LX11599
A New Look at Proton Conduction in Oxides
The conduction of hydrogen ions, or protons, is the underlying
mechanism behind the key technologies of hydrogen production, storage,
and energy conversion. In this study we have found that the proton-
tunneling rate - a key component of proton conduction - is closely
related to the vibrational dynamics of the hydrogen in the host
material. We find that when the proton's vibrational motion is
excited by infrared light of a specific, resonant frequency the
tunneling rate is increased dramatically. This colossal enhancement
could potentially improve proton conduction in important clean energy
devices such as fuel cells where a proton-conducting electrolyte
permits the migration of hydrogen ions and forces the electrons to
drive an electrical load such as a motor. Such an improvement in
conductivity would allow a lower operational temperature, a wider
choice of materials, longer cell life, and improved device
reliability. Our study presents a new method for observing proton
tunneling in solids and will be a starting point for future
investigations, both experimental and theoretical, relating proton
conduction mechanisms to hydrogen vibrational dynamics.
***
EU10299
New results for critical indices of fluids.
Fluids consist of a network of interacting particles on the microscopic level. In physics and chemistry it is a challenge of key interest to understand and determine the properties of such many-body systems as accurately as possible. Especially interesting are the properties close to the critical temperature and density, where determining these properties is a demanding task. The thermal properties in terms of temperature and density close to this point are expected to have so-called scaling behavior, which mathematically can be expressed through critical indices or exponents.
In this paper, a new method has been applied to obtain new values for the critical indices of fluids. Two accurate and related liquid state theories have been unified. Then, through the analyzes a new relation between the critical indices was found. Also, it was found that the critical index for the critical isotherm should be an integer odd number. Taken together, the critical indices became simple fractions and integer numbers. The novelty of this result is the simple numbers obtained for the indices. However, on one hand these numbers deviate somewhat from previous estimates, while on the other hand it is not ruled out that they are exact. Thus a central question is: Are these new numbers inaccurate, or is there need to modify previous estimates based on earlier analysis, simulations, and demanding experiments?
***
LZ11710
*Lane formation in driven systems: From pedestrian zones to complex
plasmas*
The formation of “particle lanes” is a ubiquitous phenomenon occurring
in nature when two different species are driven against each other.
When the driving forces are strong enough, different particles exhibit
a remarkable self-organization – they start moving collectively,
forming interpenetrating “stream lanes”. The phenomenon – which is
commonly known from pedestrian dynamics in highly populated pedestrian
zones – occurs in very different systems of driven particles, ranging
from colloidal dispersions to molecular ions. In terms of the
individual particle dynamics, complex plasmas bridge the realms of
classic fluids and colloidal suspensions, and therefore can provide us
with invaluable insights into the new dynamical regimes of laning.
Recently, lane formation was studied in experiments performed on the
International Space Station using binary complex plasmas. By combining
the experiments and particle-resolved Langevin simulations, the
dynamical onset of laning was investigated. Furthermore, based on the
anisotropic scaling index analysis that is exceptionally sensitive to
symmetry changes occurring in particle ensembles, a universal order
parameter was proposed for the lane characterization. The use of such
an order parameter could be very useful for studying the onset of non-
equilibrium phase transitions occurring in various driven systems.
***
LV11730
Feeling photon forces with optical tweezers
Optical tweezers is a phenomenon where highly focused laser light
results in micron scale objects
(microbeads, biological cells, and even single proteins) being suspended
at the laser focus.
However, optical tweezers can also be used to feel the force that comes
from the momentum transfer
of photons. The photon as the quantum of the electromagnetic field also
acts as a particle that carries
momentum which it can impart on to any object. Such a feat is realized
in this work, where an
optically trapped bead covered with nano-sized silver islands is excited
by the laser in the presence
of probe molecules. The number of inelastically scattered photons from
the molecules is greatly
enhanced due to coupling to the metal. The result: the emitted photons
transfer momentum which
pushes the trapped bead off its equilibrium. The recoil forces are in
the range of 100 femtoNewtons
which is comparable to forces as tiny as the gravitational attraction
between two small glass spheres.
The technique demonstrates the use of optical tweezers as an alternative
measure for quantifying
light by relating it to mechanical force and can be applied to all forms
of spectroscopy.
***
LP11356B
Is disorder that messy?
Optical quasicrystals are structures with a modulation of the
dielectric function which
does not show any translational periodicity. These kinds of structures
are furthermore
typified by a high-order rotational and mirror symmetry. We
demonstrate that the optical
properties of quasicrystals find their origin in the optical behavior
of translationally
ordered crystals. In fact, it is possible to identify sub-structures
of the quasicrystal
which, arranged in a translational configuration, reproduce the same
transmission spectrum
of the quasicrystal itself. A number of questions arise then from this
finding: do quasi-
disordered structures differ that much from ordered ones? Can we still
assume that high-
order rotational symmetry of quasicrystals causes their typical wide
optical gaps? In order
to answer these questions we have analyzed the quasicrystals showing
the highest rotational
order known so far, namely 12-fold rotational symmetry. Our results
demonstrate that some
sub-parts of these quasicrystals, typically super-lattices with the
same local rotational
symmetry of the quasicrystals, show the same zero-transmission regions
of the quasicrystals
themselves. Hence, we can safely conclude that the typical wide optical gaps of
quasicrystals do not originate from their high rotational symmetry but
from the optical
properties of super-lattices. Moreover, in view of this finding, we
should also reconsider
the general misconception of quasicrystals as structures with exotic
optical properties."
***
LZ11271
Electrons with opposite spins move in opposite directions
In one dimension, there are only two ways to move: left or right. This
leads to some peculiar properties for one-dimensional systems on the
atomic scale. In our paper we present a one-dimensional conductor
forming on a bismuth surface, which effectively separates the
electrons going through it according to their spin, a kind of rotation
around the electron's axis. It turns out that electrons going to the
left have exactly the opposite spin as electrons going to the right.
Such a situation could have useful applications in the field of
spintronics, a novel type of electronics which is based on the
electron's spin rather than its charge and which could lead to more
effective computers or even quantum computing. The state reported here
is in several ways similar to so-called edge states appearing in the
recently discovered quantum spin Hall effect but instead of being
found for a sandwich structure of semiconductors at very low
temperatures, it is found on a simple, clean surface, is truly one-
dimensional and, most remarkably, even exists at room temperature.
Thursday, January 29, 2009
Wednesday, January 28, 2009
January 28, 2009
LW11682
A New Schrodinger Equation for Hadron Physics
One of the triumphs of theoretical physics of the twentieth century
was the development of Quantum Electrodynamics (QED), the fundamental
theory of electrons and photons. QED not only describes the physics
of the atom with extraordinary precision, but also the basic
properties of the electron itself. The corresponding problem in
particle and nuclear physics is to accurately describe the structure
and interactions of hadrons, such as the proton and neutron, in terms
of their fundamental constituents: the quarks and gluons of Quantum
Chromodynamics (QCD). QCD is much more complicated to solve than QED
because of the strong interactions of the confined gluons and quarks.
The most successful theoretical approach thus far has been to employ
lattice gauge theory computer simulations. In this paper we derive a
new quantum mechanical bound-state equation of quarks and gluons which
has many similarities with the Schrodinger wave equation for atomic
systems in QED. The bound-state solutions of this single-variable
relativistic wave equation give a very good representation of the mass
spectrum and wavefunctions of hadrons for general spin and internal
orbital angular momentum. It thus serves as an excellent first
approximation to QCD which can be systematically improved. Our
derivation is based on two remarkable theoretical developments: (a)
Maldacena's AdS/CFT correspondence between solvable gravitational
theories in a curved higher dimensional space-time (Anti-de Sitter
Space) and quantum field theories in ordinary physical space-time;
and (b) "Light-Front Holography", which allows one to map information
in the fifth dimension of AdS space to hadronic wavefunctions
describing the separation of the quark and gluonic constituents - not
at a fixed time, but at a time set by the front of a light wave.
Because of light-front holography, the description of hadrons obtained
from the new light-front Schrodinger equation is equivalent to the
solutions obtained in AdS space. The AdS representation of a proton in
light-front QCD is illustrated in the figure. For example, a proton
with its three quarks close together corresponds to the boundary of
AdS space at large circumference; conversely, a large-size proton
with far-separated quarks is represented at the inner sphere in AdS
space.
Figure caption: AdS representation of a proton in light-front QCD
***
BX10891
Verwey transition explained
The Verwey transition between the low-temperature charged ordered (CO) and the high temperature valence mixed (VM) modification can now be understood by density functional theory (DFT) calculations illustrated for the double cell perovskite YBaFe2O5 . In CO two types of iron ions appear, Fe2+ and Fe3+, whereas in VM both iron sites have the same non integer oxidation state (namely 2.5). Only by going beyond the conventional DFT (in our case GGA+U) we find the charge order and the insulating phase. The orbital ordering in this correlated system is the main reason for the orthorhombic distortion that occurs during this phase transition. The calculations agree with available experiments for the magnetic moments, the charge ordering and Mössbauer data.
***
ly11758
Graphite Lubricant 2.0
Graphite is one of the best solid lubricants. It is made of stacks of atomically thin sheets of carbon, named graphene. The excellent lubrication is believed to originate in the easy shear of graphene layers with respect to each other. In our work, we have grown large terraces of single and double layer of graphene on silicon carbide. We find that even single layers graphene exhibit very low friction. Furthermore, we find that friction on a double layer is half of the friction on a single layer graphene. The friction contrast can be explained by a difference in the way how lattice vibrations are coupled to electrons in graphene. We discovered this difference in coupling by studying electron emission under illumination with the light of a synchrotron source. Double layer graphene outperforms even graphite as a lubricant due to reduced adhesion.
***
LW11526
Teaching Plasma a New Tune
Recent experiments in the non-neutral plasma group of the University of
California, San Diego, have demonstrated the existence of a new kind of
plasma wave, the Electron Acoustic Wave (EAW) a nonlinear wave that
propagates at much lower frequencies than regular electron plasma waves. For
most of the last 50 years, physicists expected that EAWs, if they existed,
would be heavily damped and insignificant. But in the 1990¹s new theories
emerged that suggested otherwise. These were, for the most part, ignored -
until now, when we have observed these EAW plasma waves directly. At low
amplitudes the observed waves match the EAW dispersion relation predicted by
Dorning and collaborators. But when driven to large amplitudes, the waves
³train² the plasma so that it resonates at the driver frequency even after
the driver is turned off, for any frequency chosen by the experimentalist.
Put another way, it¹s as if a grandfather clock could be taught to speed up
by changing the length of its pendulum. We present the first detailed
measurements showing the intimate interaction between acoustic waves and
plasma particles. Such detailed measurements illustrate how university-scale
basic plasma physics experiments can study complex wave phenomena relevant
to large-scale plasma experiments conducted in national laboratories.
***
ES10464
*New AC-field pumps for micro-systems*
New electro-hydrodynamic micro-pumps are devices of much potential for
micro-fluidic systems. The pumps have no movable parts, a simple
electrode design and are operated by AC fields in the kHz and MHz
ranges. Media of various conductivities can be pumped, from distilled
water to cell culture media. The pumping effect is based on AC field
forces acting on spatial charges induced in a medium with inhomogeneous
dielectric properties. In aqueous media, such inhomogeneous properties
result from temperature gradients generated by external heat sources or
Joule-heating by the AC pump field inside the pump medium. Effective
fluid motion is induced when the temperature or field distributions are
asymmetric. Unlike electro-osmotic pumps, the new pumps do not exploit
the low-range electric double-layer polarization but instead, structural
polarization effects occurring throughout the volume of the pump-medium.
This also makes them superior to traveling-wave pumps that can only
effectively generate travelling electric fields within a specific range
of an electrode array driven by phase-shifted signals. The new pumps
generate constant pump forces in broad frequency bands by the
interaction with the in-phase section of the polarization, while
travelling-wave fields interact with its out-of-phase element, so
generating a Lorentzian force-peak at the fluid's charge relaxation time.
*Figure caption:*
Temperature distribution inside a directly heated micro pump with
platinum structures on a glass carrier for heating and AC-field
application.
***
LX11295
How to manipulate atoms beyond the laser domain
In this letter, we demonstrated a polarization control in crystal-
assisted coherent excitation of atoms. This method enabled the
ingenious probe and manipulation of atoms in the X-ray domain where
the laser equipments are not available.
Energetic atoms flying through a crystal lattice experience a
temporally-oscillating field by traversing the periodic array of
atomic planes, which can resonantly excite the atoms just like a laser
field (three-dimensional resonant coherent excitation; 3D-RCE). Our
experiment used a thin silicon crystal and Ar$^{16+}$ ions accelerated
to 70% of the speed of light. We controlled the polarization direction
of this oscillating crystal field by selecting the direction of atomic
planes, and succeeded in the alignment of atomic orbital into a
specific direction. Furthermore, we took advantage of the unique
possibility to use two different atomic planes simultaneously. The
response of the atomic system to one field was probed by the other
field under the polarization control with respect to each other,
demonstrating the so-called pump-probe experiment in the X-ray domain.
Our novel technique opens a way to the study of quantum systems in the
short-wavelength region as an alternative to the optical methods.
***
LU11676B
For refrigeration problems, a magnetically attractive solution
Your refrigerator's humming, electricity-guzzling cooling system could soon
be a lot smaller, quieter and more economical thanks to an exotic metal
alloy discovered by an international collaboration working at the National
Institute of Standards and Technology (NIST)'s Center for Neutron Research
(NCNR).
The alloy may prove to be a long-sought material that will permit magnetic
cooling instead of the gas-compression systems used for home refrigeration
and air conditioning. The magnetic cooling technique, though used for
decades in science and industry, has yet to find application in the home
because of technical and environmental hurdles - but the NIST collaboration
may have overcome them.
Magnetic cooling relies on materials called magnetocalorics, which heat up
when exposed to a powerful magnetic field. After they cool off by radiating
this heat away, the magnetic field is removed, and their temperature drops
again, this time dramatically - enough that scientists have attained
temperatures of nearly absolute zero via the effect. Two factors have kept
magnetic cooling out of the consumer market: most magnetocalorics that
function at close to room temperature require both the prohibitively
expensive rare metal gadolinium and arsenic, a deadly toxin.
But many gas-compression refrigerators employ hydrofluorocarbons (HFCs),
greenhouse gases that can contribute to climate change if they escape into
the atmosphere. In addition, it is becoming increasingly difficult to
improve traditional refrigeration. "The efficiency of the gas cycle has
pretty much maxed out," said Jeff Lynn of NCNR. "The idea is to replace that
cycle with something else."
The alloy the team has found - a mixture of manganese, iron, phosphorus and
germanium - is not merely the first near-room-temperature magnetocaloric to
contain neither gadolinium nor arsenic - rendering it both safer and cheaper
- but also it has such strong magnetocaloric properties that a system based
on it could rival gas compression in efficiency.
Working alongside (and initially inspired by) visiting scientists from the
Beijing University of Technology, the team used NIST's neutron diffraction
equipment to analyze the novel alloy. They found that when exposed to a
magnetic field, the newfound material's crystal structure completely
changes, which explains its exceptional performance.
"Understanding how to fine-tune this change in crystal structure may allow
us to get our alloy's efficiency even higher," said NIST crystallographer
Qing Huang. "We are still playing with the composition, and if we can get it
to magnetize uniformly, we may be able to further improve the efficiency it
already has."
***
LU11568
Graphene with superperiodicity effects
Epitaxial growth of graphene on solid surfaces is a relatively
simple and reliable way to prepare this novel material which has
the potential to replace silicon in future electronics. Experiments
disclose in particular that graphene grown on iridium exhibits
exceptional structural quality, extending over micrometers large
areas of iridium surface, including steps. Our study of such
graphene clearly shows that there is a mismatch between iridium
and graphene lattices. Mismatch induces long-range corrugation
exposing graphene to an additional periodic potential which is
responsible for the modification of graphene's electronic structure.
Opening of the gaps in the band structure is one of the most prominent
features of this effect. Recent theoretical research has suggested
that, due to the chiral nature of charge carriers, an additional
periodic potential applied to graphene can alter their propagation in
a very peculiar way. The potential of this modification in engineering
desired properties of graphene-based electronic devices is yet to be
proved. The simplicity of investigated system and its desirable
properties make it attractive for further model experiments on charge
carrier manipulation.
***
EX10334
Liquid Crystals Fall Into Line
As evaporated liquid crystal (LC) molecules gently rain down on an optical waveguide surface we measure their average molecular axis orientation and layer thickness as the surface is slowly covered. The waveguide surface is part of a dual slab waveguide and the technique, dual polarisation interferometry (DPI), first introduced by us in 1999, resolves changes in layer thickness of less than 1 angstrom and sub-degree changes in average molecular axis polar alignment. The electrostatic interactions among the molecules provides the mechanism that drives the cooperative behaviour and ordered layer structure. Such detailed insight into LC layer ordering, easily seen now in real-time in the laboratory, provides opportunities to answer previously difficult questions in this area. By comparison, to uncover detailed stuctural data such as this has required samples to be taken to centralised facilities such as neutron sources. Verifying DPI results using neutron reflectance and pushing the work further into optically pumped LC layer reordering are the next steps.
***
LT11089
Universal behavior of a BEC "car" in a Y-shape road
In our real world, a car will always go along one branch of a Y-shape road (see the attached image). Fantastically, in the quantum world, a car may go along both two branches of a Y-shape road at the same time. In our paper, we explore the universal behavior of a BEC "car" in such a Y-shape road, which associates with spontaneous symmetry breaking. A Bose-Einstein condensate (BEC) is an intrinsic many-body quantum systems of bosons (such as photons and integer-spin atoms) staying in a same single state. Spontaneous symmetry breaking occurs if the mean-field states do not possess symmetry of the original many-body quantum system. In a coupled two-component BEC, the symmetry breaking transition from single- to bi-stable states forms a Y-shape road (see the above panel of Fig. 1 in our paper). In dynamical transitions, the mean-field dynamics obeys an universal Kibble-Zurek mechanism, which also characterizes the universal properties of the early universe and superfluids undergoing thermodynamic phase transitions. The symmetry breaking transitions also cause an anomalous mean-field breakdown dependent on approaching directions. The dynamical mechanism of symmetry breaking transitions connects with the quantum adiabaticity, which provides various applications in atomic physics, condensed matter physics and nonequilibrium dynamics, and particularly in adiabatic quantum computation.
A New Schrodinger Equation for Hadron Physics
One of the triumphs of theoretical physics of the twentieth century
was the development of Quantum Electrodynamics (QED), the fundamental
theory of electrons and photons. QED not only describes the physics
of the atom with extraordinary precision, but also the basic
properties of the electron itself. The corresponding problem in
particle and nuclear physics is to accurately describe the structure
and interactions of hadrons, such as the proton and neutron, in terms
of their fundamental constituents: the quarks and gluons of Quantum
Chromodynamics (QCD). QCD is much more complicated to solve than QED
because of the strong interactions of the confined gluons and quarks.
The most successful theoretical approach thus far has been to employ
lattice gauge theory computer simulations. In this paper we derive a
new quantum mechanical bound-state equation of quarks and gluons which
has many similarities with the Schrodinger wave equation for atomic
systems in QED. The bound-state solutions of this single-variable
relativistic wave equation give a very good representation of the mass
spectrum and wavefunctions of hadrons for general spin and internal
orbital angular momentum. It thus serves as an excellent first
approximation to QCD which can be systematically improved. Our
derivation is based on two remarkable theoretical developments: (a)
Maldacena's AdS/CFT correspondence between solvable gravitational
theories in a curved higher dimensional space-time (Anti-de Sitter
Space) and quantum field theories in ordinary physical space-time;
and (b) "Light-Front Holography", which allows one to map information
in the fifth dimension of AdS space to hadronic wavefunctions
describing the separation of the quark and gluonic constituents - not
at a fixed time, but at a time set by the front of a light wave.
Because of light-front holography, the description of hadrons obtained
from the new light-front Schrodinger equation is equivalent to the
solutions obtained in AdS space. The AdS representation of a proton in
light-front QCD is illustrated in the figure. For example, a proton
with its three quarks close together corresponds to the boundary of
AdS space at large circumference; conversely, a large-size proton
with far-separated quarks is represented at the inner sphere in AdS
space.
Figure caption: AdS representation of a proton in light-front QCD
***
BX10891
Verwey transition explained
The Verwey transition between the low-temperature charged ordered (CO) and the high temperature valence mixed (VM) modification can now be understood by density functional theory (DFT) calculations illustrated for the double cell perovskite YBaFe2O5 . In CO two types of iron ions appear, Fe2+ and Fe3+, whereas in VM both iron sites have the same non integer oxidation state (namely 2.5). Only by going beyond the conventional DFT (in our case GGA+U) we find the charge order and the insulating phase. The orbital ordering in this correlated system is the main reason for the orthorhombic distortion that occurs during this phase transition. The calculations agree with available experiments for the magnetic moments, the charge ordering and Mössbauer data.
***
ly11758
Graphite Lubricant 2.0
Graphite is one of the best solid lubricants. It is made of stacks of atomically thin sheets of carbon, named graphene. The excellent lubrication is believed to originate in the easy shear of graphene layers with respect to each other. In our work, we have grown large terraces of single and double layer of graphene on silicon carbide. We find that even single layers graphene exhibit very low friction. Furthermore, we find that friction on a double layer is half of the friction on a single layer graphene. The friction contrast can be explained by a difference in the way how lattice vibrations are coupled to electrons in graphene. We discovered this difference in coupling by studying electron emission under illumination with the light of a synchrotron source. Double layer graphene outperforms even graphite as a lubricant due to reduced adhesion.
***
LW11526
Teaching Plasma a New Tune
Recent experiments in the non-neutral plasma group of the University of
California, San Diego, have demonstrated the existence of a new kind of
plasma wave, the Electron Acoustic Wave (EAW) a nonlinear wave that
propagates at much lower frequencies than regular electron plasma waves. For
most of the last 50 years, physicists expected that EAWs, if they existed,
would be heavily damped and insignificant. But in the 1990¹s new theories
emerged that suggested otherwise. These were, for the most part, ignored -
until now, when we have observed these EAW plasma waves directly. At low
amplitudes the observed waves match the EAW dispersion relation predicted by
Dorning and collaborators. But when driven to large amplitudes, the waves
³train² the plasma so that it resonates at the driver frequency even after
the driver is turned off, for any frequency chosen by the experimentalist.
Put another way, it¹s as if a grandfather clock could be taught to speed up
by changing the length of its pendulum. We present the first detailed
measurements showing the intimate interaction between acoustic waves and
plasma particles. Such detailed measurements illustrate how university-scale
basic plasma physics experiments can study complex wave phenomena relevant
to large-scale plasma experiments conducted in national laboratories.
***
ES10464
*New AC-field pumps for micro-systems*
New electro-hydrodynamic micro-pumps are devices of much potential for
micro-fluidic systems. The pumps have no movable parts, a simple
electrode design and are operated by AC fields in the kHz and MHz
ranges. Media of various conductivities can be pumped, from distilled
water to cell culture media. The pumping effect is based on AC field
forces acting on spatial charges induced in a medium with inhomogeneous
dielectric properties. In aqueous media, such inhomogeneous properties
result from temperature gradients generated by external heat sources or
Joule-heating by the AC pump field inside the pump medium. Effective
fluid motion is induced when the temperature or field distributions are
asymmetric. Unlike electro-osmotic pumps, the new pumps do not exploit
the low-range electric double-layer polarization but instead, structural
polarization effects occurring throughout the volume of the pump-medium.
This also makes them superior to traveling-wave pumps that can only
effectively generate travelling electric fields within a specific range
of an electrode array driven by phase-shifted signals. The new pumps
generate constant pump forces in broad frequency bands by the
interaction with the in-phase section of the polarization, while
travelling-wave fields interact with its out-of-phase element, so
generating a Lorentzian force-peak at the fluid's charge relaxation time.
*Figure caption:*
Temperature distribution inside a directly heated micro pump with
platinum structures on a glass carrier for heating and AC-field
application.
***
LX11295
How to manipulate atoms beyond the laser domain
In this letter, we demonstrated a polarization control in crystal-
assisted coherent excitation of atoms. This method enabled the
ingenious probe and manipulation of atoms in the X-ray domain where
the laser equipments are not available.
Energetic atoms flying through a crystal lattice experience a
temporally-oscillating field by traversing the periodic array of
atomic planes, which can resonantly excite the atoms just like a laser
field (three-dimensional resonant coherent excitation; 3D-RCE). Our
experiment used a thin silicon crystal and Ar$^{16+}$ ions accelerated
to 70% of the speed of light. We controlled the polarization direction
of this oscillating crystal field by selecting the direction of atomic
planes, and succeeded in the alignment of atomic orbital into a
specific direction. Furthermore, we took advantage of the unique
possibility to use two different atomic planes simultaneously. The
response of the atomic system to one field was probed by the other
field under the polarization control with respect to each other,
demonstrating the so-called pump-probe experiment in the X-ray domain.
Our novel technique opens a way to the study of quantum systems in the
short-wavelength region as an alternative to the optical methods.
***
LU11676B
For refrigeration problems, a magnetically attractive solution
Your refrigerator's humming, electricity-guzzling cooling system could soon
be a lot smaller, quieter and more economical thanks to an exotic metal
alloy discovered by an international collaboration working at the National
Institute of Standards and Technology (NIST)'s Center for Neutron Research
(NCNR).
The alloy may prove to be a long-sought material that will permit magnetic
cooling instead of the gas-compression systems used for home refrigeration
and air conditioning. The magnetic cooling technique, though used for
decades in science and industry, has yet to find application in the home
because of technical and environmental hurdles - but the NIST collaboration
may have overcome them.
Magnetic cooling relies on materials called magnetocalorics, which heat up
when exposed to a powerful magnetic field. After they cool off by radiating
this heat away, the magnetic field is removed, and their temperature drops
again, this time dramatically - enough that scientists have attained
temperatures of nearly absolute zero via the effect. Two factors have kept
magnetic cooling out of the consumer market: most magnetocalorics that
function at close to room temperature require both the prohibitively
expensive rare metal gadolinium and arsenic, a deadly toxin.
But many gas-compression refrigerators employ hydrofluorocarbons (HFCs),
greenhouse gases that can contribute to climate change if they escape into
the atmosphere. In addition, it is becoming increasingly difficult to
improve traditional refrigeration. "The efficiency of the gas cycle has
pretty much maxed out," said Jeff Lynn of NCNR. "The idea is to replace that
cycle with something else."
The alloy the team has found - a mixture of manganese, iron, phosphorus and
germanium - is not merely the first near-room-temperature magnetocaloric to
contain neither gadolinium nor arsenic - rendering it both safer and cheaper
- but also it has such strong magnetocaloric properties that a system based
on it could rival gas compression in efficiency.
Working alongside (and initially inspired by) visiting scientists from the
Beijing University of Technology, the team used NIST's neutron diffraction
equipment to analyze the novel alloy. They found that when exposed to a
magnetic field, the newfound material's crystal structure completely
changes, which explains its exceptional performance.
"Understanding how to fine-tune this change in crystal structure may allow
us to get our alloy's efficiency even higher," said NIST crystallographer
Qing Huang. "We are still playing with the composition, and if we can get it
to magnetize uniformly, we may be able to further improve the efficiency it
already has."
***
LU11568
Graphene with superperiodicity effects
Epitaxial growth of graphene on solid surfaces is a relatively
simple and reliable way to prepare this novel material which has
the potential to replace silicon in future electronics. Experiments
disclose in particular that graphene grown on iridium exhibits
exceptional structural quality, extending over micrometers large
areas of iridium surface, including steps. Our study of such
graphene clearly shows that there is a mismatch between iridium
and graphene lattices. Mismatch induces long-range corrugation
exposing graphene to an additional periodic potential which is
responsible for the modification of graphene's electronic structure.
Opening of the gaps in the band structure is one of the most prominent
features of this effect. Recent theoretical research has suggested
that, due to the chiral nature of charge carriers, an additional
periodic potential applied to graphene can alter their propagation in
a very peculiar way. The potential of this modification in engineering
desired properties of graphene-based electronic devices is yet to be
proved. The simplicity of investigated system and its desirable
properties make it attractive for further model experiments on charge
carrier manipulation.
***
EX10334
Liquid Crystals Fall Into Line
As evaporated liquid crystal (LC) molecules gently rain down on an optical waveguide surface we measure their average molecular axis orientation and layer thickness as the surface is slowly covered. The waveguide surface is part of a dual slab waveguide and the technique, dual polarisation interferometry (DPI), first introduced by us in 1999, resolves changes in layer thickness of less than 1 angstrom and sub-degree changes in average molecular axis polar alignment. The electrostatic interactions among the molecules provides the mechanism that drives the cooperative behaviour and ordered layer structure. Such detailed insight into LC layer ordering, easily seen now in real-time in the laboratory, provides opportunities to answer previously difficult questions in this area. By comparison, to uncover detailed stuctural data such as this has required samples to be taken to centralised facilities such as neutron sources. Verifying DPI results using neutron reflectance and pushing the work further into optically pumped LC layer reordering are the next steps.
***
LT11089
Universal behavior of a BEC "car" in a Y-shape road
In our real world, a car will always go along one branch of a Y-shape road (see the attached image). Fantastically, in the quantum world, a car may go along both two branches of a Y-shape road at the same time. In our paper, we explore the universal behavior of a BEC "car" in such a Y-shape road, which associates with spontaneous symmetry breaking. A Bose-Einstein condensate (BEC) is an intrinsic many-body quantum systems of bosons (such as photons and integer-spin atoms) staying in a same single state. Spontaneous symmetry breaking occurs if the mean-field states do not possess symmetry of the original many-body quantum system. In a coupled two-component BEC, the symmetry breaking transition from single- to bi-stable states forms a Y-shape road (see the above panel of Fig. 1 in our paper). In dynamical transitions, the mean-field dynamics obeys an universal Kibble-Zurek mechanism, which also characterizes the universal properties of the early universe and superfluids undergoing thermodynamic phase transitions. The symmetry breaking transitions also cause an anomalous mean-field breakdown dependent on approaching directions. The dynamical mechanism of symmetry breaking transitions connects with the quantum adiabaticity, which provides various applications in atomic physics, condensed matter physics and nonequilibrium dynamics, and particularly in adiabatic quantum computation.
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