11-26-07

LF11266
Spacecrafts and artificial satellites orbiting around the Earth, the Chernobyl
sarcophagus,
and the organic tissues in radiotherapy for cancer treatment are all subject to
intense radiation
damage induced by highly energetic ions (cosmic rays, solar wind, particles
originating in
radioactive transitions,...). Materials swell and crack when subjected to such
ordeals, but they do it differently depending on their chemical nature.
Theoretical simulations complementing indirect experiments
are extremely important to understand and hopefully predict these behaviours. A
key for these
simulations is knowing how hot electrons get in matter
while a projectile traverses it, since that crucially determines how atoms
interact with each
other, and thus the response of matter. The rate of this energy uptake by
electrons depends on the speed of the
projectile. It happens to be very poorly characterised for insulating matter at
relatively
low velocities due to experimental difficulties. So much so that even the fact
on whether
there is a velocity threshold is unclear, meaning whether the electronic
stopping is quite
suppressed below a given velocity.
The present work proposes a direct way of obtaining the needed information using

time-dependent first-principles calculations. In its first application it is
tested for
protons shot through lithium fluoride, the best studied system in the field,
obtaining
promising agreements with what is experimentally known (like the ratio between
the
stoppings of protons and antiprotons), and support for the velocity threshold
idea,
including fair quantitative estimates. The study opens the field for analogous
studies on
materials of interest for nuclear engineering, waste containment, and even
biomedicine.

***

LJ11619

Photonic amorphous diamond --- an amorphous structure with 3D photonic
band-gap



The photonic crystals possessing 3D photonic band gap (PBG) have attracted
much attention and been studied extensively because of their wide potential
applications in optics. Since Bragg scattering of photons by a periodic
lattice is considered as the origin of the gap formation, it has been widely
believed that the 3D PBG should be realized only in ‘photonic crystals’
having lattice periodicity. In this paper, we report that the 3D PBG
formation is, in actual fact, possible in a photonic amorphous structure in
spite of complete lack of lattice periodicity. We have numerically confirmed
that the structure ‘photonic amorphous diamond’ possesses a sizable 3D PBG
and that it can confine light at a defect as strongly as conventional
photonic crystals can. These findings present an important new insight into
the origin of 3D PBG formation and open new possibilities in developing 3D
PBG materials.


***

LG10962
Amoebae can anticipate periodic events.

Single-celled organisms might be cleverer than previously thought.
Anticipating events are higher functions performed by the brains of higher animals;
their evolutionary origins and the way they self-organize, however, remain open questions.
Here we show that an amoeboid organism can anticipate the timing of periodic events.
The plasmodium of the true slime mold Physarum polycephalum moves rapidly under favourable conditions,
but stops moving when transferred to less-favourable conditions.
Plasmodia exposed to unfavourable (low-temperature and low-humidity) conditions,
presented in three consecutive pulses at constant intervals, reduced their locomotive speed in response to each episode.
When subsequently subjected to favourable conditions,
the plasmodia spontaneously reduced their locomotive speed at the time point when the next unfavourable episode would have occurred.
This implied anticipation of impending environmental change.
After this behaviour had been evoked several times, the locomotion of the plasmodia returned to normal.
We explored the mechanisms underlying these behaviours from a dynamical systems perspective.
Our results hint at the cellular origins of primitive intelligence and imply that simple dynamics might be sufficient to explain its emergence.

***

LD11736

HOW SOFT IS MATTER AT HIGH DENSITY?


When heavy ions are smashed together in high energy scattering
experiments, matter at high density and temperature is produced, with
typical densities a few times the density of atomic nuclei and
temperatures of the order of a thousand billion degree. In these
extreme conditions new particles are produced and matter becomes
softer because the energy can be distributed also among the
new degrees of freedom.

In our paper we have investigated how easily that type of matter can
be compressed and we have shown that, even if a mixed phase of quarks
and hadrons is produced, the pressure does not remain constant, at
variance with what happens in more normal mixed phases, and therefore
the incompressibility of matter is not vanishing.

This result is important when analyzing high energy experiments,
because it indicates that no dramatic change in the mechanical
properties of matter is expected when quarks start being produced and
that only a gradual softening should take place. Therefore it cannot
be ruled out that a mixed phase of hadrons and quarks was already
produced in experiments in which densities of the order of 3 times the
density of normal nuclei were reached, as for instance in the
experiments performed in the past at the Alternating Gradient
Synchrotron of Brookhaven.

In the Figure we show the bulk modulus of matter, which is also called
incompressibility, as a function of the density in units of nuclear matter
saturation density (the central density of nuclei). Clearly, in the mixed
phase the incompressibility is reduced but it is not vanishing.

***


LH11802

PROBING NON-EUCLIDEAN SPACE OF MULTIWALL NANOTUBES

Carbon nanotubes differ from crystals in the same way a spherical globe
is distinct from the flat Earth of ancient myths. In nanotubes,
following an atomic plane can bring one back to the starting point, and
two parallel atomic lines may cross. This "non-Euclidean geometry"
profoundly affects the structural relaxation dynamics, as researchers at
Sandia National Laboratory and Rice University have shown.
High-resolution microscopy at elevated temperature makes it possible to
monitor and model with atomistic precision the real-time evolution of
atomic planes in the bodies of tubes, and to see the conventional rules
of physics violated. Burgers' vector, always invariant along a
dislocation line, according to the textbooks, must change to accommodate
the built-in curvature of nanospace. The screw dislocations, connecting
multiple tube walls, prevent slippage between them and change their
electrical connectivity. Apart from the charisma of curved spaces and a
peculiar mathematics which is bound to emerge, the research should
impact significantly the applications of nanotubes as structural
materials and electronic interconnects.

***

LD11792

Regionalism versus Globalism in Quantum Clusters

What is the character of correlations in large clusters of particles?
This is one of the central questions in statistical mechanics and the
physics of condensed matter. Here we give a method for dealing with
this question in interacting quantum clusters, by using techniques
from the newly developing field of quantum information. We apply our
method to a certain strongly interacting cluster of quantum particles,
known as the resonating-valence-bond state, the usefulness of which
ranges from the description of bonds in organic molecules to
fault-tolerant quantum computation. We find that the WHOLE bunch of
particles in the resonating-valence-bond state have strong quantum
correlations. On the other hand, small groups of the same cluster have
only a negligible amount of quantum correlation.

***

LH11440
What is spinning inside the nucleon?

This simple question have haunted physicists since the discovery of the
composite structure of the nucleon in the late sixties. Forty years of
experiments have established that the nucleon spin is not only composed of
the spin of its constituents, the quarks and the gluons. The present work
demonstrates the possibility of experimentally accessing their orbital
angular momentum (OAM) which may constitute the missing piece of the
nucleon spin puzzle. Our experiment at the Jefferson Laboratory (JLab)
studied the rare events where a high energy gamma ray scatters on a quark
inside the neutron and the neutron remains intact. Similarly to the
scattering of light by a material, the energy and angular distribution of
the gamma rays is sensitive to the momentum and position distributions of
the quarks and the gluons, allowing for a femto-tomography of the nucleon.
The probability of these events is related to the contribution of the OAM
of quarks to the nucleon spin. Our measurements provide constraints on the
OAM of quarks within a model based on a recent formalism describing the
nucleon structure in a universal way. These pioneering measurements are
part of the ongoing experimental effort to understand the nucleon
structure at JLab. With the planned upgrade of JLab to 12 GeV, systematic
studies of the scattering of gamma rays by neutrons and protons will shed
new light on nucleon structure and may bring a definitive answer to the
exciting mystery of the nucleon spin.

***

LD11496

Atoms in the focus of an extreme-ultraviolet laser

At the new free-electron laser facility FLASH in Hamburg, unexpected high degrees of photoionization were observed on xenon atoms. By generating a microfocus in the extreme ultraviolet (EUV) with the aid of a multilayer mirror at the photon energy of 93 eV, irradiance levels above thousand terawatt per square centimeter could be achieved and xenon ion charges up to 21+ showed up. Although the experiments were performed within the regime of the classical photoelectric effect, the explanation of the results seems to be beyond the scope of the perturbation theory and photons as the light particles. As such, the study may be the starting point for new theoretical work on photon-matter interaction at short wavelengths and high photon intensities with strong impact on x-ray laser science. In the near future, those facilities may open the doorway to totally new experiments of materials research, e.g. to study ultra-fast chemical reactions on surfaces or within biological systems.

***


LJ11471

Echo formation prior to expectation

Everybody is familiar with echoes. One can hear them, and they play an
eminent role in many measuring techniques and in all fields of spectroscopy.
Often, echoes are delayed, but can an echo be formed prior to what is
expected? If so, the interpretation of echo-experiments and echo-devices
must be rethought from scratch. Precisely such echo-signals have been
observed by Zaenker et al. by NMR spectroscopy in hyperpolarized 3He gas and
analyzed quantitatively. These echoes originate from constructive
interference between the coherent and the stochastic motion of the spins in
a magnetic field gradient and occur at a time sqrt(2) tau rather than 2 tau,
where tau is the pulse delay. These pseudo spin echoes substantially broaden
the dynamic range over which translational motion can be studied and may
prove very important in the emerging field of magnetic resonance imaging
with hyperpolarized gases. Similar effects are expected in related echo
experiments with photons and neutrons in presence of stochastic dynamics on
the timescale of the echo formation.

***

LH11692.

Furry black holes

The ¿no-hair¿ conjecture states that black holes are simple objects,
characterized by just a few quantities, such as mass, angular momentum
and charge. In this paper we present new black hole solutions of
Einstein¿s equations, which require an unlimited number of pieces of
information in order to completely describe them. The black holes are
coupled to a particular type of matter, Yang-Mills fields, which provide
the black hole with ¿hair¿ ¿ extra structure outside the event horizon
of the black hole ¿ and live in anti-de Sitter space, a type of
space-time which is particularly important in string theory. We call
these ¿furry¿ black holes because they have large amounts of black hole
hair. Many hairy black holes have been found previously, but few of the
known solutions are stable, in other words the ¿hair¿ tends to fall off
the black hole. At least some of the black holes in this paper are
stable, so that the ¿fur¿ remains attached to the black hole. These are
the first stable black holes with unlimited amounts of hair to be found.

***


LH11179
Experimental demonstration of a vital noise protection scheme for quantum computers

Computers based on quantum mechanics promise to revolutionise traditional
information technology. Unfortunately large quantum systems are very sensitive
to decoherence, which is the uncontrollable interaction of the system with its
environment. We have experimentally demonstrated a computing protocol using
specially designed environment-insensitive quantum states that protect the
encoded quantum information from decoherence. In our experiment we have used a
quantum system known as a photonic cluster state. Even when this system is
exposed to the highly damaging effects of decoherence, remarkable protection of
the information is achieved, delivering protocol outcomes that are extremely
close to the ideal ones. Our results demonstrate the first successful
realization of an error-tolerant one-way quantum information protocol and open
up a way toward the construction of noise-resilient, large-scale quantum
computational devices.


Attached Photo 1: This image shows the pump beam preparation stage with the
various laser beams (note the different colours) that are employed in the
preparation of the pump beam. After this stage the ultra-violet pump beam enters
an interferometric setup which serves to create the photonic cluster state.
(Picture credit: Robin Riegler)

***


LG11224
"Skinny" Pions Sail Through Nucleus

Scientists have observed that the nucleus becomes invisible for
"skinny" pions, demonstrating, for the first time, the onset of color
transparency for pions. This result also further elucidates the onset of
quark-gluon effects in nuclei.

Like protons and neutrons, pions are built of smaller subatomic
particles called quarks. Normally, pions traversing the nucleus feel
the strong force of the protons and neutrons they encounter, causing
a fraction of the pions to be reabsorbed by the nucleus. If the
strong force is described in terms of the underlying quarks and
gluons, however, such reabsorption is predicted to disappear. This
vanishing act is a result of small-sized, point-like or "skinny" pions
being produced in sufficiently energetic collisions between beams of
particles and atomic nuclei. Under these circumstances, the escape
probability of the pions increases towards unity, just as a pair of
oppositely charged particles, when brought close together, can
traverse undisturbed through a field of other charged particles.
This unique phenomenon is better known as color transparency, and is
necessary to describe high-energy scattering reactions well. Witnessing the
onset of this phenomenon has, however, remained elusive.

In an experiment at the Department of Energy's Jefferson Lab, a beam
of electrons was used to knock pions out of various atomic nuclei ranging
from deuterium to gold. The escape probability of the produced pions
was found to increase with higher-energy collisions, i.e., the
nucleus became increasingly transparent to the pions. This rate of
increase, as well as the variation in transparency with different
nuclear targets, was found to agree with predictions which assume the
pions were produced in a small-size, ie. "skinny," configuration and
remained so while traversing the nucleus. The Jefferson Lab pion experiment
thus observed for the first time the turning-on of the color transparency
phenomenon in the most basic quark-antiquark system.

***

LF11031
Cylindrical invisibility cloaks designed by the coordinate
transformation technique (Science 312, 1780, 2006) can be perfectly
invisible but however with an experimentally unrealizable inner
surface. The cylindrical cloak recently demonstrated by the experiment
(Science 314, 977, 2006) uses simplified material parameters. The
simplification procedure not only avoids the singular inner surface
problem, but also eases metamaterial engineering at other parts of the
cloak. In this study, we quantitatively show that such a simplified
cylindrical cloak is however still a distance away from achieving
perfect invisibility. Especially, the zero-th order cylindrical wave
component will see such a cloak similarly as a transparent tube.
Therefore field penetration and scattering are inevitable. Although
high-order cylindrical wave components are unable to penetrate into
the cloak, they experience fairly heavy scatterings, even when the
cloak's outer radius is kept much larger than the inner radius. Our
findings suggest that achieving near-perfect invisibility may require
metamaterials fabricated with strict adherence to the derived material
parameters according to the coordinate transformation.

11-13-07

LG11351
Ultrafast acoustics at ultrahigh pressure

The most famous direct application of the experimental determination of how sound speed evolves under extreme condition is to provide a crucial test for Earth and planetary models. It also has potential for application in areas ranging from fundamental problems in physics of solid and liquid state as for example crucial test for modern simulations, which in turn could be beneficial for various other scientific as material research. However in spite of many efforts to improve the pressure range, several experimental constraints still preclude studying the elastic properties in laboratory of most of materials at very high pressure. We have developed an innovative high pressure method combining the diamond anvil cell device with the technique of picosecond ultrasonics. This method overcomes all the drawbacks of traditional techniques and can be easily extended to elastic investigations of all materials up to several Mbar and thousands of K. We believe that the technique of ultrafast acoustics in diamond anvil cell, which is described for the first time in this paper, is a critical step forward to the study of elastic properties under extremes conditions.
As a case study we studied the elasticity of the prototypical quasicrystal (QC) AlPdMn. While it is well known that phasons dynamics are at the origin of the quasicrystalline long-range order at high temperature, the question of why QCs state is still stable at ambient temperature remains. Our results on the pressure dependence of the acoustic attenuation shed lights in this on-going debate. Intrinsic local excitations (as tiles flip or tunneling states) are expected to actively participate to the stabilization of the quasicristalline phase without long-wavelength phason fluctuations.

***

LF11092
Weeding out extra unwanted reference frames solves some serious
problems for stirred-up fluids


A powerful method for studying the physics
of randomly stirred fluid flow and turbulence
uses the concept of "summing over all fluid histories".
However, this history sum, introduced over thirty years ago,
has the undesired and embarrassing side-effect of also
summing over an infinite number of inertial frames of reference,
thus rendering this calculation method defective.
Clearly, the sensible thing to do is to study the fluid
dynamics in just one reference frame, which in fact is the
solution to the problem. Picking out one reference frame in the
fluid "history sum" turns out to bear remarkable resemblance to
so-called "gauge-fixing" in modern theories of high energy particle
physics. Moreover, once we repair the sum by this "gauge-fixing",
we discovered that the dynamical description of the fluid flow has a
deeper "hidden" symmetry, which relates the nonlinear and
linear parts of the Navier Stokes equation and supplies exact information
on how these two parts of this equation can change as one changes the
scale or resolution at which the system is observed (renormalization).
The method discovered in this Letter works in general
for systems that do not depend on a particular inertial
reference frame, including random surface growth (Kardar-Parisi-Zhang
equation) and fluids with magnetic fields (magnetohydrodynamics).


***

LE11110

Ferromagnetism in dilute electron gas

Ferromagnetism has been one of the most fascinating fields of research in condensed matter physics since the last century due to many interesting fundamental issues and technological applications. Recent technological advances indicate dominance of spin-based technology in the future. Ferromagnetism is characterized by a temperature called Curie temperature. Below this temperature, the magnetic moments exhibit long range order, which helps to align all the magnetic moments parallel in the presence of external magnetic field. Thus, ferromagnetism depends on two parameters; the local magnetic moment and the entities that mediate coupling among these moments. Almost all the magnetic materials consist of elements having partially filled f or d bands. In the f band systems, the highly localized f electrons exhibit magnetic moment and the long range order appears via the interaction of these moments with the conduction electrons. On the other hand, the d electrons have both; local character leading to magnetic moment and itineracy that mediates magnetic coupling. Interestingly, some recent studies discovered ferromagnetism in lightly La doped band insulator, CaB 6 which does not contain partially filled d or f band element. Highly diluted electron gas loses its itineracy forming a Wigner crystal. It was proposed theoretically that an enhancement of electron density, just above the Wigner crystallization limit leads to polarization of the electron gas. This system, thus, was considered to be a realization of such effect on real systems. Subsequently, some other studies on this system suggested that extrinsic reasons such as presence of small Fe impurities may be introducing ferromagnetism. In this study, we have investigated the temperature induced evolution of the electronic structure of CaB 6 and LaB6 employing photoemission spectroscopy with state-or-the-art energy resolution. We find that small impurity and/or disorder in the B sublattice introduces local character among the B 2s 2p conduction electrons. Such localized features could be revealed in the high resolution spectra at low temperature, when the electron density at the Fermi level is highly diluted (small conduction electron density due to imperfections in the B sublattice in CaB 6). This leads to ferromagnetism in this system with Curie temperature much higher than the room temperature. An enhancement of conduction electron density (in LaB6) smears out such features and no long range order is observed. This study thus adds a new dimension in the field of ferromagnetism by establishing that local character can be introduced in a dilute electron gas by introducing impurity and/or disorder. Interestingly, the Curie temperature is significantly high (much higher than room temperature) in these hexaborides and hence good candidates for technological applications. We hope material research for spin based technology would be highly benefited by this knowledge.

***

LG11620
Rho mesons cling to mass:

In the crushing atmosphere at the core of some stars, the
building blocks of matter are thought to boast less mass than
those trapped in the less-dense matter found here on Earth.
A recent experiment at DOE's Jefferson Lab, tested that idea
by comparing the masses of rho mesons in dense nuclei to those
in a light nucleus. They found that rho mesons have the same
mass and only a slightly shorter lifetime as a result of being
in a dense nucleus. The result may lead to an improved
understanding of the fundamental principles of the theory of
quantum chromodynamics at low energies.

The theory of quantum chromodynamics successfully describes
high-energy experiments involving quarks and gluons. Unfortunately,
QCD isn't as successful at describing low-energy experiments
involving particles built of quarks and gluons, called hadrons.
However, a number of QCD-inspired predictions suggest that the
masses and lifetimes of hadrons will be modified when produced
inside dense and/or hot matter. For instance, these QCD-inspired
calculations (based on chiral symmetry restoration) predict a
decrease in the mass of a two-quark particle called the rho meson
in dense nuclei. Other predictions of rho meson properties in
dense nuclei are based on effects that one typically expects in a
system of particles that interact with each other; these models
predict a shorter lifetime and a change in the mass of the rho
meson (some predict smaller masses, while others predict larger).

Researchers tested the predictions by producing and measuring
the properties of rho mesons in ever-denser environments. They
compared the masses and lifetimes of free rho mesons with those
in various nuclei (deuterium, carbon, iron and titanium).
The researchers observe no significant mass shift and some shortening
of the rho meson lifetime due to normal medium effects. The result
from this precision experiment provides some of the first data
that can be used to constrain theoretical models of medium
modifications of rho mesons.

***


LG11731
Magnetic fields permeate the universe, and with the increased resolving
capabilities of instruments, a flurry of details on highly complex flows
emerge. It is now clear turbulence is ubiquitous in geophysical and
astrophysical flows. However, we still don't understand how waves and
eddies interact in these flows to give rise to the scaling properties
that are observed.
In this letter, we present results from a numerical simulation of
magnetohydrodynamic (MHD) turbulence at unprecedented spatial resolution
(1536^3 grid points). Detailed analysis shows the emergence of an
isotropic flow at large scales with a scaling compatible with the
weakening of interactions between eddies due to the presence of waves,
and an anisotropic flow at smaller scales with a scaling consistent with
wave turbulence. The behavior differs from scaling laws observed in
previous numerical simulations or predicted by some theories of MHD
turbulence. It gives also the first observation in simulations without
an external magnetic field, of an isotropic spectrum first predicted by
Iroshnikov and Kraichnan in 1967.


***


LD11485

Light Localization in Disordered Photonic Crystal Waveguides

Similar to matter waves, the propagation of electromagnetic waves can be stopped in ordered crystal structures that exhibit photonic band gap - as well as in disordered, random media where the phenomenon is known in theory as Anderson localization. By combining the seemingly disparate principles we demonstrate a method for photon localization that relies on the interplay of the two mechanisms, one based on order and the other one based on disorder.

Localization by random scattering was first predicted for electron-matter-waves in disordered atomic crystals. Anderson localization has been shown difficult to attain, the concept, however, can be extended to any wave phenomenon and light in particular. In this regard, random scattering media and disordered lattices have attracted considerable interest as experimental systems for testing localization concepts. It has been proposed that the localization conditions should be easier to fulfill in disordered periodic structures with a photonic band gap. We present observation of this idea in photonic crystal waveguides operated at optical frequencies where localization can find various applications in optical sensing systems and random nano-lasers.

11-7-07

LH11000
Open systems dynamics in closed quantum systems:
Exact relaxation in quenched quantum many-body systems

Why do systems dynamically relax to a statistical equilibrium state?
This intriguing but old question is enjoying a renaissance recently,
with new experimental techniques becoming available: The non-
equilibrium dynamics of atoms in optical lattices can be experimentally
observed. Specifically, following a quench - that is, a rapid change of
the system's parameters - the many-body system undergoes
complicated dynamics. So what happens? There is no environment, so
how could it possibly relax?

Recent work published in the Physical Review Letters [1] answers this
question rigorously for a class of models that are idealized instances
of the Bose-Hubbard model that takes center stage in this discussion
of atoms in optical lattices. They demonstrate that while the information
on the initial condition is of course stored in the system at all times, it
becomes diluted with time. Locally, for any subsystem, one obtains a
maximally entropy state compatible with the constants of motion.
Remarkably, this is true without a time average: The system just
smoothly and nicely relaxes. So when locally looking at the system,
we think that the system has reached its equilibrium. But only apparently
so, as one day, arbitrarily far in the future, a recurrence will show that
all the time, the initial condition was not forgotten.

***

LH11172

Use these quantum bits if you can count to one.

Scientists are striving to build large quantum computers for simulating quantum systems and for solving difficult numerical problems such as factoring large numbers. Qubits, the bits of quantum information, differ from classical bits since according to the rules of quantum mechanics they can be both 0 and 1 at the same time. When many qubits are combined, a large number of states can be processed in parallel, giving quantum computing exponentially increased power compared to a classical computer. The task of manipulating and interconnecting a large number of spatially separated qubits is daunting, and the present state of the art is limited to less than 10 qubits. We propose a new approach which uses an ensemble of atoms, each with N internal states, to encode an N qubit register. Register values of 0 and 1 are associated with 0 and 1 unit of excitation which is shared in a collective entangled state of all atoms in a small ensemble. The register is prepared and manipulated via excitations of high-lying Rydberg states which entangle the atoms. This method encodes N qubits in N internal states in sharp contrast with previous approaches that require 2N physical states, an exponentially large number, to encode N qubits. With this approach a 14 qubit register can be simply encoded in a single cloud of 100 cesium atoms, and more exotic atomic species have the potential to extend this to registers of more than a 100 qubits.

***

LJ10880

What's inside a black hole
---the prediction of string theory
confirmed by supercomputer


Black holes not only swallow up infalling matter,
but also emit thermal radiation as discovered by Hawking.
This suggests that the black holes have a certain interior
structure, but what precisely it is has long been a mystery.
By using supercomputers, we have successfully confirmed
a theory clarifying the interior structure of the black hole.
The key was provided by string theory, which was invented
to unify Einstein's general relativity and quantum mechanics
in a consistent manner. We were able to compute accurately
the energy of a system composed of strings, which is
conjectured to be described by a black hole macroscopically.
As the temperature of the system is lowered, our data indeed
approach the energy of the black hole.
This result shows that the thermal properties of the
black hole such as the Hawking radiation can be explained
microscopically in term of strings. There are many other interesting
issues such as the black hole evaporation and the early universe,
in which string theory is expected to play a crucial role.
Therefore the establishment of a new method in string theory
utilizing supercomputers has great significance. It is expected
to boost our understanding towards profound issues such as
the origin of the universe, matter and its interactions.

***

LE11199
Why are enzymes so big ?

In our paper,
localization of high amounts of energy for long periods of time
is shown to occur in rigid parts of protein structures.
Since it was recently shown that
catalytic sites of enzymes are often found in the rigid
parts of their structure, this result strongly supports the hypothesis
that enzymes may use such high amounts of energy in order to
achieve their function. Indeed, enzymes need energy to break
and form chemical bonds. It is usually assumed that they
use chemical energy only (coming from labile chemical bonds
like those of the ATP molecule, used for energy storage
by all living cells) and that they "wait" for favourable thermal
energy fluctuations otherwise. What our work suggest is that
enzyme structures are set up so as to be able to catch and
store much more energy than "standard" thermal fluctuations
normally allow. This would also provide an explanation for
the following, puzzling question: why are enzymes so big ?
The answer being: in order to have large stiff parts, far
away enough from the water environement and its dissipative effects.

***

LH11482
Quantum Description of "Real" Black Holes Found

We have found the quantum states of a maximally rotating black hole
using string theory techniques. The number of such states agrees
exactly with an earlier prediction of Stephen Hawking. Unlike previous
work on black holes in string theory, the black holes we study are
very similar to those observed by astrophysicists. We are able to
identify and count the quantum states of these physical black holes by
using symmetries of the theory to mathematically relate them to
previously studied cases. This is exciting as it is a step towards
understanding the quantum gravitational physics of black holes.

***


LY10632

Can aerosols be trapped in open flows?

This paper describes an important and intrinsically appealing new
phenomenon in chaotic fluid flows, which is counter-intuitive and likely
to stimulate new discoveries in several fields. We feel that this paper is
worthy of news coverage by APS, AIP and the media. Below is our attempt to
provide a one-paragraph summary consistent with the editorial style of
Physical Review Focus. The text is accompanied by a color figure (see
caption below) and a movie that illustrate the phenomena described in the
paper (available at www.pks.mpg.de/~rdvilela/leapfrogging.html ). We will
be happy to accommodate changes that would make this more readable and we
will be available to answer any questions you may have about the work.

At this time we also provide the names of three noted researchers that
would be able to comment on the importance and broad impact of this work:

1. Distinguished Prof. Edward Ott (University of Maryland, edott@umd.edu )
2. Professor Zoltan Toroczkai (University of Notre Dame, toro@nd.edu )
3. Doctor Manuel A. Matias (CSIC-UIB - Spain, Manuel.Matias@ifisc.uib.es )


************************************
TRAPPED BY SCATTERING: MOVING OUT, YET NEVER LEAVING

Place a heavy particle in a vortex of a fluid flow and the centrifugal
force will move it outwards, essentially in the same way a fast car slides
in a curve. Now imagine the particle in an open flow (the flow of a
channel, say): it would possibly encounter some vortices, spiral outwards,
and eventually move away as the fluid itself does, right?
Not necessarily, shows our upcoming Physical Review Letters paper.
In a rather counter-intuitively fashion, the motion outwards successive
vortices can drive the particles back to the point where they started,
creating a condition of permanent trapping that leads to accumulation
of particles in specific regions of the flow.
The consequences of this newly discovered phenomenon can be important
in various fields, including astrophysics, atmospheric and environmental
research. They have clear implications for the accumulation of aerosol
pollutants in the air. But they also shed new light on the longstanding
problems of rain formation and planet formation
as a mechanism for coalescence of smaller "particles" given that cloud
droplets in the atmosphere and planetesimals in primitive nebula are
essentially heavy particles in open flows.
Leaving aside potential applications, the fact that the particles
transported by a fluid can remain confined even when all the
particles of the fluid move away is very important in its own right.
It was previously assumed that only light particles
could be trapped, in that case by moving to the center of the vortices.
The heavy particles considered in this study are trapped for exhibiting
precisely the opposite behavior: each individual vortex scatters the
particles away, but does so towards the other vortices. The particles
remain trapped by continuously escaping from the vortices!
************************************




************************************
Figure Caption (figure attached):

Trapping of aerosols (heavy particles) in the leapfrogging vortex flow:
physical space projection of the attractors (red X symbols) and
corresponding basins of attraction (colored regions) at a given instant.
Also shown are the velocity field of the fluid (black arrows) and the
positions of the vortices (black dots) at the same instant.
The red curves indicate the orbits described by the attractors.
************************************



************************************
Movie Caption (movie available at
www.pks.mpg.de/~rdvilela/leapfrogging.html ):

This movie illustrates the dynamics of aerosols in an open fluid flow
consisting of two leapfrogging vortex pairs. To allow for flow visualization,
fluid particles initially placed in a rectangular region are painted (brown dots).
In the reference frame we use, fluid particles come from the right-hand side,
are scattered by the vortices close to the origin, and eventually leave the domain.
Aerosol particles (orange dots) initially inside the same rectangular region
have a different fate. In contrast with the fluid particles, they are permanently
trapped around the vortices.
************************************

***

LD11147
Vortices breakup in newly discovered superconductors

In a Nobel Prize winning paper, Alexei Abrikosov realized that magnetic fields
create vortices that run through superconductors like tubes, each containing
the same fixed amount of magnetic field. In this paper we show that these
vortices can break up into half vortex pairs, or ¿vortex molecules¿, in a
recently discovered type of superconductivity predicted forty years ago by
Fulde, Ferrell, Larkin, and Ovchinnikov. The consequences of these vortex
molecules on the resulting physical properties of the superconductor are quite
dramatic. This vortex molecule state twists and turns in all three space
dimensions, an intricate structure that could not have been guessed without the
insight that the vortex molecules provide. The prediction of this new state
presents a challenge to create experiments to go and find it.

***


LF11513 Wang

PERIODIC ORBITS DRIVE DIRECTED MOTION OUT OF CHAOS


D'ailleurs, ce qui nous rend ces solutions périodiques

si précieuses, c'est qu'elles sont, pour ainsi dire,

la seule brèche par où nous puissions essayer de

pénétrer dans une place jusqu'ici réputée inabordable.

(Henri Poincare', Méthodes nouvelles) (*)


A great challenge for the future technology is to extract transport from random

fluctuations. Brownian motors rectify the random motion of particles, thus

generating a current. Such phenomenon, known as the ratchet effect, may be

the key for understanding molecular motors, that is, tiny biological engines

which transform the energy produced in chemical reactions into unidirectional

motion along macroscopically flat periodic structures. Moreover, the

rectification of fluctuations is potentially useful in technological

applications such as new electron pumps, molecular switches, and transistors.



This beautiful effect takes place in presence of lattice asymmetry, external driving, thermal noise and friction.



The fascinating question then arises: what about a deterministic world in which only dynamical chaos is present? Can directed transport emerge in a purely deterministic system?



In our paper, we show that dynamical chaos alone is of little help to generate directed motion. But, here is the surprise: periodic orbits come into play. Indeed we show that large currents can be generated thanks to the presence of stable islands located around periodic orbits and embedded in the chaotic sea. Chaos, friction and external driving are still needed but periodic orbits play the decisive role.



Once again the old sentence of Poincare’ turns out to be true!





(*) English translation: …what makes these periodic

solutions so invaluable is that they are, so to say, the only breach throughout

which we can try to penetrate inside a region so far considered unapproachable.



P.S. We can provide some nice colour figure of a mixed phase space structure

embedded in a chaotic sea. Here is an example in black and white.

***

LE11272
Electron Billiard with a Laser Cue

For centuries, the classical three-body problem has fascinated scientists. The motion of a system of three planets has no closed form solution, and can exhibit chaotic behavior. The quantum mechanical three-body problem is of even more importance, since all molecules and virtually all atoms are many-body systems. The simplest such system, the helium atom, provides a test bed for understanding multielectron systems. We report a study of electron correlation in Helium using a strong laser field. The laser field serves two purposes -- it drives an electron to collide with the atom, and it provides a clock with which to observe the double ionization dynamics. We concentrate on a unique feature in the correlated electron spectrum. We employ a classical model and a quantum simulation to relate the field driven three-body system to its field-free analogue.

***

LB11532

NO-CLONING, NO-BROADCASTING ARE NOT PECULIARLY QUANTUM

Copying restrictions are generic in non-classical theories

***************************************************************

One of the fundamental results of quantum information theory, the
"no-cloning" theorem (due to Wootters and Zurek and to Dieks), states
that no physical process can produce two independent copies of either
of two pure quantum states, unless those states can be distinguished
by a single measurement. A generalization, the ``no broadcasting"
theorem, extends this result to pairs of mixed states: it says that
even correlated copies are forbidden, unless the states
commute---behaving, essentially, like a set of classical states. In
the present work, researchers Howard Barnum, Jonathan Barrett, Matthew
Leifer and Alexander Wilce show that these results generalize to
essentially any probabilistic theory. A broadcastable set of states
must behave as an effectively classical set of states, and clonable
sets of states must be distinguishable---that is, behave effectively
as a set of classical pure states. So, all nonclassical theories in a
broad probabilistic framework must contain sets of states---for
example, the set of all states---that cannot be broadcast, and sets of
pure states that cannot be cloned.

No-cloning and no-broadcasting have often been considered "peculiarly
quantum" features of a theory, closely related to the possibility, in
quantum theory, of classically impossible feats such as distribution
of secret cryptographic key in such a way that any eavesdropping on
the key will be guaranteed to be detected. If all states could be
copied, an eavesdropper could make a perfect copy of the state of all
systems being used to distribute the key, without making her presence
known through any disturbance to the original state. The present work
shows that no-broadcasting and no-cloning are, in fact, generic
features present in any non-classical theory within a very broad
framework, suggesting that all such theories may permit
eavesdropping-evident key distribution. The program to characterize
quantum theory in terms of information-processing properties, then,
must bring to bear properties beyond no-cloning and
no-broadcasting---such as, perhaps, the impossibility of bit
commitment, the possibility of teleportation, or the nontriviality of
communication complexity, to name three issues under active
investigation by researchers engaged in this program.

***



LHK1043

COMMENT TO PRL: CONTROVERSIAL CONSTANT OF NATURE MAY VARY AFTER ALL
The laws of physics are supposed to be remain solid, unchanging throughout the Universe. But astronomical observations peering back through its 14 billion year history have suggested the opposite, that electromagnetism may have changed subtly with time. Although this surprising effect wasn't seen in more recent observations, our new paper demonstrates faults in the analysis, leaving wide open the possibility of varying laws of physics. The observations focus on galaxies seen in silhouette in front of bright background beacons of light called quasars. As the quasar light travels to Earth, the galaxies absorb some in a barcode-like pattern (see Figure 1 attached) which, when decoded by astronomers, reveals the strength of electromagnetism in the galaxies 10 billion light years away. Our new work shows that recent observations from the Very Large Telescope in Chile were decoded incorrectly, giving physicists too much confidence in electromagnetism's solidity. In fact, when we correct the decoding errors and replicate the analysis, there is broad agreement with previous observations from the Keck Telescope in Hawaii: one of physicists' cherished constants of Nature may actually be varying after all.

Figure 1 caption: The experiment. Extremely distant quasars are used as bright background beacons to probe galaxies which happen to lie along the line of sight to Earth. Metallic gas in the galaxies absorbs light in a pattern of very specific wavelengths. Much like reading a barcode, the spacing between the different metal absorption lines can be decoded to measure the strength of electromagnetism at distances up to 12 billion light years away.

10-31-07

LG11619,

Using rings to increase the current in quantum electronics.


Many of the electronic properties of today circuits depend on the
ability to induce and control the currents along its basic components.
In this work we show that at the nanoscale and when the electronic transport
is ruled by the laws of quantum mechanics, the circulating currents along
wires are ruled not only by the material's properties but also by the
mechanism employed to induce the transport.

The conductance of a one-dimensional wire connected to source
and drain electrodes can not overcome the fundamental quantum
(G=2e²/h). However, we show that when the wire is bended into a ring
and the transport is promoted by time-dependent magnetic fluxes,
additional effective transport channels are generated
that allow for a spectacular increment of the induced dc current and
to achieve conductances much greater than the fundamental quantum.

***


LF11779
First evidence for truly out-of-equilibrium phase transition found in
liquid crystal topological turbulence

Spreading phenomena can be seen everywhere, e.g. in epidemics, forest
fires, rumors, calcium signaling in cells, and even galactic evolution.
Over decades physicists have modeled them and found that, in spite of
the apparently different processes at play, they often obey the same
universal laws prescribing how activity grows, how it is distributed,
how it fluctuates, etc., in the critical region marking the onset of
unbounded spreading. Known under the name of directed percolation
universality class, this out-of-equilibrium universality is now well-
established like universality in equilibrium, except in one essential
aspect: no experiment so far has been able to convincingly show evidence
for the directed percolation universality class despite substantial
efforts. This long-standing puzzle is solved here, in the context of
topological turbulence in driven liquid crystals. This opens the door
towards establishing experimentally out-of-equilibrium universality
classes in natural phenomena.

***

LH11645

Increased Fusion Efficiency with Particle Beams

A theoretical approach to fusion that was dismissed by physicists 50 years ago as impossible has been revived by in-depth simulations. So called beam fusion works by firing beams of particles known as ions into ion targets at just the right energy so that ions in the target “prefer” to undergo a fusion reaction with them, rather than simply deflect them away as would normally happen. Fusion reactions are a CO2-neutral route to energy production and they occur when two ions are given enough energy that they can fuse together. The new simulations show that the beam ions bounce around inside the target allowing them to have multiple chances of fusing with target ions. The result is that more energy can be released than was put in and this may help pave the way to more efficient routes to fusion energy production using lasers.

***

LF11842
Microscopic Saturn Rings and Satellites

When studying tiny oil droplets and air bubbles rising in a liquid
crystal, we observe defect rings and points that closely resemble their
celestial namesakes. But the droplets and bubbles are merely several
hundredths of a millimeter in diameter, and the cause of the defect
rings and points are molecular misalignment in the liquid crystal. We
also discover that subject to disturbances in the ambience, the
satellite point can spontaneously open up into a Saturn ring, and a
Saturn ring can be swept by flow of the liquid crystal into a satellite
point defect. Using computer simulation, we have reproduced these
interesting transitions, and demonstrated that they are due to molecular
rotation driven by the flow on the one hand, and alignment among
neighboring molecules on the other. These findings open the possibility
of using liquid crystals to control the assembly of microscopic
particles into regular crystalline structures, with potential
applications as photonic crystals.

***

Small Particles Big Motions

The movement of small particles like bacteria, red blood cells and environmental contaminants are important
in sensor design for health care, understanding human physiology and cleaning up the environment. Recent
research by the Magnetic Resonance Microscopy Lab at Montana State University indicates that these motions
are more complex than previously thought and can be modeled with ideas from nonlinear dynamics or chaos theory.
In particular their research shows that motions in dilute suspensions of these particles flowing in small
tubes are irreversible, an effect thought to be dominant only in concentrated particle systems. When only
two particles interact through a fluid the motion can usually be reversed by reversing the fluid flow. However
when 3 or more particles interact the system behavior is more complex and the motions less predictable.
Understanding these complex motions has the potential the enhance design of microfluidic biosensors for blood
and bacteria as well as model contaminant motion in the earth’s subsurface.

***

LE10878

What makes a rod stiff?

Can surfaces modify the mechanical properties of nanoscale devices?
Measurements say yes, but theoretical models proposed over the past 30
years are found to violate Newton's third law. A rethink is required.

Tunability of the mechanical properties of nanoscale devices lies at the
core of potential applications in nanoelectromechanical systems. By tuning
stress at the surface of micro-cantilevers, researchers have observed that
they are able to modify cantilever stiffness. This observation has been
purportedly explained using theoretical models that describe measurements
remarkably well.

Surface stress is like adding cling wrap to a surface and shrinking it.
Current theoretical models describe surface stress as if one is pulling
the end of the cantilever, like stretching a rubber band. However, there
is nothing there to pull the cantilever so the rubber band stretches
itself! As Newton pointed out, every action has an equal and opposite
reaction. As such, all these models are in violation of Newtons third law
and unphysical.

John Sader and Michael Lachut from the University of Melbourne examined
this surface effect, by taking into account Newton's laws, and discovered
a new surface stress law that is particularly relevant to
nano-cantilevers. Their research will be published in Physical Review
Letters.

The outcome is that this controversial physical phenomenon defies
explanation despite more than 30 years of research. It remains to be seen
whether the problem lies in the measurements themselves, their
interpretation or in an alternative mechanical process. Further
miniaturization to the nanoscale and use of this new law are expected to
shed light on this intriguing effect.

***

LF11629
Solving the Schroedinger equation of atoms and molecules without
analytical integration based on the free iterative-complement-interaction
wave function

Schroedinger equation has now become soluble for general molecules,
which is a firm basis for building up an accurate predictive science.
The Schroedinger equation governs chemistry, biology and physics of matter. However, for 80 years after its birth, exact solution of the Schroedinger equation has been thought to be impossible, except for some very special simple cases. Recently, we have developed a general theory to construct an analytical wave function having exact structure, so that by applying variation principle, we have been able to calculate very accurate solutions of the Schroedinger equation of small atoms and molecules. However, a problem still remained since integrations over the complement analytic functions necessary in the variation calculations are extremely difficult for general atoms and molecules, which hindered the solution of the Schroedinger equation for general systems. In this paper, we have proposed a new simple general method of solving the Schroedinger equation analytically without doing integrations. This method has been applied to several atoms and molecules, giving very accurate energies and wave functions. This has opened a road leading to accurate predictive methodology in chemical, physical and biological sciences.

***


LE11181

Microscopic bodies that swim in the computer

Microrobots that swim through the bloodstream to deliver their medicinal
payloads to errant cells are still a futuristic application, but given the
rate at which nanotechnology is progressing the time is ripe to begin
studying microswimmer behavior in order to learn what makes a good design.
The present study of these minuscule swimmers employs molecular dynamics
simulation, a method in which the individual molecules of both the swimmer
and the fluid through which it swims are represented by interacting
particles; the motion of these particles is followed by solving Newton's
equations on the computer. Swimmers propel themselves using a variety of
mechanisms familiar from nature, such as rotating limbs, changing body
shapes or fluid jets. The power expenditure of different swimmer designs
can be compared to determine their relative efficiencies, and the wakes
generated by the swimmers in the fluid can be visualized. Owing to the
extremely small sizes involved, inertial effects are very small, so that
the efficiency of the swimmer designs can be very different from the more
familiar macroscopic world. This initial study treats the problem in two
dimensions, but extension to the more realistic three-dimensional case is
underway.

[The figure shows a three-dimensional swimmer with a rotating tail, also
simulated using molecular dynamics; the fluid particles are rendered
partially transparently.]


***

LG10977
A new link found between biological and computational descriptions of the
brain


Brains are often thought of as being analogous to computers in that
they process information and perform calculations.� Much of our
understanding of how the brain works, is however in terms of
biological elements such as channels and chemical transmitters.� The
relationship between these computational processes and the biological
elements is very unclear. We have given a mathematical proof that
provides a specific link between the biological and computational
descriptions of the brain.� We also have shown using computer
simulations and recordings from real neurons that this relationship
can be applied to understand how these biological elements implement
computational processes. �Such work is essential if we are to relate
our increasing knowledge of molecular aspects of the brain's workings
to its functional properties.�

10-24-07

LG12004

Pulsed Laser Deposition" proved to be the best deposition technique for the preparation of ultra-thin film-based devices

Forthcoming generations of data storage devices based on magnetic tunneling junctions MTJs (e.g., hard-disk read heads) designed to operate in the not-so-distant Tbits/inch^2 technology (current density limit ~100 Gbits/inch^2) demand tunneling junctions optimized in regard to: (i) sensitivity to the magnetic field as high as TMR=200% (described as the relative change of the tunneling resistance with the magnetic field) and (ii) low resistance-area product for the magnetic state of highest conductance (<1 Ohm-micron^2). These requirements are only available using tunneling junctions based on single-crystal barrier layers with homogeneous thicknesses where the coherent tunneling becomes feasible. Recent advances in the fabrication by sputtering of CoFeB/single-crystal MgO/ CoFeB MTJs exhibiting large-area TMRs so high as 400% have been achieved. Despite these progresses, current thin-film technologies based on physical vapor-phase deposition (PVD) techniques present severe limitations for the preparation of layers with homogeneous thicknesses of only a few atomic monolayers. At this thickness scale (<2 nm), the layer roughness, which is mostly determined by the growth kinetics of the PVD technique used, is of the same order of magnitude as the layer thickness. This produces a large dispersion of behaviors for the thickness-dependent properties (as large as 1000% for the tunneling current through a 2 nm-thick MgO layer with a standard roughness of 0.3 nm) that compromises the tunneling device reliability. In this work, the growth kinetics of the mostly used PVD techniques -molecular beam epitaxy, sputtering, flash evaporation and pulsed laser deposition- is theoretically investigated with the aim of testing the suitability of such techniques for preparing ultra-flat ultra-thin layers required for reliable tunneling devices. We demonstrate that Pulsed Laser Deposition is the best technique for preparing the flattest layers due to a combination of two unique key features [use of (i) a supersaturated pulsed flux of (ii) hyperthermal species] that promote the coarsening of the surface species by a distinctive mechanism, baptized as kinetically limited Ostwald ripening.

***

LD11076

We provide important insight on a very well-known problem, namely the relation between gravitational dynamics and thermodynamics of horizons. The thermodynamics of black holes suggests a deep connection between gravitation and thermodynamics. Then the connection between Einstein gravity and thermodynamics of horizons was proved in 1995. We showed that the cosmological equation in other theories of gravity can be thought as a thermodynamic equation at the apparent horizon. Our work has important consequences in cosmology because gravitational dynamics and thermodynamics are totally different problems in physics. The connection between the two sheds new light on gravity.


***

LE11491
Electrons suffer from the heat in an ion track

A team of physicists from Germany and Brazil found that
the absorption of fast electrons escaping from hot regions
inside solids is significantly influenced by the electronic heat
- similar to long-distance runners who are slowing down or
may even stop as a result of excessive heat. In their
experiments, they irradiated aluminum and beryllium
samples with fast ions and observed the resulting angular
intensity-distribution of the ejected Auger electrons.
Such angular distributions are known to be largely
independent of the type of excitation, following roughly a
cosine law at low ejected-electron energies. A very specific
distribution of excitations, however, is produced by a highly
charged fast ion at around 10% the speed of light. Such an
ion is fast enough to favor high charge states inside solids
and slow enough to interact strongly with the electrons of
the penetrated material. Thus, it leads to extremely strong
inner-shell ionization and electronic heat of some 10000 K
inside a nanometer-sized cylinder surrounding each
individual ion path. The authors of this work have shown
that Auger intensity ratios for different degrees of inner-shell
ionization vs. angle are sensitive to the high energy-deposition
density, consistent with enhanced inelastic electron-energy
losses or electron absorption, respectively. This proves that
the transport of fast electrons is significantly influenced by
the spatial electronic excitation distribution.

***


LF11017

Imagine a device that is half submarine and half weather balloon, but shrunk down to the size of a marble. We report novel measurements of thermally driven fluid flows using just such a device: a miniature, submersible, mobile, wireless temperature sensor. This "smart particle" is carried along with the fluid, reporting temperature with a tiny radio transmitter as it goes. In this way, we obtain the first quantitative and direct measurements of how the flow transports heat from point A to point B, i.e. of the dynamics of "thermal plumes". Our results impact understanding of thermally driven fluid motions as found in the atmosphere, the oceans, the molten metal core of the Earth , or the interior of stars. We anticipate that similar devices, outfitted with capabilities for measuring other quantities (e.g. acceleration, pressure, concentration) will open new doors for investigating the physics of mixing and transport by fluid flows.



***

LK10890.
Foaming of magnetic alloy boosts shape-memory effect


Researchers at Boise State University and Northwestern University have
boosted the magnetic shape-memory strain of polycrystalline Ni-Mn-Ga
sixty-fold (from 0.002 to 0.12 %) by casting the material into a foam.
This strain is developed as a magnetic field moves crystal defects
(known as twin boundaries) in a fully reversible manner over millions of
magnetic cycles. This shape-memory strain in the foam is comparable, in
terms of magnitude and response time, to the elastic strain exhibited by
the best commercial magnetostrictive material, Terfenol D, when exposed
to a magnetic field. However, Ni-Mn-Ga foams have lower density and
contain less expensive metals than Terfenol D. Their open porosity may
lead, beyond light-weight actuators, to micro-pumps without moving parts
or magnetic refrigeration near room temperature.

The researchers believe that the strain improvement in the foams is due
to their struts which are spanned by entire grains, thus reducing the
internal constraints present between adjacent grains in non-foamed bulk
polycrystalline Ni-Mn-Ga. Thus, the foam struts are similar to single
crystals, but they are constrained at their nodes, so they exhibit lower
strains than single crystals with achieve up to 10%. Foamed
polycrystalline Ni-Mn-Ga can be easily cast by conventional methods,
unlike Ni-Mn-Ga single crystals.

***

LG12016

Liquid-liquid phase transition in supercooled silicon.

Despite the occurrence of advanced materials in recent years, silicon
still keeps the leadership in semiconductor technology. Most
technological applications begin with crystalline silicon wafers
elaborated from the melt, and amorphous silicon made from slightly
supercooled melts. The properties of normal and supercooled liquid
silicon are therefore of importance for the manufacturing process.
Unusual behavior of the density in the supercooled liquid states, as
deep as 200 K below the melting point, was observed leading researchers
to consider plausible the existence of a liquid-liquid transition at
even lower temperature, unfortunately out of reach to state-of-the-art
experimental facilities. Therefore, the absence of direct evidence from
experiments has prompted to look for it using numerical simulations. We
have proposed a new mixed approach by combining efficiently classical
and first-principles molecular dynamics, giving a strong support to the
existence of a transition between a high density liquid to a low density
liquid near 1050 K, regardless of a specific empirical interaction
model. We were able to clarify the nature of atomic and electronic
structures of both the high and low density phase, which were
incorrectly described from early classical simulations. Our finding
indicates the liquid-liquid transition is accompanied by an enhancement
of the local tetrahedral structure and is not characterized by a
semimetal to semiconductor transition.

***


LG12007

“Extinction of the Giants”

The birth secret of the carbon buckyball has now been caught on tape. The nanometer-sized soccer ball was discovered and christened Buckminsterfullerene C60 two decades ago, but the intimate details of its formation remained a mystery. One “hot giant” hypothesis was that the atoms assemble first into graphitic planes, crumpled into large distorted polyhedral boxes, but then shed the loosely bound threads and chains. Eventually only the spherical buckyballs survive, thanks to their high symmetry and stability. This hot evolution is so rapid that no one could either prove or disprove it by observation. Now it has been documented for the first time, captured on high-resolution TEM video, matched by the atomic simulations, in a new joint study by scientists at Sandia Laboratory and Rice University. Thanks to a controllable heat bath inside the 10-nm-wide nanotube, they see how initial angular “giant fullerenes”, comprised of thousands of atoms, gradually shrink and perfect their shape. Honed and polished by great heat, like a raw diamond in the hands of a jeweler, they turn into the gem-quality symmetric buckyballs. If heat is sustained, the fullerenes undergo a further shrinking, ingeniously conjectured by the C60 discoverers, and vanish entirely, giving in to the entropy paramount at these conditions. The researchers believe this reveals the fundamental stage in fullerene formation, and also suggests a way to possibly engineer the nanocages for a variety of applications.

10-15-07

LE11531
We demonstrate an on-chip, frequency-tunable single photon detector operating in
the microwave range. The device consists of a quantum point contact and a double
quantum dot, which can be considered as an artificial version of a diatomic
molecule such as oxygen or nitrogen. Just like a normal molecule it can absorb
light by an electronic transition. The main difference is that the artificial
molecule is engineered in a semiconductor material, and can easily be integrated
into a complex circuitry.

Here we combine the artificial molecule with an ultra-sensitive charge detector
based on a quantum point contact. The charge sensor will detect the
single-electron transition that follow the absorption of a photon. The electronic
properties of the artificial molecule are controlled with gate voltages, making it
possible to tune the frequency of the absorbed radiation. The device may thus be
viewed as a frequency-selective single-photon to single-electron converter
operating at microwave frequencies.

The sub-micrometer size of the detector allows it to be mounted on-chip next to
another device, which opens up the possibility to investigate radiation emitted
from other nanoscale structures. We use the technique to study photons created
when electrons scatter in a nearby conductor.

***

LG11369
Atomic Landau-Zener tunneling in Fourier-synthesized optical lattices

Interfering laser beams can hold atoms in a precise array, forming a
so-called optical lattice. The atoms here are like marbles trapped in
the potential dimples of an egg carton. Whether the atoms now can move
through such a periodic array, following the theory of quantum
mechanics, depends on the atom¿s band structure. So far, researchers
have investigated the movement of atoms only in sinusoidal shaped
lattices, as can be created by placing atoms in an optical standing
wave. On the other hand, nature provides us with quite different
potential forms for electrons in solid state crystals and it is well
known that the electron transport properties, as one observes by
measuring the electrical conductivity, here much depend on both the
spatial symmetry and the form of the crystal potential. We have
developed a technique for the Fourier-synthesis of optical potentials
for atoms, aiming at a rebuilding of the variety of potential forms
experienced by electrons in natural crystals. Variable egg cartons for
atoms are formed by combining lattice potentials of different spatial
periodicities, similarily as in the Fourier-synthesis of electrical
engineering waveforms. Our fundamental spatial frequency of half the
wavelength of the used laser light is created with a usual optical
standing wave lattice potential. The higher spatial harmonics are formed
by so-called multiphoton processes. Effectively, each atom absorbs
several photons as a whole. For the first higher harmonic, two photons
act together as a single effective ¿giant¿ photon of double spatial
frequency. The spatial periodicity of the formed egg-cartons have half
the size of a usual lattice, i.e. a quarter of the used laser
wavelength. This technique can be extended to even smaller egg-cartons
for the atoms when working with more photons in whole. By combining
lattice potentials of different periodicities, now e.g. asymmetric,
saw-tooth-like egg-cartons can be formed! When slightly shifting the
position of the different lattice harmonics against each other, also
lattices consisting of a periodic sequence of dimples, or an egg-carton
with a periodic sequence of hills can be formed. We have studied the way
atoms move through such egg-cartons of variable spatial symmetry and
shape. It turns out that the atoms move best in the
¿hill-configuration¿, while being most hindered in their motion in
dimple-type egg-cartons. The saw-tooth-like geometry results in an
intermediate behaviour. The results allow to determine the
quantum-mechanical band-structure of the atoms in the egg-cartons of
variable spatial symmetry, and are in accordance with theoretical
calculations. More in the future, tailoring of lattices should allow for
the development of novel forms of quantum matter with novel physical
properties.

***

LC11219
Light diffuses through Mie resonances with different speeds.

Propagation of light through complex media often becomes diffusive much
like the transport of heat or the process by which two gases mix. This
is a phenomenon that very often escapes our understanding but not our
perception: light scatters by fat in a glass of milk or sunlight is
dispersed through dust in air. The past decade has seen great strides
toward fabrication of artificial materials to control the propagation of
light, of which periodic iridescent structures (photonic crystals) are
the most beautiful example.

In this work we have achieved a strong control of light transport even
in presence of strong diffusion and we have observed oscillations of the
speed of the diffused light as the result of microscopic ¿Mie¿
resonances which trap and delay the light at each scattering event. At
variance with chalk our new material, we have dubbed /photonic glass,/
is composed of identical micrometric spheres all having the same
spectral features, therefore certain frequencies of the light are slowed
down while others are accelerated relative to each other. The result is
that white light diffusing through this photonic glass is transmitted in
a spread of colour.

The news is that the microscopic resonances of the single spheres
survive even if the billions of spheres are tightly packed in the
photonic glass. Disorder, which is usually regarded as an uncontrolled
weakening factor in photonic devices plays here a central role and
allows engineering of the flow of the diffused light. This experiment
can open the way to obtain novel devices based on /perfect/ /disorder/
rather than careful order.

***

LE10997
Title: Can Dark Matter be Decaying Now?

Dark matter, the so far unidentified matter that binds galaxies
together, has long been believed to be comprised of stable elementary
particles that interact very weakly with ordinary matter. However, we
have shown that the stability of dark matter is not a requirement, and
in fact cosmological observations may demand that it is not stable. In
this scenario, the dark matter in the Universe is composed of two
types of particles. The heavier of these particles is unstable and
decays into high energy radiation and a ``daughter'' dark matter
particle, which is absolutely stable. The average lifetime of the
``mother'' particle is about 10 trillion years, or about a thousand
times the present age of the Universe. Although this lifetime is long,
a small fraction of these particles will be decaying now, and the
radiation produced in the decays would leave an imprint in the diffuse
radiation background at high energies. The predicted energy of the
radiation turns out to exactly match the excess of radiation energy
seen in previous experiments. Future experiments are being developed
and will be able to definitively confirm or refute the hypothesis that
dark matter is decaying now.

***


LG11588
Biodiversity and Complex Spatio-temporal Patterns in Ecosystems

Identifying the mechanisms allowing to maintain the earth's biodiversity is
a major challenge in theoretical biology and ecology. Recent experiments on
microbial populations have shown that the existence of local and cyclic
interactions promotes the long-term coexistence of different species
through the formation of spatial patterns. Motivated by these observations,
we have recently demonstrated [Nature 448, 1046 (2007)] a crucial influence
of individuals' mobility: There exists a critical value of the mobility
below which all subpopulations coexist and arrange in evolving structures,
while for higher mobility there is loss of biodiversity. In this Letter, we
investigate the influence of mobility and noise on the spatio-temporal
patterns. We show that individuals' movement leads to a fascinating
self-organization of subpopulations in entangled, rotating, spiral waves
(see figure). The emergence of these kaleidoscopic structures stems from a
subtle interplay between the deterministic dynamics and noise. We
quantitatively characterize the spiral patterns by devising an analytical
description taking stochasticity into account. In particular, we obtain
expressions for the velocity and wavelength of the propagating spirals. Our
methods can be broadly applied, e.g. to chemical reactions, epidemic
outbreaks, or in behavioral sciences.

***

LF11705
Reversing time changes right into left.

Chirality—or “handedness’—is quite common in nature. It is a geometric property of many molecules, in particular of complex biomolecules. Chiral molecules exist in a right-handed and a left-handed form (enantiomers), which are related by reflection on a plane. For example, all terrestrial life uses only right-handed sugars and left-handed amino acids. More than 50 percent of the world's top 100 drugs are chiral, including familiar brand names such as Lipitor, Paxil, Zoloft and Nexium.

Particle physics is another domain of chirality, where it describes a kinematical feature of massless particles: the spin of a fermion can either be in the same or in the opposite direction to that of its momentum. Both in chemistry and particle physics space inversion changes left-handed into right-handed systems.

Nuclei have been considered as achiral, because their shapes are, generally, too simple. However, Frauendorf [1,2] pointed out recently that a triaxial nucleus becomes chiral if it rotates about an axis that lies outside the three planes spanned by the principal axes of its triaxial ellipsoidal shape. The short, intermediate and long axes form a screw with respect to the angular momentum vector. The left-handed configuration is converted into the right-handed one by the time reversal operation, which changes the sign of all linear and angular momenta. In contrast to molecules and massless particles, space inversion leaves the nuclear chirality unchanged. Since both chiral structures have the same energy, one expects to observe two identical rotational bands (sequences of quantum levels) of the same parity. A number of such pairs of bands have been identified which were suggested as candidates for chiral partners [3,4,5]. A small observed energy difference between the states of the same angular momentum I in the chiral partners indicates rapid conversion between the left- and right-handed configurations (chiral vibration). With decreasing energy split between the partner bands, the left-right mode changes from soft chiral vibration to tunneling between well-established chiral configurations (static chirality).

In a new experiment at Gammaphere, led by researchers at the University of Notre Dame, electromagnetic transition probabilities have been measured for the transitions in the “chiral” bands in the nucleus 135Nd. These measurements affirm the chiral character of these bands (there were some doubts expressed recently [6] about the chiral nature of the observed bands in the nucleus 134Pr because the electromagnetic transitions did not conform to expectations from chiral bands). Further, the authors report novel calculations, combining the tilted-axis cranking model (previously used to describe chiral behavior) with the well-established technique of random phase approximation (RPA), to establish that this nucleus exhibits a transition from chiral vibration to static chirality with increasing angular momentum.

***

LH10936
Quasi-bound States in Continuum in a Two Channel
Quantum Wire with an Adatom

We have discovered an unexpected behavior for electrons (which we
call the quasi-bound state in continuum (QBIC)) in the conduction
band of a nano-scale one-dimensional quantum wire constructed by
alternating layers of metal alloys attached with an impurity atom
(called an adatom). The one-dimensional nature of the device results
in an anomaly called the van Hove singularity at the edges of the
electron energy band. We have found that by attaching two quantum
wires together along their length in a ladder shape, there appears a
quasi-bound state (QBIC) with very large lifetime due to this
singularity, even though the electron is inside the conduction band.
In other words, an electron can be trapped by an impurity for a very
long time, although electrons with almost the same energy can move
freely. Moreover, the singularity allows the QBIC to exist over a
wide range of values for the impurity electron energy. This effect is
in contrast to another well-known effect called the bound state in
continuum (BIC) that has an infinitely long lifetime. Contrary to
the QBIC, the BIC exists only at special values of the electron
energy; this is because the mechanism of the BIC effect is due to the
geometry of the devise and requires the adatom to be placed at an
extremely precise location in the wire. Therefore, it may be much
easier to detect the QBIC effect in an experiment. The QBIC effect
may also be useful in the construction of a laser that operates at a
moderately large energy scale.

***

LD11015
Novel superconducting phase in a model for quasi one dimensional strongly correlated systems

The search for a mechanism of superconductivity in systems with only repulsive interactions is a central subject for the physics community since the discovery of cuprate superconductors in 1986. One dimensional interacting electron systems at low temperatures are ideal laboratories because very powerful techniques are available to deal with them. Unfortunately the simplest example, the repulsive Hubbard model at half filling, is an antiferromagnetic insulator and the superconductivity is then ruled out. In this paper we show that when the effect of charge density on the hopping of electrons along the chain (bond-charge interaction) is added to the latter model superconductivity becomes possible; interestingly, the period of the charge modulation is not a multiple of the lattice parameter. The model has a rich phase diagram which also includes two insulating phases, one with breakdown of the translational symmetry and the other with dominant antiferromagnetic correlations. The above results were obtained by means of a plethora of cutting edge analytical and numerical techniques.

***



LG11196
Ultracold Onion Rings and the Coldest Bull's-eye Patterns in the Universe
>
> We normally think of everything being absolutely frozen and
> rather uninteresting at temperatures as low as -273C. Yet,
> Bose-Einstein Condensates defy the conventional wisdom and
> produce beautiful waves and patterns at nano-Kelvin temperatures,
> usually referred to as the lowest temperatures in the Universe.
> In our work, we illustrate the robust formation of ring-like
> 2D bull's-eye patterns and 3D onion-ring like dynamics
> for a two-component system of dilute alkali gases at such
> temperatures.
>
> The principal reason for this remarkable pattern formation (see the
> attached JPEG figures and link to movies) is that the inter-particle
> interaction introduces an effective repulsive nonlinearity in the system.
> In our system of two gases (two spin states of Rb 87), these
> repulsive interactions are such that these two species "dislike
> each other more than they dislike themselves". As a result, they
> induce phase separation and form remarkably robust ultracold
> rings (in 2D projections) which mirror the 3D onion-ring structures
> of the two condensates. In our work, we present novel experiments
> in this system and develop a remarkably accurate theoretical
> model describing, in excellent agreement with the experiments,
> the dynamics and unraveling the fundamental physics of this system.
>
>
> Figures:
> --------
>
> The attached figure contains 3D renderings of the density distributions
> in a binary Bose-Einstein condensate of 350,000 atoms of Rb 87 computed
> from the model at 54, 72, 144 and 151 ms. Each component is depicted by
> a contour slice at about half of its corresponding maximal density. Red
> and green surfaces correspond to components |1> and |2> respectively.
> The bottom (side) projection corresponds to, as it is observed in the
> laboratory, the z- (x-) integrated density for the |1> component in our
> model.
>
> Movies:
> -------
>
> Movies of the interpenetrating dynamics for the binary condensate can be
> found at:
> http://www-rohan.sdsu.edu/~rcarrete/noticeboard/DH/movie_paper_iso_60_30_long.mpeg
> http://www-rohan.sdsu.edu/~rcarrete/noticeboard/DH/movie_paper_iso_50_55_long.mpeg
> The movies depict the same information as in the figures mentioned above.
>

***


LG11477

Digital photography reveals secrets of rare nuclear decays

Photographic techniques have played a key role in many
triumphs of early subatomic physics. The discovery of the
positron is among well known examples. With time, optical
methods were superseded by electronic recording and processing.
However, modern technology has enabled the marriage of electronic
and optical techniques. In this work we introduce digital photography
to nuclear physics and we study the most exotic nuclear decay mode
to date: the emission of two constituent protons from the
ground state of an atomic nucleus. Employing a new type of
gaseous detector (Optical Time Projection Chamber) we recorded
convincing and vivid images (see an attached figure) proving
that a very exotic, artificially synthesized isotope 45Fe
disintegrates by the two-proton radioactivity.
Moreover, the analysis of many such decay images revealed for
the first time the mechanism of this rare nuclear process and
shed light on the structure of the 45Fe nucleus.
This technique opens a new field in nuclear spectroscopy
but it may also find applications in other branches of science
and in education. A message conveyed by an image can be
often understood without any special technical knowledge or
scientific background.

Figure caption:
An example CCD image of a two-proton decay event of 45Fe.
A track of a 45Fe ion, entering the chamber from the left, is seen.
The two, bright, short tracks are protons which were emitted
0.6 ms after implantation of the ion.

***

LF11542
Dancing at the nano-scale

or

Teaching tiny dance steps to magnetic flux quanta


In the 1950's a Latin American dance, called the "Cha Cha Cha", became a
very popular new dance craze around the world. It originated from the
sensual and older "Mambo", which eventually gave rise to the faster "Salsa"
in the 1970's. The beat in a Cha Cha Cha song is hard to miss: You hear two
slow beats and three quick beats. These three quick beats gave the dance
its name Cha Cha Cha.

New experiments [1] have been able to get quanta of magnetic flux to follow
a variety of dance steps, including the "Cha cha cha".

In these experiments [1], an externally applied current pushes quanta of
magnetic flux (also known as vortices) inside high-temperature
superconductors. When the applied current oscillates symmetrically back and
forth (like an "ac" current, with one harmonic or one oscillating
frequency), the vortices obediently follow the imposed rhythm, and also
oscillate symmetrically back and forth. The applied current acts as the
leading dance partner, and the vortices follow the steps imposed by the
current.

Now let us consider more complicated rhythms. When the applied current (the
imposed musical rhythm) has two harmonics (i.e., two superimposed
sinusoidals), it can get vortices to produce far more interesting dance
steps [1]. Moving, on average, in any desired part of the sample, their
microscopic "dance floor".

For instance, in one case, vortices can follow these dance steps: a slow one
to the left and three fast steps to the right (i.e., acting like some sort
of diodes). See, for instance, the animation
http://dml.riken.go.jp/fluxtronics/NL-rectifier.

More interestingly, these experiments [1] have produced the first very
nonlinear and efficient rectifiers (producing a net motion in one direction
even though the force applied by the leading dancing partner is symmetric).
In one set of experiments [1], the quanta of magnetic flux follow the Cha
Cha Cha steps.

Animations illustrating these effects are available by clicking
http://dml.riken.go.jp/fluxtronics/NL-rectifier.
Many research groups are currently exploring novel ways to control the
motion of flux quanta in superconductors, because their uncontrolled motion
can create noise in superconducting devices. Their controlled motion can be
used to sculpt microscopic magnetic profiles in small devices, to manipulate
spins in nearby magnets, and to produce nano-scale step-motors. These new
methods are also applicable for controlling the motion of particles,
interacting with optical tweezers, in colloidal suspensions, ions in ion
traps, electrons in Wigner crystals, and for the separation of different
types of very tiny particles (e.g., electrophoresis) based on their response
to applied forces.

***

LG11532
Extraction of weak transition strengths via the
(3He,t) reaction at 420 MeV

By studying the (3He,t) nuclear charge-exchange reaction at 420 MeV on
target nuclei over a wide mass range, an international collaboration of
researchers from the U.S., Europe and Japan have found that very simple
relationships exist between the strength of Gamow-Teller and Fermi
excitations and the cross section data obtained via this reaction as a
function of mass number. The results fill a gap in our understanding of how
to use the (3He,t) reaction to extract Gamow-Teller strengths and put it on
equal footing as (p,n) reaction, with the additional benefit that much
better resolutions, and hence more detailed information, can be obtained
with the (3He,t) reaction.
Besides providing detailed information to understand the structure of
nuclei, the Gamow-Teller strength distributions are important in
astrophysical studies of stellar processes (in particular supernovae) and
neutrino nucleosynthesis and for fundamental neutrino studies, such as
neutrinoless double beta decay. In addition, the improved understanding of
the (3He,t) reaction are very beneficial for studies with the inverse
charge-exchange reaction (t,3He), which is used for similar purposes
The experiments were carried out at the Research Center for Nuclear Physics
in Osaka, Japan and were led by Dr. R. Zegers from NSCL and Michigan State
University.

***

LG11011
Excited ions in intense femtosecond laser pulses: Laser-induced
recombination

Electron-ion recombination in a femtosecond laser field, the final step
in the
process proposed by Corkum over a decade ago to explain the production
of
attosecond pulses, has been finally isolated and studied. In this
‘3-step’
model electrons liberated from atoms during an intense laser pulse (step
1) are
energetically driven back towards their parent atoms by the electric
field of
the laser (step 2) before recombining with the simultaneous emission of
a very
short duration X-ray pulse (step 3). The duration is so short (less
than one
million billionth of a second) that the pulses can be used to study the
motion
of electrons in an atom. This has led to a massive worldwide investment
in
attosecond pulse technology, with attosecond pulse experiments expected
to lead
to many exciting discoveries and a quantum leap forward in our
understanding of
atomic physics in the next decade. The isolation of step 3 in the
present
experiment, using an excited ion beam target and charged particle
detection
techniques, is not only an important step in this quest but also a
pioneering
development in the study of fundamental electron-ion collisions.



**


LF11310

Polymers in a Vacuum

A polymer chain such as a protein molecule in a vacuum might
seem like an unlikely idea. How could it get vaporized in the first
place without having its structure completely obliterated? But the 2002
Nobel prize in Chemistry was awarded, in part, for the discovery of how to
do this. This was a key element in doing mass spectrometry on biological
samples which has had an enormous impact on biology and medicine.

But getting the mass of the molecule may not be the only interesting thing
one can do with this technique. In this work, I found that there is a
lot of new phenomena related to the internal dynamics and structure of
polymers flying around in empty space. It turns out that they behave
very differently from polymers in solution. Instead of being heavily
damped, in many situations they oscillate as they damp out. The size of
them is substantially altered by the conservation laws that don't exist
in solution.

This work has the potential for profound impact. Probing protein's
internal dynamics, by radio waves, should give a lot more information than
just the mass, allowing for a better identification of the actual molecule.
Other potential applications include examining a DNA molecule suspended
over a lithographed trench, and polymers in interstellar space.

***

LG11442


Is the Bragg peak flat?

In cancer treatment, the clinical aim of maximizing damage in the
tumor sites while minimizing damages to healthy surrounding tissues
has lead to the use of protons and, more recently, carbon ions to
provide high spatial specific damage profiles. In particle therapy,
it is widely assumed that the radiation-driven production of highly
reactive OH radicals from water molecules is largely responsible for
the main cell damage. The planning of this type of therapy is based
on the premise that tumor death follows closely the energy loss
profiles of C-ions along their whole trajectories including the Bragg
peak, the region where the major damage is supposed to occur. The
results reported by "LG11442" challenge the assumption of placing
entire reliance on the sharpness of the Bragg peak. It is shown that
ion production from water is not as sharply localized in energy as
suggested by Bragg peak energy loss profiles. In fact, primary ion
production from water is far from being localized and is actually
relatively uniform at a region where the C-ions come to rest. This
finding could have important implications in treatment planning of
C-ion therapy, mainly when tumors are located close to critical
organs such as optical nerves.