April 15, 2009

BB10867

NANOMETRIC-SIZE WIRES CAN BE TUNED EITHER AS INSULATORS OR AS METALS AT WISH

In this paper we report the electrical transport properties of nanometric
wires created by using a very promising nanolithography technique,
focused-ion-beam-induced deposition (FIBID), much simpler than standard
techniques. In FIBID, a gas is decomposed by its interaction with focused
accelerated ions. The grown nanowires are formed by a mixture of platinum
and carbon and, depending on its composition ratio, they behave either as a
metal or as an insulator. As the composition can be tuned by the growth
parameters, nanostructures with completely different electrical
characteristics can be created in a simple one-step process. With this work
we have unified a wide range of previously observed phenomena, and it has
been possible to correlate the electronic properties of the material with
the carbon-platinum ratio. Besides, some of the samples present an
interesting conduction property, hardly found in other systems, as it is the
decrease of the differential conductance with the applied voltage. These
nanowires could be used to create new nano-circuits with several
functionalities.

***

LA12227ER

The Power of Harmony in Vortex Streets

When a fluid flows around a long slender object - called a bluff body - then
impressive structures of contra-rotating vortices appear in the wake behind
it forming a vortex street. A recent study in Physical Review E reveals that
fascinating vortex streets appear in bluff-body wakes if the flow is
perturbed.

A bluff-body wake is essentially a wave-maker which is the source of Aeolian
tones. Thus, changes in the flow speed will be similar to the differences
between pure musical tones, called harmonics, so knowing how much and how
fast it changes (amplitude and frequency) is enough to determine the
characteristics of the wake. It turns out that nonharmonic perturbations,
i.e., changes in the waveform quality but not the main amplitude or
frequency, have a remarkable effect on the vortex dynamics, and
consequently, on the magnitude and phase of the forces exerted on the body.
Understanding this phenomenon is important because it determines the
transfer of energy between the fluid and the body. This knowledge may hold
the key to harnessing the energy from ocean currents and atmospheric winds
more efficiently, or avoiding catastrophic vibrations of offshore
structures.

***

EBR1036

Universal dynamics of jamming particles

We find that a dense assembly of macroscopic particles in a viscous medium
exhibits an exotic cooperative phenomenon, the length and time scales of
which diverge at a certain density in the same manner as that of a
super-cooled liquid. To make the situation more accessible, just imagine
walking across a crowded party room or a rush-hour train, which is almost
impossible unless the people are cooperative. This is actually what happens
in amorphous solids, where the particles are so densely-packed that they
cannot rearrange themselves unless the particles are "cooperative". The
nature of such cooperative motion is the central question in the physics of
glassy materials, as it involves important quantities such as viscosity or
heat capacity. In this paper, using molecular dynamics simulation, we
investigate the nature of the cooperative rearrangement of a dense
particulate system; i.e., how much time does it takes and how many particles
must be cooperative upon the rearrangement. The results bear striking
similarities with a super-cooled liquid.

Figure(EBR1036.jpg): Colored in red is the cooperatively rearranged
particles, which is heterogeneously distributed.

***

LX11486BR

Building Fullerenes Out of Boron

The soccer-ball-shaped C60 buckyball can be viewed as the
smallest molecule in a family of icosahedral carbon fullerenes
that approach the flat graphene sheet when the number of atoms
becomes large. In an article to be published in Physical Review B,
researchers have used computer simulations to show that boron,
carbon's neighbor in the periodic table, can form an analogous family
of stable fullerenes that ranges from the buckyball-like B80 to a
flat boron sheet. Each boron fullerene has a precise structural
relationship to its carbon counterpart, and also has the same number
of valence electrons. In both the icosahedral fullerenes and their boron
analogs, the electronic band gap, which arises from quantum confinement
of electrons, decreases with increasing cluster size. However, unlike
the carbon clusters, for which the band gap approaches zero only in the
limit of infinite size, the boron fullerenes are predicted to transition
to a metallic state at a cluster size of about 2000 atoms.


***

LW11626

A hidden process of sleep

Sleep, an essential part of our lives, commonly consists of
frequent transitions between various sleep states
(including spontaneous awakening periods). In this paper,
we propose a new method, based on a Markov transition matrix,
to quantitatively analyze this time course of sleep.
The transition matrix is determined by statistical analyses of
113 obstructive sleep apnea (OSA) patients who suffer complete or
partial pharyngeal obstruction during sleep. We find that
duration probabilities of each sleep state fit to a modified
exponential distribution, in contrast to recent reports of
a scale-free form for wake and an exponential form for sleep.
This result suggests that sleep can be understood in a unified framework,
providing important constraints for theoretical modeling of sleep.
OSA patients are often aided with continuous positive airway pressure
(CPAP) treatment to improve their sleep quality. We further analyze
sleep of the same subject, but treated with CPAP, and compared with
the pre-treatment ones, suggesting potential applications of our method
in sleep clinics.


***

LX11217

Atomic-scale imaging with ultrasound

In Medicine, ultrasound is successfully used as a non-invasive
tool to image an unborn baby in the mother's womb. As its
counterpart in nanotechnology, we introduce the non-invasive
Damping Force Spectroscopy (DFS) technique capable of imaging
subsurface structures and vibrational modes on the atomic scale by
observing the damping of an oscillating atomic force microscope
(AFM) tip in the "non-contact" regime.

We apply DFS to peapods, consisting of carbon nanotubes filled
with metallofullerenes (hollow C_82 'buckyballs' containing a Dy
atom inside). Spatial maps of the damping signal show atomic-scale
features superior to state-of-the-art topographic AMF images. Not
only can DFS clearly distinguish between empty and filled peapods,
but can also reveal the location and packing of the
metallofullerenes in nanotubes of different diameter as well as
changes of the local vibrational modes.

We trace back the microscopic origin of the damping signal to a
hysteresis in the interaction between the AFM tip and the elastic
peapod. First principles total energy and molecular dynamics
calculations allow us to provide a quantitative interpretation of
the DFS signal by identifying which vibrational modes may be
excited by the AFM tip at a particular location.



***


LV11546


Low power magneto-optical recording in a ferromagnetic semiconductor

Continuing miniaturization is a constant and important goal
of the information industry. A primary obstacle is that smaller bit
sizes require higher coercive field in order to provide stable and
low-noise recording. The strong and highly localized magnetic fields
then needed to switch individual bits are difficult to produce. One
promising approach to address this issue is heat assisted magnetic
recording (HAMR), where a magnetic medium is locally heated by light,
leading to a rapid reduction of its coercive field. This method however
requires large powers and is thus not efficient to commercial. An
alternative approach using light of much lower power would be highly
desirable. In this Letter, we have realized a concept for non-thermal
magneto-optical (MO) recording using very low illumination power. Our
approach is based on light-induced de-pinning of domain walls, rather
than modification of the magnetic interaction. Using this concept we
demonstrated a complete cycle of MO recording with non volatile data
storage. We believe that our findings have the potential to evolve
into an important paradigm for use in information storage technology.


***

LB11686

Violation of mirror symmetry in atoms confirms properties of electroweak vacuum

In physics, the vacuum is never still. Each particle carries a cloud of continuously sprouting virtual particle-antiparticle pairs. The strength of the mutual interaction between two particles becomes dependent on their relative collision energy: at higher energies, the collision partners tend to penetrate deeper inside the shielding clouds. For feeble electroweak interactions, the Standard Model of elementary particles yields an answer for such an energy-dependence (or "running"). Particle colliders provide reference points at high energies. Relatively inexpensive table-top experiments on violation of mirror symmetry in atoms probe the vacuum at low energies. However, for these low energies, where the shielding clouds are penetrated the least, previous analyses were consistent with no running.

Here we improve the accuracy of probing the least-energetic electroweak interaction. We extract the strength of the parity-violating interaction of atomic electrons with quarks of the caesium nucleus by combining previous measurements by C. Wieman group with our calculations. The refined analysis required detailed understanding of correlated motion of 55 electrons of cesium atom. This is not an easy task as the number of memory units required for storing full quantum-mechanical wavefunction exceeds the estimated number of atoms in the Universe. Special tools and approximations were developed. Overall, compared to previous analyses, reaching the next level of accuracy required a factor of 1,000 increase in computational complexity.

Our precision result confirms the fundamental running. Together with the results of high-energy collider experiments, we demonstrate the validity of the predicted running of the electroweak force over an energy range spanning four orders of magnitude (from ~ 10 MeV to ~ 100 GeV).


***

BZ10920

Progressively induced superconductivity in graphene

Graphene, the 1 atom thick carbon crystal, has striking electronic
properties. In particular, current is carried by seemingly mass-less
relativistic particles. This has a lot of interesting consequences, one
of these being the way charges are transferred from graphene to
superconducting electrodes. In graphene, as in any normal metal,
electrons (or holes) are scattered by impurities, giving rise to a
resistance. In a superconductor carriers condense into a coherent
many-body state of electron pairs (Cooper pairs). This leads to the
famous zero resistance state with a supercurrent at zero applied
voltage. What happens if we put a normal metal (N), or graphene, between
two superconductors (S)?

An electron from the normal metal that approaches the interface is
reflected as a hole, so that a pair of electrons can enter the
superconductor. This process depends on the probability of crossing the
interface between the two materials, the so called interface
transparency, which is rarely perfect . Whereas at zero voltage high
interface transparencies enable a large supercurrent through the normal
metal, at finite voltages and moderate transparencies, another process
takes place whereby an electron from the superconducting electrode
enters into the other after bouncing back and forth through the normal
metal between the two S electrodes (multiple Andreev reflections).

In this experiment, by improving the interface transparency between the
superconductor and graphene, we could favor one transport process over
the other. This was done by running a large current through the sample
for a short time. We first observed an increase of multiple Andreev
reflections with no supercurrent, and further annealing led to a high
supercurrent with barely visible multiple Andreev reflections.


***

EY10345

Nanoimprinted Polymer Films Control the Alignment of Rod-like Molecules

Polymer films nanoimprinted with checkerboard patterns of square wells of size varying from 200 nm to 800 nm align calamitic (rod-like) liquid crystals (LCs) vertically, horizontally or tilted depending on the depth/width ratio of the wells. Alignment of LCs is necessary for uniform optical properties of display pixels or enhancement of electrical conductivity of LC devices. Compared to other alignment methods such as mechanical rubbing or chemical modification of substrates, this method enables us to control the alignment of LCs accurately in a variety of direction solely varying the scale and depth of topographic patterns. The LCs prefer to lie down on polymer films that are smooth but when the films are topographically patterned, the increasing elastic energy density as the wells become narrower eventually overcomes the surface anchoring of the polymer and the average orientation of LCs makes a transition from planar to vertical. Nanoimprint uses polymerization of liquid while it is pressed by a mold with very fine patterns and this technique can produce many replicas at low cost. This work demonstrates great potential of using topographically patterned polymer films for the control of optical and electrical properties anisotropic soft matter for advanced and novel devices.

***

LZ10928

Slicing and dicing electronic wavefunctions

How much information does one need to describe the state of a physical
system? This question is intuitively simple in the realm of classical
physics: the information needed is roughly proportional to the size of
the system (at worst, one needs to specify the position of each atom).
But things become more intricate when quantum mechanics is at play. The
system is then described by a /wavefunction/, a complex mathematical
object whose information content can in principle grow exponentially
with the size of the system. Within some commonly used approximations of
quantum theory, the information needed appears to grow like the /square/
of the system size. A natural question then arises: when can a
wavefunction be described with an amount of information that is simply
/proportional/ to the system size?

This paper proposes a practical answer to that question by providing a
new way to decompose a wavefunction into pieces of decreasing sizes.
This process reveals to what extent a wavefunction can be "compressed"
in order to be represented with a minimal amount of information, while
preserving accuracy. An important practical implication of this result
is that it may simplify the way we model quantum mechanical systems
using computers. The vast amount of data needed to describe such systems
has severely limited the size of what can be "simulated" numerically.
Although this problem has been the subject of intensive research in the
past two decades, the present paper offers a promising new way to
describe quantum mechanical systems using as little information as possible.


***

AB10404

Why are ultracold atom Fermi superfluids so special?

Dilute atomic Fermi gases appear to be an unusual example of systems where
the attraction between fermions which leads to Cooper pairs is active over
a range of energies orders of magnitude larger than the Fermi energy.
Equivalently, this attraction, which is tuned by a magnetic field induced
Feshbach resonance, is active over a momentum range which extends to
nearly a thousand times the Fermi momentum. This is known to lead, for
example, to universal high frequency features in the spectroscopy of cold
atomic gases, a phenomenon which has been playfully dubbed the case of the
`tail wagging the dog'. Our recent work on superfluidity of cold Fermi
atoms in an optical lattice appears to provide yet another simple and
remarkable example of such `high energy' physics dominating the behavior
of the superfluidity of these systems. We show two striking results for
such optical lattice superfluids which follow from the assumption that
Cooper pairing between atoms in the lattice only occurs between fermions
in the same band. We show that the Hartree energy shift which is
important for inhomogeneous superconductors is absent for cold Fermi
superfluids in an optical lattice even though their density is nonuniform.
Second, we show that even though the optical lattice Hamiltonian is only
periodic under discrete translations, the pair field is completely uniform
in space. Our results indirectly cast serious doubt on the validity of the
enormous number of theoretical proposals over the years which attempt to
model such atomic superfluids in an optical lattice through the one-band
attractive Hubbard model or its variants.


***


BYR1092B


Manganese ion off-centrality breaks the "d-zero-ness rule" in perovskites.

First principle calculations confirm that Mn ion substituting for Sr
occupies the off-central position in SrTiO3. So, we have proved the
existence of a dipole impurity which possess a magnetic moment and
thus couples both with the magnetic and with the electric field. The
calculations evidence that off-central Mn ion induces a polar state in
the host lattice. This state violates the apparent mutual exclusion of
magnetism and ferroelectricity in perovskites that is called the
"d-zero-ness rule". The rule acknowledges the observation that
transition metal ions can cause a ferroelectric instability only when
they have empty electronic d-shell (see e.g. D.Khomskii, Physics 2, 20
(2009)). Now it seems to be restricted to the d-ions occupying B
position in a perovskite compound ABO3.


***

BV10605

Computer simulation of carbon nanotubes immersed in a fluid under high
pressures.

Due to outstanding elastic properties, carbon nanotubes have been termed
as nature’s ultimate springs. However, some studies found that under
pressure (~2×109 Pa) the tubes loose their shape and collapse to become
ribbon-like whereas others found the tubes retain their shape till much
higher pressures (~1×1010 Pa). To understand the different experimental
results, we theoretically investigated the high pressure behavior of
carbon nanotubes immersed in argon fluid using computer simulations
viz., classical molecular dynamics. We find that when the tubes are
empty they collapse at quite low pressures and form different
configurations of slightly different energies depending on the
conditions in the simulations. On the other hand, when the fluid is
present inside and is surrounding the tubes, their behavior under
pressure is substantially different. Surprisingly, at lower densities of
argon fluid, the pressure required to cause radial collapse is lower
than that of the empty tubes. At higher densities the fluid supports the
tubes and the collapse pressure is found to be higher. Moreover, we find
that the Ar fluid inside the tubes becomes increasingly ordered at
higher pressures. Moreover, the order of argon atoms is constrained more
by the tube-Ar interactions than the inter-atomic interactions amongst
Ar atoms.

***

LA11898A

What is the wave function of a single free particle with a given energy?

Quantum mechanics does not give a procedure to answer this very basic and
fundamental question. It could take a form of a single plane wave, a
Gaussian, or infinite number of other forms. This ambiguity is also part of
the great debate between Einstein and Born. In this paper, we attempted to
answer this question in the frame work of Madelung fluid dynamics as the
generalization of the Schrodinger equation, by allowing a rotational quantum
flow. “We ask for the most probable wave functions of a single particle
with a given energy, by constraining the quantum probability density to
maximize the Shannon information entropy”. We show that there is class
solutions which are self-trapped, rotationally symmetric, spinning yet
stationary. The stationarity is shown to come from the balance between the
attractive force of a self-generated quantum potential and the repulsive
centrifugal force of the spinning velocity field. We also showed that in the
asymptotic limit, the wave function is no more spinning yet is still
stationary and turns out to be equal to the lowest stationary state of the
Schrodinger equation of a single particle trapped in a cylindrical tube
external potential.

April 7, 2009

LW11712

Long Distance, High Intensity Laser Beams One Step Closer;
Made Possible by Crossing Two Beams in Air

Shooting a focused laser from the ground to the clouds for controlling
lightning and delivering power may now be one step closer to reality,
physicists from The University of Texas at Austin show.

The scientists demonstrated that they could cross two laser beams in
ambient laboratory air and transfer seven percent of the energy of one
of the beams to the other. They controlled the energy exchange by
simply adjusting a time-delay between the two laser pulses.

The beams were intense enough to produce “optical filaments,” which
are dramatic structures resulting from an interaction between the
light and air. The filaments exhibit a focus of high intensity light
that can persist for hundreds of meters. Controlling these filaments
could lead to advances in lightning control, remote sensing and power
delivery.

Laser beam control has usually meant changing the pulse launch
conditions. The Texas physicists’ innovation is a new "knob" they
used to control filaments during their actual flight.

Their work implies that filaments could be made to propagate further
before having their energy depleted. A beam could be amplified
repeatedly by crossing other beams at different points along its path,
almost as repeaters do with conventional fiber optic technology.


***

LY11094

A Superheated State in Granular Matter

Melting a solid substance is almost a trivial phenomenon: one only needs to increase the temperature of whatever we want to melt in order to reach the melting point. Nevertheless, some times we heat quite beyond this temperature and solids do not melt. This strange phenomenon is called superheating. We have found that granular matter behaves also in this bizarre way. We confine a granular layer made of hundreds of spherical beads, inject vibrational energy to the point where the system should be a gas, and yet, it does not melt. Since there is no cohesion between the grains, it is dissipation what holds the granular layer together, until a fluctuation appears and makes it to “explode”.


***


LB12524


Decoding Quantum Information of Black Holes by 3d Crystals

We developed a new method to use crystal melting models in three dimensions to identify quantum states of black holes in superstring theory.

In 1974, Stephen Hawking showed that black holes, though they are completely dark as classical solutions to the Einstein equation, emit heat and evaporate by quantum effects. If this phenomenon obeys the standard laws of statistical mechanics, there must be an enormous amount of quantum information stored in a black hole1.

In this paper, we showed that each quantum state of a particular class of black holes in superstring theory corresponds one-to-one to a molten crystal in three dimensions. For example, an ice is a crystal of water molecules. When it melts, it starts losing molecules from its corners. Similarly, the space-time without a black hole corresponds to a perfect crystal. As the crystal loses molecules, the black hole grows larger. In the thermodynamic limit, where the size of individual atoms becomes negligible, we showed that smooth space-time emerges and Hawking’s prediction is reproduced.

***

EA10620

Breaking-rate minimum predicts the collapse point of overloaded
materials

As a model of composite materials, we choose a bundle of fibers with
stochastically distributed breaking thresholds for the individual
fibers. The fibers are assumed to share the load equally, and to obey
Hookean elasticity right up to the breaking point. We study the
evolution of the fiber breaking rate at a constant load in excess of
the critical load. The analysis shows that the breaking rate reaches a
minimum when the system is half-way from its complete collapse.


***

LW11744

Ultracold Fermi-condensate sensors for dynamic imaging of
electro-magnetic fields

Ultracold gases provide micrometer size atomic samples whose
sensitivity to external electro-magnetic fields may be exploited in
future sensor applications. In this paper, we propose theoretically
that a Fermi condensate of atoms provides an excellent system for
dynamic imaging of the fields. Bose-Einstein condensates of bosonic
atomic gases have already been demonstrated to perform excellently as
magnetic field sensors in atom chip experiments. As such, they offer a
combination of resolution and sensitivity presently
unattainable by other methods. These sensors are based on
modifications of the atom cloud density due to the magnetic field that
they are sensing. The Fermi condensate sensor proposed in our work is
based on a different principle: creation of single particle
excitations in the gas. The Fermi condensate has an excitation gap,
i.e. a forbidden energy range, therefore only high enough frequencies
of the magnetic field the sensor is monitoring can cause excitations.
By tuning the excitation gap, one can thus resolve the
frequencies present in the magnetic field, as well as its spatial
distribution. Sensitive and high-resolution magnetic field detection
is needed for instance in non-invasive characterization of weak
electrical currents in micro- and nanosystems.

April 3, 2009

LR11774BR

Dissipationless flow of electrons---one by one

Electrons can flow without dissipation between two superconductors in
close proximity by transferring a pair of electrons---known as Cooper
pairs---at a time, a phenomena known as Josephson effect. We
theoretically discovered a dissipationless one-by-one electron
transfer from one superconductor to the other giving rise to
``fractional Josephson effect''. This occurs when the two neighboring
superconductors are connected by a new state of matter called a
topological insulator, which was theorized in 2005 and experimentally
discovered a year ago. The usual two-charge transfer occurs because
two electrons tend to bind together and form a Cooper pair inside a
superconductor. The new single-charge transfer that we predict is
possible because an individual electron finds an extra zero energy
"Majorana bound state" to stay at the interface between the
superconductor and the topological insulator. Our prediction can be
readily tested in future experiments

***

LX11765

Imaging Beyond the Diffraction Limit by Resistive means

Diffraction sets a fundamental limit to the resolution of an imaging system, restricting the ability to discriminate objects smaller than a wavelength. Here we present an approach for subwavelength imaging using a mundane conducting film as a natural optical superlens. It is theoretically predicted that near field sub–diffraction-limited imaging is possible as the film allows the recovery of critical evanescent waves that define a sharp image. This happens because space acts like a low pass filter for highly evanescent field components, and if a sheet or thin layer of imperfectly conducting material is placed adjacent to a source, such that the layer overcomes the larger impedance of the spatial low pass filter, no relative attenuation of evanescent components is experienced at the location of the sheet, resulting in a very sharp image (spot sizes of roughly 5% of the illumination wavelength are observed). The conducting layer enables us to trade definition for amplitude. Impedance sheets are commonplace in RF/microwaves, hence the phenomenon identified here is widespread, and can be easily extended into the Infrared and Terahertz regions, as well as to other areas of Physics where wave motion exists.

***

LW11257B

Spontaneous localization of dynamic energy in a simple ionic crystal

It has been shown over the last decade that driving a discrete
nonlinear lattice can cause dynamical energy to spontaneously
localize. A fundamental question in condensed-matter sciences and
nonlinear dynamics is whether or not such intrinsic localized modes
(ILMs) can appear in an atomic lattice in thermal equilibrium.
Neutron scattering measurements of He-4 and in alpha-U at high
temperatures have indicated new modes, possibly attributable to ILMs,
but these interpretations remain speculative since realistic models
of the nonlinear lattice dynamics are not available. These systems
are also exceptional in that both exhibit many exotic phenomena;
alpha-U is the only element to exhibit a charge density wave and
solid bcc He-4 is a quantum solid. The occurrence of new modes in
either of these systems, while interesting, does not have broad
implications since the underlying cause is related to rather unique
properties. By contrast, here we report the experimental observation
of ILMs in a remarkably simple ionic crystal, NaI, at high
temperatures and further show that these results are consistent with
realistic molecular dynamic simulations. Our work presents the first
observation of intrinsic 3-D localization requiring only discreteness
and nonlinearity in an atomic solid.

***

LZ11265AJ

Can relativity bother quantum cryptography?

Modern physics is dominated by quantum mechanics and relativity.
This is fair to say that Bell inequalities probe one of the deepest
aspects of quantum mechanics. The genesis of the Bell's discovery
can be traced back to the Einstein, Podolsky and Rosen seminal
paper about the completeness of quantum mechanics. They have
argued there that quantum mechanics would not provide a complete
description of nature if locality is assumed. Quantum mechanics
had to wait thirty years to be vindicated by John Bell who introduced
the inequalities which allow to test quantum mechanics against
competitive local hidden variable theories. If the spooky quantum
mechanical effect named entanglement [where non-causally related
particles can influence one another (see PHYSICAL REVIEW FOCUS,
27 December, "Spooky at any speed")] were correct, Bell inequalities
would be necessarily violated. Remarkably Bell inequalities have
been shown to be violated by 30 standard deviations, which strongly
supports quantum mechanics.

On the other hand, the fact that causally disconnected particles
can influence one another if they are quantum mechanically entangled
has raised an intense debate on the interplay between relativity and
quantum mechanics. In our work "Influence of detector motion in Bell
inequalities with entangled fermions", we investigate how relativity
influences the spin correlation of entangled fermions measured by
moving detectors. Suppose the physical situation where two entangled
spin-1/2 electrons described by wave packets fly in opposite directions.
At some point when they are far away one from the other (and thus
causally disconnected) each particle finds a spin detector. Although
actual experiments confirm that Bell inequalities are violated as
predicted by quantum mechanics when the detectors lie at rest,
we show that quantum mechanics will predict a quite different
output if the left and right spin detectors are set in fast enough
relativistic motion, namely, the CHSH Bell inequality will be
*satisfied* rather than violated.

Entanglement of quantum systems is currently used in many applications
including quantum cryptography protocols which is beginning to be
commercially traded. As technology develops, we expect that
quantum cryptography will be used to exchange messages around
the globe with the help of satellites. Because they move fast with
respect to the Earth surface, our work anticipates that relativity
should play some role here. This is difficult to anticipate at this
point whether or not this is going to be a protagonist one as in the
GPS case.


***

BZ10846

Listening to Underground Phonons

Ultrathin metal films on stiffer substrates can guide various kinds of
sound waves, some travelling at the surface, some underneath, and some at
the interface with the substrate. Not just the surface waves, largely
exploited in surface acoustic wave (SAW) devices, but especially their
sub-surface companions promise a future in novel electro- and
opto-acoustic devices, thus widely extending their application spectrum.
However, the rich family of sub-surface phonons remained so far elusive to
current surface probes such as electron energy loss spectroscopy. It comes
now as a surprise that the gentlest of all surface probes, helium atom
beams, can actually measure the dispersion of most sub-surface phonons.
Although He atoms merely tickle the surface a few Ångstroms above the
topmost atoms, they perceive the motion of the underground atoms via the
electron density oscillations at the surface. This mechanism, first
pinpointed in a previous study on the surface of copper, is now found
to work best with ultrathin lead metal films. Its electrons are highly
responsive to atomic motion making lead the element with the second
highest superconducting transition temperature (7.23K). Since this
responsiveness, shared by most metals, governs many thin-film transport
phenomena, measuring underground phonons is not only a significant step in
surface spectroscopy, but also points the way towards new nanometric
devices.

April 1, 2009

LX11061

New insight into how atoms move in highly-ordered binary compounds

Atom movement in solid compounds has many applications in technology. Most
often, atom movement takes place through the presence of a very small number
of lattice vacancies into which neighboring atoms can jump, like a slide
puzzle. In a highly ordered compound of two elements, A and B (think of
sodium chloride), different sequences of jumps are possible depending on
whether the vacancies are on A-sites or B-sites. In this paper, we show a
way to determine whether A-site or B-site vacancies are primarily
responsible for long range atom movement. Furthermore, we carried out
measurements on a series of compounds of rare-earth elements with indium and
found that A-vacancies are responsible for long range movement at one end of
the series while B-vacancies are responsible at the other end. This
remarkable finding is unexpected in light of the great chemical similarity
of rare-earth elements.

***

LU11918

A Zoo of Carbon and Oxygen

As the use of carbon-based nanotechnology is becoming ever more
prevalent, insights into how graphene, carbon nanotubes, and other
carbon-based materials behave in an oxygen atmosphere is ever more
important. This work investigates the oxidation mechanism of graphene,
specifically how oxygen interacts with its basal plane. The results
indicate that the perfect regions on the basal plane are inactive, but
that oxygen is able to attack vacancies. The reaction path proceeds in
two stages: all dangling bonds and those under stress are firstly
saturated with oxygen, and large oxygen functional groups subsequently
evolve. This work also shows that these groups also follow a strict
energetic hierarchy, which is split between the two stages. The dominant
reaction mechanism is suggested by the nature of the hierarchy, and is
expected to be generally valid for a number of sp2-bonded nanomaterials.
This detailed knowledge about possible oxidation sites on graphene and
the resulting functional groups is a critical piece of information for
nanoscience - just about every device comes into contact with the
atmosphere.

***

LR11546E

Is Jarzynski's Equality Practical For Free Energy Reconstruction?

The atomic force microscope (AFM) is a tool that uses a tiny,
needle-like tip to catch and manipulate single proteins. When a
protein is caught and stretched like a rubber band we can measure the
amount of force needed to stretch the protein. Jarzynski's equality is
a formula used to analyze this data to determine the amount of energy
needed to stretch and unfold a protein. This is important because this
information can help us understand the way proteins fold and unfold,
as well as tell us how certain proteins may function and behave in the
human body.

***

LW11013B

Optics Clues to Pairing Glues

One of the hottest questions in condensed matter physics is: What
causes superconductivity in cuprate superconductors with an
exceptionally high Tc (the temperature above which the material
becomes a normal conductor)? We obtained a precise answer to this
question by measuring and analyzing optical spectra of a large number
of high Tc superconducting materials.

Supercurrents are carried by bound pairs of electrons. Binding of two
electrons can occur if an electron polarizes its surrounding medium,
and a second electron is trapped inside this polarization cloud. This
induced polarization depends in general on the motional energy of an
electron. We deduced from our optical spectra the amplitude of the
induced polarization as a function of electron energy.

The main novelty is that we prove that aforementioned formalism
enables us to make detailed predictions: Firstly, if for a given
sample this relation was determined from the optical data taken at,
say room temperature, we can tell exactly what the optical spectra
will look like at all other temperatures. Secondly, knowing these
polarization amplitudes we were able to predict Tc and compare it to
the actual value. These predictions are ‘only’ two times higher than
the actual Tc’s, which is very good considering that, among other
things, we have not taken into account the effect of impurities which
are know to strongly reduce Tc.

A related hotly debated issue is, whether aforementioned polarization
is an elastic deformation of the crystal structure, or a so-called
spin-polarization (a magnetic effect). While we obtain indications in
our data for both types, we also observe in the so-called overdoped
samples that the elastic deformation effect is much too weak to
explain Tc, while Tc is still very high. We therefore can eliminate
elastic deformation as “the mechanism of high Tc” in favour of the
other possibility: High Tc superconducting pairing is mediated by
spin-fluctuations.


***

AA10400

Orienting molecules with a one-two laser punch

Spatial orientation of molecules is a pervasive issue in chemical physics
and, by breaking inversion symmetry, has major consequences in nonlinear
optics, allowing sum frequency generation for example. In this paper, we
propose and analyze a new approach to create an oriented sample of
molecules. Present laser techniques are good at aligning molecules, i. e.
making each molecule stand parallel to the rest. Still, out of a large
ensemble, half the molecules will stand on their feet, and others will
stand on their heads. In this work, we show how a combination of tailored
and timed laser pulses hitting the molecules in a one-two punch manner can
be used to selectively remove molecules with an unwanted orientation.
Specifically, subjecting an aligned molecule to a tailored infrared laser
pulse creates a pair of coherent wavepackets that correlate vibrational
phase with the head-up or head-down orientation. Subsequent, suitably
phased ultraviolet pulses dissociate molecules that have their bonds
stretched, thereby "weeding out" one but leaving intact the other
orientation.

March 30, 2009

LX11472


Strained layers curl up into hyperlenses

Rolled-up three-dimensional metamaterials with tunable plasma frequency
might pave the way to the realisation of hyperlenses working in the visible
regime.

We propose and demonstrate that three-dimensional radial metamaterials can
be created in a strain relaxation process by rolling-up planar
metal-semiconductor double layers with multiple rotations. The walls of the
resulting rolled-up-carpet like structures represent high quality
three-dimensional radial superlattices with accurately tunable unit cells
and lattice constants. Transmission experiments through these superlattices
reveal that they can be described as radial metamaterials with an effective
plasma frequency which is, in contrast to natural metals, not restricted to
the ultra violet but tunable over a broad range in the visible regime by
adjusting the ratio between metal and semiconductor layer thickness.
Effective-medium-picture-based considerations to use these radial
metamaterials as easy to process freestanding hollow hyperlenses for the
visible are confirmed by finite difference time domain simulations.

***

LW10937

Studying Spectacular Exploding Stars

Using the Daresbury Recoil Separator and Holifield Radioactive Ion Beam Facility at Oak Ridge National Lab, we have directly measured a nuclear reaction with radioactive nuclei crucial to our understanding of exploding stars. In stars that are denser, hotter and smaller than the sun, like the "Pup Star" Sirius B, thermonuclear runaways can occur once the temperature and pressure on the surface are high enough. These nuclear explosions, known as novae, release huge amounts of energy, and create lots of radioactive isotopes which are expelled into space. In order to understand just how powerful these explosions are and how much of any given isotope they produce, we have to know the rates of the nuclear reactions taking place. Usually, we have to measure these rates indirectly because of the almost insurmountable difficulties involved; but for the first time we have measured directly the rate of proton capture on radioactive fluorine-17, an important step in the chain of nuclear reactions during these explosions.

***

LW11137

Quantum Ghosts Are Useful

The idea that far distant particles can somehow 'talk' to each other so
perturbed Einstein that he called such weirdness 'spooky action'.
Scientists today are learning how to use the quantum entanglement that
gives rise to spooky correlations as a resource and now a team of
physicists at the University of Bristol have harnessed this phenomenon to
shed light on another unusual and previously intractable aspect of quantum
physics - that of distinguishing between two similar quantum operations.
In the everyday world any process can be visualised as some black box with
an input and an output; if you would like to identify a box you simply
apply some input, measure the output and deduce what happened in between.
But quantum black boxes are different. Distinguishing between two quantum
black boxes that are similar can be impossible with only single particle
inputs since it is not generally possible to then distinguish the different
outputs. The Bristol team demonstrate how distinguishing between two
similar quantum boxes becomes possible when quantum entanglement is used.
Apart from providing insight into the fundamentals of quantum physics, this
work is also crucial for future quantum technologies - how else could a
future quantum engineer build a quantum computer if she can't tell which
circuits she has!

***

LB11950


A new quasiparticle state found in oxide nanostructures

Oxide heterointerfaces frequently produce unexpected and unusual
electronic and magnetic states: magnetic, orbitally ordered, charge
ordered, and conducting behavior that is borderline between insulating
and magnetic, with the metal-insulator transition be triggered by
subtle effects. Nanolayers of VO2 provide a distinctly different class
of novel phenomenon in oxide nanostructures: (1) the ions are not
pushed away from their formal valence state, so there is no impending
"polar catastrophe," (2) a topologically distinct electronic structure
results for a specific thickness of VO2 slab encased in insulating
TiO2: a single point in (momentum) k space separates filled and
unfilled states. This point Fermi surface is analogous to the Dirac
point in graphene, but is even more unusual. First, the VO2 slab is
half metallic -- only majority spin states appear near or at the Fermi
level. Second, the (two dimensional) dispersion of the two bands away
from the point is Dirac-like (linear) along one principal axis, while
it is effective-mass-like (quadratic) along the other. This
"semi-Dirac point" carries with it different transport properties than
a Dirac
point, different behavior with doping, different behavior in a
magnetic field. This unique new state displaying a novel type of
quasiparticle behavior is a theoretical discovery, but there is strong
reason to expect it to be realizable with current technology.
Especially because it is a magnetic, conducting, nanoscale system, it
could well have applications in the spintronics arena.


***

LZ11496

Bouncing atoms off of light

In this paper, we demonstrate the ability to “bounce” atoms off of
light. Whenever atoms scatter light, they get a momentum kick that
can be used to manipulate their motion. With careful control, the
kicks can be made very precise and repeatable. We applied this
technique to atoms that were falling in gravity. We dropped the atoms
and, when they were moving at the correct speed, applied a laser pulse
that reversed their motion just as if they had bounced off a surface.
Eventually gravity pulled the atoms back down, and the pulse was
applied again. This could be repeated up to a hundred times before
the atoms were lost due to imperfections in the laser... many more
bounces than you can get with a rubber ball. The bouncing technique
has a number of potential uses. We explored the measurement of
gravity, which has applications in geophysics, energy exploration, and
inertial navigation. We could obtain the strength of gravity simply
by measuring the time interval between the bounces. Other possible
applications include simulating zero-gravity (without the expense of
spaceflight), cooling the atoms to the lowest possible temperatures,
and improving the accuracy of atomic clocks.

***

LV11622B

Information content in x-ray emission spectra of liquid water.

Ab initio molecular dynamics simulations are used to evaluate the claims
from x-ray spectroscopy of liquid water.
Recent high-resolution oxygen x-ray emission spectra show two distinct
lone-pair peaks for liquid water.
Does the fine-structure in oxygen K-edge x-ray emission imply that
liquid water is a two-component mixture
or is it the signature of a transient OH species arising in the
core-excitation process?

Just as the interpretation of x-ray absorption of liquid water, this
question is intensely discussed in the x-ray
spectroscopy community, because x-ray emission is an independent probe
of the electronic structure containing
complementary information. In this paper, ab initio molecular dynamics
simulations are used to show that the water
lone-pair features are of fundamentally different origin. One is
primarily due to the lone-pair of the intact water
molecules, the other is assigned to a transient OH species formed by
ultra-fast photo-dissociation.
Hence, x-ray emission cannot be taken as evidence of a two-component
mixture model of liquid water with classes
of molecules in distinctly different H-bond environment. Instead x-ray
emission is a unique technique to study
the ultra-fast response to high-energy radiation.


***

EX10270

But, just where is the interface?

For several years now, molecular computer simulations have been providing a
wealth of information about phase coexistence: the conditions at which two
phases are simultaneously stable, and the details concerning the molecular
region that separates them: the interface. For instance, "explicit
simulations of interfaces" are able to directly model interfaces, such as the
liquid-vapor interface. However, it turns out that the exact location of the
interface is delicate to define, at least in mathematical terms. In this
paper, a new proposal employs concepts taken from the field of
computational geometry, where the definition of the "shape" of a set of
points is a well-known problem. In particular, the alpha-shape construction,
which was originally introduced in order to identify the boundary of a set of
points, is used in order to define our interfacial molecules.

Image Caption: A snapshot from a simulation of liquid-vapor coexistence of a model
for noble gases. Solid spheres are the atoms that are identified to be "at"
the surface by our method. Also shown, the interface is modeled by the
triangulated surface (red triangles and blue ridges).


***

LY11398B

Photoionization can be a useful tool for studying single-electron
transistors and other nanodevices

In this paper, we demonstrate for the first time, based on theoretical
results, that photoionization can be a useful tool to investigate
single-electron transistors, and encourage experimentalists to use it for
these and other devices of interest for nanoelectronics. The reason is
that photoionization permits to obtain information on how many electrons
occupy a quantum dot and the charging energy in a direct manner. This is
very important, because experiments carried out up to now, which measure
the electric conductance, only allow to determine these quantities
indirectly. It is worth emphasizing that in the photoionization processes
considered by us, an electron absorbs a photon with energy of the order of
the work functions (typically, 1 eV) and is ejected into the vacuum. This
phenomenon is completely different from the widely studied photo-assisted
tunneling considered by previous investigators, involving much lower
photon energies (typically, a few meV). We give concrete suggestions on
how to conduct experiments using photoionization alone or in combination
with transport measurements. Monitoring zero kinetic energy (ZEKE)
photoelectrons is especially recommended, because ZEKE--spectroscopy
offers a better resolution than standard photoemission.

***

BZ10479

Graphene Stays Cool No Matter What

It was recently discovered experimentally that graphene, which is just a
single layer of carbon atoms arranged in a honeycomb pattern, exhibits far
better thermal conductivity than any known material including diamond and
carbon nanotubes. This discovery opened a new window to graphene
applications in electronics and thermal management. At the same time, the
physical mechanisms behind this superior thermal property of graphene
remained a mystery. In this paper, we have shown theoretically that the
dynamic properties of graphene crystal lattice and its strictly
two-dimensional nature are responsible for extremely high thermal
conductivity of graphene. Phonons, quanta of crystal lattice vibrations,
which carry heat in graphene, propagate with high velocities and do not
scatter as strongly as they do in conventional three-dimensional bulk
crystals. We also explain why the thermal conductivity of graphene depends
on the width of graphene flakes. The results of the paper help to pave the
way for graphene applications in heat removal from electronic chips. The
chip overheating is now one of the most serious problems faced by the
electronic industry.

***

LT11155BR

The parent compounds of high-Tc cuprates become superconducting!

The parent compounds of high-Tc cuprates have long been
considered to be antiferromagnetic Mott insulators. For example,
La2CuO4 with the K2NiF4 structure is an insulator with no doubt.
R2CuO4 (R: rare-earth element) with the Nd2CuO4 (abbreviated usually
as T’) structure has also been believed as a Mott insulator since
the discovery of “electron-doped” superconductors, T’-(R,Ce)2CuO4
in 1989. Our recent work, however, has demonstrated T’-R2CuO4 to be
superconducting. The origin of the sharp contradiction between the
past and our results can be traced to impurity oxygen at the apical
site. Impurity oxygen atoms in T’ cuprates play the role of a very
strong scatterer as well as a Cooper-pair breaker. Therefore the
generic behavior of T’-cuprates can be reached only after complete
removal of impurity oxygen atoms. We employed a new thin-film
process, low-PO2 firing followed by low-temperature reduction, to
clean up impurity oxygen atoms, then achieved superconductivity in
the parent compounds, T’-R2CuO4. Our results, although further
works are required, throw strong skepticism on the currently accepted
“doped Mott-insulator” scenario for high-Tc superconductivity:
high-Tc superconductivity develops upon doping of either holes or
electrons in Mott-Hubbard insulators

***

LA11699

The Knizhnik-Polyakov-Zamolodchikov Formula Finally Proven

The famous KPZ formula appeared in 1988 as a striking application of
string theory to two-dimensional statistical mechanics: it predicted the
existence of a precise relation between the fractal dimension of a
random subset of the plane and its dimension in the presence of
two-dimensional quantum gravity. More than twenty years after its
discovery, this formula is finally rigorously proven in this paper,
within the realm of probability theory and Liouville quantum gravity.

In Liouville quantum gravity, the usual Euclidean area element dxdy of
the standard xy plane is replaced by a quantum area element,
dA=exp[h(x,y)] dxdy, where h(x,y) is the so-called Gaussian free field,
a stochastic two-dimensional generalization of Brownian motion.
Measuring geometrical sets with this quantum metric is tantamount to
randomly exploring very high mountains and very deep valleys at all
(infinitesimal) scales. In this paper, the proof rests on a fine
mathematical analysis of the local averages of the 2D Gaussian free
field. By focusing on the average values of the field restricted to
concentric circles, we reduce the problem to a calculation involving a
standard one-dimensional Brownian motion.

Perhaps most surprisingly, our method shows that the KPZ formula holds
for all planar fractals, and not only for restricted classes of
conformally invariant sets, as originally assumed. Several of the most
fundamental open problems in 2D quantum gravity (such as proving that
"discrete quantum gravity" based on random triangulations has Liouville
quantum gravity as a continuum limit, and identifying the geometrical
nature of quantum geodesic paths) can now be precisely formulated and,
we hope, settled using the framework we introduce here.

***

LC12080

From three to four: a quantum leap in few-body physics

Already in the 1970's, the Russian theorist Vitaly Efimov found a
stunning solution to the quantum three-body problem, predicting a series
of ultra-weakly bound trimer states. It took more than 35 years until
first evidence of the mysterious "Efimov" three-body states was found in
ultracold samples of optically trapped Cs atoms (Kraemer et al.,
Innsbruck, 2006). After this long time the field is now taking off at an
amazing speed, and an increasing number of ultracold atomic and
molecular systems reveal traces of Efimov states.

The addition of a further particle, the step from three to four bodies,
leads to an enormous increase in complexity with great challenges for
its theoretical description. Recently, two theory groups (Hammer and
Platter, Bonn & Ohio, 2007; von Stecher, D'Incao, and Greene, Boulder,
2008) predicted the existence of pairs of four-body states being closely
tied to Efimov trimers. The experiment of the Innsbruck group now
confirms the central theoretical predictions. The results on
recombination in an ultracold gas of cesium atoms show a pair of
resonances caused by four-body processes as fingerprints of the
predicted pair of four-body states.

***

LV11317A

Linking algebra of qubit pairs to projective geometry

A coupled pair of quantum spins ("qubits") describes many
interesting phenomena in quantum cryptography, quantum teleportation,
and quantum computing. These topics are also increasingly of applied
interest. The mathematics that physicists use in these descriptions
is called Lie and Clifford algebras. However, other branches of
mathematics such as projective geometry and design theory have not
been associated with qubits. But this paper points out such links,
and also to a set of "hypercomplex" numbers called octonions (which
are generalizations beyond reals, complex numbers and quaternions,
all of which have applications throughout physics). A striking
feature of projective geometry is a duality between points and lines
so that any valid theorem remains so upon interchanging points and
lines (not true of ordinary geometry). Thus, a diagram of seven
points and seven lines, with each point lying on three lines and each
line containing three points, occurs both as the smallest projective
plane and for describing the multiplication table of the seven
octonions. This diagram is now linked in this paper to one of the
algebras involved in quantum logic gates built out of two qubits.
These connections may be mutually exploited in both Lie algebras and
projective geometry.

***

LZ11585A

SUDDEN DEATH AND SUDDEN BIRTH OF ENTANGLEMENT WITH MEMORY

Entanglement is "the characteristic trait of quantum mechanics", as
Schrödinger stated almost a century ago. It constitutes a key resource
for a number of applications of modern physics, offering a new way of
transmitting information and performing controlled interactions on
quantum bits. Entanglement will certainly be an essential ingredient in
the realization of quantum computers.
However, quantum properties are very fragile, decoherence is
omnipresent, and sometimes entanglement can be completely destroyed in a
finite time. So in order to control decoherence and preserve
entanglement, a deep understanding of the disentanglement process in
realistic situations must be achieved.
In our paper we study the exact entanglement dynamics of two quantum
bits (qubits) in a common environment characterized by memory, for
example two atoms in a leaky cavity. Such a system exhibits interesting
features as the resurrection of the qubit-entanglement after a period of
death, or the revivals of disentanglement after the sudden birth of
entanglement. Our results shed new light on the role that the
environmental memory plays in the entanglement dynamics, and might help
to understand how to exploit such a crucial resource in the future.

***

EX10283

Capsule in micro channel flow

In this paper, we investigated the initial motion of capsule in micro channel flow just after release by a novel numerical simulation method, which combines two methods, one for solving the mechanical problem of fluids inside and outside the capsule membrane, and the other for tracking the capsule membrane. Studying the motion of capsule in micro channel flow is quite important in physics, physiology, and pharmaceutics. For example, it may deepen our understanding of red blood cell behavior in blood vessels. Our results show that the capsule behavior depends on initial capsule shape, initial capsule position, and membrane mechanical properties. Off-center capsules, whose initial center positions are not on the center-line of tube, tend to migrate towards the tube center-line because of the influence of fluid flow. This leads to the development of a cell-free layer around the tube wall. At the same time, off-center capsules experience tank-treading motion, i.e. the membrane rotates around the interior fluid.

***

BZ10683

Magnetism at the interface between non-magnetic oxides.

In this paper we report the appearance of magnetism at the interface between two non-magnetic oxides due to a surface reaction. Mixing Co3O4 with TiO2 we found room temperature ferromagnetism despite the antiferromagnetic and diamagnetic character of both oxides respectively. The nice point of our work is that we do not need to claim a new kind of magnetic interactions to account for it but we can explain it with the well known theories of magnetism in oxides developed in the 60’s but applied to surfaces and interfaces instead of bulk materials. The key idea is that Co3O4 is very similar to Fe3O4 (which is magnetic at room temperature). The slight differences among them that make Co3O4 non-magnetic disappear when the Co3O4 is mixed with TiO2 due to a surface reaction with transfer of some electrons from one oxide to the other. Thus, a thin layer at the surface of Co3O4 grain becomes ferromagnetic. This surface magnetism “was always there” but now we are able create materials with a large fraction of surface atoms (for which the effect is significant) and we are now able to measure with a extreme precision that was not possible 40 years ago. Similar effects have been recently reported in epitaxial films (explained in terms of new magnetic ordering mechanisms) with a deep control of growing conditions but the possibility to arise them by simple mechanical milling we show here, will increase the possibilities to use and explode it for applications.

***

CB10204

The “middle-of-the-road” nucleus 106Zr

The article predicts the spectroscopic properties of 106Zr, an atomic
nucleus with 106 nucleons of which 40 are protons. As such, it is rich in
neutrons and short lived, and lying just at the limit of current detection
capabilities. Its interest resides in the fact that it sits right in the
middle between the ‘magic’ neutron numbers 50 and 82—numbers that define
the usual shells of a nucleus. According to novel theories, this
traditional shell structure may well be modified in very neutron-rich
nuclei that are increasingly being probed with radioactive-ion beams. So,
whether 106Zr behaves as a nucleus at mid-shell will ultimately depend on
the character—magic or not—of its far-away sibling 122Zr with 82 neutrons.
The comparison of our prediction with future experimental studies of 106Zr
might help to indicate whether the magic number 82 persists in the heavy
zirconium isotopes, long before the nucleus 122Zr itself will become
experimentally accessible.

***

LL11329

Does the zero-bias anomaly always signify Kondo physics?

Summary: Once a conductance peak at zero bias, the so-called zero-bias
anomaly (ZBA), is observed in various mesoscopic systems, it is immediately
associated with the intriguing Kondo effect. Here, in contrast, we show
this is not always correct and the ZBA observed in quantum wires is in fact
very different. This paper presents experimental evidence showing that the
temperature and magnetic field characteristics of the ZBA peaks in quantum
wires are inconsistent with Kondo physics. In addition, it is found that
the single ZBA peak still occurs in a fully spin-polarised regime in which
the Kondo spin-flip is prohibited. We demonstrate that a shift in
one-dimensional (1D) energy levels with source-drain bias can reproduce the
ZBA and, unlike the other systems, the 1D system does not need to have the
Kondo mechanism to give rise to a zero-bias conductance peak. This
manuscript is thus expected to have a significant impact on the general
understanding in this active field and to generate substantial interest in
the community.


***

BAR1150

Electrostatic Noise on the Nanoscale

As it becomes more common to fabricate and study electronic devices
on the nanoscale, techniques that can characterize and predict performance
become essential in understanding new phenomena and can lead to improvements
in fabrication and design. We show
that by using electrostatic force detection techniques one can pinpoint
areas over semiconducting surfaces where certain types of electric
charge noise are prominent.
Fluctuating charge is detected via the resulting electrostatic force by
measuring the resonance frequency of a small conducting cantilever placed
only a few nanometers away from the surface. This method of detection has
been shown capable of detecting the electrostatic force from single
electrons, and in this study has shown lateral resolution of at least 20 nm.
Cockins et al. also show that the noise characteristics can be influenced
by light. As charge noise is almost always detrimental to device operation,
and as the mechanism responsible for this particular type of charge noise is
instrumental in photovoltaic operation, the combination of electrostatic
force detection with surface mapping and optical excitation provides a means
to link nanostructure to device performance.

March 23, 2009

LZ11793

The drunkard returns, coherently.

A group of physicists from the Weizmann Institute of Science and the
Technion - Israel Institute of Technology have shown that one can "see"
the effective dimension of a random walk by looking at its spectrum. In
an experiment exploiting electromagnetically induced transparency, the
random walkers are Rb atoms diffusing in a buffer gas that kicks them
randomly. The atoms are prepared in a coherent superposition of their
two ground states and starts to oscillates like tiny clocks. The atoms
are probed whenever they return to their starting point. Interestingly,
it was proven by G. Polya back in 1932 that a random walker always
returns to the origin in dimensions d=1,2. As the authors explain, by
measuring the average time of return to the origin, one can learn about
the dimensionality of the diffusion. Surprisingly enough, this average
diverges in dimension one and two – a consequence of Polya's theorem.
The authors have measured a related quantity called the "critical
exponent" of the spectrum, which determines this divergency. Using this
spectroscopic probing technique it is possible to learn about the
underlying dynamics of other systems such as quantum billiards or
spintronic devices.

***

LX11147AR

Quantum information processing in optical fibers

Quantum information science aims to harness uniquely quantum effects
to gain advantages in information technologies; photons with their
high-speed transmission and low noise properties appear destined for a
central role. A group of physicists and engineers from the University
of Bristol report a two-photon quantum logic gate implemented entirely
in optical fiber. This all-fiber implementation is of crucial
importance to future applications since it allows the logic gate to be
miniaturized and admits high performance operation. Crucially, such a
fiber approach is compatible with optical fiber communication, which
is expected to form the backbone of future quantum networks that offer
security based on the laws of physics. The controlled-NOT logic gate
reported is also the fundamental building block for many quantum
technologies, including quantum information processing. It was
constructed from specially fabricated fiber couplers (in which two
optical fibers are joined in a short section, enabling photons to hop
from one fiber to the other) that affect the different polarizations
of light in different ways.

***

LA11792

Entanglement, at the heart of quantum computers, can also be a
curse

Entanglement is the key property which enables quantum
mechanical particles to perform some tasks - for example
unconditionally secure cryptography or certain otherwise
intractable computations - which are impossible in the everyday
classical world. Our recent work points to a new aspect of
entanglement: it must come in the right dose to be of use.

In quantum mechanics, two particles are said to be "entangled"
if their behavior is correlated more strongly than classical
physics would allow. While entanglement is a prerequisite for
powerful quantum computers, it also gives rise to the intrinsic
randomness of quantum experiments ("God playing dice").

Any computational scheme must find ways for compensating for
this probabilistic nature. While it is known how to efficiently
cope with the quantum mechanical uncertainty in some specific
cases, no scheme has so far been discovered which could utilize
any quantum system with sufficient entanglement for
computational purposes. Such a universal scheme would have been
very desirable: physicists would like to take advantage of the
states they naturally find in their laboratories, instead of
having to carefully engineer one of the few states known to be
computationally powerful.

Our recent work shows that such a general scheme cannot exist.
We establish that, if the amount of entanglement in a system
grows too large, the intrinsic quantum randomness becomes so
pronounced that no useful information can be extracted.

So while it remains an important and fruitful task to identify
new quantum systems with computational power, researchers must
bear in mind that this property is more elusive than previously
thought.

***

LY11340

NSCL researchers constrain symmetry energy at low density

By analyzing data from several combinations of collisions of tin nuclei,
researchers at Michigan State University National Superconducting Cyclotron
Laboratory (NSCL) have refined understanding of symmetry energy. Their work
marks the first successful theoretical explanation of the common symmetry
energy-related observables – including isospin diffusion and differences in
neutron and proton emitted spectra – in heavy-ion experiments. The result
should help in discerning the properties of neutron stars, particularly in
the crust region

***

BB11168

Thermodynamics goes nano!

A recent study has shown that nineteenth
century thermodynamics can still provide useful insights into
twenty-first century nanosciences; and all this is done with pencil and
paper rather than an expensive super-computer! When the size of
materials becomes smaller than one thousandth the width of a human hair,
matter begins to behave highly exotically. By shrinking the size of
materials, the “surface-to-volume ratio” increases; considering this, we
can study size effects on material properties from macroscopic laws, the
so-called “top-down approach”. In thermodynamics, the Gibb’s energy
concept is particularly adapted to describe the liquid-solid phase
transition, what we mortals call the melting temperature. Interestingly,
Dr Guisbiers, working at IEMN in Northern France, has expressed the
size-dependent relation determining the melting temperature of
nanoparticles. Combining the nano-scale melting temperature with the
bulk phase diagram equations, it has been possible to predict the phase
diagrams of perfectly miscible nano-alloys. Moreover, as the
surface-to-volume ratio governs the size-dependent materials properties;
size effects on the energy bandgap and the melting temperature are
linked. In terms of applications, Dr Guisbiers says that his model can
be used to predict the energy bandgap of nano-semiconductors and to tune
the thermo-optical properties of semiconductor nano-alloys especially
those used in nano-optoelectronics applications.

***

BA11323

RELIEVING THE STRESS OF BEING SMALL

The surface of any material is different from its interior. The atoms in
the surface are half-exposed, so they tend to get closer to and bond
more strongly with their nearest neighbours. This makes surfaces harder,
a phenomenon known as surface tension and familiar to anyone who has
seen an insect walk over water. Small objects have a large surface to
volume ratio, so they can experience considerable stress due to surface
tension.

New research shows that certain materials known as
ferroelastics (a group that includes many ferroelectrics and also
martensitic steels and shape memory alloys) can help relieve the stress
of surface tension by dividing into small regions with different
crystallographic orientations (“domains”). While domain formation (also
known as “twinning”) is common in ferroelastics subject to external
stress (e.g., thin films clamped to rigid substrates), it was not known
that it could be self-imposed in the absence of external forces. Surface
tension twinning is expected to affect not just individual nanodevices,
but also macroscopic samples made of fine-grained powders, including
ceramics.

***

CX10076

“Perfect fluid” observed in RHIC collisions may be more hadronic than
partonic.

Many researchers who study relativistic heavy-ion collisions at the BNL
Relativistic Heavy Ion Collider (RHIC) have concluded that the particles
which take part in these collisions behave as though they make up a
“perfect fluid” since fluid-flow-like effects are seen in measured
observable quantities. The key question about this fluid relates to the
type of particles which mainly composes it: are they partons (i.e.
quarks and gluons) which are fundamental particles, or hadrons (e.g.
pions and nucleons) which are themselves composed of quarks and gluons?
In recent work which is scheduled to be published in Physical Review C,
the author shows results from a simple model which strongly suggests
that the “perfect fluid” may be composed primarily of hadrons rather
than partons. By assuming that the early stage of relativistic gold on
gold collisions is a system of hadrons in a simple geometry and allowing
the hadrons to scattering with each other, the author is able to
qualitatively describe many of the experimental features of RHIC
observables thought to be of fluid flow origin. The author also
extrapolates the model to higher energies to make similar predictions
for lead on lead collisions at the CERN Large Hadron Collider (LHC),
which is scheduled to begin running this Autumn.

March 19, 2009

LX11410

Helium nanodroplets ignited from inside

Clusters can be tailored to contain from a few atoms to some
million atoms. When exposed to strong short laser pulses,
electrons are released from these atoms and trapped by the
cluster forming tiny nano-scale plasmas. Thereby, they absorb
extremely efficiently energy from the laser, outperforming
single atoms and bulk material under similar conditions. We have
discovered a dramatic ignition effect. It turns a naturally
transparent helium cluster, so-called nanodroplet, into a fast
and strong absorber of laser light when doped by only a handful
of xenon atoms. These few seed atoms in the center of the
droplet spark the plasma which grows to an unusual cigar shape.
Such a shape allows for a tremendously strong resonant
absorption within a few femtoseconds only. The energy absorption
is so strong, that a "hole" forms in the middle of the laser
pulse. This leads to a startling possibility, which awaits
exploration as a promising technological application of our
work: The creation of short and intense dark laser pulses

***
LY10902BR

Perfect lens and compensated media unified by transformation optics

Transformation optics has provided us a convenient guideline for designing
exotic electromagnetic devices, such as invisibility cloaks. In this rapid
communication, it is shown that transformation optics also opens new horizon
for interpreting in a unified manner several optical meta-phenomena, namely
Pendry's perfect lens, indefinite media lens, and compensated bilayer media.
The proposal of these devices has created a lot of excitement in recent
years and motivates current metamaterial technology. Here we reveal that
these devices share the same physical root: they are all bilayer media
obtained with the coordinate transformation technique. As an extension of
our finding, we predict and numerically confirmed that, by incorporating the
obstacle in the electromagnetic space, perfect imaging beyond passive
objects or active sources is possible. Such transformed bilayer system can
be naturally extended to arbitrary geometries, such as cylindrical and
spherical ones.

***

LZ10950

RESEARCHERS FIND A NEW VIBRATION

Researchers discovered a new atomic oscillation that could affect a
range of phenomena in solids.

When hydrogen atoms diffuse into the crystal aluminum antimonide, they
form bonds with the aluminum atoms. The bond stretching and bending
vibrations of the hydrogen showed a bizarre isotope effect. While
aluminum-deuterium pairs have one stretch-mode frequency, the
aluminum-hydrogen pairs have two frequencies.

To solve this puzzle, the scientists performed calculations on massively
parallel supercomputers. The computations revealed that there is a
transverse mode where the aluminum and hydrogen atoms oscillate
together, as a single unit. The transverse mode plus two bending modes
just happens to equal the hydrogen stretch-mode frequency. This
accidental resonance causes the stretch mode to split in two.

Normally, a stretch mode is localized. Only the hydrogen atom
oscillates. The neighboring atoms barely move. The accidental resonance,
however, causes its spatial extent to increase dramatically. Instead of
only one atom moving, hundreds do.

The researchers discovered a new quasi-particle that exists somewhere
between a sound wave and a localized vibration. In the future, it is
possible that these strange quasi-particles will be found in many
condensed-matter systems.

***

EZ10345

Mathematical modeling of fluid-particle behavior in ureteral peristalsis

Transport of body fluids in humans, animals and plants generally occur by
peristalsis. This refers to successive waves of contraction along the walls
of a hollow muscular structure that push their contents forward. In the
urinary system, urine flows from the kidney to the bladder by peristaltic
action of the ureteral wall. Sometimes this is accompanied by bacteria or
calcium oxalates. Bacterial attachment to the ureteral wall can produce
inflammation, and calcium oxalates can precipitate and form ureteral stones.
A mathematical model of peristaltic flow with particles is developed in this
work. An analytical solution of the fluid
velocity field is first obtained. This is then used in conjunction with an
equation of motion for a small rigid sphere in nonuniform flow under the
action of several forces to calculate particle motion. Retrograde motion of
particles, such as bacteria or stones, can occur in
the upper urinary tract when there is a partial occlusion of the peristaltic
wave. Some of the particles participate in the formation of a recirculating
bolus, and some are delayed in transit and eventually reach the walls.

***

LT11994

Absence of fundamental length scale explains dynamical dark energy.

Fundamental theories without an explicit length scale are invariant
under a change of scale. In a world with more than three space
dimensions scale invariance can have profound consequences for the fate
of the dark energy in the universe. We discuss the presence of two
phases for possible stable cosmological solutions, somewhat analogous to
the phases in many body physics as vapor and water. Within a given
phase, certain physical properties do not depend on the details of the
unknown fundamental theory. For one of the phases we find that
Einstein's cosmological constant vanishes by a mechanism of
self-adjustment. Due to quantum fluctuations, this phase is approached
only as the cosmological time goes to infinity. Therefore the dark
energy vanishes only in this limit. In the present very old universe a
tiny dynamical dark energy density remains, which is typically of the
magnitude required to explain the cosmological observations.

***


LA12142

When superconductivity meets ferromagnetism in ferropnictides

Summary Text: Superconductivity and ferromagnetism are antagonistic. On one hand, a superconducting state tends to expel magnetic fields. On the other hand, ferromagnetism, which produces strong internal magnetic field, generally kills superconductivity. It is fundamentally interesting to find when and how the two phenomena live together. In this paper we report the coexistence of superconductivity and ferromagnetism induced by isovalent phosphorus doping in a ferropnictide system of EuFe2(As1-xPx)2. On cooling, superconductivity associated with the d-electrons in iron atoms appears first at 26 K, and ferromagnetism due to the f-electrons in europium atoms then comes below 20 K. Strikingly, the zero-resistance superconducting state is robust against the ferromagnetism at low temperatures, making the material as a true ferromagnetic superconductor. Besides, the isovalent phosphorus doping, which generates chemical pressure, provides an alternative route to realize superconductivity in ferropnictides.

***

LX10931

How does the Earth's magnetic field reverse?

The magnetic field of the Earth is roughly a dipole aligned with its
axis of rotation. Paleomagnetic measurements have shown that the direction
of the field is not constant: from time to time the magnetic field reverses and
the poles shift in an apparently random way. In this paper we present a model
that explains how these reversals occur.

We propose that the reversals result from the competition between the dipolar
mode and a second unstable dynamo mode. This explains many features of the
Earth's magnetic field. Not only the existence of reversals that thus can
be triggered by a small amount of fluctuations but also their shape: the
dipolar field first slowly decays to zero and then grows with the
opposite sign on a much faster time scale. Aborted reversals, also called
excursions, are predicted. The statistical properties of the duration
between reversals are calculated and this allows to understand the
existence of long durations without reversals, also named superchrons.

If the second mode is a quadrupole, which is likely from numerical
simulations, we show how reversals of the magnetic field are correlated
with the flow in the Earth's inner core: they require breaking of its
equatorial symmetry. Thus, paleomagnetic records can provide informations
about the history of the internal structure of the flow in the inner core
of the Earth.

***

LX11825

Rise and fall of black hole entropy

We have shown analytically that the rise of the entropy of
a black hole with the area is strongly modulated.
Black holes, which show intriguing thermodynamic features,
have the area in Planck units behaving like an entropy.
The theory of loop quantum gravity has a
way of counting states corresponding to discrete eigenvalues of
the area operator, involving square roots of the
familiar eigenvalues of the squared spin operator.
By counting states with the area approximately fixed,
it had been seen earlier that the spins are distributed
in a Boltzmann fashion, involving an analogue temperature and
the area instead of the energy, while the entropy increases
linearly with the area. In the present work, by recognizing the
irrational nature of the exact area eigenvalues, we have found that the
spins have several segregated classes, each with a Boltzmann distribution
governed by its own analogue temperature,
while the linear rise of the entropy is modulated so that it
rises to peaks lying on the linear curve and falls
drastically between the peaks.
This structure is prominent for small black holes and
recent numerical calculations were puzzled by it.

***

BA11272

Relating superconductivity to structure.

This work indicates that strontium nickel phosphide (SrNi2P2) is a
conventional superconductor with a transition temperature of 1.4 K
(similar to more than 10,000 other compounds which have been
discovered). However, the surprising aspect of this work is that we
establish a trend among this and other related nickel based
superconductors, which mimics that of the structurally similar
iron-arsenide-based cousins. The transition temperatures of the
iron-based systems, which are widely believed to be unconventional
superconductors, reach a remarkable 55 K, second only to the
copper-oxide based superconductors. It is completely unexpected that
structural trends would be identical between a class of
unconventional and conventional superconductors. Whether our work
indicates that the nickel-based systems have a similar pairing
mechanism to the iron-arsenide systems and are simply not as well
optimized, or a deeper relationship between crystal structure and the
general phenomenon of superconductivity is an open question.

***

LA11995

Dancing Algae

Scientists at the University of Cambridge have discovered that freshwater
algae can swim about each other in intricate dances, held together only by
the fluid flows they create. The researchers studied the organism Volvox,
which has thousands of cells arranged on the surface of a spherical matrix
about half a millimeter in diameter. Each of those cells has two hair-like
appendages known as flagella, whose beating propels the organism through the
fluid and simultaneously makes it spin about an axis. When two nearby Volvox
swim close to a surface, the deflection of the flow by the boundary leads to
an attractive interaction that pulls them together. Once the individuals are
close, they can orbit around each other like waltzing dancers, or oscillate
back and forth like a minuet. This behaviour has been explained by
mathematical models, which also suggest that the fluid flows set up by the
individuals could assist with fertilization during the sexual phase of their
life cycle.

***

LY11325

A Chemical Quorum

A quorum, in legal terms, refers to the minimum number of members required at a meeting before
business can be done. In biology, quorum sensing (QS) refers to the ability of cells to "switch on"
behavior in response to an increase in group size or density. QS can be thought of as an example of
emergent or collective behavior, the macroscopic properties of a system that arise as a result of
the interactions of its components. We investigated the collective behavior of catalytic
micro-particles that individually display nonoscillatory steady state behavior when immersed in a
catalyst-free Belousov-Zhabotinsky solution. When the particles are gathered into groups larger
than a critical group size, however, spatiotemporal oscillations are exhibited. The activity of the
particles is regulated by the exchange of species with the surrounding solution. The transition from
steady state behavior in small groups of particles to spatiotemporal oscillations in groups larger
than a critical size has the features of a dynamical quorum sensing transition.

***

LZ11313

Magic-Sized Diamond Nanocrystals

Diamond is known to be the hardest of materials and the strongest of
electrical insulators. Making diamond metallic is possible by doping
this material: the process of inserting into diamond lattice large
amounts of atomic impurities. When the number of such dopants exceeds
the critical limit diamond becomes a metal. Such insulator-to-metal
transitions are well known in physics. The researchers from Air Force
Research Laboratory and North Carolina State University found that
metallic diamond possesses very unique structural properties. Using
scanning tunneling microscopy they found magic-sized nanocrystals of
metallic diamond. These tiny nanocrystals are parallelogram shaped and
are closely-packed into a continuous solid film. The AFRL/NCSU
researchers have noticed that the height-ratio of magic nanocrystals is
extremely close to 2/3, the observation which brought them to the
conclusion that quantized electrons play crucial role in the growth
process of these nanocrystals. Indeed, since the discovery of quantum
physics it was known that quantum effects manifest through quantized
sizes: quantized electron states inside atoms and quantized electron
orbits in magnetic field. Thus, the recent discovery reveals the new
phenomenon: the quantized sizes of "artificial" diamond atoms. Each of
these "artificial atoms" contains 7 electrons.

***

LA12237

Understanding the ashes of supernovae and nuclear reactors

Whether one considers supernovae events or the operation of nuclear fission
reactors, extremely exotic neutron-rich nuclei that beta decay back to the
stable isotopes around us will be created. As these nuclei decay, some can
proceed by the process of beta-delayed neutron emission which changes their
mass thus altering the path to stability and the elemental composition of
the ashes of the supernovae event or nuclear fission reactor. Precise
measurements carried out at the Oak Ridge National Laboratory's Holifield
Radioactive Ion Beam Facility, a DOE national user facility, have revealed
yields for beta-delayed neutron emission in the copper and gallium isotopes
which are two to four times higher than those previously reported. Our
measurements, which have increased accuracy and precision in comparison to
the previous measurements, were made possible by new and improved techniques
in beam purification. Revised theoretical calculations are able to
reproduce our results. Additional measurements are needed to determine if
the lack of accuracy in the previous experiments is systematic or isolated
only to the copper and gallium isotopes.

***

LZ11375

Neutron spins visualize magnetic fields

With the presented novel neutron imaging technique it is possible to visualize quantitatively magnetic fields in- and outside of magnetic samples with a sub-millimeter resolution.
In conventional neutron radiography a neutron beam passing through a sample gets attenuated according to the material properties and one obtains an image similar to an x-ray picture. However, due to the spin with its associated magnetic moment, the neutron also senses the strength of magnetic fields. This interaction results in a subtle change in its Larmor precession frequency, which can be detected with a spin echo method similar to the ones used in neutron scattering or NMR. From this measurement a two dimensional magnetic field map can be reconstructed.
The technique offers a wealth of new possibilities in real space condensed matter research, which could help to shed more light on various macroscopic magnetic phenomena.

The attached image shows the two dimensional projection of the characteristic shape of a dipolar magnetic field in the vicinity of a ferromagnetic steel rod of 9 mm length, obtained with this new technique.

***

Two-way traffic: Coexistence of Melting and Crystallization in Polymers.

Everyday experiences show that substances either melt or crystallize
with an increase or decrease in temperature. In contrast, polymers can
undergo both transitions simultaneously at the same time during a
heating process. Using Molecular Dynamics simulations of long chain
molecules, we have shown that melting and crystallization processes
coexist. Furthermore, we were able to reveal some of the molecular
details which lead to this unusual behavior. In particular, we have
shown that at intermediate stages of heating some micro-crystalline
domains disappear while others continue growing (see picture). Because
polymer crystals are far from thermodynamic equilibrium, heating can
increase their thermal stability. Thus, during a heating process,
micro-crystalline domains "decide" whether they are going to melt or
to become more stable. Based on this effect, treatments with
particular temperature-time characteristics might be developed to grow
polymer crystals with designed structural properties.

***

LZ10969

Spin-wave goes further

Anyone who ever threw a stone into a lake could observe generation of wave whose amplitude rapidly attenuates as it propagates, because of energy dissipation. Spin-wave is the magnetic analogy of wave and thus also rapidly attenuates in general. In this paper, we theoretically show that spin-wave attenuation can be suppressed by injecting an electric current and thus spin-wave can go further. It is caused by nonadiabatic interaction between conduction electron spins and spin-wave. We also find that spin-wave can be amplified at a sufficiently high current (see figure). Our finding is important from the viewpoint of fundamental understanding of spin transport mechanism since it provides a new way to experimentally estimate the magnitude of nonadiabatic interaction, which is highly controversial at this moment. Furthermore, our finding will be potentially useful for spin-wave-active devices such as spin-wave logic devices and spin-wave interconnect buses which require as high signal output as possible.

***

LU11904

Ants hate traffic jams

Have you seen ants marching on a trail in a platoon? The ants are
almost as disciplined as soldiers marching in a line. Sometimes,
analogies have been drawn between the traffic of vehicles on busy
highways and ants on a crowded trail. But, which one is a better
analogy? In their forthcoming letter in PRL, John et al. report
empirical results which demonstrate striking differences between
ant-traffic and vehicular traffic. John et al. have analyzed their
empirical data by computing quantities which are known to characterize
vehicular traffic on highways. Surprisingly, unlike vehicular traffic,
free flow of ants on trails can take place even at high number
densities at which vehicles would be stuck in a jam! In other words,
phenomenon of jamming is practically non-existent in the world of ants!