid, and the frictional
stress resulting from grain-grain contact networks within the compacted
granular layer. The granular labyrinth is visually striking, with
characteristic length-scales up to several centimeters. An understanding
of the pattern formation process is developed along three complementary
paths: experiments, simulations and theoretical considerations. LC10922
***
Control and dynamics of vortices in a turbulent flow
Vortices and similar structures appear continuously in nature: from
tornadoes to vortices at the tip of plane wings,
from the Jupiter's Great Red spot to the smoke of a cigarette, the fluid
elements tend to form stable structures.
The understanding of their dynamics and stability are important both
from the basic and applied points of view:
alterations of the environmental conditions can produce structural
changes on these flows.
The mechanisms at the origin of these changes and the role played by the
structures themselves are still subject of debate.
That¿s why experiments carried out in controlled environments are so
important: the parameters that regulate
the evolution of the fluid (i.e. pressure, propellers) can be easily
modified, in such a way that the flow can be
destabilized and new structures or configurations can arise. This work
shows that the vortices in a cylindrical
cavity can present a rich dynamics, with random or forced flow
inversions. This dynamics can be explained
using an astonishingly simple toy model. LB11142
Attached figure: Picture of an experimental vortex in the cylindrical cavity
***
New type of electron-nuclear spin polarization
The electron spin in semiconductor quantum dots is currently extensively investigated in the context of quantum information processing. The interaction with the nuclear magnetic moments of the lattice atoms constitutes the major source for the loss of the electron spin. We, for the first time, demonstrate the formation of a strong nonequilibrium dynamical nuclear polarization in semiconductor quantum dots under continuous optical pumping of the electron spin at zero magnetic field. Unlike the vast amount of previous work on semiconductors based on the standard spin cooling regime, the formation time is as short as 20 microseconds. We also uncover a life-time of about 200 microseconds due to nuclear dipole-dipole decay. Specific of our study is the use of quantum dots where the electron spin is coupled to only a few hundred nuclear moments and where the Overhauser field does not exceed markedly the spectral broadening of the Zeeman levels. The nuclear polarization is almost complete and survives up temperatures of about 100 K. In this way, the electron spin is stabilized and the coupled electron-nuclear system exhibits a variety of novel features that might be used for quantum bit implications. LC11325
Researchers at the University of Electro-Communications (UEC) in Tokyo,
led by Professor Ken'ichi Nakagawa, have used the weirdness of quantum
mechanics as a tool to build a new type of ratchet.
A ratchet is a device which converts back and forth movements into
forward motion. Scientists are interested in such behaviour on a
microscopic scale to explain movement in biological systems and also as
a novel means to transport atoms in a desired direction. However, for
atoms, so-called "quantum interference" becomes important, usually
leading to the ratchet effect stopping after a certain time. Now, a new
approach pioneered at UEC has created an atom ratchet which keeps on
going. The trick is to prepare atoms in a so-called "superposition"
where objects may be in two different states at once due to the strange
laws of quantum mechanics. Atoms are initially prepared to have zero and
non-zero speed simultaneously. If light pulses are then applied from
both sides to this state, movement occurs predominantly in one
direction. Unlike previous atom ratchets, in this case quantum effects
allow the motion to continue, rather than eventually stopping it. The
team plans to extend their technique to allow the storage and readout of
information using atoms. LC11385
***
An Elastic Pattern Switch
Nature makes extensive use of periodic and layered structures to achieve different properties and attributes. The brilliant coloring of many birds, butterflies, beetles and fish as well as the hydrophobic character of desert beetles and lotus leaves originates in their surface structures. Mimicking the complex periodic forms of the natural world in the manufacture of physical devices is a significant challenge at the required length-scales. The results of our investigation show that the application of a simple load to a periodic structure can trigger an unexpected global pattern switch above a critical point. This enables the possibility of creating prescribed complex patterns on currently available periodic lattices and also switching certain properties on and off with deformation. The effect is both reversible and repeatable. Its origin is an elastic instability so that it occurs over a narrow range of the applied load making precise switching practical.
Our results were obtained in samples at the millimeter-length scale relevant to phononic crystals but the effects should persist at the micro- and nano-scales. At this level, the new elastic pattern switch provides the exciting prospect controlling photonic crystals to manipulate light. LC10939
***
Capillary filling - a long-known effect with novel applications
Whether designing a modern ⿿lab-on-a-chip , DNA electrophoresis chips, or
nanowires of 1 nm diameter ⿿ one resorts nowadays to a long-known
phenomenon: if a capillary is inserted into a liquid, the liquid will rapidly
flow into it (fluid imbibition). Described and understood 90 years ago in terms
of well-defined macroscopic quantities such as surface tension, meniscus
curvature, and contact angle of wetting, this phenomenon needs reassessment in
the modern world of tiny (nano)-scale lengths where a nanotube diameter may
span several atom diameters only, and the discrete nature of matter cannot be
ignored. Now, checking the validity of classical laws within the realm of
nanoscales is most efficiently done by computer experiments, most notably by
Molecular Dynamics simulations of model systems.
To this end we simulate both a simple liquid, such as a liquefied noble gas,
and a model of polymer melt, like silicon oil, letting them fill a super narrow
capillary(with diameter of about ten times the atom size), immersed in the
liquid. In both cases, after a transient period (of a few nanoseconds), a
meniscus is observed to move up the tube on a distance which grows as a square
root of the time elapsed, exactly as predicted by Lucas and Washburn nearly a
century ago. As expected, the speed of filling increases as the square root of
the tube diameter. The flow velocity is maximal in the middle of the capillary
and goes gradually down to zero at the capillary wall.
For the polymer melt, however, it turns out that the melt slips with non-zero
velocity even immediately at the wall! The hypothetical distance from the wall,
where the extrapolated flow velocity would have reduced to zero, defines what
is called a slip length, and in our computer experiments this slip length is
comparable in size with the nanotube radius. Therefore, a straightforward
application to nanotubes of the classical Lucas-Washburn law for capillary
filling would not work. Our computer experiments show, however, that a
consistent description of the imbibition process in nanotubes is still possible
upon a simple modification of the Lucas-Washburn law whereby the capillary
radius is incremented by the observed slip length of the flow.
Thus a knowledge, accumulated over decades of scientific research, may now be
readily transferred to modern nano-applications and nano-technology.
Snapshot of a capillary being filled by a simple liquid upon immersion
into a reservoir. The magnifying glass is focused on the meniscus area
and shows the velocities of particle between tube axis and wall against
a density profile of the liquid. LC11834
***
Anderson localization goes oscillating
Have you ever thought how much "periodic" can be a random system?
Does natural disorder hide anything strictly periodic? A simple
model of a disordered optical system suggests that its mean
transmission should oscillate with sample length. The model,
supported by numerical experiments, shows for the first time that
the transmission periodically rises and drops as the system gets
thicker. Physicists know since half a century that Anderson
localization is the phenomenon ruling wave transport mechanism in
disordered systems. It is also widely accepted that the transmission
drops exponentially for thicker samples. Now the new results
demonstrate that in a generically produced random mixture of two
different units, e.g. special-sized "black" and "white balls", the
exponential law appears modulated by periodic oscillations.
Responsible for these unexpected oscillations are rarely forming
special states, known as "necklace states", which induce direct
transmission channels even through very thick samples. Such
necklaces periodically appear to be efficiently transmitting each
time the system reaches special thicknesses. The edifying feature of
the effect found adds a new insight into the physics of Anderson
localization. LC11146
***
.JPG)
Modified Thermal Emission from Self-Assembly: Opals Meet the Light Bulb
Han et al. predict that if a heated metal is first structured through simple self-assembly techniques, the color of its “glow” can be altered in useful ways. A heated object emits light according to its temperature and optical properties. This phenomenon, known as thermal emission, is utilized in conventional light bulb filaments. Unfortunately, light bulbs emit a large quantity of heat, resulting in inefficiency. Photonic crystals — materials that are structured on a micrometer length scale — are being explored for eliminating this heat, potentially providing more efficient thermal emission sources. However, the study of this effect has been limited due to difficulties in making the appropriate structures. Previously, photonic crystals have been obtained by emulating the natural process that leads to gemstone opals. Unfortunately, experiments on these self-assembled structures suggested that they were ill suited for modifying thermal emission. Han et al. considered the physics of this problem and found small, experimentally realizable changes to the structure that should lead to modified thermal emission. Thus, these new self-assembled photonic crystals, which are easy to prepare, have great potential for modifying thermal emission both for fundamental understanding and applications.
LC11750
***
Single-Molecule Force Spectroscopy Reveals Protein Folding Free
Energy Landscapes
Understanding biomolecular interaction is one of the most important
question of biological physics. Atomic force microscopes allow one
to unfold proteins or stretching molecules essentially by hand and,
with the help from modern statistical theory, to reconstruct the
free energy landscape of such interaction.
Kiang's group used a microscopic cantilever attached to the
atomic force microscope tips, to pull a protein while measuring
the protein's reaction force by the bending of the cantilever.
Using the modern nonequilibrium work theorem, the group
mapped the protein's free energy landscape along the entire unfolding
trajectory, which had not been accomplished to date by any other
methods. The new technique can be applied to most any biomolecule,
complex, or molecular interaction. LU10757
***
Magnetic Explosions
We studied the way microscopic spins in magnetic materials move upon
magnetization reversal, such as materials that are used in hard disk
drives. These systems can be designed to have avalanches where only
a few spins are needed to start a cascade, leading to many flipped
spins, for example, as you see with dominoes. However, we show that this
process is much more analogous to an explosion than had been previously
thought. Instead of spins just flipping once from up to down, they
continue to spin around, that is, the spins get hot, and in many materials
they stay hot for a substantial amount of time. This greatly alters the
microscopic physics of the system, which means that a change in one part
of a magnet can affect the behavior much further away. The hot region
forms a growth front and the heat from it recruits neighboring spins
into the explosive region before they eventually cool down. We analyzed
this behavior in detail using simulations and theoretical models and
were able to determine under what circumstances an explosion takes off.
The attached graphics shows a simulation of spins in a film during an
avalanche for: (a) high damping, and (b) low damping. The brightness
is a measure of the amount of spin motion. LR10732
***
From geometry class, we know that we can think about shapes that live
only in a two-dimensional world, but can real materials behave as if
they live in Flatland? New measurements of current flow in a special
type of copper-oxide high-temperature superconductor provide evidence
for two-dimensional superconductivity coexisting with one-dimensional
rivers of charge. The superconducting state, involving resistance-less
flow of electrical current, requires that conduction electrons pair up
and be able to flow in three-dimensional (3D) space. In copper-oxide
superconductors, the interactions that cause the pairing occur in 2D
planes; weak interactions between the planes allow the supercurrents to
flow in 3D. In the special copper-oxide compound considered here,
experiments indicate that the charge carriers tend to segregate into 1D
rivers known as "stripes". It had been common belief that the ordering
of charges into stripes within the Cu-O planes would prevent or compete
with superconductivity. The new measurements of current flow
contradict these expectations and are consistent with theoretical
predictions for superconductivity in Flatland. The observation of 2D
superconductivity within a 3D crystal over an extended temperature
range is extremely unusual, and the coexistence with 1D stripes
indicates the surprising nature of superconductivity in copper-oxide
compounds. LC11292
***
Face-to-face with correlated wave functions
The microscopic description of strongly interacting systems is
one of the latest frontiers in condensed matter theory.
Indeed, several materials which show spectacular - and still debated -
properties, like high-temperature superconductivity, superfluidity
and quantum Hall effect, go with a strong repulsion among particles.
The aforementioned phenomena generally manifest themselves at very low
temperatures, where the quantum-mechanical nature of particles comes
into play. As a consequence, all information related to their
collective quantum behavior are encoded into the ground-state
wave function of the system.
Nonetheless, the derivation of the ground-state wave function in presence of
strong correlation is a complicated issue, which so far has been solved in
few cases.
From the experimental point of view, the recent developments on cold atoms
trapped in optical lattices allow one to have a direct control on the
various couplings entering into the game, giving the unique opportunity to
make a close connection with theoretical models and to answer
fundamental questions.
In particular, by tuning the lattice potential depth on trapped-bosonic atoms,
it is possible to control the ratio between the kinetic energy (that tends to
delocalize the particles) and their local repulsion (that instead tends to
localize). It is therefore possible to drive the system from a superfluid to
a Mott insulating state. This behavior, that is also expected in electronic
systems, reflects the dramatic change in the electronic/bosonic arrangement
across a quantum phase transition.
In this work, we tackle the problem by introducing a wave function, which
turns out to be very effective to model a system of interacting bosons in
one, two, and three dimensions. We find that
the essential ingredient in order to capture
the physics of a strongly-interacting system is a two-body correlation term,
known as Jastrow factor.
In presence of this term, particles ''feel each other'' over all possible
distances, making it possible to correct the mean-field wave function and
describe a superfluid-insulator transition.
Remarkably, we find for the first time that the long-range
character of the Jastrow terms is essential in order to correctly reproduce
both the superfluid and the Mott insulating state.
Moreover, our accurate description of the correlated ground state allows us to
explain the peculiar excitation spectrum found in the
experiments of Stoferle and collaborators.
Indeed, the different behavior of the Jastrow correlation factor
across the transition naturally induces high-energy excitations in the
superfluid state in two and three dimensions, but not in one dimension.
These high-energy excitations signal that the superfluid state already contains
the main fingerprints of the correlated insulating state.
This is an interesting and also surprising result, that bears a
lot of similarities with the metal-insulator transition in electronic systems,
but is not accounted for by most accepted theories for bosonic systems.
Moreover, this general scheme indicates that this
class of wave function permit to have access to the Mott
transition in three-dimensional fermionic models, too,
all the more reason when realistic Coulomb interaction is taken
into account. LY10312
***
The addition of randomly scattered waves gives rise to a speckled intensity
pattern. The evolution of such patterns as the frequency scanned is observed
for microwave radiation, however speckle patterns arise for any type of
wave, including light, sound and electrons. Though the pattern is random, it
is not devoid of structure. A network of phase singularities appears about
points of vanishing intensity. We show that the motion of these points of
darkness provides a diagnostic of the nature of wave transport within the
scattering object. The probability distributions of the displacement of
phase singularities together with the variations of phase and intensity
throughout the speckle pattern are governed by a single parameter, which
provides a precise measure of the degree of spatial localization of the wave
within the sample. For exponentially peaked localized waves, the speckle
pattern seems to move in fits and starts as opposed to the smooth
development of speckle for extended waves. This reflects the degree to which
modes within the sample overlap. The paper provides sharp measures of
speckle evolution which can be exploded to monitor motion of hidden objects
and the development of structural defects. LY10404
***
Femtosecond field ion emission by surface optical rectification
Recent experimental evidence from separate fields of laser physics converges into indicating
that ultra short laser pulses may remove atoms from a metal surface within a time as short as
few hundreds of femtoseconds. This effect might lead to interesting applications such as nano
machining of materials with a control of the surface roughness at the atomic level. One
example is the recent advent of the atom probe tomography technique in which laser pulses
are used to remove a tip like shape sample material atom by atom allowing a full three-
dimensional image of the material to be obtained in the real space. Another example is the
possible strong anisotropic ablation of suspended gold nanoparticles induced by 100 fs laser
pulses. Finally a non-thermal ablation regime of planar metal surfaces has been reported. In
all these cases the underlying mechanism of this ultrafast emission is obscure and might be
ascribed to some yet unidentified nonlinear surface optical effect. In this article a possible
explanation for these observations is presented based on the optical rectification of the light
wave at the surface of materials. Moreover, we put forward the working hypothesis that the
same effect may be also at the root of the reported ultrafast ablation phenomena. LB11245
***
Unraveling a granular labyrinth
We describe a new pattern formation process where beautiful labyrinth
structures emerge as a mixture of grains and fluid slowly dries out. Key
to the pattern formation is an interesting interaction between granular
and fluidic influences. The granular-fluid system is confined between
two glass plates, and the receding fluid/air meniscus compiles a layer
of compacted grains and becomes unstable, whereby fingers of air invade
the system in a random fashion. We show that a uniform characteristic
length-scale in the pattern develops as a compromise between two
opposing forces: the surface tension of the fluid, and the frictional
stress resulting from grain-grain contact networks within the compacted
granular layer. The granular labyrinth is visually striking, with
characteristic length-scales up to several centimeters. An understanding
of the pattern formation process is developed along three complementary
paths: experiments, simulations and theoretical considerations. LC10922
***
Control and dynamics of vortices in a turbulent flow
Vortices and similar structures appear continuously in nature: from
tornadoes to vortices at the tip of plane wings,
from the Jupiter's Great Red spot to the smoke of a cigarette, the fluid
elements tend to form stable structures.
The understanding of their dynamics and stability are important both
from the basic and applied points of view:
alterations of the environmental conditions can produce structural
changes on these flows.
The mechanisms at the origin of these changes and the role played by the
structures themselves are still subject of debate.
That¿s why experiments carried out in controlled environments are so
important: the parameters that regulate
the evolution of the fluid (i.e. pressure, propellers) can be easily
modified, in such a way that the flow can be
destabilized and new structures or configurations can arise. This work
shows that the vortices in a cylindrical
cavity can present a rich dynamics, with random or forced flow
inversions. This dynamics can be explained
using an astonishingly simple toy model. LB11142
Attached figure: Picture of an experimental vortex in the cylindrical cavity
***
New type of electron-nuclear spin polarization
The electron spin in semiconductor quantum dots is currently extensively investigated in the context of quantum information processing. The interaction with the nuclear magnetic moments of the lattice atoms constitutes the major source for the loss of the electron spin. We, for the first time, demonstrate the formation of a strong nonequilibrium dynamical nuclear polarization in semiconductor quantum dots under continuous optical pumping of the electron spin at zero magnetic field. Unlike the vast amount of previous work on semiconductors based on the standard spin cooling regime, the formation time is as short as 20 microseconds. We also uncover a life-time of about 200 microseconds due to nuclear dipole-dipole decay. Specific of our study is the use of quantum dots where the electron spin is coupled to only a few hundred nuclear moments and where the Overhauser field does not exceed markedly the spectral broadening of the Zeeman levels. The nuclear polarization is almost complete and survives up temperatures of about 100 K. In this way, the electron spin is stabilized and the coupled electron-nuclear system exhibits a variety of novel features that might be used for quantum bit implications. LC11325
Wednesday, June 27, 2007
6-27-07
Mott Insulator Structure
A metal can be defined as being a good electrical conductor, whereas an insulator acts as a poor electrical conductor. Nevertheless, there are materials which, according to theoretical predictions, should be metals, but when measured behave actually like insulators. The latter class of materials is referred to as Mott insulators which represent prime examples for strong electronic correlations in solids. The insulating state in a Mott insulator, in contrast to the one in conventional insulators, can come about purely by electron-electron interactions. In real materials, however, the actual role of the crystalline lattice vibrations for the Mott metal-insulator transition is still in debate. In this paper, we report on directional-dependent ultra-high-resolution thermal expansion measurements across the Mott metal-insulator transition in a quasi-twodimensional organic conductor. We find strong indications for the intricate role of the lattice degrees of freedom for the Mott transition in these materials.
***
New type of electron-nuclear spin polarization
The electron spin in semiconductor quantum dots is currently extensively investigated in the context of quantum information processing. The interaction with the nuclear magnetic moments of the lattice atoms constitutes the major source for the loss of the electron spin. We, for the first time, demonstrate the formation of a strong nonequilibrium dynamical nuclear polarization in semiconductor quantum dots under continuous optical pumping of the electron spin at zero magnetic field. Unlike the vast amount of previous work on semiconductors based on the standard spin cooling regime, the formation time is as short as 20 microseconds. We also uncover a life-time of about 200 microseconds due to nuclear dipole-dipole decay. Specific of our study is the use of quantum dots where the electron spin is coupled to only a few hundred nuclear moments and where the Overhauser field does not exceed markedly the spectral broadening of the Zeeman levels. The nuclear polarization is alm!
ost complete and survives up temperatures of about 100 K. In this way, the electron spin is stabilized and the coupled electron-nuclear system exhibits a variety of novel features that might be used for quantum bit implications.
***
Overcomming atomic-scale mismatch of materials
Interfaces, which lock into registry at the atomic scale, provide a
fundamental building block in device technology. However, if the atomic
structure of the two materials is mismatched, there had been no
possibility of connecting them in registry - not even using interlayers
of conventional periodic materials.
In this paper we show that quasiperiodic materials can provide
interlayers, which allow to in-registry interface alignment even for
mismatched materials. This opens up exciting new possibilities for
devices combining materials, which up to now had been thought to be
structurally incompatible.
The Figure illustrates the concept. The yellow and orange materials in
Fig. 1a are mismatched, as their atomic structures do not match at the
interface (they can slide freely along each other). The quasicrystalline
(non-periodic) interlayer in Fig. 1b, however, provides an atomic match
with both materials (at the interfaces the green atoms are locked
between neighboring yellow and orange atoms, respectively such that the
materials cannot slide freely with respect to each other).
***
New type of electron-nuclear spin polarization
The electron spin in semiconductor quantum dots is currently extensively investigated in the context of quantum information processing.
The interaction with the nuclear magnetic moments of the lattice atoms constitutes the major source for the loss of the electron spin.
We, for the first time, demonstrate the formation of a strong nonequilibrium dynamical nuclear polarization in semiconductor quantum dots
under continuous optical pumping of the electron spin at zero magnetic field. Unlike the vast amount of previous work on semiconductors
based on the standard spin cooling regime, the formation time is as short as 20 microseconds. We also uncover a life-time of about 200 microseconds
due to nuclear dipole-dipole decay. Specific of our study is the use of quantum dots where the electron spin is coupled to only a few hundred nuclear
moments and where the Overhauser field does not exceed markedly the spectral broadening of the Zeeman levels. The nuclear polarization is almost complete
and survives up temperatures of about 100 K. In this way, the electron spin is stabilized and the coupled electron-nuclear system exhibits a variety
of novel features that might be used for quantum bit implications.
***
The presence of air makes it easier to penetrate deep into sand.
When an object impacts on a bed of very loose, fine sand, a fascinating
phenomenon occurs: The object is instantaneously engulfed and a surprisingly
vigorous jet shoots out from the surface of the sand, just as in a liquid1.
Even more intriguing is the recent observation that, when the ambient
pressure is reduced, the jet is found to be less vigorous2. What we found in
our laboratory is that the size of the jet changes with the ambient pressure
because the object penetrates considerably less deep into the sand in vacuum
than at atmospheric pressure. It turns out that, when the sand grains are
small, the airflow into the sand created by the moving object can be strong
enough to partially levitate the grains and reduce the friction on the
object. The details of how this happens are still unclear, but what is
certain is that, when designing spacecrafts and robots that should land or
walk in sandy surfaces like in Mars, you better take into account the
ambient pressure at which the event occurs!
1 Lohse et al., Phys. Rev. Lett. 93, 198003 (2004)
2 Royer et al., Nature Phys. 1, 164 (2005)
Thursday, June 21, 2007
6-21-07
NEW CLUES ON THE TRACK OF LORENTZ VIOLATION
Violations of Lorentz invariance are allowed in many theories of
quantum gravity, but have never been detected. Now a collaboration of
researchers at Stanford University, University of Western Australia
(Perth), Observatoire de Paris, and Humboldt-University (Berlin,
Germany) has followed a new lead:
The team combined two Michelson-Morley (MM) experiments to
simultaneously observe the times-of-flight of light propagating back
and forth in dissimilar interferometers. Berlin used optical
Fabry-Perot cavities made of quartz, whereas Perth used sapphire
whispering-gallery resonators, which enclose microwaves into a
refractive medium.
The individual experiments are sensitive to Lorentz violation in the
physics of light (the photon sector) as well as the physics of the
material of the cavity walls, which acts through distorting the
cavities (the matter sector). Since the effects could cancel, this
leaves a loophole for Lorentz violation.
Holger Müller (holgerm@stanford.edu) of Stanford explored how these
influences depend on the geometry, material, and index of refraction
of the cavities. He calculated them in a Lorentz-violating extension
of the standard model and disentangled them in the data analysis. The
result: Einstein?s relativity triumphs, in spite of a new lead and the
very best sensitivity ever achieved in MM experiments, a part in 10^16.
***
On making viscous “veins” and “swirls” in microchannels
Microfluidic devices are used to create novel and well-controlled
environments to study the behavior of viscous stratifications. In these
systems, due to the importance of confinement, a viscous liquid swept
along in the flow of a less viscous liquid can form viscous “veins” or
threads. By taking advantage of the laminar flow conditions, the
position and the size of the threads can be readily manipulated. This
Letter presents an experimental study of two miscible viscous threads
that flow off-center in microchannels. In particular, the shear-induced
buckling of thin threads near the channel walls is investigated.
Subjected to this instability, the viscous columns develop into
swirling patterns that lead to their eventual rupture into array of
viscous swirls, the miscible counterpart of droplets. The swirling
instability provides a means for passively producing discrete diffusive
microstructures in a wide range of fluid environments.
***
Micro Swimmer with a ``Diffusive Jet Engine''
An artificial micro-swimmer that propels itself by asymmetric distribution of
reaction products around it is made and fully studied experimentally. The
observed properties of the swimmer are in perfect agreement with the
theoretical predictions [PRL, 94, 220801 (2005)].
The directed propulsion of micro- and nano- scale objects in water is
problematic
because of the combination of low Reynolds number and Brownian motion on
these length scales. In order to achieve an artificial micro- or
nano- scale swimmer
that is able to propel itself in a purposeful way, one needs both a
swimming strategy
that works in the environment of low Reynolds number, and a strategy
for steering
and directing the motion that can overcome the ubiquity of Brownian motion.
Common bacteria, such as E. Coli, achieve propulsion by non-time-reversible
motion of long flagella, and employ a "run and tumble" strategy to be able to
swim towards or away from environmental stimuli. Here we fully characterize
the motion of an artificial micro-scale swimmer, which uses diffusio-phoretic
effects for autonomous propulsion. We show that at short times, the motion
has a substantial component of directed motion, with a velocity that depends
on the concentration of fuel molecules. At longer times, the motion reverts
to a random walk, in which runs of directed motion are interrupted by random
changes of direction. Our results suggest strategies for designing artificial
chemotactic systems.
***
Zigzag coherent structures produce rogue waves.
A specific, genuinely three-dimensional mechanism of rogue wave
formation has been recognized through numerical experiments.
As the present simulations show, freak waves, one of the most
striking phenomenon of the ocean, can appear through interaction of
coherent structures, consisting of obliquely oriented stripes
with increased wave amplitude. Spontaneous formation of zigzag
patterns takes place in a nonlinear stage of the so called
Benjamin-Feir (or modulational) instability from a planar wave.
If initial wave steepness is sufficiently high, these coherent
structures produce giant waves. The most tall waves appear in
``turns'' of the zigzags. A simple explanation for this mechanism
has been suggested. This work is a first-time systematic study of a
late stage of the Benjamin-Feir instability, based on fully nonlinear
explicit equations of motion for long-crested water waves.
Unlike previously used weakly nonlinear models, the present model
is sufficiently accurate to simulate very steep waves.
***
Ultra-high energy density capacitors
Capacitors in current use have low energy storage capacity as the
polarization saturates at low electric field. This limits the
application of such materials as energy storage device. We uncovered
a new mechanism for storing unprecedented amount of energy in
capacitors, i.e. through reversible transformation between non-polar
and polar phases in nanoscale materials.
Like batteries, capacitors are energy storage devices. Capacitors
provide quick bursts of energy, necessary for power conditioning,
high-density electronics, or quick acceleration. Unlike batteries, no
chemical reactions are involved in their energy storage mechanism,
rather polarization, the electric field equivalent of "magnetization",
is involved. Our supercomputer simulations explain how certain polymer
mixtures store energy at very high densities and tailor it to specific
applications. These mechanisms are universal and can be exploited in
other systems through, for example, controlled deposition of
nanodomains that undergo a non-polar to polar transition at different
values of the electric field. In general, our work provides a
systematic route, not limited to polymers, for obtaining
nanostructured materials with very high energy density. Polymers are
also nontoxic and environment friendly. In contrast to batteries,
capacitors can be charged quickly, charged many more times, and energy
can be extracted much faster.
Figure Caption (figure attached with the mail):
The left side of the diagram shows the nonpolar alpha phase and the
polar beta phase of PVDF. In the nonpolar phase the polarization on
the alternate polymer chains within the unit cell cancel the total
polarization, whereas in the polar phase there is a non zero
polarization within the unit cell. In the right hand side, the red
shaded area is the energy density for a single phase material (here
for a polar beta phase). The blue shaded area is the energy density
when a non polar alpha phase transforms to polar beta phase under the
application of electric field. A single phase material has a given
polarization at zero electric field and the polarization saturates
(Dsat) at very low electric field. In our calculations we show that it
is possible to convert an impurity modified polymer from non polar
alpha phase to polar beta phase under electric field. Polarization
saturates at large electric field and leads to higher energy storage
in the mechanism proposed by us.
***
The glass transition in tight situations
Are glasses merely extremely slow liquids? Liquids take on the
shape of their container. In this paper, we show that unlike
liquids, glasses aren't comfortable in confined spaces.
We study pastes composed of tiny plastic particles in a
liquid, a model system which acts like a glass when the
particle concentration is increased. Using an optical
confocal microscope, we directly view particle motions in
three dimensions. When the sample is confined between
two parallel plates, particles move slowly and the
sample becomes even glassier. As the sample particle
concentration is increased toward the glass transition
point, we observe confinement-induced slowing at larger plate
separations. Previous research has shown groups of particles
in dense suspensions move cooperatively. Our work suggests
glasses are solid-like because these groups can't move when
the sample chamber is thinner than the typical size of these
groups. These experiments help us understand earlier work done
with thin polymer films and other glassy materials, but as we
use particles rather than atoms, we get to directly see how
confinement influences the glass transition.
***
Nonlinearities predicted to be strengthened by photonic crystals
Typically, photons can pass by one another unchanged; however, a
number of important scientific and technological applications can be
enabled by using matter as a medium for photons to talk with one
another. These interactions are generally weak; however, in this
paper, we discuss a scheme that can strongly enhance the strength of
these interactions in a completely new way. It consists of placing a
nonlinear material inside a photonic crystal. The latter is
characterized by its photonic bandgap, a range of frequencies for
which photons are almost perfectly reflected. Its presence allows
more time for nonlinear processes to take place. However, the
nonlinear material is probed at a frequency just below the photonic
bandgap. For certain special materials, such as single nanocrystals
of cadmium selenide, the degree to which this lifetime can be
increased may be as much as a factor of forty at room temperature.
For other materials, an enhancement of at least a factor of two is
expected. Enhanced optical nonlinearities should allow much lower
powers and volumes to be used in nonlinear devices, which will have
implications for telecommunications, optical computing, and
ultimately, quantum computing.
***
Electrons in ferromagnets invited to the quantum dance!
Condensed matter physicists have for a long time been intrigued by the
manner in which electrons in metals assume a quantum wave-like character
as the temperature is lowered. This character trait becomes manifest when
electrons remember their travels and interfere with themselves, much like
water waves building up in amplitude when suitably channeled. The
resulting tendency of the electrons to stay in one place, or equivalently
to be "localized", causes an increase in electrical resistance that has a
non-classical quantum origin. This signature quantum resistance is easily
observed in ordinary metals but is not expected to be present in magnetic
metals (i.e., ferromagnets) such as iron, since intrinsic magnetic fields
severely compromise the ability of the electrons to self
interfere. Contrary to these expectations, this paper shows that electrons
in ferromagnets can behave much like their brethren in ordinary metals and
execute a dance in which they revisit their former coordinates and self
interfere. In the experiment, atomically thin films of iron are fabricated
and then measured at low temperature and high magnetic field without being
exposed to harmful air oxidation. Theoretical analysis takes into account
electron-electron interactions and explains the observed localization
behavior.
***
Luttinger liquid in the core of a screw dislocation in Helium-4
On the basis of first-principle Monte Carlo simulations we discover that the screw dislocation in He4 crystal features a superfluid at its core. This is the first example of a regular quasi-one-dimensional supersolid -- the phase of matter featuring both crystalline and superfluid orders.
The supersolid state of matter in ideal quantum crystals was hypothesized about by Yang, and later by Andreev, Lifshitz and Chester about half a century ago. In this regard He4 crystals were considered to be the best candidates due to their extreme quantum properties. It appears, however, that this idea does not materialize, as recent numerical studies have shown. Supersolid state of He4 is anticipated now to emerge from extended structural defects, i.e., contrary to the common wisdom, disorder promotes superfluidity. Previous studies found that grain boundaries with high degree of frustration between the crystallites and fully disordered “glassy” states in He4 have superfluid properties. In this respect, dislocations constitute a special system which is characterized by translational order along their cores, as perfect crystals do. Here we find that in the case of screw dislocations the resulting state is a superfluid in the core. This is the first example found in Nature of a regular supersolid structure formed in continuous space.
***
Complex Yet Similar Networks
Imagine that suddenly all the molecules of air in your office move to
one corner of the room. It is interesting that this possibility is
actually not prohibited by any law of physics but instead by statistics:
the probability of observing this, or even any noticeable fluctuation in
the pressure, is just too small to scare any statistical physics
practitioner. In this paper we demonstrate that a similar effect is at
work in a class of much more complex systems, namely those modeled by
complex networks. Previous studies had difficulty in defining
unambiguously the thresholds of dynamical processes such as epidemic
spreading and synchronization, because of the huge number of possible
configurations of the underlying networks. We have solved this problem
by showing that the network spectra, which encapsulates the relevant
information about the thresholds, are remarkably similar in ensembles of
large complex networks. Therefore, within these ensembles, the
probability of strong deviations in the network dynamics is negligibly
small, much in the same way as the probability of having all the
molecules at the corner of the room.
***
NONABELIONS APLENTY
The so-called "NonAbelian
matter", a.k.a. the stuff of quantum computers,
may be commonplace in semiconductor devices. Condensed
matter physicists have
studied since two decades the behavior of electrons under intense magnetic fields
when they are confined in an atomically thin interface between semiconductors.
Tuning the applied magnetic
field leads to various states of matter for the electrons
that are understandable only by quantum mechanics. The
systematics of these phases
has been obtained by various theoretical arguments in the past. Recently in 2003
a team of experimentalists lead by Wei Pan from Sandia national Lab has observed
a new series of states that
needed contrived but plausible explanations to be
accomodated in the established theoretical framework. However
the present theoretical work
shows that, contrary to previous belief, all these new states may be of the
"non-Abelian" kind,
a very delicate state of matter - the Holy Grail of topological quantum
computation.
Such states support several kinds of particles replacing the ususal electron as an elementary
particle and braiding these particles by electrostatic means can be used
to perform all the operations needed
in a quantum computer based on topology.
So we may not have to wait for complex atomic physics devices to create
these
much awaited states, they may be already in abundance in (some) semiconductor devices
when operated in
the proper regime.
RELATED RECENT PAPERS ADVERTISING SAME TOPIC:
"devices
based on the fractional quantum Hall effect may fulfill the promise
of quantum computing"
charles Day
in Physics Today, 58, 21 (2005).
"Anyon There ?"
David Lindley, Physical Review Focus, 16,
story 14, Nov 2005
"Computing with quantum Knots"
Scientific American, p.57 (2006).
***
Acceleration of nuclear spin polarization is found possible by a short
laser pulse!
Regardless of the extreme difficulty to polarize nuclei, spin polarized
nuclei are highly needed for the efficient utilization of radioactive
isotope (RI). Use of laser beams to polarize nuclei is one of the efficient
methods, but the efficiency is completely hampered if the nuclei need to be
polarized within a few microsecond, which is a typical lifetimes of unstable
isotopes. We have theoretically found that the combined use of short laser
pulses and a DC field results in the acceleration of nuclear spin
polarization, and the polarization can be completed within a few to tens of
ns. This is about 3 orders of magnitude shorter than the time needed by any
known optical methods to polarize nuclei and the achieved polarization turns
out to be surprisingly high.
***
Diving below the surface: Dynamic force microscopy reveals ordered arrays of sub-surface vacancies in ceria
Ceria is a most important material in oxidation catalysis promoting and regulating chemical reactions by taking up and releasing oxygen and is used, for instance, in automotive catalytic converters. Oxygen transport is facilitated by vacancies, i.e. single ions missing in the atomic lattice, that may diffuse from the bulk to the surface of the oxide or vice-versa. Dynamic scanning force microscopy (DFM) now allows to unambiguously identify sub-surface oxygen vacancies buried in the third surface atomic layer of a well prepared CeO2(111) surface. In this work we present evidence for subtle attractive and repulsive interactions between these sub-surface vacancies forcing them into well ordered arrays having an open structure. Such an arrangement reflects the topology of highest symmetry at high vacancy densities. This letter demonstrates by means of an impressive example the capability of advanced force microscopy to characterize atomic details not only at the surface of an insulator but also to reveal and identify complex sub-surface structures that may have a major impact on surface chemistry.
***
The nature of the mysterious dark matter may very soon be revealed through its
emission of gamma-rays. A very striking smoking gun signal is robustly
predicted for one of the simplest models for dark matter. This feature could be
detected with the GLAST satellite telescope to be launched later this year.
Astronomical observations have established that ordinary visible matter only
makes up a minor part of our Universe. Instead the mass of the Universe is
dominated by so called dark matter, believed to be made up by unknown exotic
particles. The precise nature of these particles remains one of the great
unsolved questions in cosmology. A recently proposed solution comes through a
new type of particle, called the inert Higgs. Although normally not emitting
any light at all, such particles traveling through space will occasionally
annihilate each other and produce high energy gamma-rays. We have shown that
this dark matter candidate is capable of providing an extremely characteristic
gamma-ray spectrum with good prospects for detection with the space-borne GLAST
telescope to be launched in December 2007. Such detection would unambiguously
confirm the existence of dark matter particles in our Universe.
***
All optical magnetic recordingWe have experimentally demonstrated controlled magnetization reversal
induced by a single 40 femtosecond circularly polarized laser pulse in
the magnetic alloy GdFeCo, a material relevant for data storage. No
external magnetic field is required for this opto-magnetic switching,
and the stable final state of the magnetization is unambiguously
determined by the helicity of the laser pulse. This finding,
previously believed to be fundamentally impossible, reveals an
ultrafast and efficient pathway for writing magnetic bits at
record-breaking speeds. With the recent development of compact
ultrafast laser systems and the successful incorporation of lasers in
magnetic storage devices, the present demonstration of ultrafast
all-optical magnetization reversal might spur the realization of a new
generation of ultrafast magnetic recording devices.
Figure caption:
Demonstration of compact all-optical recording of magnetic bits,
achieved by scanning a circularly polarized laser beam across the
sample and simultaneously modulating the polarization of the beam
between left- and right circular.
***
Dissipation in Shock Waves in the Solar Wind
In this paper we describe the first statistical results of electric field waveform captures taken in the ramp
region (transition region) of interplanetary (IP) shocks. Near 1 AU IP shocks result mainly from explosive
eruptions of massive amounts of plasma ( 1011–1013 kg) known as Coronal Mass Ejections (CMEs). CMEs
propagate out into the solar wind at velocities in excess of 1000 km/s dragging the sun’s magnetic field with them.
Their large velocities can result in shock wave formation. Unlike a regular fluid, IP shocks cannot rely on particle
collisions to prevent the wave from breaking by dissipating energy. Our paper investigates a possible method
for such dissipation, wave–particle interactions (i.e. electric field oscillations scattering particles). Because
these shock waves can accelerate particles to high energies that can damage space systems, understanding the
underlying mechanisms which govern their evolution and propagation from the sun to the Earth is important.
In addition, they provide an accessible laboratory for studying collisionless shocks.
***
Black hole radiation may reveal hidden symmetries
Black holes are hot: as Hawking showed 35 years ago, they emit
radiation at a temperature governed by their size. Just as air
temperature is a manifestation of the motion of molecules, the
Hawking temperature may reveal the underlying microscopic states
of the black hole. Until recently, the nature of these states
was completely unknown. Today, we have too many answers: at least
ten distinct theoretical descriptions are known, portraying very
different microscopic physics but all giving the same temperature.
In this paper, I explore the possibility that a hidden symmetry
of the event horizon could explain the thermal properties of black
holes, independent of other details. I show that such a symmetry
exists and yields the correct value for the black hole entropy.
I offer evidence that it may provide a "universal" explanation:
at least one string theory approach to black hole thermodynamics
occurs as a special case, and there are tantalizing hints of a
connection to a calculation from loop quantum gravity. Black
hole radiation may thus reveal hidden symmetries of the horizon.
***
Nonlocal Properties of Dynamical Three-Body
Casimir-Polder Forces
The existence of quantum nonlocal correlations between distant microscopic
objects is one of the most striking features of quantum mechanics. Also
quantum fields, such as the electromagnetic fields in quantum
electrodynamics, are predicted to possess nonlocal spatial correlations.
Our paper has shown that in a system of three electrically neutral atoms at
different locations the long-range interatomic forces, known as
Casimir-Polder forces, bear an imprint of the nonlocal correlations of the
electromagnetic field, although these correlations cannot be used for
transmitting information at superluminal speed. The importance of these
results is that it is shown for the first time that the dynamical
Casimir-Polder interaction between atoms can be used to detect nonlocal
properties of field correlations.
In our paper the three neutral atoms play the role of probes for detecting
field correlations, and an expression for the time-dependent three-body
Casimir-Polder interaction is obtained in two different ways: i) from the
interaction between two atoms in the field created by the third, and ii)
from the interaction of one atom with the field created by the other two. In
a stationary situation both ways yield the same result. In a time-dependent
situation, however, the results are different, since in case i) the
interaction energy displays nonlocal features, related to the nonlocal
properties of the field created by the third atom; in contrast, nonlocal
features are absent in case ii).
The different results obtained for the time-dependent situation can be
explained in terms of the three-body forces stemming from the interaction
energy, since the two different ways of calculation seem to correspond to
two different ways of measuring these forces. Case i) in fact involves a
correlated measurement of the forces acting between two atoms in the
presence of the third. In contrast, case ii) involves the measurement of the
force acting on a single atom arising from the other two. In other words,
correlated measurements on two atoms are necessary to show nonlocal
properties of the electromagnetic field, whereas single force measurements
are not able to reveal field nonlocality.
This conclusion seems to raise question about the physical meaning of
three-body forces (which are inherently nonlocal quantities) and how these
three-body forces should be measured.
Monday, June 11, 2007
6-11-07
Minimum-risk path finding by an adaptive amoebal network
The plasmodium of the primitive organism, the giant amoeba Physarum,
is able to find the minimum-risk path in a spatially inhomogeneous
field of risk.
Thus the amoeba has the capacity for information processing in
optimizing its physiological requirements.
When two food-sources are presented to the plasmodium in the dark,
a thick tube for absorbing nutrients is formed that connects the food-
sources through the shortest route.
When the light-avoiding organism is partially illuminated,
however, the tube connecting the food-sources follows a different route.
Defining risk as the experimentally measurable rate of light-avoiding
movement,
the minimum-risk path is exhibited by the organism, determined by
integrating along the path.
We note that the evolution of the tube network shares features in
common with Hebbian learning
found in neuronal networks, as the tubes grow or shrink and disappear
based on their level of activity.
***
A BIOPHYSICAL THEORY FOR ACTION-AT-A-DISTANCE IN THE BRAIN
Brain-imaging studies have shown that---during directed mental tasks
and also during resting wakefulness---separated regions of the cortex
"light up" to generate waves of synchronized electrical activity.
But what causes these different brain centres to become active
simultaneously?
In this paper [1], we propose that this "action at a distance" co-
ordination of neuron behavior arises naturally from the gap-junction
connections that couple the neurons and glia (neuron support cells)
within the central nervous system. These electrical synapses are
found in all mammalian cells. In the brain, they provide a direct
electrical connection allowing fast inter-neuron communications that
supplement the well-studied primary communication channel involving
chemical synapses driven by action potential "spikes."
Our theory suggests that---provided electrical diffusion via gap-
junctions is sufficiently strong---the brain will spontaneously
organize into distinct regions of high- and low-firing activity that
will exchange contrast on slow time-scales. These spatial patterns
are called Turing structures, named for mathematician Alan Turing,
who, in 1952, first demonstrated that competing diffusion reactions
could explain pattern-formation in biology. The figure below shows a
sample of the range of Turing patterns generated by our cortical model.
***
Colloidal traffic-control on a magnetic lab-on-a-chip
If you have ever worried about times to commute from one place to another
in a big city you will appreciate a smart traffic system. Pietro Tierno,
Tom Johansen and Thomas Fischer from the Florida State University and the
University of Oslo have designed a smart magnetic traffic control for small
particles on a lab on the chip that might be used as fast drug delivery
system. A magnetic field rotating with a frequency slightly faster than the
particles on the lab-onthe-chip can follow significantly reduces their time
of commuting through array of magnetic domains.
49eaefd9.jpg
***
With a second dimension of time, 2T-physics appears to give a better
description of the natural world.
Evidence has been accumulating that the ordinary formulation of physics
(1T physics) is insufficient to describe certain aspects of our world,
just like shadows on walls alone are insufficient to capture the true
essence of an object in a three dimensional room. According to Two-Time
Physics (2T physics), there is more to space-time than what can be
garnered with 1T physics.
The Standard Model of Particles and Forces (SM), which captures all the
physical realities that are experimentally confirmed up to now, happens
to be a particular 3+1 dimensional shadow of a 2T-physics field theory
in 4+2 dimensions, as shown last year. Among the successes of the
2T-physics approach to the Standard Model is the resolution of the
strong CP problem of QCD without an elusive axion.
To help grasp the relation between 1T-physics and 2T physics, consider
the many possible shadows of a 3 dimensional object projected from
different perspectives on the surrounding walls of a 3-dimensional room.
Just like a flatlander, that can crawl and measure only on the surface
of the wall, would think that the shadows of different shapes are
different "beasts" and move differently, so does 1T-physics presents
different dynamical systems in terms of different Hamiltonians, although
according to 2T-physics these apparently different systems are clearly
related to each other since there is a unique dynamical system in 4+2
dimensions that generates all of the 1-time "shadows". Thus 2T-physics
provides a new kind of unification.
So 2T-physics claims that there are other "shadows" of the 4+2 version
of the Standard Model that have different interpretations from the point
of view of 1T-physics in 3+1 dimensions. A program for studying these
"duals" of the Standard Model has been launched recently in a separate
paper (arXiv:0705.2834, accepted for publication in Phys. Rev.)
In this letter the formulation of the general supersymmetric version of
2T-physics field theory in 4 + 2 dimensions is outlined, for fields of
spins 0, 1/2 , 1, with N = 1 supersymmetry (SUSY). This will be a
starting point for physical applications in the form of the
supersymmetric version of the SM in 4 + 2 dimensions. Due to symmetry
constraints that arise through the 4+2 higher spacetime, it is
conceivable that there will be measurable consequences that can
distinguish 2T-physics from other approaches in experiments at the Large
Hadron Collider at CERN that will begin to produce new data in 2008.
For more information see links at my homepage:
http://physics1.usc.edu/~bars/
***
A twist in the search for the Higgs boson
The emergence of the Higgs boson decay mode into two pseudoscalar
bosons, which can relieve the so-called little hierarchy problem and
reduce the LEP2 Higgs boson mass bound, may affect the golden search
modes (h -> gamma gamma, b b-bar) of the Higgs boson significantly.
The LHC may not be able to find the Higgs boson if the
two-pseudoscalar decay mode dominates. Our Letter explicitly shows
that the associated production of the Higgs boson with a W or Z boson
can recover the loss of sensitivity in the golden modes. With the
Higgs boson decaying into two pseudoscalar bosons, which further decay
into 4 b jets, together with at least a charged lepton from the W or Z
boson decay, a significant Higgs boson signal is observable at the
LHC. The ultimate goal of the LHC is hunt for the Higgs boson, which
is responsible for electroweak symmetry breaking and mass generation.
***
The new structure of mother-of-pearl
Biominerals are of interest to material scientists, physicists,
chemists and engineers because of their remarkable mechanical
properties and their fascinatingly complex, self-assembled architecture.
Biominerals, including mollusk and crustacean shells, bone, teeth and
eggshell, are composites of micro-crystals and organics. Metzler et
al. (PRL, accepted for publication) discovered that the orientation
of each micro-crystal in the nacre (or mother-of-pearl) layer inside
red abalone shells can be revealed by imaging nacre with synchrotron
spectromicroscopy. With this high-resolution and sensitivity
microscopy approach the micro-crystals composing nacre stand out, and
their relative orientations are revealed by different gray-levels.
The contrast arises from a mechanism called x-ray linear dichroism,
discovered by Joachim Stöhr (J. Stöhr et al., Phys. Rev. Lett. 47,
381 (1981)), now director of the Stanford Synchrotron Radiation
Laboratory. Linear dichroism has been widely used to study man-made
structures, including superconductors and magnetic materials, but
this is the first discovery of such mechanism in a natural, biogenic
material.
The experiments were conducted at the University of Wisconsin
Synchrotron Radiation Center, where the principal investigator in
this work, Prof. Pupa Gilbert, of UW-Physics, led a group since she
joined UW in 1999. She also served as Scientific Director at the SRC
from 2002 to 2006.
The new dichroic contrast mechanism revealed individual stacks of co-
oriented tablets, and how these are packed in situ, in pristine
nacre. Comparing previous data and their new data, Gilbert and
physics graduate student Rebecca A. Metzler, in collaboration with
theorist Prof. Susan N. Coppersmith, also at UW-Physics, found that
the data are most consistent with a specific model for nacre
formation: randomly distributed nucleation sites are pre-formed in
organic matrix layers, and tablet n+1 nucleation and growth only
starts when the underlying tablet n has reached its final height.
More experiments, currently underway, will make all observations
quantitative. More theoretical models, in three dimensions, will
further refine the nacre formation mechanism, which has puzzled the
scientific community for decades.
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