March 6, 2009

LY11562

Evolution and self-assembly of tetrameric proteins

Proteins are molecules that serve as building blocks of cells and comprise much
of their machinery. Although some proteins function as single molecules, most
work in small groups, or complexes, such as pairs (dimers) or groups of four
(tetramers). The clusters may be seen roughly as being held together by sticky
patches on the proteins’ surfaces; dimers are thus held together by a single
patch on each protein, while tetrameric proteins, the concern of the present
work, have two patches.

In this paper, the authors use a simple model of proteins to provide an
explanation for several aspects of the evolution and self-assembly of a large
class of proteins. In particularly, by studying the thermodynamic constraints on
this evolution, we were able to rationalize the findings of Levy et al. recently
reported in Nature. Using the case study of the evolution of tetramers from
dimers by the growth of a new sticky patch on a dimeric protein, we were able to
show that the newer patch is likely to be weaker than the older patch, and that
as a consequence the resulting tetramers are likely to assemble in two steps: by
pairing into dimers, and these dimers then pairing into tetramers.


***

BA11045

A Lattice approach to graphene

Graphene, a sheet of carbon atoms arranged in a honeycomb structure,
holds great promise for its potential technological
applications, as the electrons can flow through the sheet almost
unimpeded. However, applications such as microelectronics
require a semiconducting material, in which the electrical conduction
can be controlled by varying the temperature or by
applying an external voltage. Thus, the issue of whether graphene
could become semiconducting or even insulating is of
great significance.

In our recent article, we provide evidence in favor of the
semiconducting scenario in freely suspended graphene sheets,
where the strength of the interaction between electrons is maximal.
We also remain consistent with the known properties of
graphene on a substrate (such as silicon dioxide), where this
interaction is much weaker. Our results are based on Monte
Carlo simulations of Lattice Field Theory, a calculational technique
familiar from nuclear and particle physics. This method
is applicable to strongly coupled problems, and fully accounts for
the quantum nature of the electrons and their
interactions. Our results represent a timely prediction, as the
experimental situation concerning suspended graphene is
rapidly evolving.

***

LW11067

Isospin dependence of incomplete fusion reactions at 25 MeV/nucleon

When two heavy atomic nuclei undergo a head-on collision, the so-called
nuclear fusion phenomenon may occur, resulting in the production of a
heavier nuclear specie. This fascinating phenomenon has attracted the
interest of a large of community of scientists because it opens
important opportunities in producing super-heavy elements, not yet
identified on the periodic table of the elements. The study of fusion in
nuclear reactions has however hardly addressed the effects induced by
the asymmetry in neutron and proton numbers of the initial colliding
projectile and target atomic nuclei. This article shows, for the first
time, that fusion and the production of a heavy nucleus at high
excitations is more likely to occur if projectile and target colliding
nuclei with a large neutron number excess are used, as compared to the
case of collisions between symmetric nuclei made of roughly the same
number of neutrons and protons.

The obtained results also provide quantitative information about the
equation of state of neutron-rich nuclear matter, key to understanding
the structure of exotic nuclei as well as important phenomena occurring
in astrophysics environments such as the inner crust of neutron stars
and supernovae explosions.

The discoveries described in the present article open important
perspectives at the forthcoming accelerators facilities for radioactive
beams that will be available all over the world in the next decades
(FRIB in USA, SPIRAL2 in Europe and RIKEN in Japan). These facilities
will indeed provide unique tools to improve our investigations aimed at
producing super-heavy nuclei and better explore the role of the equation
of state of asymmetric nuclear matter.


***

LY11084

Charged atoms make helium freeze

Quantum mechanical effects prevent superfluid helium from solidifying, even at a temperature of absolute zero, a property unique among all elements. Pressures above 25 bar are needed to overcome the repulsion of He atoms and force them into a crystalline structure. We have shown that the injection of atomic ions (Cs+, Rb+) into pressurized superfluid helium induces He crystal growth along a direction imposed by an applied electric field. The ions act as heterogeneous nucleation centers for the crystallization, in the same way as dust condenses water vapor to form clouds. It has been known for 50 years that positive ions trap He atoms into a solid crust, a structure known as snowball. Our observation is the first manifestation of a macroscopic conglomerate of snowballs. Recently we have discovered that doped solid helium forms a macroscopic solid - which we call an iceberg - at pressures where pure helium is liquid. Our new observations support the hypothesis that icebergs contain snowballs and so-called electron bubbles, bound by Coulomb forces. It is likely that helium icebergs have an ionic crystalline structure with a nanometer-sized lattice constant, larger than in usual ionic crystals such as table salt.

***

BW10496

Spin fluctuations become soft in unexpected places in a triangular magnet

AgNiO2 is a very unusual material. Built of stacked, two-dimensional
nickel-oxygen planes, glued together by silver ions, it is both a metal and
a magnet. This, in itself, is not unusual. However the way in which
metallicity and magnetism combine is unique, with the magnetic ions in each
plane forming a perfect triangular lattice, nested within a honeycomb
network of conducting sites.

Quantum magnets on a triangular lattice have long been a source of
fascination because, for antiferromagnetic interactions, the geometry of the
lattice is incompatible with any simple form of magnetic order. This
property is known as geometrical frustration, and can lead to many
interesting new effects, including completely new states of magnetic matter.

In this paper we study the excitations of the magnetic nickel ions, using
inelastic neutron scattering. We find that the magnetic spectra can be
described using a model with competing antiferromagnetic interactions and
strong easy-axis anistoropy, which supports a "stripe-like" magnetically
ordered ground state, and dispersing "spin wave" excitations. However there
are a number of surprises. Not least among these is that the lowest energy
spin fluctuations at low temperatures occur at a completely different
crystal momentum from the stripe-like magnetic order. This raises the
prospect of new types of magnetic order if, for example, these fluctuations
control phase transitions in magnetic field.


***

LX11133

Aging and effective temperatures near a critical point

This paper presents the first experimental study, which relates the rather well-understood critical dynamics, of a second order phase transition, with the much less understood non-equilibrium dynamics of genuinely aging systems, such as glasses, spin glasses, colloids and polymers.

In order to make such a comparison we study the dynamics of the director fluctuations of a liquid crystal (LC) after a quench close to the Fréedericksz transition, which is a second order transition driven by an electric field. We analyse the data using concepts like aging originally introduced for glasses. Several theoreticians have pointed out previously that similar concepts should apply to dynamics after a quench in the vicinity of a critical point. The main virtue of the present paper is to test such an idea in an experiment. As a result we find the existence, in this rather simple and well-defined system, of a fluctuation dissipation relation with an effective temperature higher than that of the thermal bath. Furthermore the LC presents several properties of an aging system after the quench, such as power law scaling in times of correlation and response functions.

***

BZ10542

Selective Coupling between pi Bands and Optical Phonon Modes in Multilayer Graphene

The electron-phonon coupling is the main mechanism accouting for conventional superconductivity. Understanding of this scattering process generally requires an extensive study of both the electronic and phonon properties in the system. In this paper we have shown that in graphene and multilayer graphene, the selective coupling between the carbon $\pi$ bands and the in-plane optical phonon modes can be intuitively described based on a tight-binding model and the mode symmetry.


***

LT11565

Stimulated scattering and lasing of intersubband cavity polaritons

In this Letter, we show a new mechanism of lasing, involving not photons,
but an exotic type of excitations in a two-dimensional electron gas, the
so-called intersubband cavity polaritons. Our theory predicts that lasing in
such a system can occur well below electron population inversion, i.e., with
the
majority of electrons still in the ground level. The threshold in such
inversionless laser can be orders of magnitude lower than in normal lasers
based on
population inversion, with obvious consequences on lasers efficiency.
The scattering of bosons from an initial to a final state is enhanced by the
occupation of the final state. This remarkable property, usually referred as
"stimulated scattering" is
reminiscent of the stimulated emission of photons, which is the cornerstone
of lasing. However, intersubband cavity polaritons are not elementary bosons
and thus their ability
to undergo stimulated scattering has been, untill now, unclear, if not
controversial. In this letter, we have derived and solved an analytic theory
that, fully accounting for their
non-perfect bosonicity, demonstrates that intersubband cavity polaritons
scattering can indeed become stimulated, paving the way to the realization
of a new kind of room temperature
solid-state inversionless laser.


***

ES8922

Levels of complexity in scale-invariant neural signals

Recent investigations have demonstrated that integrated physiological
systems under neural regulation do not remain in equilibrium but rather
exhibit continuous complex fluctuations even under healthy basal conditions.
These fluctuations are often characterized by self-similar (fractal)
temporal
organization and long-term memory structure resembling the behavior of
physical systems away from equilibrium. The neural feedback mechanisms
leading to such non-equilibrium fluctuations are not known.

In this study we apply modern methods from statistical physics to quantify
linear and non-linear aspects of the dynamics generated by two physiologic
systems --- the human gait and the human heartbeat. The dynamics of both
systems originate in oscillatory centers and are under multiple component
neural control and thus, are believed to exhibit similar features, and to be
governed by similar mechanisms. We find that while the fluctuations in the
output of both gait and heartbeat processes are characterized by similar
power spectra and two-point correlation properties, they belong to different
classes of complexity --- human gait fluctuations exhibit close to linear
monofractal properties characterized by a single scaling exponent, while
heartbeat fluctuations exhibit non-linear multifractal properties, i.e.,
fractals within fractals, which in physical systems have been associated
with turbulence and related multi-scale phenomena.

This study provides new insights into the nature of the non-linear feedback
loops involved in the regulation of these key physiologic systems, which
will facilitate the development of adequate models of integrated neural
control as well as of novel diagnostic and prognostic markers of pathologic
deviations.


***

LY11114BR

Dance of silver in a glass

Solid electrolyte materials are the basis for batteries, chemical sensors,
and promising alternatives to conventional FLASH computer memory devices.
The physical process that enables these applications is the fast motion of
ions in the glassy matrix. It has long been a puzzle how ions could move so
quickly through the "jumbled" structure of a glass. One of the best-known
solid electrolytes is germanium selenide glass heavily doped with silver.
Chaudhuri and coworkers explored this material with ab initio techniques,
successfully forming structural models in agreement with diffraction
experiments, and determined where the silver is trapped in the network
(always two-coordinated between special pairs of host atoms). Unusually for
an MD simulation, the authors show that it is possible for this material to
directly simulate ion diffusion, and track the progression of the silver
ions through the disordered network. Between trapping events, the motion is
quite ballistic. The authors show that beside individual traps, there are
also "supertraps" present, which rapidly exchange silver ions. The
trajectories of hopping silver ions is reported, as is the temperature
dependence of the hopping.


***

lx11733

Teaching Quantum Dots how to Amplify Light

By squeezing the electrons in a semiconductor into a one dimensional sheet, the quantum well has been transformative for the generation and amplification of light. This one-dimensional confinement creates quantum mechanical effects which underpin the development of efficient lasers for telecommunications.

It was anticipated since the 1980’s that this squeezing effect would render even more dramatic effects in a three dimensional box - the quantum dot. The “colloidal” form of the quantum dot should theoretically have the most pronounced benefit since they are the smallest: they should enable broader spectral tuning based upon size, as well as offering ease of manipulation. Despite the anticipated benefits for laser applications, the colloidal quantum dot has not lived up to its initial promise. It was believed that interactions between electrons would block the desired amplification effect.

Kambhampati & co-workers recently showed that these semiconductor quantum dots yield universal and extraordinarily efficient optical amplification as predicted by theory. We discovered that one simply needs to “teach” the driving laser how to correctly drive the dot. While our pumping scheme nicely recovers the long anticipated benefits of squeezing electrons into a “quantum box”, we were surprised to find even more remarkable features of these colloidal quantum dots. In stark contrast to the ubiquitous quantum well or molecular gain media, we found that the spectrum of amplification may be controlled by the manner in which the dots are pumped.

***

LW11445

Nanomechanics of protein filaments adsorbed to interfaces

In the human body, cells move within filamentous protein scaffolds. To
achieve motion, the cells have to create strong adhesion points with the
scaffold in the same way that a car needs tires to grip the ground in
order to achieve traction. At the cell level, the localized traction
forces induce a re-arrangement (remodeling) of the scaffolds that are
essential for the shaping of biological tissues. In our paper we
investigate this remodeling process at the nanoscale by manipulating
single protein filaments adsorbed to a surface using atomic force
microscopy. Theoretical arguments lead ot the conclusion that the filament
adsorbed to the surface experience a very high apparent viscosity, similar
to the viscosity of honey. This is a direct consequence of the adhesion
strength between the filament and the substrate. Our theoretical model
originally set-up to explain our nanoscale observations can be transferred
to a macroscopic experiment with a rubber band stuck on a honey-coated
glass slide.

***

BZR1101

Unique pairing state of iron-arsenic superconductors

The discovery of superconductivity with high transition temperatures in
iron-arsenic based compounds last spring fueled the hopes that a look from a
new perspective will solve the puzzle of the phenomenon. Until recently,
high temperature superconductivity was observed uniquely in the class of the
copper-oxygen materials and 20 years of intensive studies that have elapsed
since their discovery have revealed that the superconducting properties of
these materials are uniquely different from all other known cases. In our
recent articles we show that superconductivity in the iron compounds is
perplexing as well as, however, intrinsically different than in the
cuprates. Superconductors are materials that, when cooled below a point
known as the transition temperature, exhibit current flow without any
measurable loss of energy. This state arises due to the electrons forming
pairs, enabling them to behave identically. The collective flow of pairs
happens in such a way that a magnetic field is able to penetrate the
superconductor only up to a certain characteristic depth, known as the
London penetration depth. The study of field penetration as a function of
temperature provides vital information about the way the pairs are formed.
The results of our experiments reveal that pair formation in the
iron-arsenides proceeds differently than in any other known superconductor.
Understanding this difference may be a key to understanding the phenomenon
of high temperature superconductivity.


***

LA11813

Are magnetic impurities in graphene magnetic?

Transition metal (TM) atoms such as Fe or Co are frequent impurities in
various compounds. They may have localized magnetic moments which give
rise to interesting physical phenomena such as the Kondo effect, that is
an increase in electrical resistance of the metal with such impurities at
low temperatures, and may offer new computing paradigms via spintronics.
In this work we theoretically studied the structure, bonding and magnetism
of various TM atoms (all atoms from Sc to Zn, plus Au and Pt, which can
also be referred to as transition metals) embedded in graphene, a
two-dimensional semi-metal, by substituting one or two carbon atoms with
an aim to answer the simple, yet important question: "Are these impurities
in graphene magnetic?" Our results indicate that the TM atom-vacancy
complexes exhibit intriguing magnetic behavior. In particular, an Fe atom
adsorbed on a single vacancy is not magnetic, while the Fe-double vacancy
complex has a high magnetic moment. Surprisingly, Au and Cu atoms at
single vacancies are magnetic, an interesting result in the context of
spintronics. We also found that, for most metals, the bonding is strong,
which suggests the use of graphene with embedded TM atoms as the catalyst
in fuel cells.

***

BY10545

Solute diffusion trends in aluminum - computational approach

The diffusion coefficients of all technologically important alloying
elements in aluminum have been computed, using Density Functional
Theory in combination with Transition State Theory. Within the
framework of the so-called five-frequency rate model, various atom-
vacancy interchanges near solute atom are taken into account. However
only one of two types of microscopic transition mechanism dominates
macroscopic solute diffusion: impurity-vacancy or aluminum-vacancy
interchanges.

We found that transition elements are slow diffusers. Diffusion is
dominated by slow solute-vacancy interchanges, which are, we argue,
due to spd-hybridization of bonds, making bonds directional.
Diffusion coefficients for these elements do not depend on impurity
size, but show strong trends with number of d-electrons.

The diffusion of other elements in aluminum is dominated by the
transition rate of aluminum-vacancy interchanges and is much closer
to self-diffusion coefficient of aluminum. Linear dependence is
observed between activation energy for diffusion and the size of the
impurity, with larger solutes diffusing faster. This observation can
be used as heuristics for impurity diffusion in other materials with
a closed-packed structure.

***

LX11587B

Decoherence Or Not: A Coherence Preserving State

Quantum coherence in terms of state superposition is one of the most
important features of quantum mechanics and key to the realization of
quantum computation. Usually, the unavoidable environment would render the
quantum object decohered. With the aid of a one-dimensional model
extracted from that an electron spin decoheres in a three-dimensional
quantum dot, in this paper, we show that, the conventional understanding
to decoherence is incomplete. Besides the bath-traced electron spin
coherence description, the entanglement of the bath is indispensible to
present a complete description of the coherence state of the whole system.
The time evolution of electron spin coherence predominated by zero value
but with periodic instantaneous coherence revival was often considered as
the realization of complete decoherence. We found that, however, this is
actually a coherence-preserving phase, sustained by highly-entangled
environment while the true decoherence state corresponds to a disentangled
environment. This result challenges the conventional understanding to
decoherence. Moreover, since decoherence is usually accepted as a
criterion of the realization of quantum measurement, our result also
challenges the postulates of the quantum measurement on the unicity of the
criterion.

***LZ11406BJ

Theory of Spontaneous Buckling of Doped Graphene

Graphene is a solid made of carbon atoms. However, contrary to other solids,
whose atoms are organized in all three dimensions (they have height, width,
and depth), Graphene is built of only one layer of atoms, making it the
ultimate membrane. In this sense Graphene is similar to a sheet of paper,
however million times thinner! We know that a piece of paper, when a force
is exerted on its sides, is buckled: corrugations and ripples are formed on
it with some characteristic size. A new theory, developed in this paper,
shows that in Graphene such buckling, i.e. the appearance of corrugation and
ripples, does not demand any force. In fact, the theory predicts that
buckling is induced in Graphene just by doping Graphene, i.e. injecting
foreign particles into it. However, this property gets even weirder. In
order for this buckling to occur, these particles have to be of positive
charge. This kind of buckling would not occur for negative charges!
The importance of this property goes beyond the theoretical interest, since
Graphene is considered one of the prospect materials for nanometer-sized
electronics. As a result, it is prominent to know its structure when being
infiltrated by external charge, for quality assurance and for the design of
these future applications.



***

BV10960

Hydrogen: the dominant impurity in zinc oxide materials

Zinc oxide (ZnO), a wide band-gap semiconductor, is a candidate for
relatively low-cost application in a broad range of electronic devices
such as light emitters and sensors. Understanding the structure and role
of lattice defects in ZnO in order to explain or to engineer its
physical properties could be a major step towards the realization of
such application. In spite of decades of study and progress in ZnO
research unresolved controversies remain, mainly related to the role of
hydrogen in ZnO and the native defects formed during crystal growth.
Hydrogen may be easily incorporated into materials and may also strongly
impact the properties of electronic devices by creating
electrically-active defects. In this paper we report on a systematic
investigation of undoped, high-quality, commercially-available ZnO
single crystals from various suppliers. Our research revealed the
presence of hydrogen in all crystals studied, in a bound state at
concentrations of at least 0.3 atomic percent - appreciably higher than
those of other impurities and the usual semiconductor doping levels.
Further, by combining positron annihilation experiments with
state-of-the-art calculations, we have identified zinc vacancy-hydrogen
complexes as an important class of hydrogen-related defects in ZnO.


***

BY10524

High spin-polarization effect in cylindrical quantum wires by reducing
symmetry

We address effects associated to tuning ground-state spin character
according to (+/-) z-propagation direction in cylindrical symmetry based
quantum wires. Cross-section symmetry reduction plus spin-orbit
interaction lead to notably high degree of spin-polarized currents at low
velocity (kz) values. In contrast to the perfect cylindrical wire case
where there are always two-fold spin degenerate levels at any fixed value
of kz, regardless its propagation direction, a semi-cylindrical
confinement gives rise to an energy splitting of two-fold degenerate wire
levels produced by different Dresselhaus spin-orbit interaction terms that
preserve spin-polarization at small values of kz. These findings open
possibilities of exploring low velocity transport in quasi-1D
nanostructures based on the existing preferential spin-channels according
to the propagation direction of spin-polarized carriers. We found
conditions with more than 90% degree of spin-polarized currents in each
spin-up and spin-down independent channel. This result, based solely on
lowered spatial symmetry of quasi-one-dimensional nanostructures, enables
the realization of spin-filters and tuned spin-polarized current densities
along parallel propagation directions.

February 27, 2009

LZ11483

What a drag - Why do magnets have friction?

When you move an object in a rough environment, there exists a a drag on
the object. In most cases, this mechanical friction can be well
understood through the roughness of the objects and surfaces involved.
Friction comes in two flavors - static friction and kinetic friction.
The latter arises
when the object is actually moving and this type is experienced by spins
- the tiny quantum compass needles which
give rise to magnetism. When spins rotate, they too experience a drag.
However, unlike in the case of mechanical friction, the universal origin
of this friction is not understood. It has been presumed to take an
empirical form in equations for over the past 50 years in order to
explain data. However, two physicists Mark Hickey and Jagadeesh Moodera
from MIT's Francis Bitter Magnet Laboratory have uncovered the origin of
this friction and it lies in the fundamental description of the
electrons themselves in the foundations of quantum field theory - the
Dirac equation. This fundamental description is coupled with the law of
electromagnetic induction (discovered by Michael Faraday and
mathematically described by Maxwell), to show that the spins see a
time-varying magnetic field and
this gives rise to the friction they experience. It may also give rise
to the interaction which allows magnets to emit light. The results are
to be published in Physical Review Letters this month.

***

LX11092E

SPECIAL RELATIVISTIC GENERALIZATION OF STATISTICAL THERMODYNAMICS

Does a moving body appear hotter, cooler or the same as the one at rest?
What is the special relativistic generalization of the celebrated Maxwell-Boltzmann
velocity distribution? These questions have been around for about a 100 years
now, and many famous physicists (including Einstein himself) have tried
to answer them. Much controversy surrounds these issues.
In this work, we propose a simple and realistic model of a
relativistic gas in order to investigate these questions. We find that Juttner
function is the correct generalization of the Maxwell-Boltzmann distribution.
Furthermore, we establish local thermal equilibrium for the moving system. Finally,
we show that standard statistical mechanical methods do not suffice to determine a
moving system's temperature uniquely. One is therefore left with temperature as a
system's parameter, same in all inertial frames, much like proper mass in mechanics.

***

LV11564

Surface potential determination for nano-structured materials

We describe a novel method for the determination of the local electronic surface potential of nano-structured template surfaces. Knowledge of the local surface potential is crucial for understanding phenomena sensitive to laterally varying surface properties such as catalytic processes and electron emission. Methods so far applied to this problem either lack of the necessary resolution or allow only a qualitative description of variations of the local surface potential. The presented method is based on the local detection of field emission resonances by scanning tunnelling spectroscopy (STS) and their numerical modelling using an appropriate model potential describing the local electrostatics. Due to their close residence at the surface field emission resonances allow the determination of the surface potential landscape with a lateral resolution below 1 nm. The method is applied to elucidate the site-specific adsorption properties of the strain-relief pattern formed by two monolayers of Ag on Pt(111). For the example of C60 fullerenes it is shown that the variation of the local surface potential is responsible for the site-selective immobilization.


***

LX11672

Squeezing an Image through a Tiny Hole

Common sense tells us that it would be hard to squeeze light through a
tiny hole in an opaque screen. Even harder, would be to transport a
complex information-carrying “image” through such a small aperture.
However, in this paper, using theory and computer simulations we show
that this bottleneck may be overcome: if the small hole is filled with a
material with near-zero dielectric constant and a bundle of metallic
wires, the image can be “squeezed” through the hole with its minute
details intact. Even more curious, counterintuitive fact in this
scenario is that even though the bundle of wires effectively contributes
to obstruct even more the tiny hole, the more closely packed the wires
are, the better the information-carrying wave can squeeze through! The
required materials may be either chosen from available natural materials
(e.g., plasmonic or polaritonic materials) operating near their plasma
frequencies, or they may be engineered as metamaterials with near-zero
permittivity. This phenomenon can provide a useful means for
transporting image-carrying waves through a very tight region and/or
sharp bends with potential applications in nanophotonics, and optical
communication and video transport at the nanoscale, to name a few.

***

LX11018

Discovery of Pathway from Graphite to Amorphous diamond

Carbon has various forms such as graphite, diamond, fullerene, and nanotubes. Amorphous diamond is
extreme one of diamond with no long-range ordering. A team of Japanese researchers has discovered
the novel transformation process from graphite into amorphous diamond. They have found that
transformation can occur if graphite was irradiated with high energy neutrons before the shock
compression. Wigner defects formed in graphite by the neutron irradiation are considered to make a
high density of diamond nucleation sites under shock compression, thereby leading to the
transformation into amorphous diamond. The team says that the combined method of irradiation and
shock compression is a promising way to synthesize new carbon materials such as the amorphous
diamond, as we can control the introduction of Wigner defects in terms of the kind, the amount, and
the spatial distribution by changing the irradiation condition of dose, temperature and incident
species. Moreover, the team says the present result is very interesting in general physics as it
has an analogy with "Rainmaking". As is well-known, rainmaking is the act of attempting to
artificially induce precipitation. The nucleation process for rain and the atmosphere for
precipitation are the keys for rainmaking, similar to the present study.

***

LK11669A

Quantum resource or classical control? The environment decides...

In this work we show that we can take a photonic device and use it
either as a quantum resources or a classical control field simply by
changing its environment. Quantum communication technologies are a
reality today, and the first steps are now being taken towards other
new technologies that sense, process and store information using
quantum resources. These new technologies get their power by
leveraging properties, such as "spooky action at a distance", only
seen in quantum systems. So we can operate them, these new
technologies must have a conventional - classical - IT interface.
Furthermore, the quantum resources need to be controlled with
classical sources, such as electromagnetic fields. However, we know
that everything is actually made of quantum parts! So this begs a
question: Under what circumstances are fields quantum - and thus part
of the technology resources - and under what conditions are they
classical - and thus part of the control interface? A "standard"
answer to this question is size: A field with one photon (one quantum
of light) is clearly quantum, and a large coherent field containing
many photons is classical. In our work we demonstrate that the actual
answer is rather more subtle than this. Indeed, it is possible to take
a field with fifty or more photons in it, and allow it to be highly
quantum (part of the resources), or force it to be classical (a
control field) by changing its environment. So size is a factor, but
it's not the only thing that matters. In the end how a system behaves
is also determined by what it interacts with.

***

BAR1138

Quantum dot probes the nanoscale magnetic environment

Nuclear spins in atoms of a solid-state material play the key part
in the dynamic magnetic environment influencing electronic properties on the
nano-scale. We show that by use of optical cooling, nuclear spins in a
layered GaAs/AlGaAs semiconductor can be driven in a highly polarized
(aligned) state with a very long lifetime exceeding one minute. The aligned
spins produce an effective magnetic field as high as several Tesla
experienced by the electrons in the layers containing the corresponding
nuclei. Light emitted by a few nanometer GaAs quantum dot formed at the
interface of the layers in our structure, carries information on the
magnitude of the nuclear field. We employ sensitive optical response from
such a quantum dot nano-probe for precise real-time monitoring of the subtle
changes in the nuclear spin system. Understanding the dynamics of the
effective nuclear field is crucially important for manipulation of the
electron spin, a promising qubit candidate for quantum information
processing.


***

LX11642

Extremely short laser pulses as an efficient terahertz source

Recent years have seen a significant advance in generation of extremely short laser pulses which contain but several (1-3) oscillations of the optical field and have very high powers. When such pulses are focused in a gas, they ionize atoms very fast and produce a dense plasma. In the process of plasma generation, the laser pulse accelerates electrons rapidly. As a result, strong electron currents are excited in the produced plasma and produce electromagnetic radiation in the ambient space. Earlier, most papers studied only the high-frequency spectral fraction of this radiation, which corresponds to the extreme-UV and soft X-ray attosecond-duration radiation. However, several recent papers reported on experimental observations and numerical simulations of low-frequency, specifically, terahertz emission from plasmas produced by few-cycle laser pulses. In our paper, we study the phenomenon of excitation of low-frequency residual currents, which generate this emission, in such plasmas. Efficiency of realization of this phenomenon is found as a function of laser pulse parameters. The role of the quantum effects associated with ionization and other stages of the electron behavior is investigated. We also find the optimal conditions, at which the efficiency of excitation of low-frequency currents can be very high, and see that the fewer oscillations fit in a laser pulse, the higher is the maximum achievable efficiency. The phenomenon studied in the paper opens the way for creating new high-efficiency sources of high-power electromagnetic waves in a challenging and underdeveloped terahertz frequency range. Another important task, which can be solved by using this phenomenon, is the development of a relatively simple way to determine parameters of extremely short laser pulses from the detected low-frequency (terahertz) radiation that they produce.

***

LX11637

Quantitative
spin-torque measurement


Among the future magnetic memories the domain wall the race track memory is one
of the most promising. The working principle is based on the fact that a
spin-polarized electric current exerts a pressure on a domain wall (spin
torque). Despite the fact that some of the spin-torque features have already
been established, up to now a quantitative direct measurement of the pressure
exerted by the current on the domain wall has not been achieved.

In this paper we report the first quantitative spin-torque measurement in
domain walls. In principle this method is similar to any quasistatic force
measurement. The domain wall is elastically pinned by a geometrical
constriction. Since the spring constant of the effective pinning is unknown, in
order to measure the force exerted by the current we use a reference force
given by an external magnetic field. By comparing the small displacement (down
to 0.1 nm) provoked by current and magnetic field we compare their forces.

Moreover from the point of view of possible applications, the spin-torque
measurement has revealed the unprecedented spin-torque efficiency existing in
one of the studied layers, approaching the maximum theoretical limit.

***

LQ11379

Ultra-High Energy Cosmic Ray May Be a Result of Plasma Wakefield
Surfing

The origin of the observed ultra-high energy cosmic rays (UHECR) is an
exciting scientific mystery. A single such particle would carry an energy
that is equivalent to that of a fast baseball. Where and how were they
produced? Recent UHECR observation data tend to be in favor of the
³bottom-up² scenario, which assumes that these are ordinary particles, such
as protons, that were accelerated by some astrophysical energetic sources.
In a recent study published in Physical Review Letters by Feng-Yin Chang et
al., the authors show, through computer simulations, that intense plasma
wakefields can be excited by the so-called magnetowaves, which are commonly
thought to exist in energetic outflows of astrophysical objects such as the
Active Galactic Nuclei (AGNs). A proton surfing on plasma wakefields can in
principle gain energy much like the speeding up of the surfer riding on an
ocean wave by the beach. Since this mechanism does not require the bending
of the accelerating particle¹s trajectory, the energy loss may be minimized.
The authors believe that this novel mechanism can accelerate cosmic rays
more efficiently than most other existing models. This may help to solve one
of the big mysteries in astrophysics.


***

BA11335

Two dimensional superconductivity in decoupled FeAs planes and large
upper critical field in iron oxypnicides*


Evidence of two-dimensional nature of superconductivity in iron
oxypnictides has been found by analysing resistivity curves in high
magnetic fields up to 28T.

Resistivity curves around the transition temperature are shaped by
fluctuations effects, as a consequence of thermally assisted formation
of superconducting Cooper pairs even above the superconducting
transition temperature. The analysis of such shape allows extract
information on the dimensional character of superconductivity, which
allows to obtain a description of the system and understand dissipative
mechanisms occurring in it.

In this paper, we apply such analysis to the newly discovered F doped
SmFeAsO superconductors.

Beside demonstrating the two-dimensional behaviour, we exploit our
analysis to obtain a thermodynamic and thus intrinsic estimation of the
upper critical field slope dHc2/dT. This parameter, difficult to be
defined according to a universal criterion in oxypnicides, is crucial
for applications, as it marks the onset of dissipative transport in a
magnetic field. A remarkable dHc2/dT value of -12T/K is found in
optimally doped samples.

***

LY11361B

Deformation of vortex lattices: making a soft material hard

We studied the emergence of irreversible deformation phenomena in vortex
lattices. Like ordinary crystalline materials, vortex matter in type II
superconductors can be deformed plastically, or irreversibly, upon
applying large enough currents. Through a combination of theoretical
arguments and numerical simulations, we demonstrated that plastic
activity is mediated by the motion of “scars” in the vortex lattices,
known as grain boundaries, which break the order of the lattice into
pieces. Upon increasing impurity in the system, the number of
topological scars increases and lattice domains become smaller and
smaller. This sort of “polycrystalline” arrangement is able to adjust to
impurities extremely easily, as every single crystallite introduces
additional degrees of freedom in the system. As a result the vortex
lattice, which is usually regarded as a “soft” material, becomes harder,
as larger currents are required to deform it. Our findings emphasize the
crucial role of grain boundary motion in small-scale deformation
phenomena, as observed in experiments on nanocrystalline materials and
disordered colloids.

In the figure: Upon applying a current to the vortex lattice (green
circles) plastic deformation is nucleated around grain boundary scars
(highlighted in blue and red)


***

LD11785B

Modification of surface electronic states by an adsorbed molecular
semiconductor layer

This report provides conclusive evidence that the electronic states in a
metal surface can be modified by the adsorption of an organic
semiconductor layer. While it is well established that molecular
orbitals -- the electronic states within molecules -- can be affected by
their interaction with a metallic substrate, e.g. in organic electronic
devices, the reverse effect of organic layers affecting metallic states
is less studied. In this paper we report about an increase in the
surface state occupation, i.e. an electron transfer to the surface
state, after adsorption of a monolayer of organic molecules, namely
pentacene leading to a shift in the binding energy of the surface state
and a modification of the effective mass describing the electronic
behavior of the substrate. Studying electronic interface phenomena for
technologically relevant materials like pentacene results in a better
understanding of the electronic properties of device interfaces and can
thus lead to improved charge injection behaviour in organic electronic
devices.

***

BY10738

The surface of a photonic crystal can determine its optics

Photonic crystals are leading contenders in the race to replace "slow"
electronic by "fast" photonic elements in optoelectronic circuits. In the
present work, it is experimentally demonstrated that by modifying the
surface of a photonic crystal one can tailor its optical properties. Among
such properties are resonant transmission and, for magnetoactive photonic
crystals, a giant resonance rotation of the polarization plane of incident
light. The change of the optical response of the photonic crystal occurs due
to the appearance of special non-propagating states localized near the
surface of the crystal. These states are analogous to the well-known surface
states in electronic crystals first predicted by Tamm [1] in 1932. In
Ref.[2] such states were predicted to arise at the interface between two
photonic crystals or a photonic crystal and a material with negative
dielectric permeability. When a surface of a photonic crystal is made of an
active material, optical Tamm states open up possibilities for tunability of
photonic elements.


***

LX11347

Vacancies that won't stay on-top

Usually, it is easier to remove an atom from the surface than from
the inside of a material: after all, fewer bonds need to be broken.

This paper shows that the opposite is the case for one particular
surface, TiO2 anatase.

This surface is rather 'stiff', with short bonds and a low surface
energy. Calculations predict that it costs more energy to form an
oxygen vacancy at the surface than deep in the bulk. Moreover, once
formed, the activation energy for a vacancy to migrate from the
surface to the inside of the sample is smaller than the other way
round.

Experiments confirmed this unusual prediction: using Scanning
Tunneling Microscopy, researchers have inspected the surfaces of
anatase (101) single crystals and found no surface oxygen vacancies.
More reduced samples showed clear evidence for ordered, subsurface
defects, however.

The result could be more than a mere curiosity: TiO2 is an important
material for the conversion of solar radiation into chemical and
electrical energy, and defects drive many surface chemical reactions.
If surface oxygen vacancies are formed, they will not survive long in
the ambient - almost immediately they will be covered by water or by
other gas molecules. When such defects hide in a subsurface layer,
they could provide a more subtle, but also more robust influence on
surface reactivity.

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.

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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.

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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.


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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.