7-3-08

LQ11254
"Stroboscopic" wavepackets make sense of the quantum transport of electrons

The flow of electricity through nanoscale wires and structures is dominated by
the quantum nature of matter, in which the charge-carrying electrons must be
described by probability waves. The need for a quantum approach makes the
accurate simulation or even qualitative theory of such problems very
challenging. In our Letter we present a formally exact approach based on
special quantum-mechanical "wavepackets" which are snapshots of the wave of a
single electron taken at regular time intervals -- as if by a stroboscope --
within a controlled energy range. When many electrons are present, each
electron passes from one wavepacket to the next as the stroboscope flashes.
Crucially, this guarantees the exact fulfilment of Pauli's famous exclusion
principle which prevents two electrons from being in the same place at the
same instant. Viewed now in terms of these wavepackets, many complex
problems in quantum transport gain a simple "common-sense" interpretation.
We illustrate this by deriving new results for how the current builds up, as
a function of time, when a voltage is applied to a nanostructure, and also
for the accumulation of electrons of a preferred spin at the edge of a
current-carrying layer.


***

LM11702
Liquid crystals on droplets: designing superatoms

Chemists and materials scientists have only 80 stable atoms available
with which to make molecules and materials. The molecules are formed
by directional quantum mechanical bonds which share electrons between
neighboring atoms. Each atom has a very limited number of possible
bonds it can form, determined by its atomic number.
In recent years attention has turned to creating a new kind of
building block (a superatom) which may be 100-100,000 times larger
than conventional atoms. The bonding between superatoms is driven by
the rich statistical mechanical physics of nanometer to micron scale
structures that undergo complex thermal fluctuations. Since they are
not restricted by quantum mechanics the number of potential superatoms
is enormous and the corresponding supramolecular chemistry and
materials science is only just beginning to be explored. One way to
create superatoms is to self-assemble liquid crystals, like those
found in your digital watch, on spherical water droplets in oil in the
same way mustard seeds can stabilize oil droplets in egg yolk (mostly
water) in the classic sauce mayonnaise. Liquid crystal molecules on
spherical droplets have unavoidable irregularities (defects) in their
patterns at which additional molecules can be attached to link droplets
into molecules and bulk materials. In our paper we show one can control
the location of the defects by varying the material properties of the
liquid crystal, thus allowing for the controlled design of a variety
of superatoms.


***

BR10744
Evidence of local superconductivity in granular Bi nanowires fabricated
by electrodeposition


Bulk Bi is a semimetal down to at least 50 mK without showing any
evidence of superconductivity. However the electrical property of Bi
films and nanowires is very sensitive to the exact conditions on how the
samples were fabricated. Recently, we systematically investigated the
electrical properties of granular Bi nanowires consisting of
rhombohedral Bi grains of a few nanometer in size, fabricated by
electrochemically depositing Bi into porous polycarbonate membranes. It
was found that the granular Bi nanowires exhibit superconducting,
insulating or even super-resistive behavior sensitively depending on the
details of its morphology or specific configurations of the nanowires.
The superconductivity comes from the interfacial structures between the
grains of the nanowires due to structure distortion or lattice strain,
but the grain itself with a rhombohedral structure may still remain its
semimetal or semiconducting character. The superresistivity is the
consequence of the nucleation of local superconductivity at the grain
boundary area without long-range phase coherence, a reminiscent of the
¿Cooper-pair insulator¿ observed previously in ultra thin
two-dimensional (2D) superconducting films.


***

LP11053
Critical dynamics of vesicle stretching transition in elongation flow

Biological membranes often develop nanotubes and form dynamical tubular networks, which connect over
long distances various cell types, including neuronal and immune cells, to provide signal communication
as well as virological transport. In this paper we report an effective way to produce nanosize lipid tethers from
tubular vesicles by elongation flow at its stagnation point. A surprising observation is that during the
stretching a sequence of transitions to various conformational states occurs (Fig.1). First transition from
tubular-to-dumbbell shape takes place at the critical strain rate. The striking features of the stretching
transition are critical slowing down in vesicle relaxation towards a steady state and increased fluctuations
of vesicle stretching, the features similar to well known in the continuous thermodynamic phase transition.
Thus, we suggest that the critical effects are a universal dynamical phenomenon, which occurs in any
microscopic and mesoscopic objects where thermal noise leads to large variety of available configurations
close to the conformation transition, where the entropic force (elasticity) is balanced by the hydrodynamic
drag (stretching force). At higher strain rates, ext order modes become unstable that manifests in a much
faster rate of a tube extension and leads to the well-known pearling state. The larger the strain rate,
the higher order mode becomes unstable, the more pearls initially appear, and the faster the rate of a tube
extension towards the even longer and thinner tube.

***

LP10964AR
Building quantum processors in a noisy environment

Quantum information processing involves manipulation of entities (atoms,
ions, photons etc.) which obey laws of quantum mechanics. Building
stable, scalable "quantum components" like registers and processors is
one of the biggest challenges in physics. The major obstacle to quantum
processing is due to the deleterious effect of the surrounding
environment that destroys correlations arising out of their
¿quantumness¿. This is called decoherence and has been known for a long
time. The schemes proposed to counter decoherence fall into two broad
categories: error correction and error avoidance. In this paper, we
investigate the latter under some realistic assumption. The
error-avoidance schemes rely on the fact that in some cases it is
possible to encode information in quantum states that do not degrade
(decohere). We show that for most realistic models this can only be
approximated. The validity of the approximations depends on the relative
strengths of the interactions. We give various conditions and
estimations on these approximation schemes. The implication is that for
succesful quantum processing components we need more detailed modelling
of the quantum system and its interactions with the environment.

It might be much harder to build a working quantum processor than we expect.

7-1-08

LQ11508B
A pulsed semiconductor disk laser shows hysteresis in the output characteristics

Hysteresis in a semiconductor disk laser pulsed with a semiconductor saturable absorber
mirror is reported for the first time. This kind of bistable laser can potentially be utilized
in optical memories and clocks for computers.
Semiconductor disk lasers combine the wavelength versatility of semiconductor lasers with
the high optical power and excellent beam quality of traditional thin disk lasers, which makes
them ideal for projector and display applications. Pulsed semiconductor disk lasers can reach
multigigahertz pulse repetition rates and are well suited for high-speed communications,
switching, sampling, and clocking. The laser studied in this paper could operate at three different
pulsing frequencies in the gigahertz range, exhibiting bistability and hysteresis between
these states in the output power as well as in the laser wavelength and pulse width.
Furthermore, the size of the hysteresis loops could be tuned by varying the design of the light
intensifying semiconductor structure, holding promise for controllable devices in the future.

***

LQ11508B
Improvements in magnetic nano-oscillators as a function of the
orientation of an applied magnetic field

In devices consisting of an alternating stack of magnetic and
nonmagnetic metal layers, a perpendicularly-flowing direct electrical
current can become partially spin polarized upon passing through one
magnetic layer and can then transfer some of that spin into the next
magnetic layer downstream. As a result, under certain experimental
conditions the magnetization of the second layer can be excited into
steady-state oscillation and it emits microwaves. This phenomenon is
under development for making frequency-tunable nanoscale microwave
sources. For these applications, it is desirable that the
oscillations be highly reproducible and periodic, i.e. that they have
narrow linewidths in their frequency spectrum. In this paper, we
study how these linewidths depend on the angle of an applied magnetic
field. We found that the most-commonly studied field orientation, in
the plane of the magnetic layers and parallel to the magnetic easy
axis of the oscillating layer, actually produces the worst microwave
properties. As the field angle is rotated by 90 degrees in the sample
plane, the linewidths can decrease dramatically, by a factor of 20-50
in some devices. This enables a simple way to improve the microwave
source. Computer simulations attribute the improvement to a
transition from spatially incoherent dynamics within the moving
magnetic layer to nearly spatially-uniform oscillations.


***

LQ11476

SLOW ANTIPROTONS UNRAVEL THE SECRETS OF ATOMIC COLLISIONS.

We have used a beam of slow antiprotons to learn more about the dynamics of
atomic collisions in general.

One of the first successes of quantum theory was the accurate description of
the structure of the isolated hydrogen atom. Due to difficulties with the
inclusion of the interaction between its two electrons, it took years to
reach a similar accuracy for the helium atom.

Today, 90 years later, similar progress has not yet been achieved in the
case of atomic collisions, where a description of dynamic systems involving
several electrons are needed. Even for the "theorists dream system", namely
the collision of a slow antiproton on a helium atom leading to the
liberation of one electron, for which a large number of advanced theoretical
calculations exist, these differ by up to 30%.

Using a new scheme for the creation of a DC beam of slow (a few keV)
antiprotons at CERN, this group have obtained benchmark data for the
development of more accurate theoretical calculations.

The use of antiprotons instead of other ions is important, since
antiprotons, due to their negative charge, do not support the process where
electrons are transferred to the slow projectile. This simplifies the
already difficult task of the theorists.


***

BQR1070
Temperature-induced spin coherence dissipation in quantum dots

Electron spins in quantum dots are an attractive candidate for
implementing quantum information technologies in a solid state
environment. This attractivity is mainly based on the long living spin
coherence in these systems, as most of the spin relaxation mechanisms
which work very efficiently in systems of higher dimensionality are
suppressed in quantum dots. The interaction which cannot be avoided at
cryogenic temperatures is the hyperfine interaction with the
background of lattice nuclei limiting the coherence time $T_2$ to the
microseconds range in GaAs based quantum dots. Despite of these
results, the understanding of the spin dynamics is still fragmentary.

For practical applications also the temperature dependence of the
coherence time is of key importance. In this work the authors report
the first measurement of the variation of $T_2$ for quantum dot
electrons with increasing temperature. Such measurements became
possible by addressing quantum dot ensembles and implementing the
recently developed technique of mode-locking by which the spin
ensemble is synchronized with a periodic excitation laser. This
technique allows a refocussing of the spins in the inhomogeneous
ensemble by which the single spin dynamics can be extracted. It was
found that $T_2$ is constant in the microseconds range up to 20 K, but
for higher temperatures shows a sharp drop. Comparison with detailed
model caluculations shows that this drop arises from fluctuations of
the hyperfine interaction due to the temperature induced lattice
vibrations. These results are a key step towards understanding the
limitations of spin relaxation which are under lively debate at the
moment. From this understanding one might develop tools to extend the
coherence towards longer times, as required only not for quantum
information but also for spin-based electronics



***

LS11288
Demystifying the Plutonium Enigma

Plutonium is an element which, while controversial, is central to our national
security, both in terms of energy production and defense. Despite almost half
a century of research, plutonium is remains a fundamental problem
of condensed matter physics. In this paper, we have performed one of the most
accurate simulations to date of the electrons in solid Pu using the Dynamical
Mean-Field Theory (DMFT) and Density Functional Theory (DFT). This achievement
was made possible by both recent theoretical developments and the massive
supercomputing resources at Lawrence Livermore National Laboratory. We
show that the electrons in Pu are delocalized, but are somewhat heavy due to
their interactions. The behavior of the electrons are then studied as the
volume of the Pu lattice is expanded, which is a feat that is not easily
achieved under controlled experimental conditions. The electrons become
progressively heavier as the volume increases, and for a 30% expansion they
behave in a localized fashion at ambient temperatures. Predicted trends of
various physical properties are in favorable agreement with experimental
observation. The success of this study is simply the beginning of a much
longer journey of discovery. Delta Pu displays many anomalies which are not
sufficiently understood, such as a density which is less than liquid Pu (like
water) and the fact that it shrinks when it is heated! Finally, it appears that
we may have a tool, DFT+DMFT, which is capable of capturing the salient physics
of Pu and revealing its secrets.


***

LP11489
The universe is necessarily unpredictable

From the flight of a falling apple to the rising of the sun, the world
around us appears to be highly predictable. However, in contrast to
these examples, we show that there are particular natural processes
which are entirely unpredictable.

A remarkable feature of our world is that numerous physical phenomena
can be calculated with virtual certainty. In fact, such certainty can
only apply to large objects; for smaller ones, our ability to make
precise predictions becomes inherently worse. This is because small
objects are described by an altogether different theory, quantum
mechanics. Consider, for example, the radioactive decay of some atom.
Quantum mechanics tells us the probability of decay within, say, the
next hour, but not the exact time of decay. This is an undesirable
feature and led several scholars, including Albert Einstein, to
speculate that there may exist a (yet unknown) more complete
description of quantum processes. However, as our article shows, this
is impossible. The work of Kochen and Specker and of Bell showed that
outcomes of physical experiments cannot generally be predicted *with
certainty*. We have extended their result to make a stronger claim,
that the outcomes of particular physical experiments are *entirely
unpredictable*.


***

BP10887
A rule of selecting preferred copolymers for organic optoelectronic devices

Organic light-emitting diodes (OLEDs) and photovoltaic cells are optoelectronic devices with contrary operation processes. Surprisingly, it has been revealed that donor-acceptor copolymers are species of promising materials that favor both OLEDs and photovoltaic cells. How to choose efficiently a copolymer for a specific purpose is a realistic issue that must be addressed by experimentalist. In this paper, by employing a dynamic simulation method, we propose a general role associated with the electronic structure of donor-acceptor copolymers, such as the ratio of level offset to polaron or exciton binding energy, and the interfacial electronic coupling, to help experimentalist on this issue. It is found that the most favored level offset for photovoltaic cells is the one with level offset greater than the exciton binding energy while for OLEDs is that between polaron and exciton binding energy. The electronic coupling at the interface is also an important factor that can affect the operation of optoelectronic devices. Our study presents a helpful guidence to experiments in the preliminary selection of preferred copolymers for organic optoelectronic devices.

6-30-08

LQ11225B
Unusual dependence of the superconductivity on the thickness of MoGe films.

Thin films of MoGe show progressively reduced superconducting transition temperatures as their thickness is decreased. The optical conductivity has been obtained from far-infrared transmission and reflection measurements for a set of such films. The absorptive part of this conductivity shows a clear energy gap whereas the dispersive part measures the superfluid density. The superfluid density decreases as thickness (and transition temperature) is reduced, whereas the ratio of the gap to the transition temperature, a measure of the strength of the pairing interaction, is unchanged. The normal state properties (e.g., the resistivity) also do not change. Although the result that the transition temperature is suppressed in thin films, even though the basic electronic properties and the strength of the superconducting pairing interactions in the metal do not change, initially seems to contain contradicitons, detailed theoretical calculations, using a sophisticated theory of superconductivity, do provide an explanation of the results. However, the measurements also find that the expected coherence peak in the optical scattering rate is lost in the thinner samples, a result which cannot be explained within conventional theories.

6-27-08

LN11370ER
Toroidal crystals: physics on the surface of a donut

Are there defects in the most effective packing of sugar grains on
the surface of a donut? Remarkably the ordered structure of many
natural systems can be related to this simple and appetizing
question and the answer is frequently yes! In the language of
geometry the surface of a donut is called a torus. Crystalline
assemblages of identical sub-units packed together and bent in the
form of a torus have been discovered in the past ten years in the
protein coats of viruses, self-assembled fatty acids and carbon
nanorings. In our article we provide for the first time a unified
description of the structural properties of toroidal crystals based
on the elasticity of disclination defects. In two-dimensional
crystals disclinations are lattice sites with more or less than the
average number (6) of neighbors. Disclinations typically have 5 or 7
neighbors in a triangular lattice. On a flat surface disclinations
are energetically prohibitive and never appear in the ground state.
As soon as the crystal is curved, however, disclinations may appear
and fundamentally alter the basic order. Disclinations may serve as
active biological sites or places for chemical linkages so that
mesoscopic toroidal surfaces can spontaneously link to form novel
molecules and bulk materials. Donuts continue to surprise.

***

BQR1070
Temperature-induced spin coherence dissipation in quantum dots

Phys. Rev. B Rapid Communication and Editor's Suggestion

Spins in ensembles of quantum dots offer one possible pathway to
implementing quantum information technologies in a solid-state
environment. Unfortunately, the spin interaction with the host lattice
leads to coherence lost of the quantum bit. In this work the authors
reported the first measurement of the temperature dependence of the
electron spin decoherence time T2 in semiconductor quantum dots and
they compared their experimental results to recent theoretical
calculations. It has been possible thanks to: (i) a refocusing
technique using laser pulses called "mode-locking technique" developed
by this research group which allows to avoid the inhomogeneities
within an ensemble, (ii) the investigation of InAs quantum dots where
the electronic confinement is much larger than GaAs dots (gate-defined
GaAs dots which allow only very low temperature measurements). It was
found that T2 remains constant up to 20 K and then it presents a sharp
drop due fluctuations of the electron spin interaction with the
lattice nuclei spins. The topic of the paper is of timely nature. The
results are quite important for the corresponding science community
considering the actual debate in the community about the mechanisms
responsible for spin decoherence.


***

LN11086B
Noise self-pumping in long Josephson junctions

The noise self-pumping effect in a spatially extended system is investigated.
This effect leads to significant degradation of noise properties of spatially
extended systems. In particular, in long Josephson junctions it is realized
in a similar fashion as usual ac self-pumping effect: fluctuating solitons,
radiating from the junction, induce fluctuating magnetic field, which in turn
modulates the dynamics of the soliton chain and increases the spectral
linewidth. Contrary to the theory for short Josephson junctions,
predicting linear decrease of the spectral linewidth with increase
of junction length, the minimum of the linewidth versus the length
is observed both for uniform and non-uniform bias feed
distributions.



***

LH11065E
Re-entrance in vitro

In a recent communication, researchers studied the effect of temperature
on single stranded DNA which can form a hairpin structure as seen in the case of
Molecular Beacon. They showed that in the constant force ensemble (appropriate for the set up like magnetic tweezers) the reaction co-ordinate i.e. extension may increase or decrease with temperature as seen in the recent experiment. Though an increase in extension with temperature is well understood theoretically, but there is no clear understanding about the decrease in the extension with temperature. Their exact solution based on the model Molecular Beacon for short chains showed that the decrease in the extension is an entropic effect. This advancement in knowledge now may resolve a long standing issue related to the prediction of re-entrance in force induced transitions where a double stranded DNA (dsDNA) goes to the zipped state from the unzipped state and again to the unzipped state with temperature. Notably now the prediction of re-entrance is not
only confined to dsDNA but also for other bio-polymers e.g. proteins and homopolymers but it experimentally remained elusive so far. Using the large conformational change
in the reaction co-ordinate and the formation of hairpin because of solvent, they showed
that Molecular Beacon is an ideal candidate to observe re-entrance in vitro.


***

LS11341
A UNIVERSAL ELASTIC ANISOTROPY INDEX FOUND
Practically all elastic materials are anisotropic, which means that their properties are directionally dependent. This calls for an appropriate universal measure to uniquely quantify the degree of anisotropy for any particular material, e.g. crystal. Three well known anisotropy measures used for years lack universality as they are non-unique and ignore contributions from the bulk part of the elastic stiffness (or compliance) tensor. In this letter we introduce a universal anisotropy index that overcomes these limitations and allows one to fully quantify the single crystal anisotropy. Furthermore, we establish special relationships between the proposed anisotropy index and the existing anisotropy measures for special cases. An elastic anisotropy diagram is constructed for over 100 different crystals (from cubic through triclinic), demonstrating that the proposed anisotropy measure is applicable to all types of elastic single crystals, and thus fills an important void in the existing literature.


***

LL11424E
Moving at a constant pace may be key to the formation of coherent
patterns in swarming organisms

Self-propelled particle (SPP) models describe leaderless pattern
formation in biology, such as fish schools and insect swarms. SPP
models are classified into two distinct categories: kinematic and
dynamic. Kinematic SPP models assume that each particle travels at a
near constant speed and senses the position and orientation of its
local neighbors to adjust their orientation. Dynamic SPP models
describe the motion of particles based on Newtonian mechanics that
involve attractive and repulsive forces between particles rather than
alignment principles. In their paper, Newman and Sayama developed a
new dynamic SPP model in which a sensory blind zone is introduced into
each particle's zone of interaction, and studied the effects of the
blind zone on the formation of coherent vortex patterns. The result
indicates that even a slight sensory deficiency makes swarms unable to
form a pattern, which is quite different from kinematic models that
can have large sensory deficits and still form vortex patterns. This
comparison presents a conjecture that certain biological swarming
behaviors may sensitively depend on the ability of individuals to
maintain a constant velocity. Specifically, for organisms that keep
moving autonomously at a near constant pace, coherent pattern
formation emerges relatively easily even with significant sensory
blind zones, but for organisms whose motion strongly depends on
environmental stimuli, coherent pattern formation requires a nearly
complete panoramic range of interaction in order for particles to gain
enough propulsion from behind.

6-25-08

BSR1113
Oxygen-free iron arsenide superconductors discovered

A family of oxygen-free iron arsenide superconductors with a not yet
optimized critical temperature (Tc) of 38 K was discovered. The material
is closely related to the recently found iron-arsenide-oxides, but
adopts an even simpler structure. It is meanwhile accepted, that the
crucial building block to superconductivity in LaOFeAs is the
iron-arsenide layer. The latter is sandwiched between lanthanum-oxide
layers, but only by barium atoms in the new material BaFe2As2. In our
recent paper (PRB, BSR1113, in press, arxiv:0805.4021), we show that the
physical properties of the new parent compound BaFe2As2 are amazingly
similar to LaOFeAs. Thus we predicted that BaFe2As2 would be a
superconductor by doping as known from the LaOFeAs materials. Very
recently, we succeed in inducing superconductivity at 38 K in
Ba0.6K0.4Fe2As2 by hole doping. (PRL, submitted, arxiv:0805.4630). This
discovery opens new avenues to find further superconductors in the large
family of BaFe2As2-related compounds. But more important, the new
materials have a simpler structure and are easier to synthesize also as
large single crystals. This gives a world of opportunity for rapid
experimental and theoretical progress and thus a fresh impetus for
finally solving the mystery of high-Tc superconductivity.

***


LF11238


Violation of macroscopic realism in every-day life?


The laws of physics are of quantum nature and microscopic systems cannot be
described classically. But the macroscopic objects around us seem to have
objective properties prior to and independent of measurement which can be
inferred without altering them. Is it possible to experience a violation of
this macroscopic realism in every-day life?
Under realistic conditions we are only able to perform coarse-grained
measurements that do not resolve individual quantum levels of a macroscopic
object. We show that this usually allows a realistic description of the
object's dynamics: At every instant of time the quantum state - even if it
is a macroscopic Schrödinger cat-like superposition - appears as a classical
mixture and the time evolution of this mixture can be explained classically.
However, we demonstrate that there exist non-classical time evolutions which
allow to see a violation of macroscopic realism even under classical
coarse-grained measurements. The question why we then do not see such
violations arises again. We finally suggest that the reason for this is that
non-classical time evolutions are of high computational complexity.
Figuratively, this means that if nature spontaneously "chooses" a time
evolution, it is much more likely that a low complex, i.e. a classical, time
evolution is realized and thus our every-day world appears classical under
coarse-grained measurements.

Figure Caption:
The two graphs on the top show the quantum states of a Schrödinger cat-like
superposition of a spin pointing to the north and to the south (left) and a
classical fifty-fifty mixture in which half of the spins is along north and
the other half is along south (right). Under every-day coarse-grained
measurements both states have the same classical description in terms of a
fifty-fifty probability distribution (bottom left and right). However, there
exist non-classical time evolutions producing time-dependent superposition
states that allow to violate macroscopic realism even under coarse-grained
measurements.



***

BQ10572

Riding the wave of silica glass

Surface dynamics measurements can provide insight into the surface structure where direct imaging techniques fail. Silica glass, the chief component of the vast variety of silicate glasses, is an excellent insulator and its surface features very low surface tension values. For these as well as other reasons, commonly applied surface science techniques did not yet succeed in obtaining high-resolution information of the melt-formed silica surface.
In this paper, we have used neutral helium atoms as probes for investigating the silica glass surface. In the course of the experiment, a coherent matter wave of helium particles is created, gets scattered at the surface and the energy distribution of the scattered particles is obtained by measuring their time of flight. General surface dynamics parameters have been determined from an analysis of elastically scattered particles. Furthermore, a new surface dynamics effect peculiar to disordered matter – the surface boson peak – has been discovered; properties of which have also been reported in two recent issues of Physical Review Letters [PRL 99, 035503 (2007) and PRL 100, 135504 (2008)].

***

LQ11728

Observing the quantization states of 3d electrons in monoatomic Cu chains.

It is well known that the behavior of d electons in solid is very important and associated with magnetism, superconductivity and transport phenomena. For the first time, this paper reported the quantization of one-dimensional 3d electrons states in monatomic metallic Cu wires. Low dimensional electronic systems with novel and tuneable properties attract high fundamental interest with challenging questions. For example, sp electrons in one dimensional Au atom chains on Si(111) step surface exhibit a number of exotic features, such as Peieris transitions, periodic lattice distortions, and charge density waves. By applying angle-resolved photoelectron spectroscopy, the authors in this paper for the first time observed an anisotropic one-dimensional 3d electron band quantization states in a monatomic array of Cu chains on the Pt(997) step surface. The 3d electrons of Cu are delocalized along the chains and localized in perpendicular direction of chains. Moreover, the authors demonstrated that the one dimensional confinement of Cu 3d electrons quantization states is introduced by the direct wave-function overlap between next-neighbor Cu atoms in the chains. These results are susceptible to have more general impact on the microscopic understanding of electronic low-dimensional systems.

***

BS10835
Sub-picosecond ac spin current pulses produced.

Conventional electronics utilize electron's charge. Recently, there is a
general interest in using spin of electrons for the next generation
electronic technology - spintronics. Spin currents, flow of electron's
angular momentum, or spin, is the counterpart of charge currents in
conventional electronic devices. Although generation of charge currents
only requires application of a voltage, spin currents are not easy to
produce. In the past, several methods have been developed to generate
spin currents that are dc in nature and steady state. Now, ac spin
current pulses shorter than 1 picosecond have been demonstrated by using
coherent control techniques. The current pulses are generated optically
in intrinsic GaAs bulk and quantum wells, without applying any external
voltage or using magnetic materials. The new scheme may find
applications in high-speed spintronic designs where ac currents are
desired.


***

BSR1064

Strong correlations and order in the iron pnictide superconductors.

One of the key questions raised by the discovery of the new iron
based high
temperature superconductors is whether their physics is akin to the
copper
based high temperature superconductors discovered over 2 decades ago.

In our paper we argue that structural and spin ordering transitions
observed in
recent neutron scattering experiments suggest that these two classes
do indeed
share important features of ``strong correlation physics''. We show
that a
model of localized, and strongly correlated, electrons naturally
exhibits a
transition at relatively high temperatures where the square lattice
symmetry of
the Fe ions is reduced to a rectangular symmetry, just as is
observed. We extend
our model to describe a number of properties of the superconducting
and metallic
states, including the pairing symmetry of the Cooper pairs.


***

BR11049

In this paper, we present for the first time a complete description of
non-specular diffracted beams, both theoretically and experimentally.


An interesting and almost unexplored phenomenon occurring in photonic
crystals at wavelengths on the order or smaller than the lattice
parameter is the opening of diffraction channels, that is, a finite
number of diffracted beams emerge from the crystal slab when the
photon energy is greater than a threshold energy or diffraction
cut-off. These diffracted beams are propagating waves that can be
projected on a screen in order to measure their intensities. However,
up to date, most of the experimental and theoretical analyses in this
high energy range have been focused on the intensities of the
specularly reflected and forwardly (or ballistically) transmitted
beams.

We found in the spectral analysis of the diffracted beams strong
intensity modulations that arise from resonances of the three
dimensionally ordered sphere ensemble. Such novel diffraction
phenomena can be put into practice in a number of optoelectronic
devices.

***

CP10108

Pygmy dipole resonance is a very important phenomenon for astrophysical
models of rapid neutron capture occuring in core-collapse supernovae
which is responsible for the production of roughly one half of all heavy
elements beyond iron in the Universe. Since strongly interacting protons
and neutrons, the constituents of atomic nuclei, respond differently to
an external electromagnetic field, their relative motion exhibits different
modes of excitation at various energies. The best-known example is the giant dipole resonance caused by an oscillation of the proton and neutron liquids
against each other. A neutron or proton excess localized on the
nuclear surface forms a kind of a soft nuclear skin and in an external
electromagnetic field this skin can oscillate against the core
formed by an equal numbers of protons and neutrons. This oscillation
is called pygmy resonance. It occurs at low energies and
resembles the motion of sea water during the tides caused by the
gravitational attraction of the moon.

In this paper, we develop an approach which allows us to describe
many-body dynamics of giant and pygmy dipole resonances as well as
other nuclear vibrational excitations. Mode coupling between the
giant and pygmy dipole resonances and other surface vibrations is
taken into account in a fully consistent way. The respective model
predictions are in a very good agreement with the available
experimental data. Due to the consistency and the relatively small
number of the model parameters, which are universal within the
entire nuclear chart, the method allows also a reliable description
of nuclei not yet accessible to the experiment.


***

BP10529
Classification and Analytical Description of States in a Photonic Crystal

Photonic crystal (PC) has revolutionized the entire field of optics. Numerous methods are proposed for the theoretical modeling of PCs. Unfortunately none of these methods is simple, as they all involves numerical method without any analytical details or a formula-like description.

This work solves this problem by reporting a systematic and generalized way of getting analytical descriptions for two-dimensional PCs. The photonic states in the limit of zero modulation were analyzed and a systematic classification system based on integers was introduced for the states with distinct frequencies. The integers can be factored and states with the same factor can be categorized as they have the same form of solution when the spatial modulation is switched on. As it is shown in the paper, such states can be easily solved and typically lead to analytical solutions.
With the presented work, it is now possible to have an analytical description or formulas for many novel optical properties of PC explored previously using numerical methods such as band gaps, refractions, group velocities and density of states. This in turn, may enable researchers to engineer PCs and PC-based devices analytically.

6-23-08

AR10216
Quantum Billiard Pocket Computer

We propose and describe for the first time how quantum computers could
be physically realized by playing billiard with single atoms. The required
miniature billiard tables must have the proportions of an A4 paper sheet.
Experimental means to create such tables with cushions made of laser light
do already exist. So far, they have been used to study the chaotic motion
of atoms confined to the interior of appropriately shaped laser corrals.
Our "A4-billiards", on the contrary, lead to perfectly predictable and
time-periodic motional patterns. Their dynamic behavior becomes particularly
transparent if observed stroboscopically, with a natural time-unit ("clock
cycle") proportional to the billiard area. The character of the resulting
motion is governed by the laws of quantum mechanics. A striking consequence
is that an atomic billiard-ball (i.e., a "blob" of quantum matter) can
"split" and --in a way-- end up at two or more locations at the same time.
Adding a controlled, minimal intervention from outside (e.g., shining short
laser pulses on individual "blobs" at appropriate instants of time) it
becomes possible to manipulate the system's quantum state at will. The
A4-proportion of these billiards allows one also to increase the system's
complexity by introducing onion-skin-like "billiard in a billiard"
architectures.


***


BS10711

Relaxor Ferroelctric-like Behaviour in Ca Doped TbMnO3

TbMnO3 is known as the first magnetic induced ferroelectric material. Many ferroelectric materials have large switching (coercive) fields. In this manuscript we show that small amounts of doping with divalent Ca2+ results in relaxor type behaviour. This behaviour is associated with low coercive fields. The manuscript present neutron diffraction, magnetic and electric properties for 2, 5, and 10% doping.


***


LG11819B

Mysterious threshold switching finally unveiled

Apply a large voltage to an amorphous semiconductor, and
the electrons hopping through traps in the material will
heat up and gain mobility. Above a certain threshold
voltage, this process culminates in a tremendous
enhancement of conductivity, which goes under the name of
⿿threshold switching⿿. Threshold switching was first
discovered in 1968 by S. Ovshinsky in chalcogenide
glasses, namely selenium- and tellurium-based alloys
lacking an atomic order. Researchers have long debated the
root cause of this mysterious effect, invoking thermal,
electric breakdown or phase transitions. A report appeared
recently in Phys. Rev. B may have unveiled the intimate
physics of threshold switching. Amorphous semiconductors
conduct electricity by electrons/holes hopping through
localized states. The higher the carrier energy, the
higher its ability to escape from traps, hence its
velocity. This is similar to what happens in a river,
where deep waters tend to remain trapped between rocks and
flow slowly, while shallow waters run more freely. High
electric fields can drive electrons/holes to high energy,
thus promoting their mobility and eventually leading to a
flash-like increase of conductivity. This discovery may
help the development of faster and less consuming non
volatile memories and cognitive devices based on
chalcogenide switching.


***


BR10880
Photodiode based on carbon monolayer.

Fabricated a few years ago, graphene, a one-atom-thick carbon
layer of surprisingly high electronic and crystal quality, is a
possible candidate for the base material in future
nanoelectronics. However, the absence of the energy gap between
the valence and conduction bands in this two-dimensional
semiconductor hinders the possibility to control currents in
graphene-based devices, e.g. diodes and field-effect transistors.
To resolve this problem
we propose to use external electromagnetic
radiation, which allows to create a dynamical gap in the graphene
energy spectrum. The value of the gap depends on the power and the
frequency of radiation. We show that the electromagnetic field of
a wide frequency range generates photocurrent in this gapless
semiconductor. Such a photocurrent arises as a result of inelastic
electron tunneling accompanied by one- or two-photon absorption.
Applying a sufficiently large radiation power, one can also fully
suppress electron transport in a graphene-based junction.


***


LG11487
Visualizing atomic-scale acoustic waves in nanostructures

The highest frequency acoustic waves in materials, with nearly
atomic-scale wavelengths, are beginning to be utilized as a new kind
of tool to measure the properties of nanostructures but practical
detection of these waves with ultrahigh THz frequencies (10^12 Hz) is
extremely challenging. We have discovered a new physical phenomenon
that enables observation of such high frequency waves.

The highest frequency acoustic waves can form spontaneously at the
front of shock waves or be generated by sub-picosecond pulse length
lasers. Under some circumstances, when such a wave crosses an
interface between two materials, tiny electric currents are generated
at the interface. These currents produce electromagnetic radiation
of THz frequencies that can be detected a few millimeters away from
the interface. Using molecular dynamics simulations, we show that
the time-history of the wave can be determined with potentially
sub-picosecond, nearly atomic time and space resolution by measuring
the electromagnetic field coherently generated. We have studied the
effect for an interface between AlN and GaN which are used in LED
(light-emitting diode) nanostructures and are piezoelectric, i.e.
electric currents are generated when they are squeezed.
Piezoelectric materials have been employed for decades as arrival
time gauges for shock wave experiments but have been limited by
electrical equipment to detection of acoustic frequencies less than
10 GHz, precluding observation of the highest frequency acoustic
waves.


***


BSR1110B

Precision STM measurements resolve atomic structure debate

Recent scanning tunneling microscopy measurements by Choi et al. have
resolved a long-running debate concerning the atomic structure of
one-monolayer films of copper nitride (Cu2N). Copper nitride has attracted
considerable interest for nanoscale templating applications, because
one-monolayer films can be grown that self-assemble into a regular array of
square islands. Despite the variety of techniques applied to study this
system over the last 20+ years, the mechanisms for self-assembly are not
well understood. Choi et al., use an STM operating in a low-temperature,
ultrahigh vacuum environment to directly measure the lattice constant of
copper nitride films. The measurements suggest that strain due to the 3%
lattice mismatch with the underlying Cu substrate contributes to
self-assembly in this system.

***

BM10809
Electron-hole drops in multivalley semiconductors


The article develops a new formalism that gives the total energy of an
electron gas in common multivalley semiconductors such as Si, Ge, and GaAs.
To demonstrate its usage, the formalism is applied to electron-hole drops.

Every electron in a solid feels forces from all other electrons, creating
correlations, which makes the total energy of the electrons analytically
tractable in just a small handful of systems. In multivalley semiconductors
the electrons can be distinguished by a number categorizing them according
to their band structure valley; the limit of many valleys permits an
approximation that allows the exact total energy to be found. The energy
penalty of a changing electron density is also found, which allows systems
with spatially varying electron density to be analyzed.

The electron density profile for experimentally realisable electron-hole
drops is derived. Drops can be experimentally probed by straining their
semiconductor, which is naturally investigated within the formalism since
strain reduces the main parameter of the theory, the number of valleys.


***

AR10399

Focusing evanescent waves to a tiny spot

Evanescent waves can be shaped and focused to tiny spots using planar
gratings. This effect was first demonstrated by Merlin (Science, Vol.
3127, 927, 2007), who coined it ?radiationless electromagnetic
interference?. Conventionally, focusing is only achieved using
propagating waves, and to date evanescent waves have been completely
forgotten in this respect. In the current work we demonstrate that
evanescent waves exhibiting certain symmetric polarization patterns
can generate focal spots or holes which are much smaller than the
wavelength of light. The work has applications in optical microscopy
and data storage, where intensity distributions smaller than the
wavelength of light are required to resolve nanoscale objects.

6-18-08

BQR1070
Spins in ensembles of quantum dots offer one possible pathway to implementing quantum information technologies in a solid-state environment.

Unfortunately, the spin interaction with the host lattice
leads to coherence lost of the quantum bit. In this work the authors
reported the first measurement of the temperature dependence of the
electron spin decoherence time T2 in semiconductor quantum dots and
they compared their experimental results to recent theoretical
calculations. It has been possible thanks to: (i) a refocusing
technique using laser pulses called "mode-locking technique" developed
by this research group in order to avoid the inhomogeneities within an
ensemble, (ii) the investigation of InAs quantum dots where the
electronic confinement is much larger than GaAs dots (gate-defined
GaAs dots which allow only very low temperature measurements). It was
found that T2 remains constant up to 20 K and then it presents a sharp
drop due fluctuations of the electron spin interaction with the
lattice nuclei spins. The topic of the paper is of timely nature. The
results are quite important for the corresponding science community
considering the actual debate in the community about the mechanisms
responsible for spin decoherence.


***

LP11210E
Rich-club vs rich-multipolarization phenomena in weighted networks

Large scale hierarchies characterize many complex networks describing
systems in different domains. Elements at their top, the rich, are
usually recognized as the most central or influential. In what manner
do these hubs relate to each other, in particular whether they are
multipolarized or on the contrary tend to club forming elites or
backbones, is an open question. The rich-club phenomenon quantifies
this tendency to form tightly intertwined communities based on
unweighted network representations. Here, we define this metric for
weighted networks. We show that in some real systems the results
provided by the unweighted and weighted approaches may differ
dramatically, implying oligarchies of rich nodes that despite being
mutually joined by a considerable number of links are nevertheless
loosely interconnected in terms of weight. We also argue that an
exhaustive assessment of the property requires the scanning of the
weighted subgraphs formed by the hubs. This examination is able to
unveil features contrary to the average behavior: the formation of
local alliances in rich-multipolarized environments, or a lack of
cohesion even in the presence of rich-club ordering. Beyond structure,
this analysis matters for understanding functionalities and dynamical
processes relying on hub interconnectedness and, in a broader context,
may help explain how primary forces such as competition and
cooperation influence collective behavior.


***


EK10349
Cliques in networks: When the friend of my friend is also my friend.

A group of closely knitted pals is denoted a clique, every member of a
clique of friends is befriended with each other. In network theory
cliques are maximal sets of vertices mutually interconnected, where
`maximal' means that there are no out-of-clique vertices connected to
all members of the clique. We propose and study a hierarchical algorithm
to generate graphs having a predetermined distribution of cliques. We
evaluate the statistical properties of the graphs generated, such as the
degree distribution and network diameters, and compare them to some
real-world graphs, such as protein-protein interaction networks. This
work is part of a general research effort aiming to understand the
emergence of non-trivial structural network properties by studying
appropriate graph evolution rules.

***

AR10377E
Microwaves, tunneling faster than light?

The mystery of superluminality in the radio range, manifested by
tunneling of GHz microwaves through the wave barrier, and the
accompanying effect ¿ complete transmission of tunneling wave through
this ¿forbidden zone¿ are discussed in our paper. The traditional
observations of tunneling electromagnetic waves are known to be impeded
by very weak percolation of these waves through an opaque barrier. To
avoid this obstacle, we propose a setup, consisting of two equal
segments of a coaxial transmission line. One segment is empty and the
other contains a thin gradient dielectric diaphragm, characterized by a
special profile of dielectric susceptibility across the diaphragm. The
microwave in an empty coaxial is travelling with a free space light
velocity c, which is considered, according to Einsteinian causality, to
be the highest limit of propagation speed of any physical signal. The
wave, tunneling through such gradient diaphragm, possesses a jump-like
phase shift, which is shown to exceed in some cases the phase shift,
continuously accumulated by the freely propagating wave. The possibility
to measure the difference between phase path lengths of tunneling and
travelling modes is expected to shed light on the fundamental physical
problem: does the Einsteinian limit remain valid for tunneling waves
too, or is there a hope to send short tunneling signals ¿Yes!¿ and ¿No!¿
faster than by ordinary light carriers in vacuum?


***

LR10945
A Surprising Connection Between Black Holes and Superconductivity

We demonstrate that the physics of black holes can describe the physics
of superconductivity. Black holes, which were among the outlandish
predictions of Einstein's equations of general relativity, are
regions of space where the gravitational field is so strong that not
even light can escape. Superconductors, on the other hand, are
materials of great technological interest that conduct electricity
without any resistance at low temperatures. Electrical currents can
persist for years in metals that have been cooled below the
temperature at which they become superconducting. Until recently, it
would have appeared impossible that there could be a connection
between these two very different physical systems. Our demonstration
builds upon a discovery in string theory called the 'anti-de Sitter/
conformal field theory correspondence', or AdS/CFT correspondence for
short, first proposed by Maldacena, Gubser, Klebanov, Polyakov, and
Witten ten years ago.

While most metals become superconducting only at temperatures below
minus 420 degrees Fahrenheit, certain copper based compounds remain
superconducting at the boiling point of liquid nitrogen, a
comparatively balmy minus 320 degrees Fahrenheit. The mechanism that
allows these 'high temperature' superconductors to work is poorly
understood because the current carrying electrons interact strongly
amongst themselves. By showing that a new approach to
superconductivity is possible, we hope that this work will ultimately
contribute to a better understanding of high temperature
superconductivity.


***

BR10647
Resonant Switching of Magnetic Devices Assisted by Microwave Current Pulses

We present experimental results of a new strategy to achieve magnetic
switching in nanoscale devices that can be used as magnetic random
access memory (MRAM) elements. The strategy employs the use of
spin-polarized electrical currents to apply a torque to the
magnetization of a small magnetic sample. Unlike previous studies of
this "spin-transfer torque effect" which employed simple square-wave
current pulses, we explore the use of an oscillating applied current
whose frequency (in the microwave range) is matched to the natural
precession frequency of the magnet. We find by using low-temperature
proof-of-principle experiments that this resonant alternating current
can efficiently drive the nanomagnet to large precession angles,
enabling switching to occur faster and more reproducibly, with less
energy consumption, than when using square-wave pulses alone. The
phenomenon is somewhat analogous to pushing a child on a swing -- it
is easier to achieve a large amplitude of swinging by using a
periodic push matched to the natural frequency of the swing, rather
than by applying a single long push in one direction. In addition to
enabling greater speed and efficiency, resonantly-excited magnetic
switching may also enable alternative, more compact MRAM circuit
architectures.



***

BSR1064

Strong correlations and order in the iron pnictide superconductors.

One of the key questions raised by the discovery of the new iron based
high temperature superconductors is whether their physics is akin
to the copper based high temperature superconductors discovered over 2
decades ago.
In our paper we argue that structural and spin ordering transitions
observed
in recent neutron scattering experiments suggest that these two
classes do
indeed share important features of ``strong correlation physics''. We
show
that a model of localized, and strongly correlated, electrons naturally
exhibits a transition at relatively high temperatures where the square
lattice symmetry of the
Fe ions is reduced to a rectangular symmetry, just as is observed.
We extend our model to describe a number of properties of the
superconducting
and metallic states, including the pairing symmetry of the Cooper pairs.



***

LQ11753

Even water cannot escape quantum mechanics

The importance of water in environmental and biological systems
arises from the unique behavior of the hydrogen bonds between
adjacent water molecules. Typically, in chemistry, quantum mechanics
is only considered important when describing the electrons. However,
due to the light hydrogen atom, nuclear quantum effects have a
significant impact upon the nature of water. Properties such as the
melting point show a dependence upon the hydrogen isotope present.
Moreover, recent experiments showed that the momenta of protons in
water are at great variance with classical behavior. Here, we
report a computer simulation in which the nuclei are treated within
quantum theory while their interactions are derived on the fly from
the quantum mechanical ground state of the electrons. These
computations were carried out on massively parallel IBM Blue Gene/L
hardware utilizing recently developed algorithms that facilitate the
calculation of the proton momentum distribution. Our results show
that the momenta of protons in water are entangled with the position,
as dictated by the Uncertainty Principle. Therefore, broadening the
position distributions corresponds to narrowing the momentum
distributions. Due to this entanglement, differences in the hydrogen
bond structure between the solid and liquid phases are reflected in
the momentum distribution. Additionally, we confirm that nuclear
quantum effects soften the structure of the liquid, in agreement with
the isotope effect on the melting point. These results suggest that
effects such as delocalization and tunneling, that are associated to
the quantum character of the hydrogen nuclei, may play a role in
biological settings.

6-16-08

AN10173
Quantum honeycomb

A new structure resembling the packed hexagonal form of honeycomb has
been predicted and observed in computer models of groups of atoms that
are cooled down to a point where they lose their individual identities.
The honeycomb in this case is built not from wax but from a regular
arrangement of spinning vortices that form holes in the pancake-shaped
cloud of atoms. The wavelike behavior of such cold atom clouds has
previously been tested by splitting atomic clouds into two parts and
allowing the parts to collide, in experiments that are analogous to
Thomas Young's famous 200 year-old experiment with light. In this paper
we show that if the cloud is instead split into three parts, the pattern
of dark stripes observed by Young and cold atom researchers changes
dramatically into the lattice of swirling vortices. For cold atom clouds
to form requires them to be magnetically confined. We show that this
confinement leads the lattice to melt as the vortices interact
chaotically, clustering into structures that migrate and scatter
throughout the cloud. Computer generated movies of the lattice formation
and vortex interactions have been produced showing the whole process.


***

EP10337
JUST FOLLOW YOUR NOSE

Levy walks named after the French mathematician Paul Pierre Levy have
been observed in a bewildering range of organisms, including plankton,
honeybees, jackals and even sharks. In this paper I show how such
movement patterns will rise in predators that locate their prey by
simply following their noses. This is a significant finding because the
key to prediction and understanding lies in the elucidation of
mechanisms underlying observed patterns. The new result indicates that
Levy movements are common place but runs counter to the long-standing
notion that Levy movements arise from the execution of an optimal
searching strategy. The analysis and interpretation of animal movement
data is, however, not wholly straightforward and some of the analyses
claiming Levy walk behaviour have recently been called into question.
This issue is hotly contested, and arguments about the reliability of
methods used to test for the presence of Levy movement patterns may
continue for some time. Central to a resolution, though, is the
determination of underlying mechanisms that can give rise to such
patterns.


***

LR11055
A Non Accelerator Probe of New Physics

Motion reversal is known in physics parlance as time-reversal, as it
can be realized mathematically by changing time, t to -t. The origin
of time-reversal violation is one of the least understood of all the
profound issues in physics. The observation of an electric dipole
moment (EDM) of any fundamental particle or of a composite system like
atom or a molecule is a direct signature of the violation of
time-reversal symmetry in Nature. Open-shell atoms will have two
dominant sources of intrinsic electric dipole moments; one due to the
intrinsic EDM of its constituent electrons and the other due to a
time-reversal violating interaction between the electrons and the
nucleus mediated by spin zero particles. Despite the relentless
experimental search for EDMs in elementary particles and as well as in
composite systems for more than five decades no conclusive result has
been obtained. However, many ongoing high precision atomic EDM
experiments are aiming to achieve better detection limits, a few
orders of magnitude lower than the current experimental limits. In
order to obtain a limit for the electron EDM, one needs both the
enhancement factors (ratios of the atomic to the electron EDMs) and
the experimental atomic EDMs to a high precision. A rigorous
relativistic quantum mechanical calculation has been carried out to
predict the EDM enhancement factors for Rubidium (Rb) and Cesium (Cs)
with a sub 1% accuracy for the first time. One of the unique features
of this work is that it deals with the interplay of two very different
interactions—the long range Coulomb interaction and the short range
time-reversal violating interaction. The new results for the
enhancement factors when combined with those of the proposed non
accelerator EDM experiments on Rb and Cs when they achieve their
desired sensitivities could open up a novel direction for finding new
physics beyond the much celebrated model of particle physics, the
Standard Model, which indeed is quite significant, in the era of the
Large Hadron Collider. This could also serve as stringent tests of
many unification models, including Super-symmetry, and provide
insights into one of the most important but unresolved questions in
cosmology: the matter-antimatter asymmetry in the Universe to which we
owe our existence today.


***

LH11279B

Electronic orbital currents and polarization in Mott insulators

Systems with correlated electrons are now at the forefront of research in solid
state physics. The standard point of view is that at low energies Mott
insulators exhibit only magnetic properties, while charge degrees of freedom are
frozen because electrons are localized. But are they really localized? We
demonstrate this is not true in general: for certain spin textures on frustrated
lattices (v.g. triangular) there exist nontrivial charge effects in the ground
and lowest excited states. We show that in some cases \textit{spontaneous
circular electric currents} exist in the ground state \textit{of Mott
insulators} and are proportional to the \textit{scalar spin chirality}. Such
persistent currents can run on the surface of these insulators. We also show
that other spin structures generate \textit{spontaneous charge redistribution}.
In some cases, this charge ordering results in a net \textit{electric
polarization}. This is a novel, purely electronic mechanism for
\textit{multiferroic behavior}. We also discuss some dynamic consequences of the
effects discovered such as dipole-active "ESR" transitions, rotation of electric
polarization by spins and the possibility of having a negative refraction.

6-13-08

LH11279B

Spontaneous currents, charge redistribution and electric polarization in Mott insulators: are electrons really localized?

Systems with correlated electrons are now at the forefront of research in solid state physics. The standard point of view is that at low energies Mott insulators exhibit only magnetic properties, while charge degrees of freedom are frozen because electrons are localized. But are they really localized? We demonstrate this is not true in general: for certain spin textures on frustrated lattices (v.g. triangular) there exist nontrivial charge effects in the ground and lowest excited states. We show that in some cases spontaneous circular electric currents exist in the ground state of Mott insulators and are proportional to the scalar spin chirality. Such persistent currents can run on the surface of these insulators. We also show that other spin structures generate spontaneous charge redistribution. In some cases, this charge ordering results in a net electric polarization. This is a novel, purely electronic mechanism for multiferroic behavior. We also discuss some dynamic consequences of the effects discovered such as dipole-active "ESR" transitions, rotation of electric polarization by spins and the possibility of having a negative refraction.


***

LN11218

Are we at the centre of the universe? - A new test

A founding assumption in cosmology is that we - defined by our local
cluster of galaxies - are not in a 'special' place in the universe.
This is the Copernican Principle and when combined with observations
such as supernovae it implies that there must exist a kind of 'dark
energy' counteracting gravity; if this actually exists then its
origins and very nature are a complete mystery. But if the Copernican
Principle is wrong it would negate the need for dark energy. It is
consequently critical to test our bold belief that we do not occupy a
special place in the cosmos. This has not been possible in general to
date: while our current cosmological observations are consistent with
the Copernican Principle, we are far from the compelling confirmation
demanded in other scientific disciplines. We provide a new simple
method to test the Copernican Principle in a completely generic way
which can be easily implemented as we try to unveil the nature - or,
perhaps, very existence - of the dark energy.


***

LK11527B

A Nuclear Spin Probe of Nanoscale Electronic Matter

Experimental advances in the generation of local nuclear polarisation in
semiconductors open up an important and powerful new experimental
technique which can be used to probe the properties of nanoscale
electronic devices. We describe how the generation and detection of local
nuclear polarisation in the vicinity of a semiconductor quantum point
contact device can be achieved by purely electrical control.
Measurements of the nuclear spin relaxation rate, made possible by this
proposed technique, provide the ideal way in which to determine the nature
of the electronic correlations responsible for the so-called "0.7 effect".
This is a key outstanding problem in the physics of nanoscale
semiconductor devices, the origin of which remains hotly debated after
more than a decade. Our work identifies experimental signatures that can
establish unambiguously the physics at work in this puzzling phenomenon.

6-11-08

LF11730B

Hidden One-Dimensional Fermi Surface in Hexagonal Potassium
Tungsten Bronze


Summary :

The electronic structure of hexagonal potassium tungsten bronze K_(x)WO_(3)
has been investigated by high-resolution angle-resolved photoemission
spectroscopy (ARPES). The experimentally determined band structure resolves
the long-standing puzzles concerning the anomalous transport properties in
this hexagonal bronze. It has been found that K_(x)WO_(3) for 0.18 < *x* <
0.32 shows superconductivity at low temperature, while exhibiting
characteristic anomalies in electrical resistivity suggestive of charge
density wave (CDW) formation at high temperature. This characteristic
temperature highly depends on the alkali metal (K) concentration, exhibiting
a maximum at 350 K for x = 0.25. We have performed ARPES measurements on
potassium tungsten bronze, for x = 0.25 (K_(0.25)WO_(3)) by a
high-resolution spectrometer at the Synchrotron Radiation Center in
Wisconsin, USA. We have also performed ab initio band-structure calculations
to compare our experimental results. From our ARPES experiments, we found
that the Fermi surface (FS) (see Figure) of K_(0.25)WO_(3) is the
consequence of one-dimensional (1D) bands, which can be well connected by
common vectors called nesting vectors. The nesting vectors, *q1* and q2
derived from our experiments matches well with the commensurate
super-lattice reflection vectors obtained from other experiments like
electron diffraction. We conclude that the CDW formation, which reduces the
conductivity in the system, is due to the presence of quasi 1D-like bands
and/or FSs and is responsible for the high temperature anomaly in transport
properties.


***

LL10932.
First quantitative evidence for evolutionary control

In a famous paper presented before the Linnean Society in 1858, Alfred Russel Wallace - often considered the co-discoverer of natural selection - proposed that evolution exploits the principles of feedback control in order to generate biological complexity. To our knowledge, no direct evidence for this contention has ever been reported. In "Mutagenic evidence for the optimal control of evolutionary dynamics" (Phys. Rev. Lett., to appear), we present what is ostensibly the first quantitative experimental evidence, since Wallace's original proposal, that nature employs evolutionary control strategies to maximize the fitness of biological networks. Protein mutagenesis experiments carried out in our laboratory demonstrate that the electron transport chain - the fundamental metabolic pathway of most forms of life - has evolved hierarchically, with evolution on long time scales steering the later stages of evolution toward optimal refinement of the network's function. Based on this observation, we develop a general theoretical framework - the optimal control theory for evolutionary dynamics - that can be used to interrogate the role of evolutionary control phenomena in other biological systems.


***


LL11107
A SUPERFLUID AT ROOM TEMPERATURE

For many years, one of the 'holy grails' of physics has been to make a
superfluid at room temperature in the lab - until now superfluidity has
only been found at temperatures close to absolute zero. This paper has shown theoretically how one can stabilise a
high-density superfluid up to room temperature in a magnet. The magnet
is the well-known 'Yttrium Iron Garnet' system - usually called "YIG"
- which has been used for over 50 years in magnetic technology. The
superfluid is made from 'magnons', ie, from the quantum-mechanical version
of the oscillations of magnetisation in the magnet. In the superfluid all
these magnons go into the same quantum state - called a 'Bose condensate'.
The paper of Tupitsyn et al. was a reaction to experiments by Demokritov
et al in Germany, who saw hints of this superfluidity in thin films of
YIG. Tupitsyn et al show that these hints must have been produced by a
very low-density unstable precursor to the superfluid, and that to
stabilise a high-density superfluid, all one has to do is apply an
external magnetic field perpendicular to the film. Now the experimental
race will be on to find this superfluid, and see how it behaves!


***

BQ10924

Remarkable electronic properties in Mg_(2)Al_(3)

The magnetoresistance of a conductor is the change of its
electrical resistance in magnetic field. With current through a
conductor in a magnetic field, a voltage (the Hall voltage)
occurs, which is directed perpendicularly to both current and
magnetic field. For simple metal alloys the magnetoresistances is
usually small (order of ppm in fields of a few Teslas), and the
Hall voltage depends linearly on magnetic field and is nearly
independent of temperature.
In this paper we have studied two closely related phases of
Mg_(2)Al_(3), a metal alloy with the large number of 1170 atoms in
the unit cell. In one phase the Hall voltage and
magnetoresistance have values as outlined above. In the second
phase, the magnetoresistance increases strongly with decreasing
temperature below 100 K and exceeds 6 % at 4 K. The Hall field
becomes temperature dependent below 100 K. At 4 K it displays a
maximum as a function of magnetic field. These results open
challenging questions about the relation between structure and
physical properties for these alloys.


***

EQ10334

Random migrations of non-Brownian neutrally buoyant particles across
the streamlines of a carrier flow in a dilute sheared suspension are
simulated on the basis of a dipole model. Both passing and reversing
trajectories of isolated particle pairs predicted earlier for
wall-bounded shear flows are obtained within the model. A diffusivity
is due to far-field collective hydrodynamic interactions rather than
long-term pair interactions on reversing trajectories. The calculated
self-diffusivity coefficient is linear in particle volume content and
agrees well with the experimental data. The self-diffusivity in a
homogeneous suspension is linear in the particle volume content. Even
small concentration inhomogeneity across the cell width results in a
significant diffusivity growth near the walls.

6-9-08

BR10547

Theory of quantum metal to superconductor transitions in highly
conducting systems.

Background:
Phase transitions occur at critical points at which some macroscopic
properties of a material change abruptly from those characteristic of
one phase to those of another. Classical phase transitions occur at a
critical temperature which separates a high temperature phase from a low
temperature one. Quantum phase transitions occur at zero temperature, as
a macroscopic parameter is varied, such as the pressure, the magnetic
field, or amount of disorder. In many cases, quantum phase transitions
are similar to classical phase transitions, although with subtle
differences such as different critical exponents. However, there is
always one fundamental difference between classical and quantum
transitions; in quantum transitions, the dynamics and the thermodynamics
are inexoriably linked, whereas classically two systems could exhibit
identical thermodynamic behaviors in the neighborhood of the transition,
but quite distinct dynamical behavior. For this reason, quantum phase
transitions in metallic systems are quite different from classical ones,
and remain mysterious and incompletely understood. The specific
dynamical character of a metal, which is responsible for the existence
of a finite conductivity in the zero temperature limit, impresses itself
on all aspects of the quantum physics. A broad array of the most
intensively studied problems in condensed matter physics are related to
this problem.

Another important issue in phase transitions is the role of ¿quenched
disorder.¿ For instance, for a classical transition, one might imagine
that the transition would ¿broadened,¿ in the sense that it occurs at
one T_c in one part of the system and a different temperature elsewhere.
However, for classical critical phenomena, the emergence of a long
correlation length in the vicinity of the critical point assures that
local fluctuations of the properties of the system are averaged out,
leading to a sharp, more or less homogeneous transition. In quantum
systems, however, and especially in metals, the long range quantum
coherence of the system calls this general line of analysis into question.

Results:
We have undertaken a systematic study of the zero temperature quantum
phase transition from a superconducting to a metallic state, under a
broad range of conditions. We have found that a generic characteristic
of this transition is that the small inhomogeneities of the material are
amplified in the vicinity of the critical point, so that the system
inevitably resembles an array rare ¿puddles,¿ which are locally
superconducting, weakly coupled to each other through large regions of
intervening normal metal. This leads to an anomalously large regime in
the phase diagram, especially at low but non-zero temperatures, in which
the system is neither a superconductor nor a normal metal, but exhibits
new, highly quantum behavior which interpolates between the two. It
leads to the existence of a superconductor to metal transition in
situations in which it was believed that only a superconductor to
insulator transition was possible. Finally, in the case of a d-wave
superconductor, we find that near the superconductor to metal
transition, there occurs at lest one additional new phase, an
inhomogeneous superconducting phase with global s-wave symmetry. This
result has important implications for the behavior of overdoped cuprate
high temperature superconductors near the point at which
superconductivity is quenched.


***

LQ11850

We describe a new phenomenon concerned with the water response to a
temperature gradient. We show that water molecules reorient
their dipole moment along the direction of the gradient. The
polarization of water can be efficiently tuned by varying the
temperature
gradient strength.

Temperature gradients are omnipresent. They can be exploited to design
smart materials that generate electricity from waste heat.
Seebeck observed such phenomenon in the 19th century by putting in
contact two dissimilar metals. Similar phenomena are
observed in solutions containing water and salt. Using non equilibrium
thermodynamics theory and computer simulations,
we have shown that pure water will "react" to a temperature gradient
by polarizing itself. This response is driven by the desire of
water to minimize the production of entropy.
Our work shows that large temperature gradients, typically 106 – 108
K/m, can strongly polarize water. These gradients can be
created in the lab by heating gold nanoparticles with lasers, and they
may occur naturally in our bodies, as a by product of the
operation of small molecular motors, P-ATPase, which plays a major
role in enabling muscular activity. These and other biological
processes occur in aqueous solutions. The effect described here should
be important to understand non equilibrium phenomena of
relevance in biology, biochemistry and biophysics.



***

156805PRA

Enhancement of photon correlation with chirped laser pulses

Photons can be produced like twins and correlated in a controllable manner, thanks to the quantum optical techniques using lasers. To what extend can we control the quantum correlation of photons? How large correlation can we produce? By exciting an atom with laser pulses that are chirped in frequencies, it is possible to produce as substantially large nonclassical correlation. The advances in femtosecond laser pulses has benefited the study of molecular dynamics. Raymond Ooi of Korea University believes that this finding opens up a new route to engineer the production of nonclassical photon pairs using shaped laser pulses.



***

EK10395

Anderson localization as a measure of structure properties

Anderson localization in disordered system is an ubiquitous phenomenon which occurs both in quantum systems such as electron and spin in disordred systems and classical systems such as optical wave and acoustical waves in random media.

This kind of localization happens in configuration space and has been studied in depth in the past five decades.

Recent years' studies in complex networks have discovered that the disorder can happen in topological space.

In a recent work published in Physical Review E, Zhu and co-workers, from National University of Singapore, proposed to use localization as a measure of structure properties. This is possible by mapping networks to large clusters, namely, the nodes and edges to atoms and bonds between them, respectively.

Zhu et al focused on the special localizations come from the connection distribution disorders in the topological structure. And due to the lack of Euclidean distance, Zhu et al extend the localization definition by using the probability distribution function of the occurring probabilities at the nodes.

Several techniques such as participation ratio, structure entropy, distribution of nearest neighbor level spacing, and wavelet analysis are employed to describe the localization properties in detail. Interestingly, Zhu et al find multi-fractal structures embedded in the ranked occurring probabilities. It is well-known that the fractal structure of a lattice can lead the self-similar properties in wave functions. Zhu et al's finding can be regarded as an evidence and quantitative measure of self-similar structures of networks.

The structure-induced localization may have potential applications in understanding the electronic and heat transfer properties of materials such as conductive polymers and carbon nanonets.



***


BR10698

Fractal worlds ruled by Fermi-Dirac quantum statistics

Neutron stars are protected against a gravitational collapse because
of Fermi-Dirac quantum statistics causing
a degeneracy pressure. The state of electrons in metals like copper is
understood by the same principle of filling
up single particle states employing the Pauli exclusion rule.
Intriguingly, this paradigm has been shattered by
the observation that the electron systems found in some heavy fermion
metals and high Tc superconductors lack
a Fermi degeneracy scale, while they show instead a scale invariant
quantum dynamics. The lack of understanding
of these fermionic quantum critical states is rooted in a deep
theoretical problem: matter formed from bosons is quite
well understood employing the analogy with classical matter following
from Feynman’s path integral, but the infamous
‘minus signs’ associated with Fermi-Dirac statistics completely block
this alley.
Krueger and Zaanen demonstrate for the first time how to reconcile
theoretically the emergence of scale invariance
with the workings of Fermi-Dirac statistics. In the alternative
‘Ceperley’ path integral the effects of fermion statistics
are encoded in a geometrical ‘nodal structure’ which is a smooth
manifold in the normal Fermi-liquid case but turns
into a fractal when the fermions become quantum critical. Employing a
wavefunction Ansatz dating back to
Feynman they present an explicit example of a such a critical state
(see Fig.) that appears to be consistent with
experimental observations.


***

BR10547

zero temperature quantum phase transition from a superconducting to a metallic state

Phase transitions occur at critical points at which some macroscopic properties of a material change abruptly from those characteristic of one phase to those of another. Classical phase transitions occur at a critical temperature which separates a high temperature phase from a low temperature one. Quantum phase transitions occur at zero temperature, as a macroscopic parameter is varied, such as the pressure, the magnetic field, or amount of disorder. In many cases, quantum phase transitions are similar to classical phase transitions, although with subtle differences such as different critical exponents. However, there is always one fundamental difference between classical and quantum transitions; in quantum transitions, the dynamics and the thermodynamics are inexoriably linked, whereas classically two systems could exhibit identical thermodynamic behaviors in the neighborhood of the transition, but quite distinct dynamical behavior.

For this reason, quantum phase transitions in metallic systems are quite different from classical ones, and remain mysterious and incompletely understood. The specific dynamical character of a metal, which is responsible for the existence of a finite conductivity in the zero temperature limit, impresses itself on all aspects of the quantum physics. A broad array of the most intensively studied problems in condensed matter physics are related to this problem.

Another important issue in phase transitions is the role of “quenched disorder.” For instance, for a classical transition, one might imagine that the transition would “broadened,” in the sense that it occurs at one Tc in one part of the system and a different temperature elsewhere. However, for classical critical phenomena, the emergence of a long correlation length in the vicinity of the critical point assures that local fluctuations of the properties of the system are averaged out, leading to a sharp, more or less homogeneous transition. In quantum systems, however, and especially in metals, the long range quantum coherence of the system calls this general line of analysis into question.

Results:

We have undertaken a systematic study of the zero temperature quantum phase transition from a superconducting to a metallic state, under a broad range of conditions. We have found that a generic characteristic of this transition is that the small inhomogeneities of the material are amplified in the vicinity of the critical point, so that the system inevitably resembles an array rare “puddles,” which are locally superconducting, weakly coupled to each other through large regions of intervening normal metal. This leads to an anomalously large regime in the phase diagram, especially at low but non-zero temperatures, in which the system is neither a superconductor nor a normal metal, but exhibits new, highly quantum behavior which interpolates between the two. It leads to the existence of a superconductor to metal transition in situations in which it was believed that only a superconductor
to insulator transition was possible. Finally, in the case of a d-wave superconductor, we find that near the superconductor to metal transition, there occurs at lest one additional new phase, an inhomogeneous superconducting phase with global s-wave symmetry. This result has important implications for the behavior of overdoped cuprate high temperature superconductors near the point at which superconductivity is quenched.