8-28-08

BS10930

A model for strange properties of carbon nanotube sheets

When most materials are pulled in one direction, they get thinner in the other
direction, similar to how a rubber band behaves when it is stretched. This phenomenon
can be quantified by Poisson’s ratio, which is the ratio of the percent lateral
contraction to the percent applied stretch. Materials like regular rubber that contract
laterally when stretched present positive Poisson's ratios. If a lateral dimension
expands during stretching, the associated Poisson’s ratio is negative and the material
is called auxetic. Examples of auxetics are re-entrant polymer foams used in some seat
cushions, membranes of red blood cells, cat skin and cow teat skin. Hall and colleagues
from The University of Texas at Dallas created auxetic carbon nanotube sheets by using
ancient methods for making ordinary writing paper. The nanotube paper is a mixture of
carbon single-walled nanotubes and multi-walled nanotubes. The increase of the amount
of multi-walled nanotubes in the paper produces a sharp transition from a positive
Poisson’s ratio to a negative value. A team of Brazilian nanotechnologists and
colleagues of the The University of Texas at Dallas have proposed a model for
explaining this transition considering the nanoscale aspects of the nanotubes and the
essential structural features of the sheets.

***

LS11651

First glimpse of the hidden sector has already been observed in the sky

In the very near future the LHC will commence searching
for new particles with masses of the order of a TeV (roughly 1000
times the proton mass). This will test many proposed extensions of
the Standard Model as, e.g., supersymmetry, large extra dimensions,
technicolor to name only a few. However, many extensions of the
Standard Model contain additional hidden sectors that interact only
very weakly with ordinary matter. Due to their feeble interactions
even light particles in such hidden sectors may be missed in such a
collider experiment (hence the label `hidden'). Yet, it may be
exactly these hidden sectors that carry crucial information on how
the Standard Model is embedded in a more fundamental theory as, e.g.
string theory. This creates the need for complementary probes. In
our paper we argue that cosmological observations can be a powerful
tool in this endeavor.

One type of particles that appears in many models with such hidden
sectors is a particle that has properties very similar to those of
the ordinary photon. However, since it lives in a hidden sector it
couples only very weakly to ordinary particles. We may call it a
hidden photon. One way to search for hidden photons is to use high
precision laboratory experiments. In this paper we find that one can
also use cosmological observations to search for hidden photons. The
presence of hidden photons could leave observable footprints in the
cosmic microwave background (the left over radiation from the hot
big bang which is currently very precisely mapped by the WMAP
satellite and will soon be measured to even higher precision by the
PLANCK satellite). Moreover, it would affect how soon after the big
bang structures, such as galaxies and and galaxy clusters, form.
Finally, it would change the relation between the number of photons
and the number of baryons (protons, neutrons) -- a number that can
be inferred from the observation of the cosmic microwave background
but also from the (measured) abundances of the elements produced in
the early universe.

Some of the present cosmological data even favor the existence of a
hidden photon with a mass in the meV range. So, maybe the first
glimpse of the hidden sector has already been observed in the sky.



***

LQ11896

Broadband cylindrical acoustic cloak for linear surface waves in a fluid

A theoretical model of cloaking for water waves has been accompanied
by the new experimental results presented and discussed in the paper.
It is shown that the cloak responds as an effective anisotropic fluid whose
characteristics have been evaluated analytically. This work leads to a
new range of designs of metamaterials in the area of fluid-solid interaction.


***

EP10348

An Axisymmetric Lattice Boltzmann Method

This paper reports a novel development of a simple
lattice Boltzmann model for simulation of incompressible
axisymmetric flows, which enables 3D axisymmetric flow
problems to be solved with an efficient 2D approach at
high accuracy. The new method is validated by typical
numerical tests. It is simple, efficient and accurate,
naturally suitable for both steady and unsteady flows
involving more physical phenomena. This greatly extends
the power of the standard lattice Boltzmann method for
fluid flows, leading to a wide range of new applications
in science and engineering.

***

LQ11472

Meta-screens: versatile structures for squeezing light into
sub-wavelength spots

Have you ever wondered why one cannot see atoms with the naked eye or
with conventional optical microscopes? In any imaging or sensing
apparatus involving electromagnetic waves (like radio waves for medical
diagnostics or light for optical microscopy), there is a perceived
fundamental limit on the smallest detail that can be resolved. This
limit is known as the 'diffraction limit' and it is on the order of one
wavelength. In a new study to be published in Physical Review Letters, a
very simple technique has been invented to focus electromagnetic waves
into tiny sub-wavelength spots, thus overcoming the diffraction limit.
The method is based on the 'meta-screen', a new concept utilizing
narrowly spaced slots each of precise length cut into a metallic screen.
Previous attempts at sub-wavelength focusing have been severely hindered
by material losses which quickly degrade performance as well as by their
inability to be scaled to any arbitrary wavelength. The newly developed
meta-screens solve both problems thus promising unprecedented levels of
resolution and flexibility from radio-waves all the way up to visible
frequencies.


***

LQ11852B


Global Order in Locally Frustrated MgTi2O4

Perfect order represents the fundamental state of matter. Sometimes, however, achieving this state is problematic due to the difficulty of propagating local order pattern through space. This phenomenon, called geometric frustration, may imply degeneracy of the ground state at the classical and quantum levels. A famous example, studied by Linus Pauling, is the ordering of protons in common water ice. The physics of the spinel compound MgTi2O4, a magnetic analog of ice with spins-1/2 instead of protons, is governed by the competition between electronic effects, which favour the localization of spin, charge and orbital degrees of freedom, and geometric frustration, which prevents such ordering on a global level. While MgTi2O4 is metallic, paramagnetic, and orbitally degenerate at high temperature, it becomes insulating, spin-paired, and orbitally ordered upon cooling. This dramatic changes result from the peculiar structure of MgTi2O4, which does not allow propagation of local up-down pairing of spins. In our work we show how a particular orbital order, preformed in the high temperature phase, gets stabilized upon cooling and controls the structural distortion and helical superstructure features, which represent long-range effects of local lattice frustrations.

***

LD11394

Quantum error-correcting codes (QECCs) provide an active way of
protecting our precious quantum data from noises. QECCs are usually
constructed via the stabilizer formalism with the resulting codes
being referred to as stabilizer codes or additive codes. In this
paper we present the first evidence of a nonadditive
error-correcting code, a code without a stabilizer structure, that
outperforms the optimal stabilizer code while correcting arbitrary
errors based on graph states. Since less structured than the
stabilizer codes, the nonadditive codes are more effective in the
sense of a larger code subspace on one hand, harder to construct and
identify on the other hand. In comparison, the nonadditive codes
constructed previously that outperform the corresponding stabilizer
codes cannot be used to correct arbitrary errors. In addition we
have figured out a complete encoding-decoding circuit including the
syndrome measurements and recovery operations for the proposed
9-qubit code by using only elementary gates. It is not hard to
envision that our method can be readily generalized to find many
other good QECCs, additive or nonadditive, binary or nonbinary, and
even the codes dealing with different error models.

***

LR11064

Playing billards with ultracold "holes":

Observation of the robustness of oscillations and collisions of the
coldest dark matter-
waves in the Universe !

In this paper, we report for the first time in any physical field the
observation of multiple
oscillations and collisions of dark solitary waves. Combining
experiments, numerical simulation
and theory, we illustrate the remarkable robustness of such nonlinear
waves in the newest
form of matter, namely Bose-Einstein condensates (BECs). BECs were
created experimentally
for the first time in 1995, a feat associated with the 2001 Nobel prize
in Physics and emerge
at the lowest temperatures in the Universe (of the order of a few
billionth of a degree Kelvin).
Dark solitons ("holes" in the density of atoms in such BECs) were shown
in our experiments
to sustain numerous collisions between them interacting almost
elastically, i.e., like billiard
balls confined (in this case through light fields) in a box.

8-26-08

BD10943

Temperature of a sub-nano particle

Although temperature is always well defined for a macroscopic system, it
may not be so for a small, isolated and low energy cluster of atoms. In
this paper, the magnetic moments of clusters containing 12 to 200 cobalt
atoms is studied using low temperature molecular beam method. It is well
know that when the size of a magnet is reduced to nanometer size, the
orientation of its magnetic moment fluctuate quickly, giving zero
average moment. In this paper, besides the reduced average magnetic
moments, we also observed all kinds of orientation of magnetic moments
in a cluster beam. This indicates that the concept temperature is only
good for the ensemble of the clusters. For every individual isolated
cluster, it is better described in terms of energy rather than in terms
of temperature.

***

LR10905

Quantum Condensing at Room Temperature

Quantum mechanics normally shows its influence only for tiny particles at
ultralow temperatures, but here we show that it holds sway at room
temperature in a solid. Over 80 years ago Einstein noticed that some swarms
of particles favour marching in close (quantum) step with each other. This
transition from random dance to correlated choreography depends on the
temperature, density and mass of the particles, and leads to superconductors
and superfluids. Now, by coupling electrons inside semiconductor devices to
light trapped inside the same space, we create new particles which weigh so
little that the Einstein condensation happens at room temperature. The
emitted light from these chips reveals the spontaneous spin alignment of
these condensates, just like magnets in which all the electron spins align
together but in a randomly chosen direction. These condensates promise the
optical equivalent of superfluid behaviour, allowing new sorts of
optoelectronic sensors to be conceived. Directly visible to the naked eye,
such quantum condensates are becoming accessible to human scales of size and
temperature.

***

LT11207

Surface surprise from transparent conductor

We have shown that indium oxide can sustain a large build-up of
electronic charge at its surface. This contradicts previous work which
indicated, as in almost all other semiconductors, an absence of
electrons close to the surface. Indium oxide, mixed with a small amount
of tin, is ubiquitous in commercial devices such as flat-panel displays
and solar cells. Despite this, it continues to surprise researchers. Its
band gap, the most fundamental quantity that characterises a
semiconductor, has recently been shown to be much smaller than
previously thought [Walsh et al., PRL 100, 167402 (2008)]. Immediately
following this result, we have found that, when you grow the material
sufficiently carefully, this semiconductors intrinsic state is
uncovered, with negative charges spontaneously accumulating at its
surface. Exploitation of this property promises novel applications of
indium oxide, such as generation of terahertz radiation and chemical
sensing. Our results also contain new insight into the reasons why this
material is a good electrical conductor, while still being transparent
to visible light, explaining the properties that stimulated its current
widespread use.

***

LR11235BR

Metallic contacts cause electron and hole transport asymmetry in graphene.

The remarkable transport properties of graphene arise from its highly
symmetrical conical band structure, but the conductance measured in
graphene field-effect transistors always exhibits a sizable asymmetry
between electrons and holes. We have measured the conductance between
both invasive and external metallic contacts on the same graphene
sheet, and have shown that this asymmetry is due to the
graphene-metallic contact interface. By varying the contact metal
used, we conclude that the workfunction mismatch between the metal and
graphene generates a potential step at the interface, which can form a
monopolar or bipolar heterojunction, depending on the carrier type in
the graphene. These results should guide future device designs and
will help improve the quality of graphene field-effect transistors for
which low contact resistances are crucial.


***

LT11694

Changing Ferroelectric Patterns by Gentle Squeeze

The interest in epitaxial ferroelectric nanoislands has dramatically increased in the past few years due to the prospects of their integration into the next generation of nanoelectronic devices, especially, high-density memories. However, the switching behavior of nanoparticles and even a clear understanding of how the polarization is distributed across them is still lacking. In tiny ferroelectric particles, the electric dipoles cannot order in a conventional (collinear) way unless external field is applied. The reason is that such an ordering would lead to the local polarization having normal component at the surface of the particle. This, in turn, would lead to appearance of uncompensated depolarizing electric field that tends to suppress the polarization that caused it. Therefore, instead of being ordered in straight lines, the local dipoles in nanoparticles choose more complex ways of ordering so that to avoid noticeable electric field in the system. The tendency to eliminate the polarization field and align the local polarization with the nanoparticle surface usually leads to two simplest choices. The first one is to arrange the dipoles in vortices or to form narrow stripe domains with the polarization directed "up" and "down"(so-called 180º domains). In the Physical Review Letter article, Ivan Naumov and Alexander Bratkovsky of Hewlett-Packard Labs show that the vortex-like ordering and 180º stripe domains formation compete in the flat ferroelectric nanoparticles and the balance can be easily changed by epitaxial misfit strains, produced by a substrate. Moreover, there are unusual intermediate patterns separating those two states, where a vortex-like ordering coexists with 180º domains. One of such intermediate structures is of the so-called "skyrmion" type, where the in-plane component of local polarization still forms a vortex, yet the out-of-plane component breaks the system into coaxial oppositely polarized domains. The results show that the "vortex-1800 domain" crossover behavior can be easily controlled by the misfit strain, which would be handy in engineering ferroelectric nanostructures with desirable properties for applications in memories, actuators, etc, that can work at a nanoscale.


FIGURE. Top: Schematic showing how the polarization pattern in a disk-like nanoparticle depends on the in-plane strains. While the tensile strains stabilize the vortex state, the compressive strains favor 180º domain phase. Bottom: Polarization distributions in the PbZrTiO3 disk nanoparticle under tensile (left, color map corresponds to preferential local direction of polarization) and compressive (right) strains.





***

ER10529

How Communicating Cars Can Cut on Congestions

Knowing the traffic situation a few kilometers or hundred meters ahead
is a prerequisite for a series of novel traffic information and driver
assistance systems improving traffic safety and efficiency. We show how,
and under which conditions, the necessary information can be transported
via wireless communication between vehicles. In contrast to services
based on radio or mobile phones, no public infrastructure is necessary.
However, the range of wireless communication is typically limited to a
few hundred meters. Since the percentage of equipped vehicles is
expected to remain well below 10% for the next decade, information loss
during communication is a serious problem. In our work, we have studied
the so-called "longitudinal hopping mode" in which messages are
transported against the traffic stream using as relays equipped cars
driving in the same direction. The question is: What is the minimum
percentage of equipped vehicles to ensure a linked chain of relay
vehicles between the sender and receiver? To investigate this, we
simulated message propagation for several traffic situations based on
simulated and measured traffic data. We compared the longitudinal
hopping mode with a "transversal hopping strategy" in which the relay
vehicles drive in the opposite direction. We found that "transversal
hopping" is most effective for very low equipment rates but one should
switch to the longitudinal mode at a later state of deployment.


***

ET10517

How does God play dice?

Tossing the dice is commonly considered a paradigm for chance.
But where in the process of throwing a cube does the randomness reside?
After all, for all practical purposes the motion is described by the laws
of deterministic classical mechanics. Therefore the undisputed status
of dice as random number generators calls for a careful analysis.
This letter is an attempt in that direction. As a simplified model of a
dice a barbell with two marked masses at its tips and only two final
positions is considered.
It is shown how, depending on initial conditions and the degree of
dissipation
during bounces, the outcome is only more or less unpredictable: the
system is not truly
random but pseudo-random -- even under conditions where it appears to be
random.


***

BT10812

Two new metamagnetic features

In perovskite manganites, the giant magnetoresistance effect---a significant decrease
in electrical resistance in the presence of a magnetic field, manifests itself as a
metamagnetic transition from an antiferromagnetic (AFM) insulator state to a
Ferromagnetic (FM) metal state. In this paper, we show that the magnetization curve
of Pr0.5Ca0.5Mn0.97Ga0.03O3 consists of two new metamagnetic features, which is
associated with two differently crystal-distorted AFM phases.

The exact crystal structure of the charge-ordered AFM state has a large influence on
the balance between Mn3+ and Mn4+, which is crucial but not yet clearly understood.
Yaicle et al. have revealed the coexistence of the CE-type and pseudo-CE-type of AFM
phases [1, 2]. By applying pulsed magnetic field up to 15 T, we observed successfully
two metamagnetic transitions in the magnetization curve. The pseudo-CE-AFM phase is
less distorted compared to the CE-AFM phase and can be easily transformed into the
FM metal phase with no or less lattice distortion. Thus, the former is responsible
for the first metamagnetic transition below ~7 T and the latter the second transition
above ~7 T. This study brings new insight into the understanding of the novel magnetic
properties of manganites

8-20-08

LK11706

How liquids hide the secret of glass cooperativity.

How can glassy cooperative dynamics take place without any apparent
or straightforward connection to structural changes?
To find an answer to one of the great unsolved problems of soft condensed
matter physics, the authors of this paper move along unexplored directions.
It was already known that the accessible potential energy surface
qualitatively changes in the supercooled liquids, due to the presence of
local minima, or inherent structures (IS). Here, with a novel perspective,
the authors are able to show for the first time that the IS
bear a signature of the cooperativity and heterogeneity displayed by
the dynamics: in the supercooled regime, their response to deformation
reveals the presence of large cooperative and heterogenous domains.
The approach proposed is similar to the one typically used for elastic solids but applied here in the fluid phase. In this way, they recognize
the static counterpart of the cooperative dynamics and support strong connections with recent studies on elastic properties of amorphous solids.

***

LU10980

An exotic state of matter could be observed using radio frequency (RF) fields

In our work, we propose how to observe an exotic state of matter,
related to superconductors, in a gas of ultracold atoms.
Superconductors are materials where electricity flows without losses.
Such materials are utilized for instance to create magnetic fields for
certain types of medical imaging and superfast trains. It would be of
great interest to find materials which are superconductors at ever
higher temperatures, to allow important applications such as lossless
transport of energy. For this goal, the physical mechanisms behind
high temperature superconductors and superfluids have to be understood
better. Ultracold atomic gases offer an ideal system to study such
mechanisms. There, it may be possible to realize exotic forms of
superfluidity: for instance the so called FFLO
(Fulde-Ferrel-Larkin-Ovchinnikov) state which is of interest not only
in context of superconductors but also, e.g., for pairing of
elementary particles such as quarks. In the FFLO state, the superfluid
and pairing characteristics vary spatially throughout the sample. In
our work, we have proposed a way to observe the FFLO state in
ultracold gases using RF-spectroscopy. The calculated response to the
radio frequency (RF) field shows clear signatures of the spatially
varying nature of this exotic state.



***

BT10758

Mechanical motion speeds up electric switching in nano-devices

Molecular or nano-electromechanical devices are characterized by a
coupling between the electronic and ionic (¿mechanical¿) degrees of
freedom. When they are traversed by an electrical current, these devices
can be pushed far from thermal equilibrium. Excitation of the mechanical
motion reduces the resistance of the device leading to a positive
feedback on the current itself. The electrical resistance can then be
found only self-consistently by the determination of the stationary
mechanical and electronic state of the device. A fully self-consistent
theory of this phenomenon for arbitrary electron-ion coupling strength
is presented in this paper.

For strong electron-ion coupling, the system can become multistable. At
low transport voltages, the current is suppressed due to a large
effective electron mass (polaronic effect), while at higher voltages the
excitation of the mechanical motion allows system to explore regions of
large currents not accessible at low voltages. The net effect is the
appearance of a rapid increase of the conductance as a function of
transport voltage in a narrow bias interval with a concomitant
acceleration of the switching time between the ¿off¿ to the ¿on¿ states
of the device. The complex behaviour is witnessed by the characteristic
forms of the mechanical and electric fluctuations. This model may help
to design nanoscale molecular and nanomechanical switches.

***

LG11596B

Quantum condensation in highly excited semiconductors

Quantum condensation phenomena like Bose-Einstein condensation (BEC) or
suprafluidity have been a highly fascinating topic in physics since decades.
While BEC has been experimentally verified in alkali gases, its observation
in semiconductors is still in demand.
In our paper, we present a theoretical approach to describe quantum
condensation in the electron-hole plasma (EHP) of excited semiconductors
using the framework of real-time Green's functions. Electrons and holes are
fermions forming bosonic bound states (excitons), similar to the formation of
molecules in atomic gases. Compared to the latter ones, however, the
description of the EHP is much more challenging since electrons and holes are
charged particles interacting via the Coulomb potential. We observe a
transition from a partially ionized EHP to a high density electron-hole
liquid connected with a change of the physical nature of the quantum
condensate (the so-called BEC-BCS crossover). In spite of this change, the
phase boundary of the quantum condensate (i.e., the critical temperature vs.
density) shows a smooth crossover from the Bose-Einstein condensate of
excitons to BCS states at high densities as expected for Fermi systems with
bound states.

***

BS11025

QUBIT DECOHERENCE BY ENTANGLEMENT WITH CLUSTERS

The quantum coherence of a spin or a quantum bit (qubit) may be lost due
to entanglement with environment such as nuclear spins of atoms in a
lattice hosting the qubit. It is nontrivial to solve the many-body
dynamics of a bath of interacting spins coupled to the qubit which induces
the entanglement. Wen and Liu discover that the qubit decoherence process
can be understood as gradual entanglement of the qubit with clusters of
bath spins of larger and larger size as time passes by. Coherent
oscillation instead of decoherence may also be observed for small spin
baths where a few finite-size clusters could dominate in the bath
dynamics. The cluster correlation theory forms a basis for decoherence
control. It may also be generalized to understand statistics of small
quantum systems at equilibrium with finite temperature.

***

LT11502

Sound Absorption in a Sandpile

Understanding the physical origin of the elastic wave absorption in earth
materials is of great importance in the fields of seismology, soil
mechanics and rock physics. Several mechanisms of intrinsic attenuation
were proposed in porous granular materials, including the Coulomb
frictional sliding between cracks and grain boundary contacts and the
viscous dissipation due to the bulk fluid flow in partially or fully
saturated rocks. In this work, we characterize the dissipations of
ultrasound in dry and weakly wet (¿humid¿) granular materials using the
multiple sound scattering. In a dry medium two distinct sources of
dissipation from the adsorbed solid films (impurity layers) are observed:
a frictional loss and a viscoelastic one. In a wet medium, we find that
the dissipation is dominated by a viscous loss due to the submicron-thick
liquid films trapped at the surface asperities. Adding more liquids
enables to form the hour-glass capillary menisci but doesn¿t increase the
energy loss. Our experiments are of considerable interest to understanding
the crucial role of grain surface properties in granular mechanics, such
as singing sands, and the effects of humidity-induced capillary
condensation in wet granular materials, including room-dry rocks.

***

LR11108

Learning Vortex Physics

Hurricanes and superconducting vortices have things in common: Both are
quite stable fluid like structures inducing catastrophic consequences when
displaced in space. In the first case the consequences are well known, in
the second one a “superconducting” wire cannot transport current without
dissipation, jeopardizing technical applications.
A large amount of actual applied research in superconductivity is devoted
to design traps to pin the core of the vortex into the material. Using
Nernst effect measurements we found a method to discover up to which
magnetic field dependent temperature the superconductor can transport
lossless electrical current. We found that by all indications in high
temperature superconductors there is a range of temperatures where vortex
elastic energy is quenched. Thus, in this region vortices are free to move
and the superconductor becomes useless to transport electrical current.
This seems not to be dramatic because this temperature region is
sufficiently high for most applications. However, the result suggests a
revision of what is accepted in vortex physics, in particular for some
oxide superconductors where the 2D vortices nucleated in layers are
assumed non-interacting with those of neighbor layers.



***

BS10890

The Best of Both Worlds

The quest for materials with ever higher superconducting
transition temperatures has been a long-standing focus of
condensed matter and materials physics. Superconductivity requires
two essential ingredients: the binding of electrons into pairs and
the establishment of long-range phase coherence of the pair
wave-function. Unfortunately, systems in which pairing is strong
typically do not exhibit a correspondingly high transition
temperature, T_c, due to their susceptibility to large phase
fluctuations. Conversely, large phase stiffness is a property of
all simple metals, but these typically have (at best) weak
pairing. An intriguing question then arises: Is it possible for a
composite system, made of a strong-pairing component and a
phase-stiff metal, to inherit the best of the two worlds leading
to a transition temperature higher than that of either of its
constituents? (Murphy's Law, of course, would imply that a
composite would combine the worst aspects of both ingredients.) We
have computed the superconducting transition temperature of a
simple model of a composite system consisting of a good metal with
no pairing, and a strongly paired system with no phase stiffness.
If the coupling between these two constituents is too weak, T_c is
reduced, as in the decoupled case, by strong phase fluctuations.
If the coupling is too strong, the pairing scale is exponentially
suppressed, again leading to a low T_c. However, for a well
defined range of intermediate couplings, we find a high T_c with a
magnitude set by the strong pairing scale of the decoupled system.
This finding complements the experimental observations of Millo et
al [1] and Bozovic et al [2] concerning the enhancement of T_c in
two-component epitaxial films of cuprate high temperature
superconductors, and suggests further avenues for finding higher
T_c's in artificial multi-component materials.


***

LN11417

Bursty-Feature of Plasma Turbulence Linked to Broadband Power Spectra

Low frequency turbulence is a major focus of plasma physics research
due to its role in transporting mass and energy across confining
magnetic fields. In this paper, we connect the measurement of an
exponential power spectrum to the presence of Lorentzian shaped pulses
in the corresponding time series data and argue that this is a
universal feature of plasma turbulence. In two different experiments,
fluctuations in plasma density and temperature produce power spectra
that exhibit an exponential frequency dependence. In one experiment
this spectral behavior coincides with the appearance of the Lorentzian
pulses as the plasma evolves away from classical transport to an
enhanced, or anomalous, transport regime. The attached figure shows a
spectrogram in which the exponential spectrum begins at approximately
5.5 ms, the same moment at which large pulses are observed in the
fluctuation measurement shown as the white trace in the contour area.
Both the pulses and the exponential power spectrum appear only during
the anomalous transport phase. The generation of the pulses is linked
to nonlinear interactions of drift-Alfven waves, which highlights the
importance of electromagnetic effects in plasma turbulence.

8-15-08

AU10206

Bare Lithium barely understood

Investigating the interactions of electrons in an atomic system is at the
heart of understanding quantum mechanics. We have studied the interaction
of lithium (Li) atoms with soft x-rays (i.e. high-energy light)
coming from a synchrotron light source. At a high enough energy of the
light a rare event can occur in which all 3 electrons of a Li atom are
ejected simultaneously due to the interactions among the electrons leaving
just a Li nucleus behind. We have precisely measured the probability to
remove all 3 electrons by counting the number of Li nuclei at several
different energies of the light. Our measurements cover a significantly
larger energy range and are more precise than previous measurements
allowing us to make stringent tests of different theoretical models even
for high energies. We find that advanced theoretical models are still
having problems to describe that probability over the whole energy range
Our study will hopefully lead to theories that are able to better describe
the interaction of electrons not only in Li but in other systems as well.

***

LM10891B

Quantum simulator for Hubbard model

Our paper proposes a semiconductor device for simulating
a complex quantum many-body problem called the "Hubbard
model", using quantum mechanical effects. Certain
computational problems are by their nature very difficult
to solve. This includes certain optimization problems,
such as the "travelling salesman problem", and also
quantum many-body problems. The Hubbard model is a
particular example of this, and is important in condensed
matter physics, since it describes the way electrons in
solids behave. High temperature superconductors are
thought to be described by the Hubbard model, making its
understanding important. In our paper, we describe a
practical method for making an artificial Hubbard model
in the lab. By measuring the properties of the artificially
created Hubbard model, it should be possible to learn many
things about the model, which are not possible to obtain on
a computer. The difference to the simulation on a computer
is that in our device the electrons behave quantum
mechanically, whereas a computer operates using the
principles of classical physics. Simulating a quantum
mechanical model with quantum mechanical particles makes
for a more efficient simulation, an idea first conceived
by Feynman, and is one of the founding ideas in the field
of quantum computation.


***

BR10717

Quantum Simulations at Micron Scales and Beyond

The ability to perform quantum simulations of materials properties
over length scales that are relevant to experiments represents a
grand challenge in computational materials science. If one could
treat multi-millions or billions of electrons at a length scale of
microns or beyond, such first-principle quantum simulations could
revolutionize materials research and pave the way to the
computational design of advanced materials. In this paper, we
propose a multiscale approach that is entirely based on an exact quantum
mechanical theory for many electrons - the density functional theory
(DFT),
and allows prediction of materials properties at micron scales and
beyond,
without empirical or experimental input. The method, termed
QCDFT, combines two novel developments in multiscale modeling: a
systematic coarse graining of degrees of freedom through finite-
element interpolation and embedding a smaller region
in a larger environment quantum mechanically. The QCDFT method
has been applied successfully to a nanoindentation study of an Al
thin film in which more than 60 million electrons are simulated; more
applications are under way.


***

BR10995

Understanding electrical noise in carbon nanotube films

Carbon nanotube films, which consist of a mesh of single-walled
carbon nanotubes, have been demonstrated as a transparent,
conductive, and flexible material for applications in
photovoltaics, optoelectronics, and sensing. However, nanotube
films have high 1/f noise, a specific type of electrical noise
present at low frequencies that determines the detection limit of
any sensor. In this work, we use Monte Carlo simulations to
understand the sources of 1/f noise in carbon nanotube films and
to analyze the parameters that affect this noise, such as device
dimensions and film resistivity. We simulate the film by
generating nanotubes with random orientations in a 3D volume. By
comparing the simulation results with the published experimental
data, we find that tube-tube junctions, and not the nanotubes
themselves, dominate the 1/f noise. These simulations not only
provide important fundamental physical insights into the complex
transport mechanisms in nanotube films, but also help improve the
performance of these nanomaterials in potential device
applications where noise is an important figure of merit.


***

LQ11413

Onset of ferrielectricity and the hidden nature of nanoscale polarization
in ferroelectric thin films


Using calculations from first principles, we have elucidated the nanoscale
organization and local polarization in ferroelectric thin films. The
spontaneous
electric polarization exhibited by particular classes of materials, where
individual crystal dipoles orient in a fixed direction, is a property known
as ferroelectricity and it is at the basis of a broad range of modern
technological applications. Although ferroelectricity has been understood
for a long time in bulk crystals, the recent drive towards miniaturization
of electronic devices has unveiled novel characteristics when the size of
the material is scaled to the microscopic dimensions of thin films. Our
results unveil a peculiar spatial pattern of the local microscopic dipoles
where individual atomic layers acquire uncompensated opposing polarizations
in what was originally thought to be a simple domain of homogeneous
orientation. This behavior arises as consequence of the complex energetic
competition between the interface effects, the internal electric fields due
to uncompensated interface charges, and the orientation and mutual
interaction of the layer dipoles. Moreover, as the thickness of the film is
varied, we discovered the appearance of a dielectric phase transition that
we believe is a universal feature of ferroelectric materials at the
nanoscale.



***

AH10250

Atomic collisions from basic research to new technologies

In this work, we studied collisions between atoms of the alkali-metal cesium and argon gas. Such studies offer significant insights into the
atomic collisional dynamics and energy transfers important to astrophysics, the atmospheric sciences, plasma processing of semiconductor devices,
combustion diagnostics, and radiation therapy. First, a circularly polarized laser created an anisotropy in the angular momentum of the cesium
atoms-that is, the momentum now varied along specific directions. Then, a second circularly polarized laser further excited these atoms and
sensed the anisotropy. We collected the light emitted by the atoms as they decayed back to a lower energy state. This procedure was
repeated, only this time introducing the argon gas at differing pressures. In this way, destruction of the anisotropy could be studied by
slowly injecting these gas perturbers into the system. Our work demonstrated how a pump-probe pulse laser technique can be used to study
such processes, providing a deeper understanding of the collisional dynamics of excited alkali atoms.


***

LV10442B

Electron conductivity peaks at "magic angles" in organic molecular crystals

In molecular crystals of the (TMTSF)2X family, big organic molecules form
an array of parallel conducting chains. Electron conductivity across the
chains exhibits sharp peaks, when an external magnetic field is oriented
along some special "magic angles". This effect was first predicted by
Andrei Lebed in 1986, and its theoretical understanding has been developing
since then. One material shows only 3 magic angles, while another material
shows as many as 21 magic angles, but only with the odd values, whereas yet
another material shows only even magic angle. A comprehensive theory
presented in our paper explains the difference between the odd and even,
many and few magic angles in different materials. These effects result
from quantum interference in the electron motion, which is affected by the
orientation of the magnetic field. Curiously, the same mathematical theory
also describes quantum interference in a very different system: a
superconducting qubit driven by a radio-frequency electric field. The
similarity demonstrates that electrons in organic molecular crystals have
as high quantum coherence as in superconducting qubits, which are the
candidates for building blocks of a quantum computer.

***

ER10389

Converting Genetic Network Oscillation into Embryonic Backbone

Converting genetic network oscillation into embryonic somite formation
(the repeated backbone and associated musculature structure in
vertebrate embryos) is a well studied case of multicellular
morphogenesis in which complex spatial and temporal distributions of
cells are produced by genetic networks. The spatially periodic nature of
somite formation suggests that the genetic network involved must
display intracellular oscillations that interact with a longitudinal
positional information gradient, called determination front, down the
axis of vertebrate embryos to create this spatial patterning. Many of
the elements of the network have been identified, but their interaction
with the determination front remains poorly understood. In this paper we
propose a mathematical model for the genetic network that takes into
account the interaction of the oscillation clock with the determination
front to obtain a growing approximately spatially periodic sequence of
somites using the known network dynamics for the zebrafish embryo. We
simulated the transient manipulation of the wavefront in zebrafish
embryos and also propose a way to perturb somitic pattern by changing
segmentation clock period as an experimental prediction of our model
that can be used in some future experiment to test its validity.



***

LS11827ER

Exciting Hard Spheres: The Billiards Break Shot as an Explosion

It is fascinating to watch what happens during the initial "break shot" in
billiards. What happens in the ideal setting of no dissipation and an
infinite billiard table? In this work, we investigated the kinetics that
ensues due to a break shot on a d-dimensional infinite homogeneous gas of
perfectly elastic stationary billiard particles. Just as in real billiards,
progressively more particles become mobile as the collision cascade
develops.

In spite of the asymmetry associated with the initial break, the expanding
region of moving particles is nearly spherically symmetric about the initial
point of collision. Using elementary kinetic theory, we determined the time
dependence of the number of moving particles and the total number of
collisions in the cascade. The properties of this cascade are closely
analogous to those for a continuum shock wave that emanates from a point
explosion in a gas in the infinite Mach number limit. Strikingly, for an
initial break shot on a semi-infinite gas, all the initial energy is
ultimately reflected into the initially empty half-space by back-scattered
particles .



***

LM11412

Novel composite nanostructures consisting of metals and semiconductors
have exciting new properties which may allow for realizing entirely
novel nanophotonic devices such as surface plasmon polariton lasers
(SPASERs) or ultrafast switches. So far, however, designing such devices
has been hindered by a very limited understanding of interaction between
surface plasmon polaritons, the elementary optical excitations of metal
nanostructures, and excitons, the optical excitations of semiconductors.

In our paper, we for the first time demonstrate and
quantitatively explain the energy transfer processes between excitons
and surface plasmon polaritons in a prototypical composite structure
made out of a quantum well and a nano-sized metal grating. Our
experiments reveal the formation of new coupled exciton-plasmon modes
having properties which are a mixture of those of the two original
excitations.

We anticipate that our results will play a key role in designing new
metal semiconductor hybrid nanostructures which will be at the core of
novel devices like nanometer-sized laser sources or which will enable
the transfer of quantum information over mesoscopic distances.

Fig. Schematic of the prototype hybrid structruce made out of a quantum
well and a nano-sized metal grating

8-12-08

LL11289

Laser driven electron acceleration approaches the stage of
suitability for medical uses

A joint team from Italian, French and
German laboratories proved with an experiment at the CEA laser
facility in Saclay (France) that a table-top accelerator based on an
ultra-short pulsed laser and a gas-jet can deliver high energy
electrons (see right hand side of Figure) at a rate suitable for
either a) efficient generation of gamma-rays (i.e. hard X-rays) able
to produce radioactive elements via photonuclear reactions or b)
Intra-Operative Radiation Therapy (IORT) of tumors. The numerical
simulation reveals (see left hand side of figure) that the
unprecedented efficiency of this accelerator was due to the
achievement of a physical regime in which multiple electron bunches
are accelerated in the gas-jet plasma during the action of each laser
shot. Technical features of the accelerator output include
1-nanoCoulomb of electrons per each Joule of laser energy, while the
electron energy ranges between 10 and 45 Mega electron-Volt.
Comparison of the main parameters of electron bunches produced by a
commercial RF Hospital accelerator for IORT treatment and those of
the present laser driven accelerator is highly promising.

***

ER10334
Exotic orbits of two interacting wave sources

As shown recently, it is possible to create, on a vibrating fluid
interface, mobile emitters of Faraday waves [Y. Couder, S. Protière,
E. Fort and A. Boudaoud, Nature 437, 208 (2005)]. They are formed of
droplets bouncing at a sub-harmonic frequency which couple to the
surface waves they emit. The droplet and its wave form a spontaneously
propagative structure called a "walker". In the present article we
investigate the large variety of orbital motions exhibited by two
interacting walkers having different sizes and velocities. The various
resulting orbits which can be circular, oscillating, epicycloidal or
"paired walkers" are defined and characterized. They are shown to
result from the wave-mediated interaction of walkers. Their relation
to the orbits of other localized dissipative structures is discussed.


***

LP11094ER

A more complete theory for dissipative solitons

As it arises in highly diverse physical contexts, the cubic-quintic complex Ginzburg-Landau (CGL) equation plays an important role in modern science. Mathematically, it can be viewed as a dissipative extension to the nonlinear Schrödinger (NLS) equation and thus describes a more broad spectrum of spatial and/or temporal complexity appearing in nature, e.g., solitons and fronts. Unfortunately, even for stationary solitons, an accurate prediction of their dissipative dynamics at an analytical level is still a challenge. In treating such a problem, the variational approach and moment method are two oft-used theories but they approximate well only for a small portion of parameter regime of solitons. In this paper, a more complete theory for stationary solitons in CGL systems is proposed, with its suitability for parameter range significantly wider than existing theories. By using this theory, one can predict accurately the regime of existence of dissipative solitons in the five-dimensional parameter space, and for an arbitrary choice of a fixed point in space, determine all of its soliton characteristics with small errors. As expected, this work may be well adapted to both the interpretation of experimental results and the future optimization of related experimental systems.


***

AT10285
The Taming of the Shrew Molecules: Laser-Assisted Symmetry-Breaking in
Isotropic Molecular Ensembles

Breaking isotropic angular distribution of molecules in gas phase is a
crucial point for many stereochemistry studies and for controlling the
properties of media. We propose how to orient small linear molecules
in "head vs. tail" manner at high temperatures, up to room ones. For
this, short optical or UV laser pulse is employed. Such pulses had
already shown its ability to align small molecules, but without
preference between forward-backward directions. Additionally, the
higher the temperature in alignment scenarios, the stronger pulse is
necessary, which often causes collapse of molecules prior to their
ordering at room temperatures. To solve these problems we propose a
new approach based on indirect affecting the molecular rotations via
slight changes in molecular geometry (bond lengths) produced by
internal excitation of molecule by laser pulse. To achieve necessary
orientation-dependent character of an excitation we propose to use the
multifrequency femtosecond laser pulse with phase matching between its
components. Well-oriented state of the molecules occurs well after the
laser impact (i.e. in “field-free” regime), and quality of orientation
is almost temperature-independent at that. Modeling the photoinduced
dynamics of BF molecules shows high efficacy of the method.




***

ES10433
Correlations in a Nonequilibrium Assembly of Particles

Study of correlations of interacting subunits in equilibrium systems be they particles, spins or other degrees of freedom,
has been a time-honoured way of investigating them. Correlations are the key to understand their responses as well as thermodynamic
behaviour. The common wisdom about correlations is that they decay with distance. However, this expectation is belied for the so
called 'Single file diffusion', which is an assembly of particles with hard cores, diffusing on a line such that they cannot cross
each other. Such diffusion is relevent in a number of situations like ion channels in cell membranes, sliding of proteins along DNA
and diffusion of molecules in zeolites etc. This paper studies diffusion of N particles on an infinite line starting from
arbitrary initial positions. This is not an equilibrium situation, as the assembly expands with time. The correlations in the system
are found to be unusually strong. If one looks at the correlations between the displacements of the central particle with others,
they fall exponentially with distance in particle labels, but with a correlation length that is greater than N/2.
If one considers similar correlation between a particle on one edge with the others, the correlation decreases upto
the central particle linearly, for the most part, but as one goes further it changes sign and begins increasing till the other edge.
As N increases, the magnitude of the correlator decreases, but correlations extend over the entire assembly as described.
This implies that the assembly expands in a highly symmetric and correlated manner with most of the movement occuring at the edges.

***

LQ11630ER
SEGREGATION IN GRANULAR MEDIA

Segregation and mixing of dissimilar grains is one of the most fundamental
issues in granular matter both from a fundamental and a practical point of
view. In some cases it is a desired and useful effect to separate
particles of different types, but in other industrial processes it is
undesired and can be difficult to control. Due to the great relevance of
this phenomenon, many studies have been performed in the past few years to
understand the physics behind this problem. However, although there is an
extensive observational evidence of these phenomena, much less is known
form a more fundamental point of view and so, a deeper understanding of
the physical mechanisms involved in the segregation process is still
lacking. In this paper, a complete theoretical description based on
kinetic theory of granular gases is provided which covers some of the
aspects not accounted for in previous studies: (i) it goes beyond the
nearly elastic description, (ii) considers the combined effect of thermal
gradients and gravity on segregation and (iii) applies for dense systems.
The results show that the form of the phases diagrams delineating the
different segregation states depend significantly on the value of gravity
relative to the thermal gradient, so that it is possible to switch between
the different states for given values of the mechanical parameters. In
addition, the theory is in qualitative agreement with some computer
simulation results and also with previous experimental works.


***

LR10941
Helical (nano)ribbons: pull tight or give slack?

See animations

We study what happens when one stretches a coiled ribbon made of
inextensible material.
For coils of small pitch, such a ribbon turns out to behave differently
from usual springs.
Under some extension it suddenly gives slack and reshapes itself to lose
a coil.
This can happen several times depending on the number of initial coils.
Such a highly nonlinear response can help explain behaviour and shapes
of real tiny ribbons in nanomechanical experiments,
e.g. cholesterol or zink oxide crystalline ribbons. Their unusual
mechanical properties could be exploited in future nanodevices.


***

CQ10112
The Pathway from Order to Chaos in Nuclei is Explored

Most nuclei are reasonably well ordered near their ground states; i.e.,
the nucleons are in well defined shell-model-type orbitals (like the
electrons in atoms) and these orbitals, or configurations, can usually
be identified experimentally. However, as one adds thermal excitation
energy to the nucleus this order is gradually lost through mixing of the
shell-model-type levels and the nucleonic motion becomes chaotic. In
this paper the full range of this transition is studied for the first
time using the special properties of rotational nuclei. Such nuclei
generate a sequence of nearly equally spaced gamma rays, emitting sharp
gamma-ray lines of regularly decreasing energy (or frequency) as they
rotate slower and slower while remaining in a given shell-model-type
configuration. This is a signal that one can follow as the thermal
energy is increased. At first the sharp rotational lines broaden since
the mixed state can decay to the band member of any of its components.
This is a process called rotational damping. At still higher thermal
energies, this paper provides the first evidence that the width of these
rotational lines narrows again due to a curious process called motional
narrowing, well known in other branches of physics. In this case the
motional narrowing arises from the interplay of the time it takes the
nucleus to change components and the time it takes to change rotational
frequency. Throughout these processes the nucleus is becoming more
chaotic and a quantitative measure of this change was developed in this
paper by relating the probability that the configuration remains
unchanged after a transition to the probability of ordered behavior.
This probability ranges from about 100% near the ground state to less
then 1% at the "high" thermal excitation energy of about 4 MeV in the
ytterbium nuclei studied.



***

LR11485

When a material is stretched along a particular direction, the material is
expected to shrink in the lateral direction - like a rubber band. Conversely,
the sides of a material usually bulge when compressed in a vice. Most materials
change their lateral dimensions in this fashion when subjected to tension or
compression. Figure 1 (a) shows an unloaded material, while Figure 1 (b) shows
how a normal material responds to tension. However, materials can respond in
the completely opposite way - that is, they expand laterally when stretched or,
equivalently, shrink laterally when compressed. Figure 3 (c) shows this unusual
response. Such materials are said to have negative Poisson's ratio and are
referred to as auxetic materials. Auxetic materials have technological
importance. For example, they can used to improve the performance transducers,
components in microelectromechanical systems, strain amplifiers, shock
absorbers and fasteners, to mention a few examples.

We report in Physical Review Letters that under tension, standard isotropic
interactions of two- and three-dimensional many-particle systems (such as
colloids) can result in elastically isotropic (nondirectional) auxetic behavior
provided that the pressure of the system is negative. Matter characterized by
negative pressure is unusual. An air-filled spherical balloon will shrink in
size if placed deep in the ocean consisting of ordinary water. On other hand,
such a spherical balloon will expand in size if placed in liquid ocean
possessing negative pressure. Matter under negative pressure exists. A mundane
example is tempered glass. A more exotic example comes from cosmology, where
present thinking links the expanding Universe to a negative pressure.

The result reported in Physical Review Letters is an unexpected, since an
inherently anisotropic behavior (auxetic behavior) arises from isotropic
interactions. Indeed, most previously discovered auxetic materials exhibit
complex, carefully designed anisotropic interactions. We have shown the
existence of elastically isotropic auxetic behavior
at zero temperature for common crystal structures in two and three
dimensions, namely,the triangular lattice in two dimensions, and the
face-centered cubic lattice in three dimensions. These lattices provide the
densest arrangements
of spheres in these dimensions.


***

BUR1118

Making nanoparticles stick

Understanding the collision process between projectile nanoparticles and
target surfaces is of key importance because identifying conditions that can
lead to efficient sticking, i.e., deposition with preservation of
nanoparticle integrity, will open the possibility to produce novel
nano-materials. The challenge is that under low energies, sticking is
prevented by the low reactivity of surfaces. Under energetic impact
conditions, the deposited nanoparticle is significantly damaged.

Here we show via molecular dynamics simulations that sticking of
hydrogen-passivated Si nanospheres is possible even under the conditions of
poor reactivity of surfaces, as long as the impinging speed is in the
hypersonic range. The underlying microscopic mechanism is unexpected and
nanoscale specific: It involves a phase transition occurring in the particle
core, from diamond to beta-tin structure, the latter eventually evolving to
amorphous. Inducing phase transitions under low-energy impacting conditions
is a surprising finding, since in bulk they are achieved under extreme
pressures. We show that although the impacting energies are low and the
nanosphere-substrate contact forces are not large, the extremely small size
of the contact zone renders pressures high enough to induce the beta-tin
change. Besides the evident fundamental interest, this mechanism brings new
understanding to current nanotechnologies, such as hypersonic plasma
particle deposition.

8-5-08

LQ11723
Moving Quarks Help Solve Proton Spin Puzzle

The famous proton spin problem, which began with data from the European
Muon Collaboration 20 years ago, keeps producing new surprises. It was
thought that the spin of the proton would come from its quarks, but the
EMC found that the quarks' combined spin didn't account for all of the
proton's spin. Researchers began investigating other spin sources, such as
the gluons that hold the quarks together and spin generated by the
movement of the quarks, called orbital angular momentum. This Letter
explains that within the successful model recently proposed by Myhrer and
Thomas, more than half of the spin of the proton is actually carried as
orbital angular momentum by its quarks. Even more fascinating, it is
shown, as a model independent consequence of QCD evolution, that whereas
the orbital angular momentum of up quarks is much larger than that of down
quarks at low resolution, this reverses at higher resolution. The
resulting high-resolution values are in surprisingly good agreement with
state-of-the-art lattice QCD calculations, as well as with recent studies
of Generalized Parton Distributions conducted at Hermes and JLab.

***

LQ11129B
Charge and spin pairing instabilities in nanoclusters

Scientists have been trying to explain the unique superconducting, ferromagnetic, ferroelectric properties in inhomogeneous cuprates, manganites and multiferroics for many decades. However, not much progress has been reached based on studies of long range electron correlations appropriate for large homogeneous systems. In contrast, we propose exact microscopic analyses of local correlations in nanoclusters (triangles, tetrahedrons, octahedrons) that give important clues for understanding the origin of pseudogaps and inhomogeneities in respective frustrated (triangular, pyrochlore, perovskite) bulk structures. This bottom-up approach allows to unravel the details which depend on the local cluster geometry, electron interaction and temperature e.g. to investigate many body physics and spatial inhomogeneities in high Tc cuprates, manganites and multiferroics. For the first time we show that electrons' charge and spin in nanoscale undergo spin-charge separation instabilities and various types of pairings. The exact phase diagrams that we obtain provide novel insight into electron condensation, magnetism, ferroelectricity and display a number of inhomogeneous, coherent and incoherent nanophases seen recently by scanning tunneling microscopy in numerous nanomaterials, assembled nanoclusters, ultra-cold fermionic atoms.



***

BP10522
Observation of the Josephson effect in thin films YBa2Cu3O7 samples


We have observed for the the first time the Josephson modulation of the
maximum superconducting current flowing in YBa2Cu3O7 films with
non-nanometric dimensions. Josephson effect is the transport of
superconducting electrons from a superconductor to a neighboring one
across a non-superconducting barrier. A well-known manifestation of this
effect is the modulation of the maximum current flowing across the barrier
by a magnetic field. Since the discovery of the high temperature
superconductor YBa2Cu3O7, many authors have pointed out that this oxide
includes boundary planes between crystalline domains that could behave as
Josephson barriers. However, the magnetic modulation of the barrier
current had never been observed, except in samples with nanometric
dimensions and in films including artificial grain boundaries. In this
paper we establish the experimental conditions required for the
observation of the effect in thin film YBa2Cu3O7 strips with
non-nanometric dimensions and we report the modulation of the maximum
supercurrent by a low magnetic field observed in two samples.


***


LP11484
Alpha Particle Condensation in Nuclei

An atomic nucleus can have a gas-like structure composed of alpha particles.
The alpha particle, helium nucleus, is a tightly bound quartet containing two protons and two neutrons. A clump of alpha particles interacting weakly with each other is bound as a gaseous nuclear state.
The most amazing is that they can be condensed into a single quantum orbit,
reflecting Bose statistics of them with spin zero, as might be called
``Bose-Einstein condensation'', which is analogous to the ultra-cold atomic
gas condensation. A characteristic aspect inherent to the nuclear system is
that the phenomenon can only be observed in excited states not in the ground
states, because in the ground states with higher density all of the alpha
particles dissolve into nucleons in pieces, i.e. into liquid of nucleons. Ever since the possibility of occurrence of the novel structure was suggested by some of the present authors, a lot of theoretical and experimental efforts have been devoted to
revealing presence of the structure. The Hoyle state in 12C, which plays a special role in carbon production in stars, now becomes convincing to be of the dilute three alpha particle condensate.

We investigate quite a wide energy
region in 16O theoretically for finding the quantum condensate with a
gas-like structure of four alpha particles (see figure), and give strong evidence of the existence. This exotic state is obtained together with the
other well-known quantum states, and its candidate for experimental
observation is also mentioned. The present discovery is of a great
significance in convincing the existence in heavier nuclei and in
establishing the new aspect in nuclear physics.

8-1-08

LP11283
TO THE COLD OR TO THE WARM? A THERMOELECTRIC EFFECT IN COLLOIDAL SUSPENSIONS

When applying a temperature gradient to an aqueous colloidal suspension,
one observes a flow of its components, or "thermophoresis". Recent
experiments on solutions of lysozyme protein, polystyrene beads, micelles,
DNA, and Ludox particles revealed surprising dependencies on solvent
temperature, acidity, and salinity. In all cases, the solute diffuses to
the warm at low T and to the cold at higher T; a change of sign occurs at
some intermediate temperature. A similar behavior as a function of salt
content was observed for a suspension of charged latex spheres at low
acidity; the particles migrate to the cold at low salinity and to the warm
upon adding NaCl.

In the present paper, we explain these observations in terms of the
thermoelectric effect of the electrolyte: the temperature gradient induces
an electric field of the order of 100 V/m which, in turn, drives the
charged colloidal particles to the cold or to the warm, depending on the
sign of their charge.


***

LB11269
HUNTING QUANTUM BUTTERFLIES

There is a paradox at the heart of quantum mechanics, stemming from the
classical phenomenon of chaos. Large everyday (macroscopic) systems,
which are described by classical mechanics and are generally nonlinear,
can manifest chaos, including sensitive dependence on initial conditions
(the "butterfly effect"). Microscopic systems, e.g. atomic and nuclear
systems, are described by quantum mechanics which is a linear theory,
where chaos is not possible. But classical mechanics is supposed to
emerge from quantum mechanics! Recently, an understanding has evolved
that the theory of open quantum systems, which accounts for local and
random interactions with other nearby systems, is generally non-linear,
and can manifest chaos. However, it has been generally assumed that as
you go to smaller scales, chaos is suppressed. Kapulkin and Pattanayak
present theoretical evidence to the contrary: it is possible to take a
macroscopic system with regular (non chaotic) dynamics, scale it down,
and obtain a mesoscopic system which manifests chaos due to quantum
effects. Scaling the system down further results in a microscopic system
which is strongly quantum mechanical and the chaos manifested in the
transition regime gets washed away by the quantum fluctuations. Thus the
quantum to classical transition is, in general, qualitatively
non-monotonic, and quantum effects can induce chaos in a regular
classical system, contrary to folk wisdom. This should be generic and
beyond fundamental questions of theoretical principle has implications
for quantum control, quantum computing, and nanotechnology. The
transition from quantum to classical behavior continues to yield much
counterintuitive and beautiful physics.

***

LS11482
Oscillating reaction in advanced materials synthesis

The unusual phenomenon of a chemical reaction that oscillates in time has
been observed during synthesis of indium nitride (InN), an advanced
semiconductor material for optoelectronics. Previously, oscillating
chemistry had been found only in reactions of certain molecules in solution
or on surfaces, and in complex living systems such as microbe colonies
and heart muscle. This new oscillatory system is the first one discovered
that involves reactions and transformations between bulk condensed materials
(solids and liquids). When a GaN surface is exposed to a steady vapor flow
of ammonia and an indium compound at high temperature, we observe that a
film of particles forms and then repeatedly transforms back and forth
between crystalline InN and liquid elemental indium, alternately absorbing
and releasing nitrogen to and from the vapor. The oscillatory behavior
indicates that the production of active nitrogen by catalytic decomposition
of ammonia at the surface is key to forming InN. This may help solve the
longstanding problem of synthesis of this important material.

***

LT11043
NEW STRUCTURE(s) OR EFFECTS ON HADRON PRODUCTION

psi(3770) is a bound state of the c and anti-c quarks. It can be
produced in e+e- annihilation. It is believed to be the only observed
structure in the energy range from 3.700 to 3.872 GeV, and almost entirely
to decay to D and anti-D meson pair. However, the BES Collaboration found
that (15+-5)% of psi(3770) does not decay to D and anti-D pair in
assumption of that there is only one psi(3770) in the energy range.
Recently, the BES Collaboration report an anomalous line-shape of cross
sections for e+e- --> hadrons in the energy range, indicating that either
there is likely a new structure in addition to psi(3770) around 3.773 GeV,
or there are some new physics effects reflecting the D and anti-D
production dynamics. This information is important in the understanding of
the QCD and the potential models based on the QCD theory, and even more
important in guiding experimental physicists to search for new kind of
particles such as glueball, hybrid, multiquarks and molecule states
predicted by the QCD theory.



***

ES10437
Hydrostatic and frictionless behavior in loaded granular materials

Granular materials have a reputation for weird phenomena, acting out
somewhere in between solid-like and liquid-like behaviors and sometimes
elsewhere. How loose granular material such as well sorted sands, behave
under loading and unloading cycles reveals unexpected brands of behavior
such as friction free points and stress anisotropies perpendicular to
the loading direction. Hysteresis, consisting of different physical
paths for a system when you load it versus when you unload it, is a well
known phenomenon in rock and soil mechanics. Recent research results, in
two and three dimensional granular systems, have demonstrated that a
peculiar force network supports an externally applied stress. Such a
network consists of a backbone of highly strained grains, that
percolates parallel to the direction of applied stress (positive
anisotropy) and a weak network strained in the perpendicular direction
(negative anisotropy). In this work we find that the unloading stage is
considerably more complex as the two previous subnetworks release the
stress. The most unusual feature of the unloading path is that one
reaches a point, before complete unloading, where the subnetwork stress
anisotropies switch sign and the system is both hydrostatic, like a
liquid, and macroscopically frictionless as seen by an external agent
applying stress, even though the grains themselves interact through
frictional forces. An interplay between microscopic friction and the
stress pattern within the granular medium is proposed as the mechanism
for the emergence of a macroscopic friction.

***


LT11307
Short Ranged Electric Fields Emanating from Molecules: A Novel Approach for Controlling Current Flow in Nanoscale Circuitry at Extreme Packing Densities

We have demonstrated experimentally and theoretically a novel principle for controlling electric currents flowing in the smallest nanoelectronic circuits that can potentially be realized. Our new approach takes advantage of the very short range of the electric fields emanating from molecules. These fields are obvious and forceful to atoms and electrons positioned nearby (within a nanometer), but are effectively cloaked or invisible beyond that. This is not down-sizing of standard methods: The field patterns generated by molecules cannot be emulated by conventional electrodes, not even by the smallest solid state transistor nano-gates currently realized under ideal laboratory conditions. Nanomolecular circuits exploiting this principle should operate without interfering with each other even at the extraordinarily high packing densities achieved by molecular self-assembly. The short range of these electric fields also implies greatly reduced energy dissipation associated with switching of circuits and therefore much less undesirable heating of electronic devices. The extremely small sizes of molecular devices also imply extremely fast communication between active entities. This work opens the way to begin imagining new information processing architectures not previously accessible.

7-31-08

LS11058
Optical micro-structures on quantum chaos

In the last decade optical micro-structures, especially photonic
crystals, have been widely used to suppress spontaneous emissions, to
enhance the efficiency of Light Emitting Diodes (LED), and to make
high reflecting omni-directional mirrors as well as low loss
waveguides. Now optical micro-structures enter the community of
quantum chaos, where "scarred" states are very important for
controlling the electronic structure of atoms and the transport of
electrons in resonant tunneling diodes. In this paper, with a
two-dimensional photonic crystal micro-structure fabricated on the
surface of a Vertical Cavity Surface Emission Laser (VCSEL), direct
observations of different lasing modes, including whispering-gallery
and chaotic modes, are demonstrated by collecting the near field
radiation patterns. The use of optical micro-structures to tailor the
modes and resulting emission from scar-based lasers will be of great
interests to researchers using ideas from quantum chaos to develop
efficient and highly-directional microlasers. The experimental
observations and the simulation results here also provide an
alternative but effective approach to access chaotic modes in VCSELs
at room temperature.


***

LN11200
The wave-particle duality of single H-atoms and a molecular double-slit

We have performed a real double-slit experiment in which one proton at the
time passes close by either nucleus of a hydrogen molecule. Here it can
capture an electron and continue as a neutral hydrogen atom to a
position-sensitive detector, where it is registered as a single particle.
The underlying wave character is revealed in the spatial distribution of
many such hydrogen atoms. The orientation of the hydrogen molecule (the two
slits) is controlled in the experiment, but no measurement reveals which
'slit' the proton passed. Young introduced the double-slit experiment in the
early 19'th century to demonstrate the wave character of light. Much later,
the wave-particle duality became a key phenomenon for understanding the
concepts of modern quantum theory, and a clear demonstration was provided by
the double-slit experiment with electrons. Recently D. Akoury et al,
referred to the hydrogen molecule as 'the World's simplest Double-Slit'
[Science, 318, 949 (2007)]. In the present experiment this double-slit is
used to demonstrate quantum interference for an object with extremely short
wavelength (only 1/4000 of the diameter of a hydrogen atom) making it by far
the shortest wavelength object for which quantum interference has been
demonstrated in any double-slit experiment.

***

BR10843
Thermal spike model best describes the origin of ion tracks in
semiconductors

The use of semiconductor devices under extreme conditions, as, for
example, in space or in the vicinity of particle accelerators or nuclear
reactors leads to the formation of disordered tracks whose origin is not
yet fully understood. However, our recent experimental and theoretical
results on ion track formation in various semiconductors (InP, InSb,
GaAs, GaP, Ge, and Si) give support for the extended thermal spike
model, and at the same time contradict three other competing mechanisms,
such as the Coulomb explosion, the shock waves, and the athermal
melting. The radiation hardness of the studied materials depends
primarily on the target material, ion species and energy, and
irradiation temperature, but also on the ion flux and target doping.
Generally, visible amorphous or heavily disordered ion tracks occur, if
the electronic energy deposition per ion and unit length exceeds the
threshold value for melting of the corresponding material. This was
found to be possible, e.g., for elemental ion irradiation of InP and
InSb, but not in Ge and Si, which is in agreement with conclusions made
by other groups.

***


LL11635
Domino Day on a Nano Level: Magnetic Moments Topple Over in Rows

Storing increasing amounts of data on increasingly small spaces is one of
the prerequisites for further progress in information technology. A magnetic
effect, which makes magnetic switches and logic elements conceivable on an
atomic scale, has been discovered by theoretical scientists from the
German Research Center "Forschungszentrum Jülich". The results of
their computer simulations mean that nanowires made of transition metals
could be suitable for transporting and storing magnetic information.

Lounis, Dederichs and Blugel discovered a kind of domino effect in rows of
individual manganese atoms on a nickel surface. They noticed that the
magnetic configuration of these nanowires differed depending on their
length. Astonishingly, only one atom more or one less makes a drastic
difference. When the number of atoms is odd, the orientation of the magnetic
moments is antiparallel, and when the number is even, they line up in a
toppled compromise position between parallel and antiparallel. Adding an
atom at the end of the nanowire or taking one away simultaneously changes
the magnetic configuration of the entire cluster. Just like a row of
dominoes, the magnetic moments topple over. However, this is where the
analogy ends because the effect can be completely reversed in contrast to
dominoes that have toppled over.

This new quantum mechanical effect, which holds for long stretches
of at least 100 atoms, has been discovered using a multiscale approach,
mapping density functional results obtained by a refined KKR-Green function
method to a Heisenberg-type model hamiltonian. The authors hope that it
will be proven experimentally in the near future.


Caption for the included Picture:
The magnetic moments of a row of manganese atoms (red) organise themselves in an antiparallel fashion on a nickel surface (blue) when their number N is odd, e.g. 3 or 9. When their number is even, say 2 or 10, the magnetic moments take up a compromise position.

***

BQ10959
Atomic mechanism of self-diffusion in silicon

In silicon, self-diffusion, i.e. the thermally-induced mobility of the host atoms, is determined by the migration of vacancies and self-interstitials. We performed comprehensive atomistic simulations in order to study the correlation between self-diffusion and the migration of both point defects. The comparison with the recent analysis of experiments on wafer processing and on simultaneous self- and dopant diffusion in isotope heterostructures revealed that the migration of the self-interstitials should be governed by a dumbbell mechanism. A dumbbell consists of two atoms which are situated at symmetric positions relative to a lattice site. During self-interstitial migration the dumbbell moves from one lattice site to the other by exchanging an atom. Our result is in contrast to the widely-used statistical diffusion theory developed by Compaan and Haven fifty years ago. Their work based on the assumption that self-interstitial migration is characterized by the exchange of an atom between a tetrahedral interstitial site and a lattice site and vice versa. On the other hand, we found that vacancy migration is characterized by the transformation of the tetrahedral vacancy to the split vacancy and vice versa which is also assumed in the statistical diffusion theory.

APS Physics Tip Sheet: New colossal carbon tubes created; and flipping spins at the speed limit

Colossal Carbon Tubes leave Kevlar and Nanotubes in the Dust
H. Peng, D. Chen, J.-Y. Huang, S. B. Chikkannanavar, J. Hänisch,
M. Jain, D. E. Peterson, S. K. Doorn, Y. Lu, Y. T. Zhu, and Q. X. Jia
Physical Review Letters (LS11423)

A collaboration of Chinese and American physicists has discovered a way to make a new carbon structure that could lead to fabrics 30 times stronger than Kevlar and 224 times stronger than cotton. The group dubbed the structures colossal carbon tubes because they're thousands of times larger than carbon nanotubes. At 40-100 millionths of a meter across and centimeters long, they’re comparable in size to typical cotton fibers.

The structures consist of nested inner and outer tubes separated by hollow channels, making the tubes both light and strong. While they are nowhere near as strong as carbon nanotubes, the colossal tubes are much more ductile than the nanoscopic variety, making them more suited for spinning into threads and weaving into fabrics. The colossal tubes conduct electricity and show some of the properties of semiconductors, which means that they could lead to novel microelectronic components as well as super strong cloth.

The details regarding how the intricate structures form is still hazy, but the researchers propose that colossal carbon tubes could go into to improved body armor, stronger carbon fiber composites (which are often shaped into parts for high-performance and lightweight vehicles), or components in microelectronics and tiny machines. - JR

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Spin Flips Hit the Speed Limit
S. Serrano-Guisan, K. Rott, G. Reiss, J. Langer, B. Ocker, and H.W. Schumacher
Physical Review Letters (LS11409)

A team of physicists at Physikalisch-Technische Bundesanstalt in Germany has managed to flip a nanoscopic magnet as fast as the fundamental speed limit allows. Their experiment consisted of two stacked layers of tiny magnets separated by a thin barrier to form what is called a magnetic tunnel junction. Such magnetic tunneling junctions are promising candidates for future magnetic memory chips.

The researchers allowed electrons aligned in a special way to flow between the layers, developing a spin torque, or twisting force that is transferred from one layer of nanomagnet onto the other. This torque pumps enough energy to the nanomagnet to make it move faster and faster until it changes direction. Several measurements showed that the researchers were able to switch the direction of magnetization as fast as physically possible.

Their spin torque record is important for the next generation of low current, ultra fast magnetic memory chips and sensors. This new generation of electronics encodes information in an electronic spin, rather than in an electronic charge. The spin torque switching effect is a powerful new approach to controlling electronic spins. - NR

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50 Years of PRL
Martin Blume

Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal’s birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of Marty’s Milestone PRL project, visit http://prl.aps.org/50years/milestones

This week, Marty is taking a look at a milestone paper from 1982 that led to the 1998 Nobel Prize in Physics.

Two-Dimensional Magnetotransport in the Extreme Quantum Limit
D. C. Tsui, H. L. Stormer, and A. C. Gossard
Phys. Rev. Lett. 48, 1559 (1982)

Following on the discovery of the integer Quantized Hall Effect by von Klitzing and coworkers, Tsui, Stormer, and Gossard undertook studies of a two-dimensional electron fluid at higher magnetic fields and lower temperatures than had previously been done. In this Letter they presented surprising results showing a plateau of the Hall effect at 1/3 the von Klitzing conductance value, (1/3)(e2/h). Several explanations for the results were discussed in the Letter, but the authors finally concluded “At the present there is no satisfactory explanation for all of our observations”. Many possible explanations were put forward by others in a flurry of papers, but the conclusive explanation was given by Laughlin in 1983 (selected as a Milestone for that year). He showed that a new state of matter with many-particle interactions accounted for the experimental results.

The 1998 Nobel Prize in Physics was awarded to R. B. Laughlin, Horst Stormer, and Daniel Tsui “for their discovery of a new form of quantum fluid with fractionally charged excitations”. See Physical Review Focus 2, story 18 (http://focus.aps.org/story/v2/st18) for a readable description of this work.
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Nadia Ramlagan and James Riordon contributed to this Tip Sheet

Journal articles and preprints are available to journalists on request.

Contact: James Riordon
American Physical Society
riordon@aps.org
301-209-3238