7-11-08

LP11401
Chaotic Dance of Nuclear Spins

An experimental study of atomic nuclei in a substance used widely for
medical imaging of human lungs has revealed a new fundamental property of
interacting nuclear spins in solids. Radically different signals measured
by nuclear magnetic resonance (NMR) exhibit identical long-time behavior.
It has been proposed that this universality is related to the chaotic
motion of the nuclear spins, which erases the memory of the initial spin
state. Such universal behavior is extremely challenging both to establish
experimentally and to understand theoretically and had remained
undiscovered in the 60 years since the advent of NMR. In the experiment,
nuclei of xenon were "hyperpolarized" with a laser in the gas phase,
liquefied and then solidified. The resulting enormous nuclear polarization
made it possible to track the spin signal with great sensitivity. The
result of the experiment focuses attention on an unsolved 20th-Century
problem--the role and the implications of chaos in the behavior of large
ensembles of quantum particles. The result of this experiment suggests
that, contrary to conventional wisdom, collective quantum dynamics
exhibits extreme randomness even when the individual behavior of quantum
particles is not yet randomized.


***

LP11113 (Embargoed until July 23, 2008)
PREDICTION AND UNDERSTANDING OF NOVEL MATERIALS TO BE USED FOR NEW, HIGH
EFFICIENCY PHOTOVOLTAIC CELLS

A new type of material which can provide highly efficiency solar cells has
been developed. This material will have an intermediate energy band which is
one possible way of enhancing the efficiency of photovoltaic cells. The basic
operation of a conventional photovoltaic cell relies on the electron
promotion from a lower energy state (valence band) to a higher energy one
(conduction band) through absorption of photons with sufficient energy. The
intermediate band is located between these bands and helps to absorb, not
only the sun’s photons with energy higher than that of the gap width, but
also the lower energy ones. This will enhance the photovoltaic conversion
efficiency in these new cells in relation to the conventional ones. In this
paper, using quantum calculations, we have obtained structural, electronic
and optical properties of a new system based on an indium sulphide
semiconductor substituted with transition metal atoms. The computed optical
absorption of these compounds compared to the corresponding undoped material,
predicts a significant absorption below the band-gap of the parent
semiconductor and an enhancement of the optical absorption across the whole
solar-spectrum range. These systems seem promising for developing more
efficient novel optoelectronic devices. Their experimental synthesis has
already been reported.



***

BR10591
The solid solution hardening SSH is a long standing problem of statistical physics.
The early time of the SSH theory sends us back to the analytical works of the late
Sir Nevill Francis Mott and Frank Reginald Nunes Nabarro. The strength of a single
crystal was then determined through some continuous models and it was predicted
to vary as a fractional power law of the solute concentration. An important number
of experimental studies were performed and the compiling of their results reveals
the conundrums left aside by the early theory, as for instance the temperature effect.
The current revival of the SSH theory is mainly supported by of the development of
the 3 dimensional atomistic simulations. The atomistic simulations allows to shed a new
light on the SSH and to advance on questions that remain.
In our paper, we studied throughout numerical simulations an alloy extensively used
in aeronautics as matrix for wings and engines, i.e. the Nickel-Aluminum system.
This allowed us to emphasize that the common belief for the weakness
of the screw dislocation pinning strength does not hold for the solid solution we studied.
We determined which version of the SSH theory is the more adapted to predict the strength
of our system. Such a result should serve as a guide to further the theoretical
developments on the dislocation mobility in random media.

***


LR11643
Colorful approach to the fractional quantum Hall effect

The different types of fractional quantum Hall effect may be described in
terms of multi-color quantum liquids. This is the result of a trial wave
function approach proposed in a recent paper by Regnault, Goerbig, and
Jolicoeur (CNRS France), which yields a complementary vision of the
fractional quantum Hall effect. Indeed, each color group of two-dimensional
particles in a strong magnetic field forms a Laughlin liquid. However, in a
typical system, there is only one type of particles, say gray, and the colors
need to be viewed as an artificial marking. In order to get rid of these
artificial colors, the authors have proposed a procedure which renders all
particles gray again. This may be viewed as taking a black-and-white photo of
a colorful painting. Amazingly, the black-and-white photo reveals an internal
structure of the quantum liquid - it consists of distinct droplets the number
of particles of which is that of the original colors. The performed numerical
calculations indicate that this structure may be a common feature of the
different types of fractional quantum Hall states, such as the
composite-fermion states or else the 5/2 state with its exotic excitations.



***

LS11531
ORIGIN OF GIANT OCEAN WAVES

Scientists at Lancaster University have made a discovery that
illuminates the origin of rogue waves -- the giant waves that
occasionally appear on the ocean and are suspected of being
responsible for many unexplained losses of large ships. These
waves are quite different from the tsunami created by undersea
earthquakes, which in the open sea are usually so low that they
are almost invisible. In contrast, survivors describe a giant wave
as being like ``a wall of water'', perhaps 100 feet or more or
more in height. There is intense interest in the origin of giant
waves on account of the commercial importance of this
extraordinary phenomenon. The Lancaster team is studying nonlinear
wave interactions through experiments on superfluid helium. These
enable fundamental wave processes to be studied under controlled
conditions, in contrast to giant ocean waves which require
hundreds of miles of open sea to appear and disappear. The
scientists were astonished to discover that wave energy could
sometimes concentrate to create giant waves in the laboratory. If
the new understanding can be exploited to explain how rogue waves
arise on the ocean, it may be possible to predict them. If so,
there will be many grateful mariners and insurance companies.


***

LP10938A
Confined light could rotate microscopic rods on a chip

Laser light can apply mechanical pressure on microscopic objects, causing them to be trapped and even rotated. In this paper, we theoretically demonstrate that light that is being guided by a tiny rod and confined around it, could rotate the rod itself, forming a new type of microscopic machines that could be on a chip. The rod, possibly made of a glass-like material, could be a fraction of a micrometer in diameter and only a few micrometers long. Laser light with a "rotating" wave front needs to be injected into the rod, and then if the rod absorbs some the light, it will tend to rotate. This is similar to a plastic collision in mechanics, such that the rotating light particles "stick" to the rod and consequently apply a rotational force. Another effect that results from the interaction of the light with the absorptive rod is that the light tends to push the rod forward. Several light-driven motors may be conceived based on this concept, such as a microscopic drill that is being pushed forward and rotated by the guided and confined light.


***
LE10914AR
FROG-CRAB catches the light



Researchers at JILA, University of Colorado at Boulder, have made a giant leap
in simplifying the generation of some of the shortest light flashes achievable to date:
x-ray pulses that are just about one femtosecond (a millionth of a billionth of a second)
long can now be generated by simply shining high-power laser pulses into a gas-filled hollow
capillary. Now, if you generate an event this short, then not only making it, but also
measuring it is a challenge. The researchers accomplished this using a method called
FROG-CRAB. The ultrashort x-ray flash is scanned across the electric field oscillations
of a second, much longer infrared light pulse, and the combined electric field of the long
and short pulse knocks out photoelectrons from a noble-gas. The recorded “spectrogram”,
shown in Figure 1, contains all the information required to know the shape and duration of
the ultrashort x-ray flash (shown in Figure 2). This kind of ultrafast radiation can now
serve as a flashlight to track some of the fastest events occurring in our everyday world,
as they happen e.g. in molecular and materials dynamics, or in the motion of electrons in a
chemical reaction.

***

LP11298
Crackling dynamics of cracks

From broken dishes to collapsing buildings, materials failure bothers,
damages or devastates our life. While the failure of homogeneous solids
is well described by Linear Elastic Fracture Mechanics, the case of
heterogeneous materials remains far more complex. In particular, the
cracks propagation displays there an intermittent dynamics -- so-called
crackling dynamics -- with seemingly random discrete jumps of a variety
of sizes. Indeed, the distribution of energy released through these
jumps forms a power law with no characteristic size scale, as observed
for instance in the acoustic emission accompagnying the failure of
various materials or - at a much larger scale - in the seismic activity
associated with earthquakes. Here we derive - and confront to
experiments - a simple stochastic description for crack growth in
heterogeneous media which suggests that this crackling dynamics observed
in fracture exhibits statistical features insensitive to the mechanistic
details. This "universality" proves that model experiments in
laboratories and more complex failure phenomena as earthquakes share
common, and to some extent predictable, features.

7-9-08

LE10865A

An interferometer-free experiment is proposed aiming at violating a
Franson-type Bell inequality using photons generated from four-wave mixing
in an ensemble of two-level atoms. Previous research showed that when a
retro-reflected pump is passed through the ensemble, pairs of
counter-propagating photons emerge off of the pump axis. These photons can
either be produced at the same wavelength as the pump beam or at
wavelengths differing from the pump by the Rabi frequency that the pump
induces. In this paper, we show that interference (Figure 1) between these
two processes leads to oscillations in the photon correlation time since
photons emitted at the same wavelength will experience the same group
delay in the ensemble whereas those emitted at different wavelengths will
experience different group delays. The violation of Bell¿s inequality
further implies the result for non-locality. Since the energy separation
between photons with different wavelengths is tunable, this experiment
might be an interesting way of probing the quantum nature of the detection
process. The interference (Figure 1) will disappear when the separation
approaches the fundamental timescales for photon absorption in the
detector.

7-7-08

LF11471
Superconducting silane is layered

An international research team has predicted from first principles the
superconducting properties of silicon-based hydrogen rich alloy ┐ silane
(SiH4) with a layered structure, fueling up the possible realization of
metallization and superconductivity in dense hydrogen which has long been
a major driving force in high-pressure physics and remains an important
challenge in modern physics and astrophysics.

Silane offers a source for high purity silicon which is at the base of
electronics and microdevices. It is gas at ambient condition. The new
structure having layered network is formed when silane is subjected to
pressures above 600,000 times atmospheric pressure at sea level. The
researchers obtained the superconducting transition temperature in the
range of 20 and 75 K in the layered metallic phase. They demonstrated that
silane is a good example to metallize and superconduct hydrogen at modest
pressure much lower than necessary for solid hydrogen because it has been
already chemically precompressed.

The research, publishing in Physical Review Letters, suggested that the
layered feature could be essential for superconductivity in other
hydrogen-dominant compounds.


***


LQ11339
Light rulers on a microchip

A frequency comb is a laser source that emits a spectrum of many different discrete frequencies (corresponding to different colors), which are perfectly uniformly spaced and can serve as a ruler to measure optical frequencies. Frequency combs have become a universal tool for optical frequency metrology, spectrometer calibration, gas sensing and arbitrary optical waveform generation within the last decade and part of the Nobel price in physics in 2005 has been dedicated to this invention.
Recently we presented frequency comb generation in circular microresonators made of fused silica. These sub-millimeter resonators are fabricated on microchips using processes that are well known from computer chip production and are promising elements for integrated photonic computers. However, to generate a uniform frequency comb, the time a photon needs for one round-trip in these resonators has to be stabilized. In our work we present in a remarkably simple approach how this stabilization can be done by controlling the optical power sent into the resonator and using a thermal effect that controls its effective size. Using this approach we were able control the average photon round-trip time during one second down to a level of 10-23 seconds (0.00000000000000000000001 s). This represents an important step towards phase stabilized on-chip frequency comb generators.

***

EQ10358
More Bandgaps in Periodic Structures

Periodic structures featuring forbidden bandgaps have found a great many
applications in diverse areas of physics and engineering. Almost all
bandgaps found hitherto are induced by the well-known Bragg resonance which
occurs between waves of identical field profiles transverse to propagation
direction. Physically, resonance can also occur between waves of distinct
transverse wave profiles. However, this non-Bragg nature resonance was
usually overlooked until its existence was proved in electromagnetic waves
[1]. Extension was recently made to sound waves in an axially hard-walled
duct [2]. In the present work we make a further extension, both
theoretically and experimentally, to surface waves in a water trough with
corrugated sidewalls. Our results demonstrate the coexistence of both types
of resonances in water-waves, thus concluding the ubiquity of the phenomenon
for classic waves. We also find that the bandgaps are highly tunable by a
simple geometric arrangement, and the non-Bragg bandgap can even be made as
wide as the Bragg one. What surprises us most, just as in the case for sound
waves, is the impressively greater transmission loss within the non-Bragg
gap, which shows the higher efficiency in localizing wave energy. The
richness of transverse modes in waveguides implies the feasibility of
implementing more bandgaps with improved techniques, thus opening a new
avenue of control over band structures.


***

LQ11818
A Hot Union

The phenomenon of superconductivity requires two essential ingredients: the
binding of electrons into pairs and the establishment of coherence between
the phases of the pairs' wave-functions. Unfortunately, systems in which
pairing is strong typically exhibit a low transition temperature due to
significant phase fluctuations. On the other hand, large phase stiffness is
commonly accompanied by weak pairing. An intriguing question then arises:
Is it possible for a composite system, made of a strong-pairing component
and a phase-stiff element, to inherit the best of the two worlds and posses
a transition temperature higher than those of both its constituents? We have
demonstrated that such an enhancement indeed takes place at the interface
between underdoped and heavily overdoped cuprate high-temperature
superconductors. Our results corroborate the notion that the underdopd
regime of the cuprates is governed by a high pairing scale and strong phase
fluctuations, while the overdoped region is more conventional in the sense
that pairing and phase order occur simultaneously. Interestingly, by varying
the doping level of the underdoped layer we found that the maximal
transition temperature of the bilayer is obtained at the same doping level
where the transition temperature of the bare underdoped films exhibits an
anomalous suppression. This suppression is usually associated with the
spontaneous segregation of the electrons into quasi-one-dimensional
"stripes", and our findings may shed new light on their role in the
mechanism of high-temperature superconductivity. From a practical point of
view, the approach pursued in this study may offer guidance for the design
of higher temperature superconductors.


***

LE11370BR
Watching superconductivity to appear in copper oxides

Summary:
High temperature superconductivity occurs when an
antiferromagnetic insulator is doped with charge carriers beyond a certain
level by
slightly modifying its chemical composition. In this paper we show
that, while the magnetic order disappears continuously, there
are discontinuous changes in the
electronic, lattice and magnetic properties at the onset
of superconductivity. This is not at all
obvious since the new state emerges at zero absolute temperature
where transitions are often governed by quantum fluctuations
favoring continuous variations. In our light scattering
experiments we follow the evolution of spin, charge, and lattice
excitations with doping. We find that all vary continuously
up to the onset point of superconductivity. Here, all of a
sudden spin excitations and one of the lattice vibrations become
strongly damped and the charges start behaving as those of a normal metal.
In many respects this is reminiscent of a first order phase transition
such as the one between ice and water.


***

LQ11301
Biodiversity in ecological systems is often maintained by the
self-arrangement of the interacting individuals into spatial patterns.
In our article, theoretically and generally, we investigate the effects
of such self-organizing spatial patterns, and find that, in most cases,
they support species diversity. However, we also identify a situation
where an instability of patterns results in rapid species extinction.

Our work builds on recent microbial experiments. There, three strains of
colicinogenic Escherichia coli display a competition similar to the
children's game "rock-paper-scissors". Growing on a (essentially
two-dimensional) Petri dish, spatial patterns form. In each spatial
region, one of the three strains dominates. These patterns help to
protect one strain from the others, and therefore enable stable
coexistence, in other words, the diversity. In our theoretical work, we
investigate a more general situation where three species exhibit cyclic
dominance. We demonstrate that, in most cases, the formation of spatial
patterns helps the maintenance of biodiversity (see Figure). In
contrast, and for the first time, we also show that in a certain regime,
the opposite is true: the self-formation of patterns leads to rapid
extinction of all but one species. We provide a fundamental
understanding of both effects in terms of an analytical description via
a complex Ginzburg-Landau equation.


***

LQ11182
RELATIVISTIC CONDITIONS FROM LONG-WAVELENGTH LASERS

It is shown in a recent PRL paper (Manuscript LQ11182) that
long-wavelength, strong-field lasers can provide access to
a relativistic domain of phenomena that has not previously
been examined. In particular, the transverse fields created
by a long-wavelength laser can produce an environment in
which the magnetic field of the laser becomes important.
With increased wavelength, true relativistic conditions can
be established. This is contrary to expectations based on a
theoretical analysis using a so-called "tunneling method",
thought to be valid for long wavelengths. The new PRL
article: 'Limits on tunneling theories of strong-field
ionization', shows that a tunneling method is not
applicable when the laser has a sufficiently long
wavelength. Instead, powerful long-wavelength lasers - such
as existing infrared FELs - can approach and enter a
low-frequency relativistic domain about which almost
nothing is now known.

***

BN10869
Large-scale Atomistic Simulation for Ferromagnetic Materials at Elevated Temperatures

Magnetic materials, on both nano- and macro- scales, have been used for a growing number of applications of increasing sophistication, ranging from the storage, recovery, communication, manipulation and processing of information, to quantum computing, and to irradiation damage-resistant ferritic-martensitic steels for nuclear reactors. However, modeling dynamical processes in these materials at high temperature and/or under irradiation proves difficult because the correlated dynamics of motion of atoms and spins is characterized by broadly similar timescales, making the evolution of the corresponding variables inseparable. The spin-lattice dynamics approach developed in this paper links the real-space motion of the atoms and the precession of their spins in one time-dependent simulation, yielding an interactive description of the lattice and spin subsystems.

The example simulations described in the paper include spin-lattice relaxation of domain walls, equilibrium and time-dependent spin correlation functions, temperature dependent magnetization curves for both the infinite periodic and finite-size atomic systems, the analysis of short range spin order below and above the Curie temperature, and the effect of magnetism on elastic modulus and thermal expansion of the material. The results show that the method will likely replace conventional molecular dynamics simulations in iron-based alloys, steels and other materials where magnetism strongly affects structural, mechanical, and various other properties relevant to applications.




***

LR11184
Do we know what it really means by alloy?

Alloying effect is generally considered well understood, and the subject
is obviously of less interest than nanomaterials these days.
Conventional wisdom tells us that an alloy state of a disordered
structure can be connected to a well defined Bloch state belonging to an
ordered structure in a so-called virtual-crystal approximation. In this
paper, we have revealed that such a understanding is invalid for alloy
states in general, based on a first ever performed systematic
examination of a prototype semiconductor alloy system Ga(x)In(1-x)P
throughout the whole composition range (0 < x < 1) and in a broad
spectral range. This material happens to be of one of the few most
important semiconductor alloy systems, if not the most important one, in
major technology applications: telecommunications, photovoltaics, and
solid-state lighting. It is the key component that leads to the recent
breakthrough in > 40% multijunction solar cell efficiency and offers a
realistic potential to exceed 45%.


***

LQ11940BR
THEORETICAL STUDIES OF ORGANIC INTERFACES AID THE DEVELOPMENT OF
ORGANIC ELECTRONICS

The strong push to use organic materials in the electronics and
optoelectronics industries means that an in-depth understanding of the
physical processes underlying the performance of novel devices such as
flexible displays and organic solar cells is required. In the present
work, the interface formed between one of the most widely used organic
semiconductors, pentacene, and the surface of a carbon electrode
(graphite) was studied using quantum-mechanical calculations.
Understanding such interfaces is critical since electronic devices
present multiple layers of materials and the interfaces between these
layers strongly control the motion of electrical charges across the
device and directly impact device performance. A novel methodology
capable of describing interfacial electronic processes was developed
based on a molecular-level picture; its validity was assessed through a
comparison with available experimental data. Subtle effects due to the
weak electronic interactions between pentacene and graphite were
uncovered and have significant implications for charge transport. The
current work is expected to help in the development of novel
optoelectronic devices by providing a toolbox of computational methods
allowing the description of key electronic processes.