EU10388

Record-Breaking Optimization of Packing Problems

In an interdisciplinary project between physics
and computer science, Andre Mueller, Johannes
J. Schneider, and Elmar Schoemer, three scientists from the
Johannes Gutenberg University of Mainz, Germany,
developed a computer algorithm for finding the
closest packing of goods of various shapes and
sizes. They tested their algorithm for a benchmark
problem which was recently defined in an international
competition, in which 155 groups from 32 countries
took part, some of which have been working on packing
problems for many years. With their new algorithm,
Mueller, Schneider, and Schoemer are able to
match and beat each and every world record
established during the benchmark competition.
Their results for this problem are published in the leading
journal of statistical physics (Physical Review E
volume 79, article number 021102, 2009).

Figure caption:
Example of a benchmark instance considered in the contest:
50 disks with different integer radii have to be packed
in a way that the radius of the circumcircle is minimal.

***

BZR1050

Geometry Matters.

Qubits made from superconducting circuits show immense promise for
building quantum computers, but noise currently places limitations on
their ability to process information using quantum mechanics. Although
the fundamental sources of noise in such devices are not yet well
understood, in our paper we present results revealing that the level of
low frequency noise in a large set of superconducting flux qubits shows
a clear dependence on qubit geometry. Qubits with long, narrow wiring
are systematically more noisy than qubits with short, wide wiring.
Furthermore, the presence of a shielding plane under qubit wiring also
significantly lowers the amount of low frequency flux noise. This
geometry dependence strongly supports hypotheses that implicate local
impurities in the vicinity of qubit wiring as being the source of low
frequency flux noise in superconducting qubits. These results will aid
in identifying these fundamental noise sources in superconducting
circuits and in the effort to remove them, thus enhancing
superconducting qubit performance.


***

LX11065AR

Frozen Light in a Liquid

Imagine light traveling in a material and then suddenly it stops, completely. This is “frozen” or “localized” light. Nearly 25 years of intense effort has gone into finding localized light with extremely limited success. Attempts to engineer a
material that localizes light have been cursed by strong absorption, as absorption both masks the signal that localization has occurred and limits applications. The primary tool was limited to mixing strongly scattering (but not strongly enough)
powders of various sizes and uncharacterized disorder that always had large absorption. In contrast, our paper reports finding localization in a liquid (a liquid crystal) with extremely small absorption so that the signal of localization is
unmistakable.
The difference in materials is much more than just the difference between a liquid and a powder. Our scattering mechanism is completely different. Unlike random powders, liquid crystals are ordered fluids in which weak magnetic fields and small
changes in temperature have a huge impact on the order. Modifying the order provides us significant control over the light. This control, along with very small absorption, opens new directions for research and possible applications.

***

EZ10282

Physicists and the brain

Physicists at Lancaster University have shown that symmetry,
or the lack of it, plays a crucially important role in the
function of the brain.

It is well-known that the brain operates through the activity of
neuronal cells. Brain rhythms like the famous alpha-wave result
from the synchronized activity of millions of interconnecting
neurons.

The work was motivated by a wish to understand
anaesthesia, where communication between different parts of the
brain is reduced or eliminated by chemical action. The scenario
that arises is of different groups of interacting neurons that
synchronize, both between individual cells and between groups.

The researchers used a very large computer to mimic the
neurons. They found that the neurons can synchronize in an
amazingly complex manner depending on the symmetry of their
interactions, e.g. does group A influence group B more strongly
than vice versa? Or, is group A better synchronized than group B?
They have shown that two groups of neurons can
exhibit five different kinds of synchronization, depending on
symmetry.

These results not only help to illuminate brain activity and
the mechanisms of anaesthesia. They also promise to
help explain numerous other situations where groups of
oscillators synchronize, e.g. light-flashing fireflies,
chirping crickets, and hand-clapping by concert audiences.


***

LW11297

Hollow K-shell Atoms as a Probe of Electron Correlation

The single-photon double K-shell ionization process in which the two innermost electrons are removed simultaneously from an atom was investigated. This process is one of the most sensitive probes of electron correlation effects that lie in the heart of understanding atomic structure. Yet, an accurate theoretical treatment of how electrons “feel” each other in many-electron systems still remains a challenge. A team of researchers from Switzerland-Australia-Slovenia have observed the radiative decay of double K-shell vacancy states following two-electron ejection by photon impact in Mg, Al and Si. Experiments were carried out at the European Synchrotron Radiation Facility (ESRF), France. The obtained results suggest that the post-photoabsorption electron correlation effects for neutral atoms differ from those in two-electron systems. The underlying physical mechanisms are similar and lead to a universal scaling behavior of the double photoionization cross sections. This work sheds new light on how inner-shell atomic electrons interact.


***

ly11671

A New Idea for High-Energy Photon Measurements

Nowadays advanced high-energy photon sources are of worldwide use
and more advanced future sources are also being discussed and planed
intensively. Among such activities, we propose a new theoretical idea
for
the measurements using X-rays and ultra-violet (UV) lights,
focusing on "the domain dynamics." Usually a state of the solid is
uniform in its entire spatial volume. However, when a different type
of state is also stable, we can think about a "domain," which is
defined as a spatial region of the latter state in the background of
the former original state. Our new finding is that this domain can
behave as a spatially extended particle over, for example, 10-100
A(Angstrom).
This phenomenon, which is expected to appear most prominently in
quasi-one-dimensional systems (a system with a conspicuous chain
structure),
has not yet been observed until now, and therefore we strongly
encourage such experiments to open a new possibility in this field.


***

LW10991

Atomically thin layers of carbon atoms are the basis of novel
nanomaterials such as graphene, which is a single planar layer, or
cylindrical nanotubes being rolled-up sheets. Carbon nanomaterials have
received strong interest for applications in electronics and sensor
technology. Their unique electronic properties are not still fully
understood and closely related to those of graphite consisting of a
stack of planar layers.

In our article published in Physical Review Letters, we report our
observations of the behavior of electrons in graphite in real-time. We
have mapped the electron dynamics with an unprecedented time resolution
of 10 femtoseconds (one femtosecond is a millionth of a billionth
second). Ultrashort laser pulses excite electrons into states of high
energy and map their return to equilibrium. We discern the different
characteristic steps of this process in time and determine, which states
the electrons transiently occupy. Our results clearly reveal that on
these timescales, graphite behaves much more like a semiconductor such
as silicon than like a metal. This behavior has a strong influence on
the motion of electric charge through the material, the electric
current, and, thus, may have significant consequences for future
high-speed and high-field electronic devices based on carbon.


***

ET10473

The Emergence of Biology from Chemistry:
Modularity Evolves Spontaneously in a Changing Environment

Biology is a subset of all possible chemistry, a modular subset. We show
in a general setting that modular, biological-like structure arises
spontaneously in a system evolving in a changing environment. The process
of horizontal gene transfer, by which much of natural evolution occurs, is
what drives the formation of modules. To complement this general result,
we show that protein-protein interaction networks appear to have become
more modular with the progression of evolution over the last four billion
years. We also review experimental data showing that modularity is
positively correlated with environmental variability in metabolic networks
of bacteria.

***

LT11078

Researchers demonstrate efficient coupling between light and electrons in quantum dots

Spins of single electrons are promising candidates for use in quantum information schemes, which might lead to drastic computational speed-ups and provably secure communication. Light, in the form of photons, is already used for classical telecommunication and can be harnessed for use in quantum information as well. In this work, we demonstrate a device capable of efficiently coupling these two physical systems together. In particular, we embed quantum dots, an atomic-like photon emitter, in an electrically-gated, optical microcavity. The benefits of this system are many-fold. First, electrical gating of the cavity enables us to control the charge of the quantum dot, which provides access to single electron spins. In addition, the electrical gating allows us to fine tune the emission properties of the quantum dot so that it can interact with the cavity efficiently. Finally, the design of our cavity is such that laser light can be efficiently pumped into the cavity, where it can then interact with the quantum dot. Using this microcavity, we demonstrate that the coupling between our laser and cavity is nearly perfect as well as that the interaction strength between the quantum dot and the cavity is ideally suited for quantum information purposes. In this way, light from an external source such as a laser can be made to efficiently interact with the spin of a single electron.

***

LJ11496A

Big life for a small blackbody

The thermal radiation emitted by a blackbody has a long and
distinguished story in physics. First it provided
a smoking gun of the failures of classical physics. A few years
later Planck's description was the first
success of the early quantum theory. In recent times it is a
centerpiece in broad range of problems: from the definition of
radiance standards to
the physics of the cosmic microwave background . In this paper I
provide a detailed analytical description of how this radiation
depend on the size and shape of the blackbody. I also show that
these finite size corrections can be detected experimentally with
the equipment utilized to measure the cosmic microwave background.

***

LW11400BJ

Stop or not to stop this is the question

Quantum theory is full of counterintuitive features. For instance,
as consequence of interference effects, the quantum motion of a
particle in one and two dimensional random potential is restricted to
a finite region of the space even in the case that the potential
energy
is always much smaller than the kinetic one. By contrast if the
potential is periodic a for certain energies the particle can travel
indefinitely even if the potential energy is larger than the
potential.
A natural question to ask is what happen in between the limiting
cases of periodic and purely random potential. More specifically,
for what disordered potentials the
quantum motion will remain closer to the classical one and
consequently the motion will be unbounded for sufficiently energetic
particles.
In this paper we show that this is the case only in potential with a
minimum degree of differentiability. We also investigate to what
extent these results could
be tested experimentally by using cold atoms in optical lattices.

***

LY11412

Smart heating and cooling with nanofluids.

We show that nanofluids can act as smart materials that can be switched on and off to dissipate heat efficiently or poorly. Heating and cooling are of cardinal importance to attain optimal performances in any technological device. In the past the attention of scientists and engineers has been mostly focused on the dissipation of great amounts of heat, the rationale behind that being that a high dissipation prevents overheating and thus enhances the efficiency of a device: the old good “the more powerful-the better!”. In recent times the lack of abundant sources of clean energy and the widespread dissemination of battery operated devices, such as cell-phones and laptops, have highlighted the need for a smart technological handling of energetic resources. In this paper we show that a particular class of nanofluids can be used as a smart material working as a heat valve to control the flow of heat. The nanofluid can be easily configured either in a “low” state, where it conducts heat poorly, or in a “high” state, where the dissipation is more efficient.

***

LS11196

Tuning material properties by an electric field: Novel data storage concepts

Oxygen containing materials cover a wide range of properties and allow
for various innovative applications. These depend on fine peculiarities
of the material’s structure at the atomic scale. In this paper, we
outline that even at room temperature the crystalline structure can be
tuned by means of an external electric field. In particular, a change of
the electronic state of titanium atoms in strontium titanate is
observed. Theoretical modeling proves the experimental findings to be
caused by atomic rearrangements. The electric field can switch electrons
from being bonded to oxygen or not. Hence, magnetic ordering by
interaction of these electrons could be triggered. Novel concepts for
data-storage and sensing applications at the nanoscale become
conceivable. Thereby, it is of special interest, that not only the
magnetization direction could be used, but also the magnetism itself
could be switched on and off. In comparison to a binary code, this
allows for more complex states in data storage technology resulting in
higher information densities.

February 12, 2009

AZ10403


Steering and splitting light with light

In this paper we give a simple method to amplify and steer a beam of
light inside a laser and to split it
into two separate copies by using light itself. The method consists in
feeding a small
fraction of the beam back into the laser at a slightly shifted position.
This type of feedback
can be easily implemented and is conceptually similar to feedback with
temporal delay
which occurs not only in different fields of physics but also in biology
and engineering. The
signal split shown in the picture is rather surprising, as it is not
produced by an external
optical element and shows more symmetry between left and right than the
device itself (in
which the feedback removes the symmetry between left and right).
Interestingly from the
standpoint of applications, we show that these properties are robust
with respect to noise.
The reported features make the spatially shifted feedback an appealing
tool for applications
such as optical beam routing and multiplexing.

***

LV11028

For Solutions, Size Doesn't Matter.


Our recent work shows that even particles in suspension, if they have
the right properties, can act like dissolved molecules. Everybody knows
that you can dissolve sugar in water, and if you cool the solution,
sugar crystals will precipitate out; heat it back up, and the sugar will
redissolve. Everybody also knows that you can shake a suspension of fine
particles in water and they will remain suspended for a while, but
regardless of temperature, they will eventually fall out. But how are
suspensions of particles different than solutions of molecules; is it
simply a matter of size? We studied a suspension of nanoparticles, 8 nm
in diameter, and showed that their suspension ability was thermally
reversible like a sugar solution; the suspension was a solution too! Our
nanoparticles are a lot bigger than molecules yet they have many of
their key attributes in that they all have nearly the same size and they
interact with each other weakly.

February 11, 2009

BZ10577

Microwave-induced Zero-Resistance State is not Necessarily Static

A few years ago, a novel zero-resistance state was discovered when a two
dimensional electron gas, placed in a moderately strong perpendicular
magnetic field, was irradiated with microwaves. Soon thereafter, a
phenomenological domain model was proposed, suggesting that, with a
microwave power strong enough, the sample would break into domains where the
local electric fields and currents are non-zero. The domain walls would tend
to arrange themselves so that the average electric field along the sample is
zero even when there is a net current flow, leading to the observed
zero-resistance phenomenon. What we set out to explore in this paper is
whether such domain phases are characterized by static states only, or
whether nonstationary domain patterns might be possible, as well.
Studying a system on ring and on a torus, we found that inhomogeneities in
electron density and, therefore,in the Hall conductivity, might well render
some of the domain states nonstationary. For the case of density varying
linearly along the radial direction of a Corbino ring, we have shown
explicitly that a periodic time-varying state exists, we have computed its
period, and we have suggested an
experimental setup that might make it observable. On a torus, with a simple
sinusoidal density variation, we have found the numerical evidence for the
existence of periodic states, under certain conditions, and have seen that,
as is the case with the periodic states on the Corbino ring, the period is
inversely proportional to the average density gradient

***

LQ11808

Forcing a frequency split

Phase locking or synchrony has been a ubiquitous phenomenon
found in fields across science. Two different oscillators with mutual
coupling
can show a common frequency of oscillation if the strength of coupling
is appropriate. But no real system is free of the influence of external
noisy forces.
But what can an external force do if the system is already synchronized to
a common frequency? After all they are coupled strongly and hence
synchronized!

We show in our Letter that the frequencies of two such synchronized
oscillatory
systems can fall apart if they are subjected to even a shared (i.e.
common) external
input. We showed this also in a larger network of coupled systems.
This can have ramifications in coupled systems found in physics, chemistry
and biology where synchrony is important among dissimilar oscillators.
Our specific motivation comes from the neurons in the brain where
synchrony is
usually bad and asynchrony is usually good. (In Parkinson's disease
patients, motor
function neurons are more synchronized, but in normal humans they are
asynchronous.)

Our studies indicate that by increasing the frequency (equivalently the
strength)
of the random but common input received by the coupled neuron model, the
neurons
which were in a synchronized state, can now display different
frequencies of oscillation.
Thus the input can take the system in and out of synchrony.

We used neuron models both specific to motor function nuclei as well as
more generic
Hodgkin-Huxley model. Two such neuron models are coupled to achieve
spike-to-spike
synchrony, hence they exhibit identical frequency of oscillation (i.e.
firing rate).
Random external forcing is turned on in the form of inhibitory input
given identically
to the two coupled neurons. (Results are qualitatively similar if
excitatory inputs were
considered.) The synchronized regime now shows asynchrony. This state is
characterized by
frequencies of the two neurons now becoming different. The reason this
happens is that
there can always be a slight phase difference between the phase-locked
neurons. In coupled
chaotic oscillators, this is similar to a lag-synchronization. This
phase difference offers an
opportunity for the oncoming input to selectively suppress one of the
neurons' response.

We think that this may be helpful in understanding mechanisms of the
motor function disorders like
Parkinson's disease where in the disease state the network activity is
reduced due to dopamine loss.
We attribute this decrease in activity resulting in more synchrony
because now the strength of
the input is reduced leaving the system in synchrony.

We expect that lag-synchronization or phase-locking with a
phase-difference found in several
physical and biological systems could open new avenues to consider the
effect of such inputs.
It would be interesting if such asynchrony is indeed found in these models.

***

LW11428

Pushing the boundary of the glass transition

Hard sphere assemblies constitute the simplest model to tackle a
variety of fundamental questions in condensed matter physics. In
particular, they have been frequently used as model systems to
understand the formation of amorphous solids, or glasses. However,
previous experiments and numerical work reported contradictory
behaviors and distinct possible locations for the glass transition.

By extending previous measurements of the microscopic dynamics of a
dense assembly of sub-micron colloidal hard spheres by more than 2
decades in relaxation times, we have shown that the most often quoted
location for the glass transition close to a particle volume fraction
of about 58% is not there. We find that the glass transition only
occurs at a much larger concentration, close to random close packing,
the maximum achievable packing fraction for a disordered assembly of
spheres, near 64%. We discover a new dynamic regime at large packing
fraction, which shares strong similarities with the activated
dynamics of molecular liquids close to the glass transition.

It remains to be established whether the newly found location of the
colloidal glass transition simply coincides with random close
packing, or whether it represents a distinct thermodynamic phase
transition towards a true glass state.


***

LZ11540

Building a Quantum Computer One Atom at a Time

We show theoretically that alkaline-earth atoms - atoms in the second
column of the periodic table - are ideal building blocks for an
experimentally realistic quantum computer.

If ever built, quantum computers will be incomparably more powerful than
their classical counterparts and will significantly improve human
existence by solving pressing problems in fields ranging from physics to
chemistry to medicine. While precise control over small quantum systems
has already been demonstrated, it still remains questionable whether a
quantum system that is large enough to perform a useful calculation can
really be controlled experimentally to a sufficient precision. However,
the daunting task of controlling a large quantum system can be
simplified by breaking it into a set of coupled smaller identical
systems called quantum registers. In this paper, we show that individual
alkaline-earth atoms are ideal candidates for these small quantum
registers. We demonstrate that the astonishingly high degree of control
over these atoms recently achieved experimentally in the atomic clock
community can be immediately harnessed for building a quantum computer.

***

LX11301

Swimming Magnetic Snakes: self-assembled, self-propelled and ... furious

Researches from Argonne National Laboratory, Illinois, discovered a new
type of magnetic micro-swimmers (magnetic snakes) with the mechanism of
locomotion not having a direct counterpart in a living world. Like in a
science-fiction novel, the snakes form spontaneously on the surface of
water (self-assemble) from a dispersion of magnetic micro-particles
without human intervention. The energy for swimming comes from an
alternating magnetic field created by a large coil. Unlike a real
snake, magnetic snakes have two tails pumping water in opposite
directions. Swimming occurs when the flow from one of the tails exceeds
the flow from another tail, leading to a net propulsion force. The
self-assembled snakes often exhibit behavior normally thought to be
characteristic of living creatures, such as "hunting" and "feeding": the
snake often crashes into another snake and absorbs magnetic particles
into its own body. The question how biological organisms propel
themselves in air or water is attracting enormous attention in the
scientific community. The interest is excited by the need to model
locomotion of biological objects such as fish, birds, or bacteria. In
addition, there is a growing technological demand for the design of
artificial swimmers, capable, for example, of delivering useful
micro-parcels in miniature medical devices for express diagnostics and
analysis.

***

LZ11049

Gold Ion Interferometer

This paper describes a two-source interferometer consisting of two
relativistic gold ions. When two ions encounter each other, either ion
may emit a photon, which can interact with the other, producing a rho
meson in the target nucleus. These two possibilities are
indistinguishable, and, due to the symmetry (parity) of the system and
the meson, the production amplitudes at the two separate sources cancel
out (destructively interfere). This paper reports on the observation of
that interference by the STAR detector at RHIC.

The rho mesons decay almost immediately to two charged pions, which
travel in opposite directions. Since any rho production is
simultaneous, the two sites cannot communicate before the rho decays;
both the production and the decays must occur independently.
Interference occurs later, after the wave functions from the two sites
overlap. Interference is occurs between identical final states.
Since the likelihood of the decays proceeding identically is tiny, the
observation of this interference demonstrates that the system wave
function retains amplitudes for all possible decays, long after the
decay occurs; particle decay does not cause wave function collapse. The
collapse occurs later, most likely when the pions interact with our
detector. This is an example of Einstein-Podolsky-Rosen paradox.

***

BZ10780

Simple metal is not so simple after all - superconductivity in
lithium

Lithium, the lightest metallic element and arguably the simplest metal,
goes through rather complex structural transformations upon cooling or
when compressed, and remarkably, becomes a superconductor under high
pressure.
In this work, structural phase transitions and superconducting
properties in three phases of lithium ("9R", "fcc" and "cI16")
are investigated by means of first principles calculations.
In particular, it is shown that significant slowdown of atomic lattice
vibrations accompanies a pressure-induced fcc to cI16 transition and
strongly enhances the coupling of electrons and lattice vibrations
("phonons") preceding this structural transformation.
The strong electron-phonon interaction is a key to raising the
superconducting transition temperature.
The estimated superconducting transition temperature in the fcc phase
increases with pressure until the transformation to cI16 occurs and is
significantly reduced in the cI16 phase, in agreement with the trend
observed experimentally.
This variation of the transition temperature as a function of pressure
is explained in terms of the number of phonons with frequencies that are
most effective in boosting superconductivity.

***

BY10533

All that glitters is not nano: conductivity at the polar surface of
doped semiconductors

In the age of nanochemistry at the atomic scale, the silicon-rich
(001) surface of cubic SiC has received considerable attention due to
“a hydrogen induced surface metallization, by controlled creation of
spatially localized atomic defects, extending over large areas of the
surface” [M. Wilson, Phys. Today, 56, 18 (2003).] The interpretation
of partially filled states, observed at the conduction band edge, as
a row of dangling bonds [V. Derycke et al., Nature Mater. 2, 253
(2003)] seemed very surprising though, for hydrogen had been known to
saturate dangling bonds and not create them. However, since the
phenomenon has been characterized experimentally in an excellent and
exhaustive manner, testing of various models, obtained by first
principles calculations, is possible. We show here, that all the
observations can be accounted for by a completely saturated surface.
It is exactly the passivation of surface states which restores flat
band conditions, allowing free carriers to accumulate near the
surface due to the polarization field of the compound semiconductor.
This can be observed as "metallization" after hydrogenation.

February 6, 2009

LY11410

Solidification velocities in deeply undercooled silver

How fast a liquid-solid interface can move in a deeply undercooled liquid in pure metals? Despite decades of research on this problem, we still do not have a definite answer. Limited experimental results show that it moves on the order of 100 m s-1. Indeed, it moves so fast that even quenching a pure metal into a deeply undercooled state becomes difficult. In this work, we use a femtosecond laser to melt a thin surface layer (~20 nm) of Ag. We are able to quench the melt in a controllable manner to temperatures as low as 0.6 Tm (Tm – melting temperature). Using third-harmonic light generation as a probe, we measure the solidification velocity as a function of temperature. We have found very interesting results. The velocity does not increase indefinitely with undercooling; it reaches a maximum (~ 75 m s-1) at about 0.85 Tm and then decreases very slowly with decreasing temperature. By comparing the results to different theoretical models, we show that a small energy barrier is likely to exist for atoms moving across the interface. The experiments also agree with the velocities measured by molecular dynamics simulation.

***

BY10656

How carbon atoms interact in graphene

The way atoms in a material interact with each other, determines the
properties of the material. This paper presents a model potential
that gives the interaction between carbon atoms in graphene, the new
wonder material with potential for powerful new devices. The
potential has been constructed to correctly reproduce some of the
most basic structural characteristics of graphene such as elastic
constants, cohesive energy, lattice constant, and phonon spectra.
This lends credibility to the proposed potential. The potential is
expressed in terms of analytical functions, which makes it convenient
for structural and thermodynamic calculations, and atomistic
simulations. This work should be a valuable resource for a variety of
nanotechnology related atomistic simulations of graphene. Earlier
work in the field consists of numerical ab initio calculations. Ab
initio methods are, in principle, more accurate but computationally
extensive, which may not be convenient for many applications.

***

LZ11673

Type-1.5 superconductivity discovered

Richard Feynman was once asked by his host, Mrs Bethe, "what would you like to drink - tea or coffee", and he answered "both". "Surely you're joking, Mr. Feynman!", replied then Mrs Bethe.
In fact, what are 1.5-superconductors- are they type-1 or type-2 ? The answer is ..."both"!

We are reporting on the discovery of a totally new type of superconductivity, which we coined "Type-1.5 superconductivity". All superconductors known up to now are either type-1 (characterized by attractive vortex-vortex interaction) or type-2 materials (repulsive vortex-vortex interaction). Remarkably, we have discovered a novel superconducting state in the clean two-component superconductors (MgB2 is the most typical two-component superconductor; others are among new superconducting oxypnictides, etc). Remarkably, in this case we can have for the first component of the order parameter type-1 conditions, while for the second component of the order parameter we can apply type-2 conditions. This "type-1.5" superconductivity is a totally new state which combines the better of the two worlds (type-1 and type-2 simultaneously) in the same single material.

The vortex matter in these type-1.5 superconductors behaves in an extremely unusual way: combination of the vortex-vortex repulsion and attraction in the same material leads to the appearance of the exotic novel vortex patterns: gossamer-like vortex arrays with vortex voids and chains of vortices surrounding them, vortex stripes with a denser vortex array combined with simultaneously present Meissner (vortex free) stripes next to them.These novel vortex patterns have been directly visualized by us in magnetic decoration experiments on high quality single crystals. Moreover, analytical modeling and molecular dynamic simulations of the vortex patterns are in a good agreement with our experimental data.
Besides great fundamental importance, combination of the best features of both type-1 and type-2 superconductivity in a single type-1.5 material has also potential for new applications which cannot be realized by using neither type-1 nor type-2 superconductors. Interestingly, exotic vortex patterns, based on the co-existence of short range repulsion with a long range attraction in type-1.5 superconductors, have many features in common with the similar patterns (stripes, bubbles, labyrinths, gossamer patterns) in Langmuir monolayers, gels, water-oil mixtures, magnetic films, etc.

***

LX11045

Chemical Tuning of Graphene: A Road to Advanced Electronics

Lewis acid character of Boron which allows charge transfer can be
exploited to achieve intrinsic room temperature half-metallic behavior
in graphene based nano devices upon chemical modifications of hydrogen
passivated zigzag edge graphene nanoribbons (ZGNRs). In this era of
miniaturized electronic devices, graphene has opened a new dimension of
device fabrication. Experimental sofistications in both top-down
(micromechanical cleavage or lithography) and bottom-up (synthetic
chemistry) approches ensure the realization of such systems and their
device applicabilities. In this letter, we have studied edge-passivated
ZGNRs of various widths with chemical dopants, Boron and Nitrogen,
keeping the whole system isoelectronic. Doping concentrations and dopant
positions regulate the electronic structure of the nanoribbons. Our study
reveals that the zigzag edge Boron Nitride nanoribbons with terminating
polyacene unit exhibit half-metallicity irrespective of the ribbon width.
This property is sustained even in presence of large electric field at
room temperature. A careful look at the spin density profile shows the
Lewis acid character of Boron atoms which drives charge transfer from
adjacent Carbon to Boron atoms creating a potential gradient across the
ribbon width giving rise to half-metallicity. Thus our study shows new
inroads to design room temperature half-metallic materials exploiting
some basic concepts like Lewis acidity, charge transfer and edge states,
opening a huge possibility in spintronics and memory storage device
applications.


***

BZR1082

Earth's core as hot as the Sun

The core of the Earth is a ball of solid iron with a radius of 1220
km, surrounded by
a shell of almost pure liquid iron which extends up to 3480 km from
the centre.
The temperature of the core is unknown, but this is a fundamental
parameter in the
building of any thermal model of our planet. The presence of a solid/
liquid boundary
(called the ICB) makes it possible to estimate it indirectly: since
the core is mainly
formed by iron, then the melting temperature of iron at the pressure
of the ICB gives a good
indication of the temperature of the core.
In this work I have used the formulation of quantum mechanics known as
density functional
theory to perform simulations of solid and liquid
iron in coexistence, including nearly 1000 atoms in the simulation
cell, and obtained a
melting temperature at ICB pressure of 6390 +- 100 K. This result
supports earlier
density functional theory calculations based on free energies, and
confirms that the
centre of the Earth is as hot as the surface of the Sun.

***

LU11652

Society benefits from behavioral diversity

Cooperation is essential in every society, but puzzling from an evolutionary
perspective. In this work, we address the role of behavioral differences –
ubiquitous among Humans - on the evolution of cooperation. We study a model in
which individuals can either cooperate or defect. They engage in a social
dilemma of cooperation, interacting along the edges of a complex network. The
structure of the network changes in time, as individuals regularly engage in new
interactions while abandoning old ones. Social interactions may be long or
brief, depending on the individuals involved. When dissatisfied, some
individuals will try to break contact as soon as possible, whereas others will
remain in touch. We show that cooperation blooms – and society as a whole
benefits – the larger the behavioral diversity in responding to unwanted
interactions. These results support the idea that diversity, on a grand scale,
is instrumental in shaping us as the most sophisticated cooperating entities on
this planet. Mathematically, we show that taking explicitly into account the
feedback between co-evolving mechanisms – here network topology and individual
strategy and behavior - profoundly affects the outcome of those processes, a
result which has widespread consequences in many problems of natural and social
sciences.

February 4, 2009

LX11785

Atomic Resolution Images of a New High-Temperature Superconductor

This paper presents the first atomic resolution images of superconducting
gap structure and quantized magnetic vortices in a new iron-arsenide high
temperature superconductor. High temperature superconductivity, discovered
first in the cuprates in 1986, has resisted attempts at explanation and
application for over two decades. The recent discovery of a second family
of high-Tc superconductors, the iron arsenides, has generated
extraordinary excitement and raised hopes for progress on both fronts.
This paper employs scanning tunneling spectroscopic imaging to study a
single crystal iron arsenide superconductor at magnetic fields up to 9
Tesla, comparing both superconducting spectra and vortex pinning to prior
results from cuprates. The observed superconducting gap of ~6 meV is
spatially inhomogeneous but everywhere indicative of strong coupling. A
static disordered vortex lattice at 9 T, uncorrelated to the locations of
surface impurities, demonstrates that vortices are strongly pinned in the
bulk of this material, supporting the optimism that iron-arsenides may
prove technologically tractable.

Attached figure shows 100 nm atomic resolution image of the surface
topography with sparse single-atom impurities (a), and simultaneous 100 nm
image of quantized magnetic vortices (broad blue depressions against a
yellow background) at 9 Tesla (b).

***

LX11367

Extracting the free energy of kinetic intermediates

Crook's fluctuation relation, discovered in 1999 and ever
since widely used in biophysics to compute the free energy of
formation of molecules such as DNA, RNA, and proteins, is now even
more useful than before, as its conditions of applicability have been
substantially broadened. The classical recipe to measure free energy
differences goes as follows: take a molecule, hold it tight by the
ends, and pull until you break its native (i.e., the one found in
nature) structure, meanwhile measuring the amount of energy you are
expending in the process. Then progressively relax the tension,
allowing the molecule to reassemble itself, and measure the energy
thus released. Repeat many times following the same protocol. The
remarkable relation discovered by Crooks makes it possible to convert
these observations into a reliable estimate of the free energy
difference between the initial and the final states of the molecule.
In our paper, we show (both theoretically and experimentally) how the
same methodology can be applied to find the free energy of any
intermediate state, not necessarily the extrema of the pulling
process. The foreseeable applications to the study of misfolded states
(responsible of many diseases, e.g. the Parkinson's) are most exciting.


***

LS11817

Probing the Planck Length via Resonant Neutrino Transitions

The Planck length L ~ 10-33 cm whose
reality or role in nature is mysterious, is a
tantalizing doorway to a theory of quantum gravity
because it involves the universal constants of
Newton and Planck. It may be possible to
experimentally determine its presence following the
suggestion that L sets a limit on the energy width
of a long lived nuclear state, overriding that
expected from the uncertainty principle of quantum
mechanics. Neutrino resonance transitions 3H->3He
may offer sufficient sensitivity because a
conceptual breakthrough indicates that the
antineutrino line from 2-body decay of tritium in
crystals can be emitted with natural width via
motional averaging by lattice vibrations. As a
result, the energy precision is extremely high,
delE/E~10-29, in the range for testing L. The
resonance probability is enhanced as well by many
orders of magnitude. Using the method of time-
filtered resonance the quantum energy width can be
measured independent of solid state interactions. A
discrepancy of the measured width of 3H relative to
that due to time-energy uncertainty, a physics
crisis by itself, can be attributed to the Planck
length. That would presage the transition quantum
mechanics to quantum gravity at ultra-small
energies, reminiscent of classical to quantum
physics in the regime of atomic dimensions.

***

LW11008

Co-opting Noise for Computation

As computational circuits and systems shrink the effects of noise become
more central to their operation. Current approaches towards addressing this
issue have sought to suppress noise to enhance function. A counterpoint to
such avoidance of noise is the phenomena of stochastic resonance (SR) where
nonlinear systems cooperate with optimal noise levels to enhance small
signal detection. Over the years SR has been exploited in many physical
systems and devices and even detected in the natural function of biological
systems. The surprising results of this research are that SR can play a
central role in the enabling and enhancement of digital computation. The
authors have shown that the response of nonlinear systems to two input
square waves, for nonzero, optimal noise levels, can be a logical operation
on the two inputs. Such logical operations or logic gates form the basis of
all digital computation and computers. It is shown that as the noise
increases in such nonlinear systems the system response co-opts the noise to
"create" or transform itself from noisy nonlinear system into a variety of
logic gates.

The observations of this paper lead to the design of morphing logic gates.
In addition to implementing a logic gate in the optimal window of noise, the
system can also switch the logic response by changing the nonlinear transfer
characteristics of the system, for a given noise-floor. Specifically we have
shown the direct and flexible implementation of the fundamental logic gates
NOR and NAND in an optimal band of noise, from which any universal computing
device can be constructed. One can switch between these logic functions by
adjusting the nonlinearity either directly, or via a controlled dc input
signal; in effect, the nonlinearity becomes a logic response controller.
This procedure is tantamount to using the nonlinearity as a "knob" to tune
the system to select different logic truth tables. In effect, we are able to
obtain the most basic ingredients of general-purpose hardware that has
potential for reconfigurability.

In summary, this work opens up the possibilities that noise can be exploited
to morph or transform nonlinear systems into and between logic gates. This
counterintuitive result may play a central role not just in nonlinear
computational systems such as chaotic computing systems but in conventional
computational architectures that cannot avoid or suppress noise due to small
size and congested computer architectures.


***

EY10250

Pharmaceutical Protein-Ligand Affinity Computation

A new computational method was developed to calculate ligand absolute binding
affinities for pharmaceutical target proteins within error of practical useful
accuracy of 1 kcal/mol. A force field formulator for organic molecules was
developed to assign bond parameters to arbitrary organic molecules in a
unified manner including proteins and nucleic acids. With the unified force
field parametrization we performed massively parallel computations of absolute
binding free energies for a protein and ligands, using a basic physical
equality of nonequilibrium work distribution with the free energy difference
between the thermodynamic states. There are important requirements for
accurate calculations. The first is a well-equilibrated bound structure
including the conformational change of the protein induced by the binding of
the ligand. The second requirement is the convergence of the work
distribution in the massively parallel computation. Finally, the most
important requirement is the accurate force field parametrization.


***

LZ11389

Initial Details don't matter for Quantum Evolutions

Why can one produce a weatherforecast for tomorrow without knowing all
positions and momenta of all particles in the atmosphere today?
Because there is a typical evolution of the gross parameters like
pressure, wind velocity, temperature etc., the details of the initial
state are practically irrelevant. It took scientists ca. 100 years to
figure out that this holds true on the classical side, e.g. in gas
kinetics. The major step here was the clarification of how the
Boltzmann equation and the Navier Stokes equation (basis of a
weatherforecast) may be viewed to follow from the laws of Newton.
In our paper we demonstrate such a typicality (independence of initial
details) of the evolution of gross parameters of quantum states in a
quite general sense, i.e. not limited to gas kinetics, etc., from the
Schroedinger equation.


***

LX11568

Beam Echo as a Source of Powerful Radiation

The echo effect has been known for many years in various fields of physics,
e.g., spin echo in solids, photon echo in solids and gases, echo effect in
plasma. A medium that exhibits echo is characterized by excitations which decay
in time due to dephasing of different components of the excitation without
involving true dissipation processes or diffusion. The memory of the initial
excitation is actually kept in the media and, with a special arrangement, can be
recovered at a later time. An important feature of the echo mechanism is that
the frequency of the echo signal can in principle be different from the
frequency of the original excitation. In this paper we propose to introduce a
microbunching in an electron beam at a laser frequency with a help of a tuned
undulator, and then recover it downstream using the echo technique at a much
higher frequency. We demonstrate that with this method one can up-shift the
laser frequency by one or two orders of magnitude. Such a microbunched beam can
serve as a new powerful source of radiation in extreme ultraviolet or even soft
x-ray range of wavelengths.

january 29, 2009

LX11599

A New Look at Proton Conduction in Oxides

The conduction of hydrogen ions, or protons, is the underlying
mechanism behind the key technologies of hydrogen production, storage,
and energy conversion. In this study we have found that the proton-
tunneling rate - a key component of proton conduction - is closely
related to the vibrational dynamics of the hydrogen in the host
material. We find that when the proton's vibrational motion is
excited by infrared light of a specific, resonant frequency the
tunneling rate is increased dramatically. This colossal enhancement
could potentially improve proton conduction in important clean energy
devices such as fuel cells where a proton-conducting electrolyte
permits the migration of hydrogen ions and forces the electrons to
drive an electrical load such as a motor. Such an improvement in
conductivity would allow a lower operational temperature, a wider
choice of materials, longer cell life, and improved device
reliability. Our study presents a new method for observing proton
tunneling in solids and will be a starting point for future
investigations, both experimental and theoretical, relating proton
conduction mechanisms to hydrogen vibrational dynamics.


***

EU10299

New results for critical indices of fluids.

Fluids consist of a network of interacting particles on the microscopic level. In physics and chemistry it is a challenge of key interest to understand and determine the properties of such many-body systems as accurately as possible. Especially interesting are the properties close to the critical temperature and density, where determining these properties is a demanding task. The thermal properties in terms of temperature and density close to this point are expected to have so-called scaling behavior, which mathematically can be expressed through critical indices or exponents.

In this paper, a new method has been applied to obtain new values for the critical indices of fluids. Two accurate and related liquid state theories have been unified. Then, through the analyzes a new relation between the critical indices was found. Also, it was found that the critical index for the critical isotherm should be an integer odd number. Taken together, the critical indices became simple fractions and integer numbers. The novelty of this result is the simple numbers obtained for the indices. However, on one hand these numbers deviate somewhat from previous estimates, while on the other hand it is not ruled out that they are exact. Thus a central question is: Are these new numbers inaccurate, or is there need to modify previous estimates based on earlier analysis, simulations, and demanding experiments?


***

LZ11710

*Lane formation in driven systems: From pedestrian zones to complex
plasmas*

The formation of “particle lanes” is a ubiquitous phenomenon occurring
in nature when two different species are driven against each other.
When the driving forces are strong enough, different particles exhibit
a remarkable self-organization – they start moving collectively,
forming interpenetrating “stream lanes”. The phenomenon – which is
commonly known from pedestrian dynamics in highly populated pedestrian
zones – occurs in very different systems of driven particles, ranging
from colloidal dispersions to molecular ions. In terms of the
individual particle dynamics, complex plasmas bridge the realms of
classic fluids and colloidal suspensions, and therefore can provide us
with invaluable insights into the new dynamical regimes of laning.
Recently, lane formation was studied in experiments performed on the
International Space Station using binary complex plasmas. By combining
the experiments and particle-resolved Langevin simulations, the
dynamical onset of laning was investigated. Furthermore, based on the
anisotropic scaling index analysis that is exceptionally sensitive to
symmetry changes occurring in particle ensembles, a universal order
parameter was proposed for the lane characterization. The use of such
an order parameter could be very useful for studying the onset of non-
equilibrium phase transitions occurring in various driven systems.

***

LV11730

Feeling photon forces with optical tweezers

Optical tweezers is a phenomenon where highly focused laser light
results in micron scale objects
(microbeads, biological cells, and even single proteins) being suspended
at the laser focus.
However, optical tweezers can also be used to feel the force that comes
from the momentum transfer
of photons. The photon as the quantum of the electromagnetic field also
acts as a particle that carries
momentum which it can impart on to any object. Such a feat is realized
in this work, where an
optically trapped bead covered with nano-sized silver islands is excited
by the laser in the presence
of probe molecules. The number of inelastically scattered photons from
the molecules is greatly
enhanced due to coupling to the metal. The result: the emitted photons
transfer momentum which
pushes the trapped bead off its equilibrium. The recoil forces are in
the range of 100 femtoNewtons
which is comparable to forces as tiny as the gravitational attraction
between two small glass spheres.
The technique demonstrates the use of optical tweezers as an alternative
measure for quantifying
light by relating it to mechanical force and can be applied to all forms
of spectroscopy.



***

LP11356B

Is disorder that messy?

Optical quasicrystals are structures with a modulation of the
dielectric function which
does not show any translational periodicity. These kinds of structures
are furthermore
typified by a high-order rotational and mirror symmetry. We
demonstrate that the optical
properties of quasicrystals find their origin in the optical behavior
of translationally
ordered crystals. In fact, it is possible to identify sub-structures
of the quasicrystal
which, arranged in a translational configuration, reproduce the same
transmission spectrum
of the quasicrystal itself. A number of questions arise then from this
finding: do quasi-
disordered structures differ that much from ordered ones? Can we still
assume that high-
order rotational symmetry of quasicrystals causes their typical wide
optical gaps? In order
to answer these questions we have analyzed the quasicrystals showing
the highest rotational
order known so far, namely 12-fold rotational symmetry. Our results
demonstrate that some
sub-parts of these quasicrystals, typically super-lattices with the
same local rotational
symmetry of the quasicrystals, show the same zero-transmission regions
of the quasicrystals
themselves. Hence, we can safely conclude that the typical wide optical gaps of
quasicrystals do not originate from their high rotational symmetry but
from the optical
properties of super-lattices. Moreover, in view of this finding, we
should also reconsider
the general misconception of quasicrystals as structures with exotic
optical properties."


***

LZ11271

Electrons with opposite spins move in opposite directions

In one dimension, there are only two ways to move: left or right. This
leads to some peculiar properties for one-dimensional systems on the
atomic scale. In our paper we present a one-dimensional conductor
forming on a bismuth surface, which effectively separates the
electrons going through it according to their spin, a kind of rotation
around the electron's axis. It turns out that electrons going to the
left have exactly the opposite spin as electrons going to the right.
Such a situation could have useful applications in the field of
spintronics, a novel type of electronics which is based on the
electron's spin rather than its charge and which could lead to more
effective computers or even quantum computing. The state reported here
is in several ways similar to so-called edge states appearing in the
recently discovered quantum spin Hall effect but instead of being
found for a sandwich structure of semiconductors at very low
temperatures, it is found on a simple, clean surface, is truly one-
dimensional and, most remarkably, even exists at room temperature.

January 28, 2009

LW11682

A New Schrodinger Equation for Hadron Physics


One of the triumphs of theoretical physics of the twentieth century
was the development of Quantum Electrodynamics (QED), the fundamental
theory of electrons and photons. QED not only describes the physics
of the atom with extraordinary precision, but also the basic
properties of the electron itself. The corresponding problem in
particle and nuclear physics is to accurately describe the structure
and interactions of hadrons, such as the proton and neutron, in terms
of their fundamental constituents: the quarks and gluons of Quantum
Chromodynamics (QCD). QCD is much more complicated to solve than QED
because of the strong interactions of the confined gluons and quarks.
The most successful theoretical approach thus far has been to employ
lattice gauge theory computer simulations. In this paper we derive a
new quantum mechanical bound-state equation of quarks and gluons which
has many similarities with the Schrodinger wave equation for atomic
systems in QED. The bound-state solutions of this single-variable
relativistic wave equation give a very good representation of the mass
spectrum and wavefunctions of hadrons for general spin and internal
orbital angular momentum. It thus serves as an excellent first
approximation to QCD which can be systematically improved. Our
derivation is based on two remarkable theoretical developments: (a)
Maldacena's AdS/CFT correspondence between solvable gravitational
theories in a curved higher dimensional space-time (Anti-de Sitter
Space) and quantum field theories in ordinary physical space-time;
and (b) "Light-Front Holography", which allows one to map information
in the fifth dimension of AdS space to hadronic wavefunctions
describing the separation of the quark and gluonic constituents - not
at a fixed time, but at a time set by the front of a light wave.
Because of light-front holography, the description of hadrons obtained
from the new light-front Schrodinger equation is equivalent to the
solutions obtained in AdS space. The AdS representation of a proton in
light-front QCD is illustrated in the figure. For example, a proton
with its three quarks close together corresponds to the boundary of
AdS space at large circumference; conversely, a large-size proton
with far-separated quarks is represented at the inner sphere in AdS
space.


Figure caption: AdS representation of a proton in light-front QCD


***

BX10891

Verwey transition explained

The Verwey transition between the low-temperature charged ordered (CO) and the high temperature valence mixed (VM) modification can now be understood by density functional theory (DFT) calculations illustrated for the double cell perovskite YBaFe2O5 . In CO two types of iron ions appear, Fe2+ and Fe3+, whereas in VM both iron sites have the same non integer oxidation state (namely 2.5). Only by going beyond the conventional DFT (in our case GGA+U) we find the charge order and the insulating phase. The orbital ordering in this correlated system is the main reason for the orthorhombic distortion that occurs during this phase transition. The calculations agree with available experiments for the magnetic moments, the charge ordering and Mössbauer data.

***

ly11758

Graphite Lubricant 2.0

Graphite is one of the best solid lubricants. It is made of stacks of atomically thin sheets of carbon, named graphene. The excellent lubrication is believed to originate in the easy shear of graphene layers with respect to each other. In our work, we have grown large terraces of single and double layer of graphene on silicon carbide. We find that even single layers graphene exhibit very low friction. Furthermore, we find that friction on a double layer is half of the friction on a single layer graphene. The friction contrast can be explained by a difference in the way how lattice vibrations are coupled to electrons in graphene. We discovered this difference in coupling by studying electron emission under illumination with the light of a synchrotron source. Double layer graphene outperforms even graphite as a lubricant due to reduced adhesion.

***

LW11526

Teaching Plasma a New Tune

Recent experiments in the non-neutral plasma group of the University of
California, San Diego, have demonstrated the existence of a new kind of
plasma wave, the Electron Acoustic Wave (EAW) a nonlinear wave that
propagates at much lower frequencies than regular electron plasma waves. For
most of the last 50 years, physicists expected that EAWs, if they existed,
would be heavily damped and insignificant. But in the 1990¹s new theories
emerged that suggested otherwise. These were, for the most part, ignored -
until now, when we have observed these EAW plasma waves directly. At low
amplitudes the observed waves match the EAW dispersion relation predicted by
Dorning and collaborators. But when driven to large amplitudes, the waves
³train² the plasma so that it resonates at the driver frequency even after
the driver is turned off, for any frequency chosen by the experimentalist.
Put another way, it¹s as if a grandfather clock could be taught to speed up
by changing the length of its pendulum. We present the first detailed
measurements showing the intimate interaction between acoustic waves and
plasma particles. Such detailed measurements illustrate how university-scale
basic plasma physics experiments can study complex wave phenomena relevant
to large-scale plasma experiments conducted in national laboratories.

***

ES10464

*New AC-field pumps for micro-systems*

New electro-hydrodynamic micro-pumps are devices of much potential for
micro-fluidic systems. The pumps have no movable parts, a simple
electrode design and are operated by AC fields in the kHz and MHz
ranges. Media of various conductivities can be pumped, from distilled
water to cell culture media. The pumping effect is based on AC field
forces acting on spatial charges induced in a medium with inhomogeneous
dielectric properties. In aqueous media, such inhomogeneous properties
result from temperature gradients generated by external heat sources or
Joule-heating by the AC pump field inside the pump medium. Effective
fluid motion is induced when the temperature or field distributions are
asymmetric. Unlike electro-osmotic pumps, the new pumps do not exploit
the low-range electric double-layer polarization but instead, structural
polarization effects occurring throughout the volume of the pump-medium.
This also makes them superior to traveling-wave pumps that can only
effectively generate travelling electric fields within a specific range
of an electrode array driven by phase-shifted signals. The new pumps
generate constant pump forces in broad frequency bands by the
interaction with the in-phase section of the polarization, while
travelling-wave fields interact with its out-of-phase element, so
generating a Lorentzian force-peak at the fluid's charge relaxation time.



*Figure caption:*

Temperature distribution inside a directly heated micro pump with
platinum structures on a glass carrier for heating and AC-field
application.

***

LX11295

How to manipulate atoms beyond the laser domain

In this letter, we demonstrated a polarization control in crystal-
assisted coherent excitation of atoms. This method enabled the
ingenious probe and manipulation of atoms in the X-ray domain where
the laser equipments are not available.
Energetic atoms flying through a crystal lattice experience a
temporally-oscillating field by traversing the periodic array of
atomic planes, which can resonantly excite the atoms just like a laser
field (three-dimensional resonant coherent excitation; 3D-RCE). Our
experiment used a thin silicon crystal and Ar$^{16+}$ ions accelerated
to 70% of the speed of light. We controlled the polarization direction
of this oscillating crystal field by selecting the direction of atomic
planes, and succeeded in the alignment of atomic orbital into a
specific direction. Furthermore, we took advantage of the unique
possibility to use two different atomic planes simultaneously. The
response of the atomic system to one field was probed by the other
field under the polarization control with respect to each other,
demonstrating the so-called pump-probe experiment in the X-ray domain.
Our novel technique opens a way to the study of quantum systems in the
short-wavelength region as an alternative to the optical methods.


***

LU11676B

For refrigeration problems, a magnetically attractive solution

Your refrigerator's humming, electricity-guzzling cooling system could soon
be a lot smaller, quieter and more economical thanks to an exotic metal
alloy discovered by an international collaboration working at the National
Institute of Standards and Technology (NIST)'s Center for Neutron Research
(NCNR).

The alloy may prove to be a long-sought material that will permit magnetic
cooling instead of the gas-compression systems used for home refrigeration
and air conditioning. The magnetic cooling technique, though used for
decades in science and industry, has yet to find application in the home
because of technical and environmental hurdles - but the NIST collaboration
may have overcome them.

Magnetic cooling relies on materials called magnetocalorics, which heat up
when exposed to a powerful magnetic field. After they cool off by radiating
this heat away, the magnetic field is removed, and their temperature drops
again, this time dramatically - enough that scientists have attained
temperatures of nearly absolute zero via the effect. Two factors have kept
magnetic cooling out of the consumer market: most magnetocalorics that
function at close to room temperature require both the prohibitively
expensive rare metal gadolinium and arsenic, a deadly toxin.

But many gas-compression refrigerators employ hydrofluorocarbons (HFCs),
greenhouse gases that can contribute to climate change if they escape into
the atmosphere. In addition, it is becoming increasingly difficult to
improve traditional refrigeration. "The efficiency of the gas cycle has
pretty much maxed out," said Jeff Lynn of NCNR. "The idea is to replace that
cycle with something else."

The alloy the team has found - a mixture of manganese, iron, phosphorus and
germanium - is not merely the first near-room-temperature magnetocaloric to
contain neither gadolinium nor arsenic - rendering it both safer and cheaper
- but also it has such strong magnetocaloric properties that a system based
on it could rival gas compression in efficiency.

Working alongside (and initially inspired by) visiting scientists from the
Beijing University of Technology, the team used NIST's neutron diffraction
equipment to analyze the novel alloy. They found that when exposed to a
magnetic field, the newfound material's crystal structure completely
changes, which explains its exceptional performance.

"Understanding how to fine-tune this change in crystal structure may allow
us to get our alloy's efficiency even higher," said NIST crystallographer
Qing Huang. "We are still playing with the composition, and if we can get it
to magnetize uniformly, we may be able to further improve the efficiency it
already has."


***

LU11568

Graphene with superperiodicity effects

Epitaxial growth of graphene on solid surfaces is a relatively
simple and reliable way to prepare this novel material which has
the potential to replace silicon in future electronics. Experiments
disclose in particular that graphene grown on iridium exhibits
exceptional structural quality, extending over micrometers large
areas of iridium surface, including steps. Our study of such
graphene clearly shows that there is a mismatch between iridium
and graphene lattices. Mismatch induces long-range corrugation
exposing graphene to an additional periodic potential which is
responsible for the modification of graphene's electronic structure.
Opening of the gaps in the band structure is one of the most prominent
features of this effect. Recent theoretical research has suggested
that, due to the chiral nature of charge carriers, an additional
periodic potential applied to graphene can alter their propagation in
a very peculiar way. The potential of this modification in engineering
desired properties of graphene-based electronic devices is yet to be
proved. The simplicity of investigated system and its desirable
properties make it attractive for further model experiments on charge
carrier manipulation.

***

EX10334

Liquid Crystals Fall Into Line

As evaporated liquid crystal (LC) molecules gently rain down on an optical waveguide surface we measure their average molecular axis orientation and layer thickness as the surface is slowly covered. The waveguide surface is part of a dual slab waveguide and the technique, dual polarisation interferometry (DPI), first introduced by us in 1999, resolves changes in layer thickness of less than 1 angstrom and sub-degree changes in average molecular axis polar alignment. The electrostatic interactions among the molecules provides the mechanism that drives the cooperative behaviour and ordered layer structure. Such detailed insight into LC layer ordering, easily seen now in real-time in the laboratory, provides opportunities to answer previously difficult questions in this area. By comparison, to uncover detailed stuctural data such as this has required samples to be taken to centralised facilities such as neutron sources. Verifying DPI results using neutron reflectance and pushing the work further into optically pumped LC layer reordering are the next steps.

***

LT11089

Universal behavior of a BEC "car" in a Y-shape road

In our real world, a car will always go along one branch of a Y-shape road (see the attached image). Fantastically, in the quantum world, a car may go along both two branches of a Y-shape road at the same time. In our paper, we explore the universal behavior of a BEC "car" in such a Y-shape road, which associates with spontaneous symmetry breaking. A Bose-Einstein condensate (BEC) is an intrinsic many-body quantum systems of bosons (such as photons and integer-spin atoms) staying in a same single state. Spontaneous symmetry breaking occurs if the mean-field states do not possess symmetry of the original many-body quantum system. In a coupled two-component BEC, the symmetry breaking transition from single- to bi-stable states forms a Y-shape road (see the above panel of Fig. 1 in our paper). In dynamical transitions, the mean-field dynamics obeys an universal Kibble-Zurek mechanism, which also characterizes the universal properties of the early universe and superfluids undergoing thermodynamic phase transitions. The symmetry breaking transitions also cause an anomalous mean-field breakdown dependent on approaching directions. The dynamical mechanism of symmetry breaking transitions connects with the quantum adiabaticity, which provides various applications in atomic physics, condensed matter physics and nonequilibrium dynamics, and particularly in adiabatic quantum computation.

January 23, 2009

LX11118

First transmission of twisted radio beams

For the first time experiments have demonstrated that radio beams can
be twisted. Beam twisting increases the capacity of radio beams to
transmit information and provides unique possibilities to study
rotating phenomena. A team led by scientists from the Swedish
Institute of Space Physics performed the experiments at the High
frequency Active Auroral Research Program (HAARP) in Alaska, USA.
Powerful radio beams were transmitted into the ionosphere, the
overhead near-Earth space environment. Several degrees of twisting
were successfully transmitted and the interaction of the twisted beams
with the ionosphere was studied. The characteristic ring-shaped cross
section of the beams could be observed by the weak optical emissions
that the beams excited in the ionosphere, a common technique used to
study the effects of radio waves in that region. Twisting radio beams
constitutes a new dimension for information transfer. The technique
works independent of the well known amplitude (AM) and frequency
modulation (FM) techniques used today, for example, for broadcasting
radio and TV channels. Further, twisted beams provide unique
possibilities for remote sensing and interaction with rotating
phenomena, such as vortices in aurora, as well as for extracting
corresponding information from distant objects in the universe in
radio astronomy.


***

LW11444

A new way to see molecules

Researchers have demonstrated a new optical methodology that provides an instantaneous two-dimensional projection of molecular function. The method maps an arbitrary number of quantum couplings within or between molecules. Published this week in Physical Review Letters, the researchers demonstrate the power of the method in the mapping of electronic excitations in a photosynthetic protein. For the first time, they show the ability to instantaneously and directly distinguish coupled-electron motions from other electron motions, providing a new window on how energy is efficiently transported to drive a chemical reaction. The novel methodology, Angle Resolved Coherent (ARC) wave-mixing, works by imaging the angle of light emissions from a chemical sample that is illuminated by a combination of high power, pulsed laser beams. A two-dimensional map is captured without post-processing and in one ten thousandth of a billionth of a second (the duration of a laser pulse). Variations of the same methodology can in principle map molecular vibrations or bonds. With relevance to the molecular biosciences and spanning disciplines, this method provides a powerful new way to feedback to molecular simulation and opens up new avenues in relation to sensitive and rapid sample characterisation.


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LX10929

Thermal diffusion in polymer solutions: To the cold or to the warm?

Recent experiments on dilute polystyrene solutions in a temperature
gradient reported that long molecules, consisting of many styrene units,
diffuse to the cold, whereas monomers diffuse to the warm; thus the
transport coefficient changes sign as a function of the molecular weight
[Stadelmaier & Köhler, Macromolecules 41, 6205 (2008)].

Here we show that this change of sign arises from the competition of two
opposite mechanisms. The first one, pointed out by Brochard & de Gennes in
1981, stems from solute-solvent interactions and drives high polymers to
the cold side of the sample. In addition, we derive a novel term of
opposite sign; this directed Brownian motion is most relevant for short
molecules and drives the solute to the warm.


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LU11652

Society benefits from behavioral diversity

Cooperation is essential in every society, but puzzling from an
evolutionary perspective. In this work, we address the role of
behavioral differences – ubiquitous among Humans - on the evolution of
cooperation. We study a model in which individuals can either cooperate
or defect. They engage in a social dilemma of cooperation, interacting
along the edges of a complex network. The structure of the network
changes in time, as individuals regularly engage in new interactions
while abandoning old ones. Social interactions may be long or brief,
depending on the individuals involved. When dissatisfied, some
individuals will try to break contact as soon as possible, whereas
others will remain in touch. We show that cooperation blooms – and
society as a whole benefits – the larger the behavioral diversity in
responding to unwanted interactions. These results support the idea that
diversity, on a grand scale, is instrumental in shaping us as the most
sophisticated cooperating entities on this planet. Mathematically, we
show that taking explicitly into account the feedback between
co-evolving mechanisms – here network topology and individual strategy
and behavior - profoundly affects the outcome of those processes, a
result which has widespread consequences in many problems of natural and
social sciences.

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BX10909

Probing the Exchange Bias in Co/CoO Nanoscale Antidot Arrays using Anisotropic Magnetoresistance

There has been sustained interest in the exchange bias phenomenon, which is a magnetic proximity effect that typically occurs due to interfacial exchange coupling at ferromagnetic/antiferromagnetic (FM/AFM) interfaces. This effect has been used to pin the magnetization orientation of the FM layer, which then serves as the reference layer for key devices in magnetic sensors and high density magnetic data storage. In this work, we have probed in detail using anisotropic magnetoresistance (AMR), the exchange bias effect in nanoscale Co/CoO antidot arrays as a function of temperature and FM layer thickness. We have employed the Co/CoO system due to its Néel temperature TN (291 K), which is just below room temperature, thus enabling the exchange bias to be reset conveniently. Our results demonstrate that the asymmetry in magnetization reversal of Co/CoO bilayers is markedly modified due the presence of antidots and is strongly dependent on the FM layer thickness. We also observe that the exchange bias field in the antidot arrays can be either larger or smaller than the continuous film, depending on the temperature. The interfacial nature of the FM-AFM coupling in the exchange biased antidot arrays is further established from the dependence of HE and HC on the FM layer thickness.

January 21, 2009

LK11128A

Abraham vs. Minkowski: A centenary, and an answer at last.

The momentum of light in transparent media has been debated for 100
years, since Minkowski proposed (in 1908) that it increased, and Abraham
(in 1909) argued that it decreased, when entering a dielectric medium.
Although calculations based on Abraham's theory have greater scientific
rigour, Minkowski's theory leads to a number of popular, and very
effective, shortcuts. This paper shows for the first time why these
shortcuts work, how they may be improved, and, ultimately, when they
fail. With reference to a review of the last hundred years' research, we
see that the momentum of the light wave and the medium cannot be cleanly
separated, and hence any method based on such a separation must
eventually break down. In showing the limits of these shortcuts, we
reinforce that previous conclusion; with the determination and partial
experimental verification of their limits, we provide the shortcuts for
the first time with a firm theoretical basis, validating their
widespread empirical use.

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BX10585

The striped superconductor: (another) new state of matter

Superconductivity is characterized by a quantity called the
"superconducting order parameter", which evolves from zero in the
normal (metallic) state to a non-zero, spatially uniform value in
the superconducting state. In this paper, we describe a new state
of matter, the "striped superconductor", in which the
superconducting order parameter is non-zero, but rather than being
uniform, it is modulated in space (such that its average
vanishes). This state can explain recent transport experiments
in the original high-temperature superconductor
La_{1.875}Ba_{0.125}CuO_4, which reveal a remarkable cascade of
transitions and crossovers which occur above the superconducting
transition temperature. We study the properties of the striped superconductor
theoretically, and propose microscopic models which realize it.
Its most striking new property is that in the presence of weak
disorder (e.g. lattice imperfections), the striped superconductor
necessarily gives way to what is the superconducting analogue of a
glass state. This is in stark contrast to a regular
superconductor, in which weak disorder does not change much. We
propose that such a "superconducting glass" has already been
observed in La_{1.875}Ba_{0.125}CuO_4.

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EX10489

Waving motion induces physical properties changes

In this paper we study a new mechanism to hasten the aggregation process of magnetic particles dispersed in mineral oils exposed simultaneously to a static magnetic field and a low amplitude oscillating magnetic field as a perturbation. The perturbation field induces a waving movement in the chains formed by the fields. These chain movements enhance lateral interactions, which induce lateral aggregation of chains to form larger chains or chains with thicker structures, in a way remarkably more intense and faster than without waving. This mechanism allows to control to some extent the characteristics of the chains and consequently to enhance changes of the physical properties of the dispersion under magnetic fields. Analogous behavior would also happen in other “dipolar fluids”.


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LX11503

Cavity Tuning of Molecule Colors

When placing a molecule into an optical nano-cavity, one changes its
fundamental quantum-mechanical interaction with the surrounding
electromagnetic field. Decades ago, this was experimentally
demonstrated by measuring the cavity-induced modification of the
fluorescence lifetime (average lifetime of a molecule's lowest-lying
excited state). Now, for the first time, it was shown that the
emission spectrum of one and the same individual molecule can be
continuously tailored by tuning the cavity size. The nano-cavity
changes the mode density of the electromagnetic field and thus its
coupling with a molecule's internal quantum-mechanical states.
Because this is sensitively dependent on the wavelength of the field
modes, it leads to a complete restructuring of a molecule's emission
spectrum as distinct wavelengths are affected differently. The effect
is well described by a semi-classical theoretical treatment, offering
the possibility to predict molecular emission properties in complex
nano-environments. This opens an exciting field for designing
fluorescent emitters with adjustable spectral emission properties.


The attached figure shows the changed emission spectrum (red dots =
measurement, blue line = fit) of a single fluorescenct dye molecule
within a silver-mirror microcavity in comparison to the dye's free
spectrum (gray shaded area).

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AY10417

Radiosensitivity of the halouracil molecules studied trough their collision with carbon ions

A strong enhancement of DNA damage through ionizing radiation may be observed by replacement of thymine by 5-bromouracil in cellular DNA and is widely employed in radiation therapy. Such behaviour is studied in this paper through the collision of C4+ carbon ion with the different halouracil molecules. Effectively, as 5-halouracils are supposed to enhance sensitivity to ionizing radiation, the collision with ions would favour fragmentation of the biomolecule. That means that, on the contrary, the charge transfer process would be less efficient with 5-halouracils compared to the uracil molecule.
In the present paper, the charge transfer has been studied theoretically by means of ab-initio quantum chemistry molecular methods followed by a semiclassical dynamical treatment. The process appears markedly less efficient, by at least a factor 100, than the corresponding charge transfer with a uracil target. This leads to an enhancement of the fragmentation process, in complete agreement with the radiosensitization properties of the 5-halouracils, in particular for 5-bromouracil. The charge transfer appears to be an anisotropic process and the preferred orientation depends on the halouracil target considered. The mechanism appears to be driven by two effects: a global electronic effect with regard to the electronegativity of the halogen atom which induces a lowering of the charge transfer cross-sections, and a more specific steric effect relied to the size of the halogen atom which favours preferred orientations for the collision reaction.


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ET10538

Speedup through recursion

Discretized effective actions are used to substantially speed up and
improve the convergence of numerical Monte Carlo calculations of
properties of physical systems. By recursively solving the underlying
Schrodinger equation, in this paper we set up an efficient systematic
approach for deriving analytic expressions for discretized effective
actions. With this we have obtained discrete short-time propagators
for both one and many particles in arbitrary dimension to orders which
have not been accessible before. Apart from Monte Carlo calculations,
our approach can also be used to systematically improve the Numerical
Matrix Diagonalization method for calculating energy eigenvalues and
eigenstates. Furthermore, the obtained discretized effective actions
are applicable to efficiently determine the statistical properties of
Bose-Einstein condensates confined in harmonic or anharmonic traps.
The presented method is also ideally suited for dealing with dilute
quantum gases in a disorder environment where the impact of two-
particle interactions upon the recently discovered phenomenon of
Anderson localization is at present studied.

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LY11801

A Newtonian System that Mimics the Baldness of Rotating Black Holes

The rotating black hole has been described as one of nature’s most
perfect objects. As described by the Kerr solution of Einstein’s
gravitational field equations, its spacetime geometry is completely
characterized by only two numbers, mass and spin, and is sometimes
described by the aphorism ``black holes have no hair’’. A particle
orbiting a rotating black hole always conserves its energy and
angular momentum, but otherwise traces a complicated twisting rosette
pattern with no discernable regularity. But in 1968, Brandon Carter
showed that the particle’s wild gyrations nevertheless hold another
variable fixed, now called the ``Carter constant’’. The true meaning
of Carter’s constant still remains somewhat mysterious 40 years after
its discovery.

Now Clifford Will of Washington University in St. Louis and the
Institute of Astrophysics in Paris has shown that, even in Newton’s
theory of gravitation, arrangements of masses exist whose
gravitational field also admits a Carter-like constant of motion, in
addition to energy and angular momentum. What’s more, the deviation
of the field’s shape from being spherical is determined by a set of
equations that are identical to those for Kerr black holes. One
Newtonian system that exhibits this property is surprisingly simple:
two equal point masses at rest separated by a fixed distance.

One goal of this research is to build a better understanding of the
Carter constant in order to analyse the orbits of small black holes
or neutron stars revoloving around rotating supermassive black
holes. The gravitational wave signal from such events may be
detectable by the advanced LIGO-VIRGO-GEO network of ground-based
laser interferometric detectors, or by the proposed space-based
antenna LISA.

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LU10982

Hot Atoms can Freeze Images

Any image propagating in free space undergoes a diffraction spreading and
eventually blurs out. For this reason, we have lenses in our eyes and
cameras, which reverse the diffraction and recover the original image. In
this paper, we present a novel scheme to eliminate the optical diffraction
of arbitrary images all throughout their propagation. It was recently
demonstrated that arbitrary images can be imprinted on light pulses which
are dramatically slowed when traversing a medium of room-temperature atoms.
Here, we show that by carefully tuning the light-matter interaction, the
optical diffraction of such images can be eliminated completely. This is
achieved by exploiting the random thermal-motion of the atoms, which
effectively trap the light in the plane perpendicular to the propagation
direction. In an analogy to the laser-trapping of atoms,
outwards-confronting light components couple more efficiently to inwards
moving atoms, counterbalancing the natural diffraction of the light. No
other medium suggests non-diffraction of images regardless of their position
and shape. Applications of our scheme include high-resolution imaging,
slowing and storage of images, and nonlinear optics, and the experimental
conditions for its realization are readily available.


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AY10309

Entanglement Distillation with local common reservoirs

Entanglement or correlations between quantum states is an essential
ingredient for teleportation and quantum cryptography.
It is known that these correlation are lost when the system interacts
with the environment.
In this work, a method is presented on how to protect the quantum
correlations against the damaging effects of the surrounding environment,
by using common reservoirs for Alice and Bob (the two parties trying to
communicate) and performing certain measurements.