January 5, 2009

LX11263

TWO AGING REGIMES MAKE RETHINK 1/F NOISE

We study a liquid crystal in a regime where ordered
"rolls" (similar to convection rolls) are present,
due to the application of an external voltage.
Defects show turbulent dynamics yielding 1/f
(to a power) noise.

Treating the birth/death process of defects as
memory-resetting "quakes" we provide a theory
correctly describing the crystal response to
external perturbations in all regimes.

In detail, the inverse-power-law Fourier spectrum has a
power index monotonically decreasing with
the applied potential. "Pure" 1/f noise corresponds
to a threshold voltage, below which non-ergodicity is
apparent. In other words, the system never reaches
equilibrium, while averages over different experimental
runs show that macroscopic light transmittivity decreases
with time with respect to the preparation of the system.

However, a very weak form of non-ergodicity is also
present
in the more turbulent phase (above the 1/f threshold),
making it impossible to predict the average response to
perturbation with the use of the conventional linear
response theory.

We imagine that our approach may be accurate
for all systems exhibiting 1/f noise.


***

LQ11230

Observing particles inside a pile of sand

Using a laser sheet scanning technique, researchers at the University of Maryland are now able to observe the motion of all particles inside a pile of sand. Additionally, they can determine how the grains rearrange relative to their neighbors to allow such a compaction.

Sandpiles are packed tightly with one grain of sand held in place by its neighbors. Despite their initial tight-packing, they can be forced to compact into an even denser configuration. To achieve this, a simple “tapping” method is applied in which researchers cyclically expand and contract the container size. This process, which is similar to tapping a cup full of flour to increase space, allows researchers to observe the grains as they settle.

To see inside the container, researchers immerse the grains in a fluid with a matching refractive index. The fluid is dyed so that it fluoresces, as a sheet of laser light passes through it; the beads appear as dark circles. This creates a cross-sectional image of the system. The sandpile is “scanned” by taking pictures while moving the sheet through the system. Through these images, they find the centers of individual grains of sand.

Through this process, UMD and NIST researchers have discovered that the direction of motion is correlated with the positions of neighboring particles. The same correlation had been found in cooled fluids, highlighting the similarity between fluids close to the glass transition and granular matter close to jamming.



***

BU10709

"Theory of direct scattering, trapping, and desorption in atom-surface
collisions"

The famous nineteenth century scientist, James Clerk Maxwell, is
perhaps best known for his work elucidating the theoretical underpinnings
of electricity and magnetism, but he actually spent most of his
intellectual effort trying to explain thermodynamics and the motions of
particles making up gases. One of the major problems that perplexed him
during a large part of his career was the fact that a seemingly realistic
mathematical description of a gas as made up of tiny hard spheres could not
be made to come to a state of equilibrium all by itself. In a paper
published the year of his death, Maxwell showed that one way to resolve
this problem was to assume that when the gas particles collided with the
surfaces of their container, a portion of them became temporarily trapped
by the binding force at the surface. Some time later these temporarily
trapped particles were assumed leave the surface and go back into the gas
phase, but with a distribution of energies that was in equilibrium with the
temperature of the container walls. This "Maxwell Assumption" has been
used ever since to analyze experimental data for the interactions of gas
particles with well-defined surfaces, although up to now nobody has been
able to verify it with actual theoretical calculations. In this paper,
using a simple model for the surface binding force combined with a correct
description of the collisions of the gas atoms with the atoms of the
container walls, we have been able to determine how some of the gas atoms
become trapped and how these later escape the surface and rejoin the
gas. These calculations allow us to determine the conditions under which
the "Maxwell Assumption" is valid and when it is not. Important and
necessary verification of our calculations comes from the fact that they
explain recent experimental data taken by the group of Steven Sibener at
the University of Chicago for the scattering of argon atoms from silver
surfaces that were covered with a single self-assembled layer of
1-decanethiol molecules.


***

LU12089

Geometric frustration leads to peculiar morphology and glassy dynamics
in buckled colloids

We use a system of colloidal spheres to study geometric frustration
similar to that found in anti-ferromagnetic compounds and thereby
provide an important connection between hard- and soft-condensed matter
physics. Frustration appears whenever a set of constraints may not be
simultaneously satisfied. One of the prototypical physical models
exhibiting such a situation is that of anti-ferromagnetic Ising spins on
a two-dimensional triangular lattice. In this model, the magnetic moment
of each particle may point in one of two directions, and energy is
minimized when two neighboring spins are anti-parallel. Frustration
arises because it is impossible to arrange any triplet of neighboring
particles such that all pairs of neighbors are in opposite states.
Motivated by recent experiments in diameter-tunable microgel spheres
[Nature 456, 898 (2008)], we consider monolayers of spheres confined
between parallel plates. We establish the analogy between our system and
the Ising model by showing how maximization of entropy in the hexagonal
sphere packing induces effective anti-ferromagnetic interactions between
neighboring spheres. We explain the glassy dynamics characterized by
logarithmically slow relaxation as well as jamming in metastable
configurations, and show how lattice deformations favor zigzag stripes,
thus elucidating the importance of elasticity as a mechanism for
frustration relief.


***

LV11694ER

Proper time for relativistic Brownian motion

Einstein's theory of relativity tells us that the progression of time
experienced by a physical object is tightly linked to its state of
motion. Our paper discusses implications of this phenomenon for
Brownian random motions.

The paradigm of Brownian motion has proven very useful for
understanding the behavior of particle-like quantities that interact
quasi-randomly with a complex environment. Modern applications cover a
wide range of different areas in biology, chemistry, finance, and
physics. Due to vast experimental progress in high energy physics and
astrophysics, Brownian motion concepts will play an increasingly
important role in these fields as well. It is, therefore, important to
understand how the underlying ideas can be consistently embedded into
the theories of special and general relativity.

Our paper explains how relativistic Brownian motions can be expressed
using different time parameters. We are able to show that two
controversially discussed relativistic generalizations of Maxwell's
velocity distribution are, in fact, closely related to each other --
they correspond to different time parameterizations of the same
underlying process.

These results will be useful for modeling complex high energy
processes, where one must distinguish between the proper (life) time
of a particle and the laboratory time measured by an observer. In
particular, our analysis allows one to extend Brownian motion concepts
to general relativity in a straightforward manner, e.g., in order to
describe quasi-random motions of relativistic particles around black
holes.


***

BX10501

Electrically controllable magnetic fields in semiconductor quantum dots

A key ingredient for semiconductor based future electronic devices is a quantum mechanical property of electrons, called the spin of electrons. Normally, this spin can only be controlled by applying external magnetic fields. However, magnetic fields can neither be switched rapidly nor can they easily be applied locally to small nanostructures. In this paper, we predict an alternative and surprisingly effective way to control this spin in particular nanostructures. The controlling "knob" in our case is an electric voltage that is applied across the nanostructure. Electric fields can easily be switched rapidly and can be applied individually to small nanostructures. Our result applies to particularly engineered nanostructures, so-called stacked quantum dot molecules that are commonly fabricated and studied in semiconductor labs. In these structures, the electric field effect is an order of magnitude larger than previously reported. Our finding opens up new promising perspectives for quantum logic gates which are the building blocks of quantum computers.

***

BV10714

Confining Dirac electrons in graphene: a challenging
task

Graphene is widely considered as a perpective base for a post-silicon
electronics. To this end,
the confinement of electrons in designed space regions is very important
for producing the
building blocks of electronic devices. For massless chiral Dirac
electrons in graphene, this is
however a challenging task due to the so-called Klein tunneling:
electrons can perfectly propagate
through an infinite steep potential. How can we learn whether electrons
are really confined ?
While confined, electron energy spectrum becomes quantized and electrons
localize in certain
levels. Besides the position, each level is characterized by a width,
which is inversely proportional
to the electron lifetime in the level. The state is called quasi-bound
(QBS) if the level width is
so small that electrons may remain for a long (but finite) time before
tunneling away. To identify
a QBS one has to determine both the level position and the level width.
In this work, we suggest
a simple (T-matrix) approach for studying QBSs induced by any smooth
1D-potentials in
graphene. Detailed calculations have been performed for double barrier
structures and
n-p-n junctions. The suggested approach is quite general and could be
also used for calculating
different fundamentail quantities.


***

LU11583B

The mechanism for the creation of slow waves near cut-off frequencies in periodic waveguides

Slow waves are of considerable topical interest; we show that they can be created within a simple waveguiding structure, a planar waveguide with periodic corrugations, and describe the physical mechanism responsible. We show this is a universal feature of this geometry in very different areas of physics such as water waves in rigid pipelines, transverse electromagnetic waves in infinite conducting waveguides, as well as in anti-plane shear acoustic and coupled in-plane shear and pressure elastic waves in freely vibrating and clamped waveguides. The presence of slow modes within elastic waveguides has remained unexplored and so potential applications have not been exploited, we demonstrate that the physics of slow light is responsible for the elastic slow modes with a subtle nuance connected to the presence of negative group velocity modes within the elastic system (see a chain-like mode in Fig. 1) . Many new applications are possible, particularly in elasticity, to generate analogies of optical delay lines, furthermore the guiding structure we describe is simple to construct.


***

LN11295BR

Cryptoferromagnetism: a new monster or lovely child of intimate interplay?

Put together key words of superconductivity and magnetism: Cooper
pairing, London penetration depth, Meissner effect, coherence,
chirality, breaking of time reversal symmetry, cooperative phenomena,
crystallographic anisotropy, domain structure, magnetic moment, then
you will get impression what it is a
crytoferromagnetic state of superconductor. If you magnetise this
superconductor by applying magnetic field and measure it positive
magnetization, as we predict, you will be wondering, whether it is a
superconductor at all. But, a recovery of zero resistance will bring a
peace to you - you deal with unusual state of unconventional
superconductor, cryptoferromagnetic state. Then, it is up to you
decide, whether cryptoferromagnetism is a monster or lovely child of
intimate interplay between superconductivity and ferromagnetism.

***

BU11025

Carbon Nanoparticles as Closed Conductive Networks

In last two decades, discoveries of fullerenes, nanotubes, graphene and their fantastic properties successively instigated three booms in nanocarbon research. We attract attention to another fullerene-related material - large carbon nano-onions, the astralens, suitable for industrial-scale synthesis. High-Resolution Transmission Electron Microscopy, X-ray Diffraction and Raman spectroscopy revealed a polyhedral multi-shell structure of astralens. Astralens have average sizes of 40 nm, flat defect-free faces of ~15 nm and defects condensed at the polyhedral edges. The faces comprise stacking of 20-50 graphene sheets. Using electron and nuclear magnetic resonance techniques we demonstrated qualitative difference of the electronic properties of astralens compared to those of quasi-spherical and small polyhedral onions. The most unusual feature of the material is a temperature independent Pauli-type behavior of paramagnetic signals from room temperature to liquid helium. Such behavior, never reported for nanocarbon samples, is attributed to delocalized charge carriers whose quantity considerably exceeds that of spins localized in defects on polyhedra edges. We suggest that each astralen nanoparticle constitutes a closed network of delocalized electrons. Our findings may open a new avenue for further study of the electronic properties of such closed conductive networks and search for their device applications.

***


EU10295

Transition to the irreversibility: an explanation based on the second law

To understand how the behavior of many-particle systems may
become irreversible upon the action of an external driving force is one of
the fundamental problems of thermodynamics and statistical physics since their foundation. An analysis based on the second law of thermodynamics
reveals that this transition can be explained within the framework of
non-equilibrium thermodynamics.

The transition from a reversible (oscillatory) to an irreversible (chaotic) behavior of massive (non-Brownian) particles subjected to an oscillatory shear in a Taylor-Couette cell is described from the entropy production rate of the particles, derived from the second law, and the Onsager relations connecting the diffusion current to the driving force, the shear flow. The observed chaotic behavior of the trajectories of the particles whose origin is the presence of hydrodynamic interactions can be
interpreted macroscopically as a shear-induced diffusion effect. We have analyzed this diffusion process by means of non-equilibrium thermodynamics and computed the corresponding effective diffusion coefficient. Its value explains the behavior of the mean square displacement observed in the experiments. Our study shows that the irreversibility inherent to the chaotic behavior of the macroscopic motions of particles is perfectly compatible with the second law of thermodynamics.


***

LW11234

Ion generator produces a very large ion current – greater than the discharge current

Researchers at Lawrence Berkeley National Laboratory have succeeded in producing an extraordinarily high flux of metal ions in vacuum by operating a conventional sputtering magnetron in an unusual, “abusive” way. By overpowering the device with pulses exceeding the manufacturer’s average power specification by a factor of over 100, they were able to ionize the sputtered atoms so that they entered the regime of self-sputtering, characterized by a return of sputtered atoms, now ionized, to the target. Although self-sputtering has been known before, the researchers were able to demonstrate both theoretically and experimentally that the usable ion flux scales exponentially with the “power distance” from the onset threshold of self-sputtering. In fact the usable ion current can exceed the discharge current. As a side benefit, no process gas is needed. For the case of copper, 250 amperes of ions can be produced with a discharge current of only 100 amperes. This research promises applications in the metallization of semiconductors and for producing coatings in the vacuum of space.


***

LN10909

"Electromagnetic pulses squeeze through metamaterial at a fraction of the speed of light"

Electromagnetically-induced transparency (EIT) is a famous quantum interference phenomenon appearing when a control beam of light makes atomic gas transparent for a signal beam of a different colour. It also leads to a dramatic reduction of the velocity of the signal pulses. EIT is intensively studied as a solution for delay lines and light ‘storage’ needed for all-optical information processing.

Here we show that slowing electromagnetic pulses can be achieved in a planar metamaterial, a metal film artificially patterned on a sub-wavelength scale. Our approach does not require the presence of the control beam and is possible in a very thin patterned layer. The pulses induce strong resonant current oscillations in the metamaterial grid that interfere in a similarly way as the quantum states of the gas medium: the pulses slowly squeeze their way through the thin metal grid with only five-thousandth of the speed of light in vacuum.

A successive stacking of several layers of the metamaterial increases transmission and improves operational bandwidth, thus offering a radically new solution for delaying optical pulses in data processing applications.

***

LW10897

A time-dependent black hole reveals natures of expanding plasma

We reveal natures of relativistically expanding plasma through a
time-dependent black hole. The Quark-Gluon Plasma (QGP) is a new state
of matter whose internal friction (viscosity-to-entropy ratio) has
recently been observed to be far smaller than any other known fluids.
Such a tiny internal friction indicates extremely strong interactions
among the constituents of the QGP, quarks and gluons. Since the
universe was once made of the strongly-interacting QGP, it is
important to build a theory that can account for its behaviors, in
order to deepen our knowledge on the Big Bang. Theoretical
(microscopic) description of the QGP is one of the biggest challenges
in physics due to its very strong interaction. A breakthrough came
from superstring theory which suggests that a certain
strongly-interacting system, similar to the QGP, is mathematically
equivalent to a five-dimensional black hole. However, previous
attempts on a time-dependent system have suffered from an appearance
of divergence (naked singularity). We have succeeded, for the first
time, to construct a consistent time-dependent black hole that can
completely account for the relativistic flow of the QGP-like fluid. We
have proven that all the possible divergences are safely hidden by a
time-dependent horizon (from which no signals can escape). This work
provides a basis to treat a time-dependent system from both the
microscopic and macroscopic points of view simultaneously.


***

LU11507BR

Spintronic Switch

To become a reliable substitute for Electronics, Spintronics
first has to get into a possession of an important elemental
unit - a "magnetic switch". This unit is supposed to be either
ferromagnetic or paramagnetic depending on the applied voltage.
In accordance with Maxwell's electromagnetics, the electric field
does not penetrate into a metal and the conventional wisdom suggests
that the external control of ferromagnetism in metals is not possible.
The possibility to control ferromagnetic order by voltage was known
for almost a decade in materials called low-temperature dilute magnetic
semiconductors (DMS), and the scientific community has been intensively
looking for the high-temperature DMS for the room temperature operation.
In our paper, we showed that contrarily to the widespread belief, the
control of the ferromagnetism is, in fact, more pronounced in metals
than in the DMS. Thus, the "magnetic switch" can be metallic. This
finding is especially promising in the light of the fact that because
of the high electron density, metals, as compared with
semiconductors (including DMS), can be scaled down to
much smaller feature sizes.


***

LY11086B

One level to rule them all

We have experimentally shown that a single energy level determines the
behaviour of defects, impurities and surfaces of electronic materials.
This should allow scientists to predict the electrical characteristics
of all semiconductors, and even custom engineer specific materials with
desired properties for use in devices as diverse as light emitting
diodes, chemical sensors, and photoelectrochemical cells for hydrogen
production. The bulk electrical properties of semiconductors can differ
drastically from one material to another. They are known to be
controlled by a small concentration of imperfections such as missing
atoms or other defects, or foreign atomic species known as impurities,
which can add electrons to, or remove electrons from, the material.
Also, even for perfect crystals, surface properties can vary greatly,
supporting either a build up or a complete absence of charge,
significantly affecting electrical contacts to devices. Investigating
the material cadmium oxide, we have now shown that how these defects,
impurities and surfaces act electrically depends on the position of a
common energy level. From the position of this so-called charge
neutrality level, which can be estimated fairly straightforwardly, the
electrical behaviour of any semiconductor can instantly be understood
and even predicted. This will allow custom materials to be developed
with desirable electrical properties for high-performance current and
future device applications.

December 23, 2008

LV11390

Spin squeezing of a cold atomic ensemble with a nuclear spin of one-half

We report the first demonstration of spin squeezing of laser-cooled nuclear spin one-half atoms via spin quantum non-demolition measurement in Physical Review Letters. This experiment can be applied to the improvement of the optical lattice clock, the continuous quantum interface between light and atoms, and so on. In this paper, we used laser-cooled 171 ytterbium atoms. Since 171 ytterbium atoms have only a nuclear spin of 1/2 in the ground state, this system is the simplest spin ensemble and robust against decoherence. Furthermore, we used very short pulses with the width of 100 ns, so the interaction time became much shorter than the decoherence time, which is important for multi-step quantum information processing.

***

LR10995

Novel source of entangled photons.

Quantum entanglement is an intriguing quirk of quantum theory linking
the properties of spatially separated objects. Applied to pairs of
photons transmitted through optical fibre or free space, it may be used
to implement quantum communication applications such as quantum key
distribution, quantum teleportation or entanglement swapping. In 1966
John Bell famously devised an inequality to show that information stored
locally in the entangled system does not dictate the outcome of
measurements. It is considered as a rigorous test for entanglement
serving as a threshold for many applications exploiting spatially
separated entangled photons. In this work we demonstrate that a novel
source of entangled photons, semiconductor quantum dots, can achieve
sufficient quality to easily violate Bell's inequality and therefore be
of use in a plethora of new applications. The primary advantage of this
source over others is its ability to emit the photons in a controlled
fashion with a near-zero probability of emitting two pairs of entangled
photons simultaneously. Such control is required for scalability and
operation in a number of quantum communication applications.

The figure depicts the decay process resulting in the emission of a pair
of entangled photons.



***

LU11507BR

To become a reliable substitute for Electronics, Spintronics
first has to get into a possession of an important elemental
unit - a "magnetic switch". This unit is supposed to be either
ferromagnetic or paramagnetic depending on the applied voltage.
In accordance with Maxwell's electromagnetics, the electric field
does not penetrate into a metal and the conventional wisdom suggests
that the external control of ferromagnetism in metals is not possible.
The possibility to control ferromagnetic order by voltage was known
for almost a decade in materials called low-temperature dilute magnetic
semiconductors (DMS), and the scientific community has been intensively
looking for the high-temperature DMS for the room temperature operation.
In our paper, we showed that contrarily to the widespread belief, the
control of the ferromagnetism is, in fact, more pronounced in metals
than in the DMS. Thus, the "magnetic switch" can be metallic. This
finding is especially promising in the light of the fact that because
of the high electron density, metals, as compared with
semiconductors (including DMS), can be scaled down to
much smaller feature sizes.

***

LW11153

Quantum fluctuations may be larger than you think

Staring into a glass of wine may bring us close to the quantum world.
Quantum mechanics predicts that even at absolute zero, motion of atoms
molecules and fields does not cease. This residual motion is called
zero point motion, and is a necessary consequence of Heisenberg's
uncertainty principle. Zero point motion has been observed in several
contexts, ranging from X-ray scattering by crystals to the Casimir effect,
which is a force due to the zero point fluctuations of the electromagnetic
field. Usually, zero point effects are small compared to thermal
flucutations. An example of the latter are Rayleigh and Brillouin
scattering in liquids, where light is scattered by thermal density
flucutuations. In this paper, we study zero point motion in a liquid and
the resulting quantum density flucutuations. We show that these
flucutuations can also scatter light and may be large enough to observe
experimentally. We find that even at room temperture, the scattering by
zero point motion can be about 0.5% of the total scattering, and a larger
fraction at lower temperatures. Although 0.5% may seem to be a small
number, it is strikingly large for a quantum effect in everyday life. This
is an illustration that the quantum world is not always so far removed
the classical world of our everyday experience.

***

LU11736E

How the shrimp makes noise

Learning from the life of marine shrimps, we have suggested a new potential
mechanism for the generation of cavitation noise, a loud acoustic noise
from collapsing bubbles. A recent report demonstrated that the snapping
shrimp can produce a tiny bubble in water by rapidly closing their claws,
through a process called cavitation. The bubble collapses so violently that
a loud noise is emitted. The recorded noise seems to consist of a number of
positive and negative pressure pulses. However, a theory for single bubbles
failed to reproduce the negative pulses, implying the existence of something
overlooked. In this paper we have shown theoretically that the negative
pulses can be explained if sound reflection at the surfaces of "microbubbles"
is considered. The collapsed bubble breaks into many microbubbles, as
observed, and they should emit positive pulses at collapse. When the pulses
hit the surfaces of other microbubbles, negative reflected pulses must be
generated because of the soft nature of the surfaces. This pulse generation
process would repeat many times between the microbubbles and then a
complicated signal will be created. This suggestion reveals the complex
dynamic process inside the micro noise generator.

***

LU11451

Spin excitations get doubled by x rays

Perturbing the spin order in a solid is a means of studying the
microscopic magnetic properties of that material. Such elementary
excitations, often called magnons, are traditionally probed in
neutrons scattering experiments. The energy lost when the neutron
bounces away from the crystal is precisely the energy needed to
change by one unit the total magnetic moment of the target. In this
paper, we have used x rays instead of neutrons to measure elementary
magnetic excitations of NiO, an antiferromagnetic material. This is
made possible by tuning the x-ray energy at an absorption edge of Ni,
in a resonant inelastic soft x-ray scattering (RIXS) experiment.
Using a newly developed high-resolution instrument, we could for the
first time determine a magnetic excitation at 95 meV, in agreement
with the previous determinations by inelastic neutron scattering. In
addition we found a second elementary spin excitation, at 190 meV,
which corresponds to a change by two units of the magnetic moment.
This double-energy excitation cannot be triggered by neutrons for
magnetic momentum conservation reasons. Our results demonstrate that
high resolution RIXS has become be a valid complement to neutron
scattering in the study of elementary magnetic excitations.


***

LW11554

High-Speed Jet Formation after Solid Object Impact

When dropping a stone into water, a thin and very fast water jet can
shoot out from the surface - a familiar and simple everyday phenomenon
which any child has tried himself or herself and which is omnipresent in
nature and technology. However, only a closer look with high-speed
cameras combined with computer simulations can reveal the rich and
complex underlying dynamics. The initial downward motion of the
impacting object is turned into an upward motion of the liquid jet.
During impact the intruding object creates an air-filled cavity in its
wake. The subsequent violent collapse of this cavity due to hydrostatic
pressure then leads to the formation of the high-speed liquid jet. In
this paper we illustrate how the collapsing cavity wall squeezes out the
jet very much like the squeezing of a tube of tooth paste - but of
course much faster. These collapsing walls make the present jetting
mechanism reminiscent of the very violent jets of fluidized metal
observed during the collapse of "shaped charges" in military and mining
operations.

***

LV11154

Remarkable maximum current enhancement in semiconducting carbon nanotubes

We have recently broken the "speed limit" (maximum current) of semiconducting carbon nanotubes, raising it more than 50% by driving the nanotubes into avalanche behavior. Single-wall carbon nanotubes are already known to carry current densities up to one hundred times higher than copper. Despite atomically narrow nanotube diameters (~2 nm), previously measured maximum currents were a respectable 20 microAmps in tubes a few microns long. As reported in Phys. Rev. Lett., we have found that at high fields (~10 V/micron) energetic electrons and holes can create additional electron-hole pairs, leading to an avalanche or "snowball" effect. Under such conditions the free carriers multiply and the current increases until the nanotube eventually breaks down, often by oxidation if exposed to air. Interestingly, we are able to perform repeated avalanche measurements in vacuum, by limiting the highest voltage applied, and obtaining currents up to 30-40 microAmps, or almost twice higher than previous reports. The avalanche is only observed in semiconducting nanotubes but not metallic ones, indicating the role of an energy gap as required for the electron-hole multiplication. We suggest that highly non-linear devices (avalanche nanotube transistors) can be made to take advantage of this phenomenon, with switching energy potentially well below that of conventional nanotube transistors.

***

LS11722

Light through a "meta-hole"

Classical optics dictates a limit on the size of an aperture that light can
pass through. Increasing amount of efforts have been devoted on enhancing
transmission through small apertures. In our recent Physical Review Letters
article, we proposed a novel method to enhance transmission from a very
small hole, with a radius 20 times smaller than the wavelength of
electromagnetic wave. In our approach, a subwavelength metamaterial
resonator (split-ring resonator) is placed near the hole, forming a
meta-hole that lets the electromagnetic waves pass through. Metamaterials
are widely studied to yield interesting physical mechanisms such as negative
refraction, subwavelength imaging, artificial magnetism and cloaking. Here,
we combine metamaterials with a single aperture in order to increase the
transmitted light intensity, paving a way towards novel applications of
metamaterials including sensors and nanolithography. Previous methods for
enhancing light transmission require bulky structures like periodic hole
arrays or surface texturing in order to couple light to the smaller
apertures. Our approach differs from the previous methods by its simple
layout, where we used a single hole with a single resonator to achieve
740-fold enhancement in transmission. Our approach -to our knowledge - is
the first study that reveals the possibility of enhancing light by utilizing
the magnetic resonance.

***

BVR1079

Improved Electromagnetic Metamaterial Absorber Realized

Researchers at Boston University and Boston College have dramatically
improved upon previous work to create a metamaterial that resonantly
absorbs nearly 100% of incident electromagnetic radiation. Their
metamaterial “perfect absorber” is resonant in the far-infrared region of
the electromagnetic spectrum. This region of the spectrum (~300 GHz – 10
THz) is of considerable technological importance with potential
applications ranging from non-destructive imaging of biological or illicit
chemical materials to secure short-range communications. However,
realizing the potential of THz radiation for real-world applications
requires further development of sources, detectors, and component
technologies. In the present work, a metamaterial absorber operating at
1.6 THz with a peak absorptivity of 97% was realized. Further, the 16
micron thick gold /polyimide structure (shown in the figure below) is
highly flexible with a high absorptivity realized over a wide range of
angles of incidence. Such materials may find application in future THz
thermal detectors or to mitigate unwanted reflections in imaging
applications.


Figure Caption: A photograph of a portion of the terahertz metamaterial
absorber. The bottom layer is a gold film with the electric ring
resonators spaced eight microns above this using polyimide.


***

EW10323

Backbone disorder can enhance charge transport ability of DNA-like systems

The backbone disorder can enhance the charge transport ability of DNA. Direct charge transport measurements on DNA, of which its conformation is very sensitive to sample preparations and surrounding conditions, indicate that they might be insulators, semiconductors, or conductors. In this paper, we study the charge transport properties of DNA by taking into account the environmental complications which are appropriately simulated by the backbone disorder. In contrast to the localization picture that the conduction of DNA becomes poorer when the backbone disorder degree is increased, we explicitly find that the backbone disorder can enhance the charge transport efficiency of DNA when the environment-induced disorder surpasses a critical value and will give rise to a semiconducting-metallic transition. The significance of our results is twofold. First, this provides a very possible scenario to interpret the variety of transport behaviors observed in DNA, and suggests new perspectives for future experiments intending to control charge transport through DNA-based nanodevices. Second, we expect that the environmental disorder-induced enhancement of transport may be a generic feature in other quasi-one dimensional materials, e.g., carbon nanotubes and graphene nanoribbons, which are usually subjected to some randomness caused by substrates or other surrounding objects.

***

AW10356

Dynamics, please!

A swinging pendulum that is brought into its topmost position usually
will fall back again - unless it is acted on by a time-periodic force,
which can stabilize the pendulum's bob above its pivot. Similarly,
the wave packet of a quantum particle moving on a perfectly regular
lattice will usually spread out all over that lattice - unless it is
acted on by a time-periodic force, which can keep the particle
permanently localized. This curious effect, termed "dynamic localization"
when it was predicted 22 years ago, has now been observed with
Bose-Einstein condensates in time-periodically forced optical lattices.
In their article which is going to be published in the Physical
Review A, a team of experimentalists and theoreticians from Barcelona
(Spain), Oldenburg (Germany), and Pisa (Italy) argues that this
observation marks a cornerstone on the way towards a new form of
quantum state engineering: An electron which feels the spatial
periodicity of a crystalline lattice, say, is described by the famous
Bloch waves of solid state physics. However, ultracold atoms in a shaken
optical lattice are subjected to BOTH the spatical periodicity of the
lattice AND the temporal periodicity of the driving force - so that
novel quantum states appear, dubbed "spatio-temporal Bloch waves"
by the authors. Because one axis of their lattice extends in the
time dimension, these states can be systematically manipulated and
controlled, a fact which not only explains the dynamic localization
phenomenen, but may lead to several further applications.


***

LS11598

Hints of Quantum Gravity in Low Energy Physics Laboratories

Gravitation and Quantum Mechanics are both universal, manifesting
themselves at large and small distances respectively. But try to
combine the two, and one ends up with absurd and infinite results.
Various theories of "Quantum Gravity" (QG), such as String Theory
and Loop Quantum Gravity, have tried to cure this malady. However,
none have made any predictions which can currently be tested in
the laboratory or in astrophysical observations.

In this paper, we show that one QG prediction, namely that our
universe may have a minimum measurable length (of the order of
a decimal point followed by 34 zeros and a 1 in metres, also known
as the Planck length), and that the familiar Heisenberg Uncertainty
Principle may undergo modifications near that length, can have
measurable effects on almost any Quantum Mechanical system. Thus,
it can potentially be tested in various laboratories, including
those of Nuclear, Atomic and Surface Physics. We examine three
such laboratory experiments (the Lamb Shift, Landau Levels and
Scanning Tunnel Microscope), the viability of measuring QG effects
in them, and bounds that they impose on some QG parameters. In an
optimistic scenario, our results could open a much needed low-energy
window to QG phenomenology, required to test the veracity of the
various theories.

December 11, 2008

EW10383

Emergent properties of flicker noise in human behavior.

When I am driving my car, the brain obtains precise information of the scene and provides the adequate response to the muscles in pressing the brakes among many other common actions. Since Cattel in 1886 and subsequently Pieron in 1914, a power law indicates that the mean reaction time decreases as the intensity of the signal increases until reach an asymptotic value or plateau. This behavior is universal and not restricted to the visual-motor system (Pierońs law). However, the neural mechanisms responsible for this psychophysical law have remained unknown. In the paper authored by Jose M. Medina, it is proposed a new physical mechanism. The general principles of information theory are applied to define information coding by the brain. It is proposed that neurons act as biological clocks and spontaneously synchronize their responses to decode the input signal. The results conclude that internal flicker noise contributes to Pierońs law and provide the basis to divide mean reaction times into the sensory latency and the motor conduction time, a problem that has not been solved yet. The article opens a new perspective on the role of noise fluctuations in the elementary neural organization and exemplifies how physics can explain psychophysics.

***

LT11296AR

New ideas for creating bosonic fractional quantum Hall states

In this paper we demonstrate how quantum control ideas, applied to a
nano-Kelvin gas of atoms, can enable the creation of bosonic analogs
of the states responsible for the fractional quantum Hall effect.
This effect, observed in 1982 by Tsui, Stormer, and Gossard,
revolutionalized our thinking about how one classifies phases of
matter. In particular, the states responsible for the effect have no
local order parameter but are sensitive to the system's topology.
Moreover, they have excitations with "fractional charge and
statistics", which not only carry a rational fraction of the charge
of an electron but also introduce a phase $\phi\neq 0,\pi$ when two
of them are exchanged. While it has been recognized for many years
that cold bosonic atoms could form states with similar properties,
experiments have found that technical difficulties (which get worse
as the clouds get bigger) have prevented their realization. To
overcome these problems, we propose trapping small clusters of
Rubidium atoms at the nodes of a cubic optical lattice formed by
interfering several laser beams. By controlling the lasers'
amplitude and phase, one can rotate each cluster of atoms. A precise
sequence of modulations, analogous to pulses of light used to
spectroscopically drive a molecule into a predetermined excited
state, will coherently drive the atoms into a wavefunction which is
similar to the one that Laughlin introduced in 1983 to explain the
fractional quantum Hall effect.

***

LU12027A

Atoms work better in small quantum computers when they work together.

In this paper we show how ensembles of few hundred trapped atoms can
efficiently absorb weak focussed light pulses in collective atomic states
and we suggest that such states of atoms can be used in few-bit quantum
computers. Our proposal to reliably send, receive and manipulate information
in one and the same physical system offers a simple strategy to build large
quantum networks that can extend quantum secure communication to long
distances and enable distributed quantum computing on large numbers of
separate registers.
The proposal uses Rydberg state dipole forces between the atoms to form our
collective bit-encoding scheme by restricting the collective population in
different internal atomic states to the (bit-)values zero and unity. In
addition to easy qubit addressing by spectroscopic selection rather than
single-atom resolution, the many-atom character of our systems leads to a
nearly perfect coupling to single photon light fields.

***

AV10417

Making chemical bonds in a bath with a flash

With the advent of ultrashort, very intense laser pulses
it becomes possible to selectively steer chemical reactions
in a desired, perhaps previously unknown but useful, direction.
Usually, i.e., under typical chemical conditions,
these reactions happen in or close to a condensed phase
such as a liquid or at a solid surface.
In their paper, Paramonov and Saalfrank study
with the help of quantum dynamical methods
the simplest chemical reaction: The making of a bond between
two atoms (oxygen and hydrogen), embedded in a `bath'
of oscillators which mimick a condensed phase environment,
by using infrared laser pulses with a duration of about
1 picosecond (1 picosecond = 0.000000000001 seconds).
In this situation both the laser field and the coupling
to the environment which serves as an energy sink,
help to stick the atoms together.
It is found that the laser pulse enhances at low laser intensity
the reaction probabilty relative to the laser-free case,
while the reaction probability is suppressed
when high-intensity lasers are used.
Surprisingly, it is found that the reaction probability
at a certain, system-specific laser intensity
is totally independent of the strength
by which the reacting atoms are coupled to their
environment. This finding is explained by numerical
simulation and by analytical models known from
atomic physics.

***

LS11242

Joining quantum worlds: manipulating ultracold atoms without touching them

We consider the ultimate quantum limit of light-matter interaction, where the quantum natures of both ultracold matter, e.g., a Bose-Einstein condensate (BEC) trapped in periodic micropotentials, and light are equally important. We use one of the most intriguing predictions of quantum mechanics, which claims that the state of one quantum system (in our case, ultracold gas) can be changed by the distant measurement of another system (light), even if they do not interact. The key point is the concept of the "entanglement", which is possible only in the quantum world. We show, how to prepare various quantum states of matter (e.g., the macroscopic superposition, or Schroedinger cat, states) by simply measuring the photons scattered. Joining the paradigms of two modern fields of quantum physics, quantum gases and cavity quantum electrodynamics (QED), will enable unprecedented control of matter at the nanoscopic level, and will give birth to conceptually new
models and systems for quantum many-body physics and quantum information processing.

***

LM11149DR

Limits on Dark Matter at a New Mass Range

Weakly Interacting Massive Particles (WIMPs) are candidates
for the Cosmological Dark Matter which accounts for about
a quarter of the energy density of the Universe. This paper
reports a search of WIMPs with mass less than 10 GeV/c**2
unexplored in previous work. Such investigations are made possible
through a low background germanium detector with an energy threshold
of about 200 eV, which represents an improvement in sensitivities over
the other techniques. New upper limits are placed on possible
WIMP interactions with ordinary matter, covering the
untested domains implied by the positive results of the DAMA
experiment. This detector technique also opens the sub-keV
energy window for new experiments studying neutrino properties
and interactions.
The research program is performed by the TEXONO Collaboration
comprising researchers from Taiwan, China, Turkey and India.

***

BW10728

Memory dynamics in nano-devices

The past experience of human beings has a significant effect on their behaviors today. This is called memory dynamics, more scientifically, the non-Markovian dynamics. Inanimate devices are no exceptions that their memory dynamics should also play an important role in their functioning and efficiency. In a recent work (BW10728) “non-Markovian decoherence theory for a double-dot charge qubit” (to be published in PRB, 2008), Matisse W. Y. Tu and Wei-Min Zhang of the National Cheng Kung University at Taiwan looked into the memory dynamics of electrons in a double quantum dot (DQD) device which is promising for quantum information processing. They developed a theory for the electrons in DQD that can fully characterize the memory dynamics of electrons due to quantum operations on the device. This solves the problem of being unable to fully account the memory effect that theorists have been facing for the past many years. It shows that the role played by memory is significant in such devices and it is accompanied by the fast operation to the device required for high efficiency quantum information processing. This work also makes be a big step toward the theory of open quantum system since the finding of the first exact master equation by Hu, et al., [Phys. Rev. D45, 2843 (1992)] for quantum Brownian motion 16 years ago after Feynman and Vernon developed the influence functional theory in 1963 [Ann. Phys. 24, 118 (1963)].


***

EX10488

The multiscale physics of ion-beam cancer therapy

For more than ten years, ion-beam cancer therapy has been successfully used
clinically in Germany and Japan. Proton-beam therapy is performed in many
more centers around the globe. These therapies appear to be a more favorable
alternative to the conventional radiotherapy. Despite apparent experimental
and clinical successes, a comprehensive theoretical description of a
physical scenario is missing. One reason is that phenomena initiated by an
energetic ion incident on tissue happen on many scales in time, distance,
and energy. Our goal is to understand the physics of beam therapy on a
microscopic level and, while moving towards this goal, we developed a
multi-scale approach to understanding the physics related to ion/proton-beam
cancer therapy and the calculation of the probability of DNA damage as a
result of irradiation of tumors with energetic ions. This approach is
inclusive with respect to different scales, starting from the long scale,
defined by the ion stopping, followed by a smaller scale, defined by
secondary electrons and radicals, and ending with the shortest scale,
defined by interactions of secondaries with the DNA. We presented
calculations of the probabilities of single and double strand breaks of DNA,
suggested a way to further elaborate on such calculations, and also made
some estimates for glial cells exposed to radiation.

***

LM11100E

Discrete chaotic states of a Bose-Einstein condensate

We find the different spatial chaos in a one-dimensional attractive Bose-Einstein condensate interacting with a Gaussian-like laser barrier and perturbed by a weak optical lattice. For the low laser barrier the chaotic regions of parameters are demonstrated and the chaotic and regular states are illustrated numerically. In the high barrier case, the chaos is associated with a quantum Schrodinger equation whose bounded solutions which describe a set of discrete chaotic states are constructed for the discrete barrier heights and magic numbers of condensed atoms. The chaotic density profiles are exhibited numerically for the lowest quantum number, and the analytically bounded but numerically unbounded Gaussian-like configurations are confirmed. It is shown that the chaotic wave packets can be controlled experimentally by adjusting the laser barrier potential.

December 10, 2008

EX10488

The multiscale physics of ion-beam cancer therapy

For more than ten years, ion-beam cancer therapy has been successfully used
clinically in Germany and Japan. Proton-beam therapy is performed in many
more centers around the globe. These therapies appear to be a more favorable
alternative to the conventional radiotherapy. Despite apparent experimental
and clinical successes, a comprehensive theoretical description of a
physical scenario is missing. One reason is that phenomena initiated by an
energetic ion incident on tissue happen on many scales in time, distance,
and energy. Our goal is to understand the physics of beam therapy on a
microscopic level and, while moving towards this goal, we developed a
multi-scale approach to understanding the physics related to ion/proton-beam
cancer therapy and the calculation of the probability of DNA damage as a
result of irradiation of tumors with energetic ions. This approach is
inclusive with respect to different scales, starting from the long scale,
defined by the ion stopping, followed by a smaller scale, defined by
secondary electrons and radicals, and ending with the shortest scale,
defined by interactions of secondaries with the DNA. We presented
calculations of the probabilities of single and double strand breaks of DNA,
suggested a way to further elaborate on such calculations, and also made
some estimates for glial cells exposed to radiation.

***

CL10086

On the hunt for lost particles

The induced fission of compound nuclei has been investigated exhaustively for many decades but despite many years of experimental and theoretical effort our understanding of fission is still far from complete. The model description of fission point ranged from the early statistical models to the advanced dynamical descriptions. After many decades of intensive research in the field of fusion-fission reactions, various aspects of the problem still remain under debate and many theoretical methods have been developed to study dynamics of these reactions. There are several approaches to energy dissipation in heavy-ion fusion-fission reactions, but difference between particle multiplicity such as charged particles, gamma rays and neutrons in theoretical and experimental view as the main consequence of earlier investigation, show that statistical approach is not main way for definition of energy dissipation in heavy-ion reactions. In our project we find classical corrected neutron multiplicity in the base of dynamical approach in the frame work of asymmetric fission for a few typical systems and then we compare our results with experimental data. These constraints may reflect characteristic features of fusion-fission reactions.

***

LW11113

Fractal magnetic domains discovered in the bulk of Nd2Fe14B

The structure and evolution of magnetic domains play a crucial role in
ferromagnetic materials by providing a link between the microscopic
magnetic structure and the macroscopic response to an external
magnetic field. Nevertheless, for the industrial strength
ferromagnet, Nd2Fe14B, the dimension, shape and arrangement of
magnetic domains are still under debate due to the lack of suitable
methods to study magnetic domain structures in the bulk. Using
small-angle neutron scattering, scientists at Ames Laboratory/Iowa
State University and the Institut Laue-Langevin have demonstrated that
the complex domain patterns at the surface, as revealed by
quantitative Kerr and Faraday microscopy, propagate into the bulk with
structural dimensions down to 6 nm, the domain wall size. Perhaps
most interesting is that the neutron scattering intensity follows a
power law with unusual exponents over one decade in length scale,
characterizing the magnetic domains as fractals with volume filling
boundaries. Below the spin-reorientation transition at 135 K, a
weakening in the magnetic anisotropy reduces the complexity of the
domain structure to a more regular arrangement and yields an anomalous
increase in the bulk magnetization. These observations open up a rich
experimental playground for studies of fractal structures in magnetic
systems with strong uniaxial anisotropy.

December 8, 2008

LV11242

Networks, shaken and stirred!

Can one really slow down an epidemic by disinfecting those airplanes that
fly between international hubs? Many networks are notoriously difficult to
break as they are practically immune to random failure and, like internet,
even resistant to intentional attack. How can one devise a strategy for
manipulating the robustness or fragility of a network, which is useful even
when we don't know its structure?

A robust network can be rendered fragile by a procedure akin to shaking.
When links connecting hubs are selectively removed, the network is prone to
"heart failure" as its central structure deteriorates. Adopting a
depreciation strategy we call "central bias" we predict, and verify by
simulation, that a dynamic state can be reached in which the network becomes
fragile to random failure, although its static characteristics qualify it as
being robust.

Conversely, a fragile network can be rendered robust by gentle stirring.
Employing a failure scenario we call "peripheral bias" we find that
selectively removing links between poorly connected nodes can boost the
network resilience. In both procedures, the depreciation process acts in a
stealth manner. The network agents, or nodes, are left unsuspecting. Only
links are affected and all intelligence required are the connectivities of
the adjacent nodes.

***

LU11307

3D X-ray imaging of human chromosome
- Nano X-ray CT Scanner for Cell Organelles -

We observed three-dimensional structure of an unstained human chromosome
by using coherent X-ray diffraction. The observed images reveal an axial
structure with high electron-density in the chromosome, which other
microscopic methods have been unable to visualize under unstained
condition. Such mesoscopic-scale structures as cell organelle have been
difficult to be observed, although molecular structures of their
components like DNA and histone proteins could be studied by X-ray
crystallography or electron microscopy. In observing mesoscopic-scale
structures, high penetration power and high imaging contrast are
essential. In optical microscope, it is well-recognized that phase
contrast microscope (Frits Zernike: Nobel prize in physics in 1953)
provides better image contrast for unstained biological samples than
bright-field microscope. In the X-ray regime also, high image contrast
can be obtained in phase contrast imaging. Our lensless coherent X-ray
diffraction microscopy is an ideal form of X-ray phase contrast imaging,
since there is no contrast degradation due to lenses. Our result
experimentally demonstrates the high imaging ability of coherent X-ray
diffraction for unstained biological specimens, which is transparent to
X-rays, opening a novel and strong mean of exploring cellular structures.

***

BV10739

Giant Bond Length Contraction in Cobalt Nanoislands

In a bulk crystal, atoms are surrounded by neighbouring atoms arranged in
three dimensions leading to a large number of neighbouring atoms (n) such
as n=12 in the case of most metals.

By contrast, atoms in a two-dimensional nanocrystal consisting of only 10
to 40 atoms are surrounded by a significantly reduced number of nearest
neighbours (n=2 to 6). This leads to strongly modified chemical and
physical properties, one of them is the rearrangement of the atoms.
Despite its fundamental importance, the knowledge of the atomic geometry
in nanostructures is quite scarce where the interatomic distance
represents the most important parameter.

In this study, Cobalt atoms were deposited on a Copper surface to form
two-dimensional nanoislands and x-ray diffraction experiments were carried
out. They provide a quantitative experimental proof of a dramatic
reduction of the (average) interatomic distance from d=2.51 Å in bulk
Cobalt to values in the 2.35-2.45 Å range. Our study [1] is based on the
analysis of the positions of the Cobalt atoms relative to the surface
Copper atoms involving a statistical disorder approach in analogy to the
Debye-Waller type treatment of thermal vibrations.

***

LN11102

Origin of Nanorods Diameter Discovered

Ever wonder why the diameter of nanorods is on the order of 100 nm?
The origin has been discovered and reported in Physical Review Letters XX,
xxx (2008). In retrospect, synthesis of nanorods has been in practice for
decades, without knowing the origin. Surface steps are effective in slowing
down the mass transport of surface atoms, and aggregated surface steps are
even more effective. This extra effectiveness makes the diameter of nanorods
100 nm or so; without it the diameter would go up to 10 microns. This extra
effectiveness has escaped attention in scientific literatures. This
discovery provokes a revisit of surface processing theories. Beyond
scientific understanding, the discovery of the origin paves the way to
controlling the diameter of nanorods. Nanorods have numerous applications
as, for example, photonic materials and catalysts in fuel cells.


***

LX11641

Fluctuations away from equilibrium

When forced, many complex natural systems away from thermal equilibrium
respond discontinuously. Avalanche of atoms causing propagation of
fracture, jerky movements of continental plates during earthquakes,
evolution or extinction of species, or even stock market fluctuations have
the commonness in jerky response when subjected to an external force, and
often display a universal scale-free size distribution of the bursts. The
scalelessness is believed to be a manifestation of critical behavior, such
as that observed at second order phase transitions, but it was debated for
many years whether it occurs at a definite driving force, or if the
complexity and long-range interactions enforce the critical state over a
wide drive magnitude. In this work, we have developed, and demonstrated, a
scheme to identify such a drive magnitude, where the system is expected to
behave in a coherent manner over very long distances. Using a prototype
system based on Nickel-Titanium shape memory alloys, which itself is a
technologically well-known material used widely as mechanical actuators to
deployable structures in satellites, we show that fluctuations in
electrical resistance act as an excellent probe to criticality. Our
experiments not only propose a "noise calorimetric" technique to study
phase changes in non-equilibrium systems, but may also constitute a new
non-invasive approach to hazard prediction which is portable to different
fields of research.

December 5, 2008

LD11581

A Second Critical Point in First Order Metal-Insulator Transitions

For more than a century metal insulator transitions were identified by a rapid resistance drop as the fraction of metal is increased. Here we show that there can be a second separate critical point, where the dielectric constant turns negative and the metallic reflectivity sharply increases. The rapid resistance change is associated with the vanishing of metallic conductance paths, while the second critical point occurs at a higher metallic fraction when the metal is sufficient to close off dielectric tunnels through the material. The intermediate state between these two critical points is characterized by two coexisting infinite clusters of both dielectric and metal. The second critical point may be seen in changes in the polarization, eddy current losses, and the presence of two sound velocities. The figure shows a calculation of the dielectric response as a function of metal fraction and frequency. The peak corresponds to the conductivity transition while the region below the horizontal blue plane indicates the transition to metallic reflectivity as seen in recent experiments under pressure on magnesium oxide. A similar phase separation can occur in superconductors with insulating inclusions.


***

EV10459

The finite size of ions yields unexpected correlation effects in liquid

In this paper, we upgrade the classical Poisson-Boltzmann theory which describes the electrostatic properties of ionic liquids in such a way to account for the finite size of ions. Each ionic species now obeys a Fermi-like distribution formulated with the ionic volume fractions in the bulk solution. Near a charged interface for example, the ions can no longer exceed the close packing density as it is the case in the classical theory. As a result, when large counterions with different valences are present, they tend to phase-separate into layers enriched in ions of same valence. The theory can be readily implemented into existing software packages computing the electrostatic of charged objects in solution and in particular, it should help biophysicists understand better the interactions of biomolecules with their ion atmosphere.

December 4, 2008

LX11132

COMPARING APPLES TO APPLES

Does an apple dropped in the summer fall at the same rate
as an apple dropped in the winter? The answer could be no,
according to this Letter.

The work presents a new class of possible violations of
Einstein's theory of relativity that are detectable only
when gravity is involved. The intrinsic size of the new
relativity violations could be large because the weak
gravity force suppresses effects.

The new violations change the gravitational properties of
objects depending on their motion and composition. Objects
on the Earth move differently in different seasons because
the Earth moves around the Sun, so apples could fall faster
in some seasons than others.

The new violations affect matter and antimatter differently,
so an apple and an anti-apple could fall at different rates
too. Objects with different compositions like apples and
oranges may also fall differently.

***

BV10489

Heat transport in nanodevices: a road to nonequilibrium temperature.

The recent surge of interest in nanotechnology, in miniaturized systems with internal degrees of freedom, or in high-frequency short wavelength perturbations,has opened new frontiers in the analysis of heat transport.

Several experiments or simulations along nanoscale devices led to results significantly different from those of the classical Fourier law since, at the lenght scale of the mean free path of heat carriers, small temperature differences may produce very high temperature gradients.

In this paper a new dynamical temperature has been used to model hyperbolic heat transport in nanosystems. It is related to the absolute one by a partial differential equation and at the equilibrium reduces to a suitable regular function of the local-equilibrium temperature.

By using such a temperature We have studied the different speed of propagation of thermal signals along or against the average heat flux. The result indicates that perturbations of the hotter temperature or of the lower temperature in a nanosystem will not propagate at the same speed inside the system.

This work may have important technological applications since it can be useful in the dynamical studies of heat transfer in nanosystems. From the theoretical point of view, the paper may open a new road to the definiton of nonequilibrium temperature in the presence of fast phenomena.

***

BU10577

PROBING METALLIC SURFACES WITH ELECTRONS - A NEW THEORY

Electron energy-loss spectroscopy is a technique in which solid surfaces are bombarded with electrons and the scattering intensity is recorded as a function of the energy lost by the incident electrons to the solid, as a means of getting information about the electronic structure of solid surfaces. A new theoretical model has been formulated that accounts successfully for such measurements made on the transition metals Scandium, Titanium, Vanadium, Chromium, Manganese, Iron, Cobalt and Nickel. This is to appear soon in the journal Physical Review B (Condensed Matter and Materials Physics). Until now, experimentalists have relied mainly on theories based on light/lasers/photons as probes for solids to support their electron energy-loss measurements -and these involve considerable amount of computational effort to do so. The final results of the new theoretical model require only few adjustable parameters to fit theory to measurements, and can be performed even on scientific calculators. Perhaps the scope and possible impact of the new theoretical model is best summed up by a remark in the report of one of the referees: "...The theoretical results will have an impact on future experiments and their interpretation..."

***

LU12103 and LU12107

Fracture in slow motion: Observations of a crack’s singular region yield
a new theory of fracture

A detailed view of precisely how things break is achieved by using soft
materials to slow down fracture dynamics by a factor of a thousand,
while still faithfully mirroring all of the complicated dynamics of the
fracture process. This enables first-time high resolution measurements
of the deformation fields surrounding the near-singular tip of rapid
cracks. The measurements show that the canonical theory of fracture
fails to provide a consistent description of the experimental data, with
elastic nonlinearities near the singular region the culprit. This, by
itself, is not surprising since the theory is based on a key assumption
of linear elastic behavior – which is an excellent assumption except at
the smallest scales near a crack’s tip. On the other hand, as fracture
occurs precisely /at/ the smallest scales, a description of this
near-tip region is important. In an accompanying Letter we develop a
weakly nonlinear theory of the dynamic fracture of a single crack that
provides excellent quantitative agreement with the experiments. The
theory, which is based on a controlled expansion of the nonlinearly
elastic stress-strain relation for highly stretched material, is
expected to be generally applicable to any brittle material.

December 1, 2008

LV11595

Data Storage by heat

Tradition computers carry and process information by electrons. Recently Wang and Li from National University of Singapore (NUS) have demonstrated that heat pulse or phonons, can be used to perform, in principal, all logic operations.

One year later, the same team (Wang Lei is now with Renmin University of China, Beijing) has gone one step further by presenting the feasibility of data storage by heat - the counterpart of another indispensable element for computation and information process, memory. Like an electronic memory that records each bit of data by maintaining voltage in a capacitance, the thermal memory stores data by keeping temperature somewhere. Due to the unavoidable perturbation from the thermometer when the temperatures are measured (i.e., data are read), anything thermally insulated, although seems to be a good candidate at the first glance, does not work. Wang and Li thus turn to build up the thermal circuit exhibiting bi-stable states by applying nonlinear lattices with Negative Differential Thermal Resistance. Via computer simulation, they have demonstrated that those two states can both last very long time and, more importantly are self-recoverable under the not-very-small perturbation introduced by the thermometer. This means that this thermal circuit can act as a thermal memory that store data by heat. Their work will soon be published in Phys. Rev. Lett. Since the related results rely only on very general principles, and given the fact that the solid state thermal rectifier has been realized experimentally in 2006, just a few years after the theoretical models, it is thus reasonably believed that, the thermal memory should be realized, e.g., in nanoscale systems experimentally, in a foreseeable future..

***

LS11443AR

How can entanglement be preserved?

In real world entanglement between quantum systems tends to quickly
disappear because of effects of the surroundings. This work shows that
this disappearance can be suppressed by embedding the entangled systems in
appropriate environments such as photonic crystals.
Entanglement represents the spooky correlations that distinguish quantum
systems from classical ones and is an essential resource to develop
quantum computers and for quantum cryptography. Entanglement is very
sensitive to its surroundings and its fragility is a serious obstacle to
its exploitation in real systems.
There are materials, such as photonic crystals, structured so to present
photonic band gaps, that is ranges of frequencies where transmission of
radiation is forbidden.
This work shows that by placing entangled quantum systems, such as quantum
dots, in these materials, when the frequency of their transition falls
inside the band gap, then entanglement may be preserved for times long
enough so that its practical use may be allowed.

***

LQ11936

Symmetry retards a phase transition

Phase transitions play a key role in modern data storage devices like CDs and DVDs. The speed of data storage is ultimately limited by the speed of phase transitions, for instance between crystalline and amorphous phases. Even though it is known that some phase transitions are slower than others, the reasons for this are not very clear and present an interesting fundamental problem. This paper shows that symmetry relations between adjoined phases may play a crucial role. In particular it is suggested that phase transitions involving an increase of the symmetry of the material may occur on a very fast time scale, whereas in the opposite case the phase transition is intrinsically slow. The particular example discussed in this paper is the optically induced ultrafast destruction of the magnetic order in yttrium vanadate, which is contrasted to the slow reorientation of the orbital (electronic) order in this material. The proposed symmetry rule is expected to have severe consequences for the development of ultrafast rewritable phase change memories.

***

BW10545

Creating and manipulating anyons : a challenge

In two-dimensional physical systems, particles usually behave as bosons or
fermions, but they may also exhibit exotic intermediate quantum statistics.
However, such exotic particles, dubbed anyons, have never been directly observed
nor manipulated experimentally. Recent theoretical proposals of topological
quantum computation, based on the very existence of anyons, have attracted
interest from experimentalists. The possibility to design anyon-standing models
with cold atoms loaded in optical lattices is certainly one of the most
suitable ways to demonstrate that such weird objects are not merely a
theoreticians' construction. In our paper we discuss, in a pedagogical way, some
important theoretical and experimental issues concerning this problematics. We
first give a detailed explanation of how low-energy anyons, but also high-energy
fermions, emerge as collective spin-excitations in a realistic spin model known
as the Kitaev honeycomb model. This should be useful for anyone interested in
learning the physics of anyons in a very simple framework. We then show that
manipulating anyons experimentally should be rather delicate. It is indeed
difficult to create and manipulate anyons thanks to single-spin operations,
without also creating unwanted high-energy fermions, because both kinds of
particles are collective spin-excitations.

***

EUR1019E

The origin of Gaussianity of velocity distribution in homogeneous
isotropic turbulence

The velocity of fluid in turbulence is very random. An idealistic
case of turbulence,
which is homogeneous and isotropic, observed behind a grid in a water channel
was discussed to have a Gaussian velocity distribution by Batchelor
(1960), based
on the central limit theorem; while a recent claim by Falkovich and
Lebedev (1997)
that the forced (not naturally decaying) turbulence should have a subGaussian
velocity distribution attracted a considerable notice. In this paper,
a reasonable closure
of Monin-Lundgren (1967) hierarchy of velocity distribution functions
in decaying
homogeneous isotropic turbulence is presented which can truncate the hierarchy
at the first equation for the one-point velocity distribution, and
the two-parameter
family of exact similarity solutions of the equation are found with a
perfect Gaussianity.
The one parameter indicates the power law index of energy decay,
while the other
is closely related with energy dissipation rate. Thus, it would be no
doubt that
the Gaussianity (or a deformed one) must be raised up from a proper
mechanism of a
statistical hydrodynamics consistent with Navier-Stokes equation, not
by a sum of
assumedly independent random numbers such as wavelets.

***

BW10476

Polaron problem: an entanglement perspective

The polaron concept – a particle (electron, hole) surrounded by the quanta of the host-lattice vibrations (phonons) – had been conceived by Landau as far back as 1933, and remains squarely among the central notions in condensed-matter physics. As the interaction between the particle and phonons becomes stronger, leading to an increased ``phonon-dressing'' of the particle, its motion invariably changes from being rather delocalized to being restricted to a single unit cell of the crystal. This change is not accompanied by a breaking of symmetry – that is, a phase transition. In this paper, we show that quantitative measures of entanglement, describing correlations between parts of a quantum system, provide us with the means to better understand the inner workings of polaron physics. This appears possible for the so-called Peierls-type particle-phonon interaction, of importance in molecular crystals, whereby phonons directly affect particle's hopping amplitude. We demonstrate that the entanglement measures change abruptly in the physical regime where such interaction leads to the spatially-distant particle-phonon correlations. As an interesting sidetrack, our results exemplify that such behavior does not necessarily coincide with the onset of (zero-temperature) quantum phase transitions – a point of contention in the current literature.

***

AS1034

It from Bit!

Quantum theory is an extraordinarily successful physical theory. But
what exactly is it telling us about how nature works? Traditionally,
this has been very difficult to answer in any convincing way, as the
physical content of quantum theory is locked up in mathematical
language that is difficult to decipher.

In the last two decades, several physicists have proposed that the
concept of information might be the hitherto missing concept which
might unlock the physical content of quantum theory. This proposal,
perhaps most clearly made by John Wheeler under the slogan "It for
Bit", asserts that information is as fundamental --- or perhaps more
fundamental --- than the concepts of space, time, mass, and energy
that form the basis of classical physics.

In this paper, it is shown that the full mathematical language and
machinery of quantum theory can be built up by making essential use of
the primitive idea of information, thereby strongly supporting
Wheeler's contention.


***

EW10364

Predict properties of colloid systems

Colloids, nanoparticles or globular proteins in various solvents are typically charged, and weakly screened long-range electrostatic repulsion between them competes with solvent-mediated short-range attraction as well as with thermal motion. The three competing tendencies: for keeping particles far apart, or very close, or randomly distributed, lead to much richer phase diagram than in molecular systems. The new stable phases consist of spherical, or elongated, or slab-like clusters of particles. The clusters may be arranged in ordered, crystal-like structures which occur for low volume fractions of particles (~0.1), because the clusters tend to be well separated to minimize the repulsion. Despite the complexity of the phase diagram, it has a universal skeleton in variables: volume fraction of particles and properly scaled temperature. Thanks to the universality, one can predict properties of many system by studying just one. The universality predicted in this work resembles the law of corresponding states in the van der Waals theory of gas-liquid separation. Since temperature is proportional to the kinetic energy, the proper temperature scale in molecular systems is the ratio between the kinetic energy of a molecule and the interaction potential at the optimal distance between two molecules. Ratio between the kinetic and the potential energy is also a proper temperature scale for the charged particles, but as found in this work the relevant potential energy is the energy associated with a formation of a dense layer followed by the depleted-density layer of optimal thickness. This is because the system tends to maximize the attraction and minimize the repulsion between the particles. The found universal sequence of bcc, hexagonal, lamellar, inverted hexagonal, inverted bcc phases for increasing volume fraction is the same as in micellar and block-copolymer systems. The universal skeleton of the diagram is decorated with more complex, system-dependent structures that occur near the coexistence of the above dominant phases for narrow volume-fraction intervals. In some systems a gyroid phase, where a regular branched cluster forms an infinite network, may occur between the hexagonal and lamellar phases.

***

LW10993

Attracted by repulsion: Exotic superfluidity in an expanding atomic cloud

Cold quantum particles can display frictionless flow, known as
superfluidity. For fermions this happens when pairs are formed by an
attractive interaction. Here we propose and study a paradoxical situation
where superfluidity of fermions occurs as a result of a very strong
repulsion, instead of attraction. The resulting superfluid state is
exotic, because the total momentum of each pair is non-zero, in contrast
to conventional superfluids. The recipe for creating such a state relies
on recent advances in atomic physics and is deceptively simple: If a dense
cold cloud of fermionic atoms is slowly expanded in the presence of a
lattice created by laser beams, then a superfluid state of doubly occupied
lattice sites emerges naturally. The reason behind this miracle is that
the pairs, although high in energy, are metastable because energy
conservation prohibits them to decay.

***

LT11699

Alfven instability in a compressible flow

A previously unknown macroscopic instability in flowing plasmas is
presented. Macroscopic instabilities modify the global structure and dynamics
in laboratory and space plasmas. Well-known examples include the
Rayleigh-Taylor and the Kelvin-Helmholtz (wind over water) instabilities. The
new instability does not have an analogue in hydrodynamics. It may only arise
in the presence of a compressible plasma flow embedded in an ambient magnetic
field. The kinetic energy of the flow is extracted and fed into transverse
disturbances that propagate along the magnetic field. The existence of such
transverse disturbances known as Alfven waves was established over sixty years
ago in laboratory conditions. There is an increasing volume of evidence to
suggest the presence of Alfven waves in space plasmas. However, so far little
has been known about their origin. The energetic and dynamic significance of
such waves cannot be underestimated. Possible implications include the heating
of the solar corona and the acceleration of the solar wind. The presented new
instability mechanism offers a unique and efficient way for the generation of
large amplitude Alfven waves. No flow shears or super-Alfvenic flow speeds are
required.

November 25, 2008

LP11468

Predicting earthquakes made possible?

A grand challenge in geophysics has been developing a method for
predicting
when a significant earthquake may occur. A team of researchers has
developed
a new method that may go a long way towards this goal. Using the method,
Manshoor {\it et al.} analyzed the fluctuations of the detrended
increments of
the time series for Earth's vertical velocity for many earthquakes.
Their
analysis reveals a significant change in the nature of the probability
density functions (PDF) of the series' increments. For a large
earthquake
the time at which the PDF undergoes a transition from a Gaussian to a
non-Gaussian is 5-10 hours. The key quantity that signals the
transition is a
parameter $\lambda_s$ that characterizes the shape of the PDF.
Far from the earthquake,
$\lambda_s\simeq 0$, but close to it $\lambda_s$ suddenly increases,
signaling
the transition. Figure 1 demonstrates this for an 7.1 earthquake that
occurred on May 21, 2003 in Spain.
The trends can also be seen in the PDF's flatness but not in the
signal itself.
Thus, the transition in the PDF, and the changes in
$\lambda_s$ and the PDF's flatness, all happening at the same time,
represent a new precursor for detecting impending significant
earthquakes.
A key insight is that, due to localization of elastic waves, only
stations
close to the epicenter provide the alert.

***

LT11399

Tuneable spin-polarized transport in amorphous CoFeB alloys

Relevant to the field of spintronics, a correlation between the spin polarization of tunnelling electrons (TSP) in AlOx/CoFeB tunnel junctions and the magnetic moments of amorphous Co80-xFexB20 alloys is reported. Such a correlation is surprising since the TSP involves s-like electrons close to the Fermi level (EF), while the magnetic moment mainly arises due to all d-electrons below EF.
The foundation of spintronics is based on the fact that s-like conduction electrons in transition metal ferromagnets get highly spin-polarized as a consequence of their interaction with localized d-electrons. Naturally, the extent to which this interaction influences the electronic structure of the conduction electrons, and the possibility of controlling the transport properties of these metals through this interaction, is an issue of vital importance. Consequently, we believe that this observed correlation and direct insight into the magnetic, electronic and transport properties of CoFeB alloys open several new possibilities to control, engineer and enhance the performance of spintronic devices

We find that the origin of this correlation is the change in the d-band exchange splitting and spin & orbital moments which force a transition from weak to strong ferromagnetism on the iron atoms. Ensuing this transition in the d-bands and due to the s-d interaction, a marked influence on the s-electron dominated TSP is observed which leads to the correlated behaviour of two entities primarily evolving from different aspects of the alloy electronic structure.

***

LY8951

A Double Look at the Resonant States of the Proton

The proton is not only the building block of atomic nuclei. As a
composite particle made of quarks and gluons, it can go to
excited states, or resonances of short lifetime. In this paper,
high-energy electron scattering at the Jefferson Laboratory
was used to study the excitation of the proton into resonances,
with their subsequent decay in two different channels: one
produces a photon, while the other produces a pion. Therefore
the first channel is governed by the electromagnetic interaction,
while the second one is governed by the strong interaction.
This double look allows new insight into the resonance behavior,
namely which physical features are similar or different between
both decays. The experiment provides an amount of consistent data
for constraining the theoretical models, which deal with the
very complex structure of the proton. At the upper end of the
measured excitation spectrum, we may even have seen hints of
a direct interaction with the quarks inside the proton. However,
to test this conjecture more experimental data will be needed,
in a specific kinematical domain that can be reached at the
future JLab 12 GeV upgrade.


***

LW10993

Attracted by repulsion: Exotic superfluidity in an expanding atomic cloud

Cold quantum particles can display frictionless flow, known as
superfluidity. For fermions this happens when pairs are formed by an
attractive interaction. Here we propose and study a paradoxical situation
where superfluidity of fermions occurs as a result of a very strong
repulsion, instead of attraction. The resulting superfluid state is
exotic, because the total momentum of each pair is non-zero, in contrast
to conventional superfluids. The recipe for creating such a state relies
on recent advances in atomic physics and is deceptively simple: If a dense
cold cloud of fermionic atoms is slowly expanded in the presence of a
lattice created by laser beams, then a superfluid state of doubly occupied
lattice sites emerges naturally. The reason behind this miracle is that
the pairs, although high in energy, are metastable because energy
conservation prohibits them to decay.


***

LT11699

Alfven instability in a compressible flow

A previously unknown macroscopic instability in flowing plasmas is
presented. Macroscopic instabilities modify the global structure and dynamics
in laboratory and space plasmas. Well-known examples include the
Rayleigh-Taylor and the Kelvin-Helmholtz (wind over water) instabilities. The
new instability does not have an analogue in hydrodynamics. It may only arise
in the presence of a compressible plasma flow embedded in an ambient magnetic
field. The kinetic energy of the flow is extracted and fed into transverse
disturbances that propagate along the magnetic field. The existence of such
transverse disturbances known as Alfven waves was established over sixty years
ago in laboratory conditions. There is an increasing volume of evidence to
suggest the presence of Alfven waves in space plasmas. However, so far little
has been known about their origin. The energetic and dynamic significance of
such waves cannot be underestimated. Possible implications include the heating
of the solar corona and the acceleration of the solar wind. The presented new
instability mechanism offers a unique and efficient way for the generation of
large amplitude Alfven waves. No flow shears or super-Alfvenic flow speeds are
required.


***

LQ11377B

Are the low-energy spectral features of the cuprate superconductors universal?

One of the hindrances in the general understanding of the cuprate superconductors
as a whole is the lack of evidences showing the universality of the low-energy spectral
features exhibited by the different cuprate superconductor families. In this paper, we
show that the low-energy spectral features like the asymmetry of the coherence peaks,
dips and humps beyond the peaks observed at the superconducting state of a 123 cuprate
system, viz., NdBa$_2$Cu$_3$O$_{7-\delta}$ are qualitatively similar to the ones repeatedly observed for
a different system, viz. Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (2212 system) [1].

The scanning tunneling spectroscopy (STS) experiments and angle resolved photoemission
experiments on cuprates have established that the dips and humps beyond the coherence
peaks are observed when the CuO$_2$ plane layer is probed [1,2]. However, several
tunneling experiments performed on a different cuprate system like YBa$_2$Cu$_3$O$_{7-\delta}$ (123 family)
do not exhibit these features [1], raising the speculation that the CuO$_$2 planes of these systems might be
influenced by the existence of the other quasi one-dimensional CuO chain
layers in the unit cell of these cuprates. Our work indicates that a direct tunneling to the CuO$_2$
plane layer (avoiding the CuO chain layer in the tunneling path) would reveal the true features of the CuO$_2$
layer in the 123 systems. Similarity of the features as observed for NdBa$_2$Cu$_3$O$_{7-\delta}$ in the present work to the ones
for the 2212 system reported by others point towards the universality of these features. From the STS data,
we also observed a very clear signature of a comparatively lower energy bosonic mode being excited by the
tunneling electrons. The work would be important for further understanding of the high transition temperature
cuprate superconductors in general.

***


LS11002A

Efficient Frequency Conversion

Frequency conversion, or conversion of light colors, is a key concept in
the field of nonlinear optics. In this process, light of two colors is
mixed in a nonlinear crystal, resulting in the generation of a third
color with their sum or difference frequency. However, there is usually
a tradeoff between the bandwidth, which is the range of frequencies that
can be converted, and the efficiency of the conversion process. In this
research, we present a novel way to achieve both. Efficient conversion
for a bandwidth that is up 100 times wider than in conventional
conversion schemes is demonstrated. This scheme was discussed after
showing that the problem of frequency conversion can be mathematically
formulated and geometrically visualized in complete analogy with the
physics of two-level systems, as pioneered by Bloch and Feynman in NMR
and atomic physics. Using this analogy, the concept of “rapid adiabatic
passage” for robust efficient population transfer in two-level systems
is applied in the context of color conversion, and the requirements of
high efficiency and broad bandwidth were reconciled. This analogy and
its geometrical visualization, can bring new physical insight into the
process of frequency conversion and to better understanding of nonlinear
optical processes.

Image caption

"Adiabatic sum frequency conversion" scheme. a) Adiabatic (very slow)
variation of the periodicity along a nonlinear crystal is required for
efficient broadband color conversion. The adiabatic constraints are
discussed in the article. b) The dynamics of the process can be
visualized geometrically on a sphere surface, where the south-pole
represents the input color, and the north-pole represents the converted
color (Efficient conversion=reaching the north pole). Usually, one can
reach the north pole only for limited colors (in a conventional
conversion scheme). Here we present the adiabatic trajectory, where
large band of frequencies can reach close enough to the north pole (we
can send an avi-movie to demonstrate this dynamics). (c) Experimental
results - conversion efficiency as a function of input color (measured
in optical wavelength).

***

ER10481

When Engineering meets Physics: a new model for semi-flexible polymers.

The mechanical properties of cells and tissue is controlled by a network of relatively stiff polymer filaments like actin, collagen, etc. In polymer physics,these filaments are descibed as semi-flexible: flexible enough to fluctuate under Brownian forces, but stiff enough for the bending energy to limit the Brownian conformations that can be assumed. This semi-flexible character causes networks of these filaments to demonstrate previously-unseen mechanical behavior, but it also necessitates the use of a computer model since the mathematical equations descibing semi-flexible behavior are complex and difficult to solve. Our idea was to simulate a semi-flexible filament as a continuous string of tiny engineering beams, as opposed to the more traditional and computationally-intensive way of simulating it as a string of beads. We showed how the Brownian forces on a curved beam can be resolved by simple statics force balances, and how engineering Beam Theory can be used to solve the bending. By such an approach, we were able to avoid the common approximation that bending deflections be small, and did not need to impose an artificial constraint to prevent filaments from lengthening while bending. However, the most
important advantage of our approach is that one beam replaces multiple beads as the modeling unit, lowering the computational cost without compromising the physics.

***

LU12009

Ferroelectricity defies high pressure

Pressure has long been considered as the enemy of ferroelectricity, the
existence of a switchable polarization in piezoelectric materials. In this
article we show experimentally that the model ferroelectric perovskite,
lead titanate, exhibits a much more fascinating behavior under pressure: a
complex succession of phase transitions leads to the re-entrance of
ferroelectricity at high pressure. This behavior was predicted
theoretically through an original electronic effect and has been supported
by our preliminary experimental results. Here, combining synchrotron x-ray
diffraction and Raman spectroscopy under pressure we show that the
accommodation of the pressure-induced strain is done through rotation of
the oxygen octahedra and/or cation displacement in successive tetragonal
phases. These results open the way to new concepts for explaining this
intruiging form of high pressure ferroelectricity.

***

LU11406

MATTER WAVES PLAY THE PHOTON GAME

In the last years, scientists have engineered materials in which light
behaves in a very strange way. In the so-called photonic crystals, photons
- the quantum particles of light - may propagate or not, depending on their
energy. The presence of this "stop"-energy band for photons, severely
affects the light matter interaction within the crystal.

We have proposed a system in which matter waves behave like photons within
a photonic crystal. This idea seems reasonable, since Quantum Mechanics
teaches us that given some conditions, matter may also propagate as waves.
But in practice in order to observe matter-waves in full performance, one
needs very well controlled environmental conditions. In our proposal, this
is achieved by considering atoms frozen in an optical lattice - an
artificial atomic crystal created by laser beams. The release of atoms out
of the optical lattice turns out to be a process analogous to the emission
of photons inside a photonic crystal. In this way, the plethora of
phenomena observed with light-matter interaction in those crystals find a
matter-wave counterpart. On the other hand, contrary to photonic crystals,
optical lattices are highly controllable devices where experimentalists can
tune the parameters to drive the system into many different regimes. This
would allow to observe in particular two extreme regimes: One in which
emitted atoms get blocked and can not propagate, and another one in which
interference effects between the matter waves give rise to an enhancement
of the emission rate.

November 24, 2008

LW11114

Spin Control: Modeling the Transistor of the Future

As transistor dimensions continue to shrink while computing demands
grow, researchers want to encode digital information in the spin of
electrons, rather than only the charge. Although "spintronics"
already underlies today's high-volume disk drive technology, building
spintronic transistors has been difficult. Twenty years ago, two
scientists proposed a highly promising design, the Datta-Das
transistor (DDT) (Fig. 1). To date, however, no one has been able to
build one; the desired spin effects are sensitive to minor
imperfections in the system. In this paper, we have proposed
creating a minutely controllable atomic version of the spintronic
transistor using, instead of electrons in a semiconductor, a beam of
ultra-cold atoms passing through a region where three laser beams
overlap (Fig. 2). The atoms in light fields precisely mimic the
physics of electrons in a DDT, with two quantum states of the atoms
playing the role of the electron's spin. Unlike the electronic DDT
-- with its myriad sources of error -- an atom-beam analogue would
offer the opportunity to carefully control the behavior of the
system, allowing physicists to determine which specific factors are
most critical to the performance of a real DDT.

***

LU11288

Shedding new light on an old problem to give cheap solar cells a boost.

In this paper a new understanding is presented of amorphous silicon
solar cells that could yield major improvements in their efficiency.
Since the 1970’s, amorphous silicon – a-Si – has been widely used as
a low-cost alternative to more traditional crystalline silicon solar
cells. However, its use has been limited to low-power applications
such as watches or calculators due to a very simple yet perplexing
effect: within just a few hours of exposure to light, the efficiency
of a-Si solar cells degrades by 25-30%. This effect has historically
been attributed to an increase in dangling bonds—silicon atoms with
less than the optimal number of four neighbors. By performing
accurate calculations of the interplay between the atomic and
electronic response of a-Si to light, this work finds that incident
light can cause regions of strain to form in the material that are
still fully four-fold coordinated and yet just as detrimental to the
solar cell performance as dangling bonds.

***

LR11694

Environmental variability enhances fitness of competing species

In our computational study of competing predator and prey
populations with spatially varying availability of resources we
have found that environmental diversity can considerably enhance
the survival rate of both species, already in the absence of any
evolutionary effects.
Even simple ecological systems display remarkably rich features.
In the 1920s, Lotka and Volterra designed a mathematical model
for predators whose reproduction requires feeding on prey that
produces regular population oscillations. Facilitated by much
improved computing power, researchers have recently refined the
theoretical description of biological systems to include spatial
spreading and environmental diversity. Then, random fluctuations
in the numbers of individuals often play a crucial role.
In this work, we have investigated the consequences of spatial
variability in predator-prey interactions: e.g., the efficacy of
predators to hunt their prey may vary due to local environmental
influences. Our numerical simulations show that such spatial
variations increase the number of surviving individuals of both
populations to a level not predictable by standard analytical
approaches. Rather, this fitness enhancement is caused by
cooperative behavior of predators and prey that cluster together
near favorable locations. Our results underscore the important
role of spatial correlations and random fluctuations in ecology.

***

LV11519

Charge ahead!

X-rays now make it possible to create detailed images of how electrons
are distributed inside a material. Utilizing recent advances in
spectroscopic techniques and a bit of mathematics it is now possible to
create detailed images of the charge distribution in a material. In a
demonstration experiment, the distribution of electrons in common copper
has been mapped out and the result shows that electrons of different
energy favour different positions within the solid.
Angle resolved photoelectron spectroscopy is a common technique for
studying the electronic structure of solids. The technique is based on
the photoelectric effect that was discovered by Heinrich Hertz in the
end of the 19^th century and later described theoretically by Albert
Einstein, meriting him the Nobel Prize in 1921. In the last decades the
technique has evolved rapidly as new sources of X-rays have become
available and detectors have improved. These advances now make it
possible to collect enough detailed data to create three-dimensional
images of the electron density in a solid. This density is intimately
related to the mechanical, optical and electrical properties of the
material.