May 6, 2009

LA11801


Reversing the colors of the rainbow

The dazzling profusion of colors observed in rainbows is one of the most remarkable optical phenomena. This effect stems from the dispersion of the refraction index of the water droplets, which, from a fundamental causality argument, must invariably decrease with the wavelength of light for any conventional material with low loss. Thus, until now, it was believed that the palette of colors refracted by a glass prism must follow a fixed pattern, showing "red" as the least refracted color and "violet" as the most refracted color. However, in this article, it is demonstrated that a meta-material prism with a suitable microstructure may overcome this bottleneck and reverse the palette of refracted colors! To achieve such effect, without contradicting causality, the structured material is designed to a have a special property: unlike in conventional media, the polarization acquired by the tiny metallic microstructures from which the material is made of depends not only on the macroscopic electric field in their immediate vicinity, but also on the electric field at long distances. Such "nonlocal" materials enable a regime of broadband anomalous dispersion with negligible loss, and may provide a useful means for the compression of optical pulses in ultrafast optics.


***

LY11529E

Why cells, foams and slurries behave alike? Unraveling simple
laws that control cytoskeleton behavior

Essential functions performed by cells depend on their capacity to
generate and respond to mechanical forces. This ability is mediated by
the cytoskeleton, which is a nonequilibrium structure in a state of
steady remodeling. Recent works on the cytoskeleton remodeling have
suggested a controversial analogy between the dynamics of the
cytoskeleton and that of inert nonequilibrium systems such as foams,
pastes and slurries. In this regard, the cytoskeletal dynamics is
described in terms of non-thermal rearrangements of stressed regions in
a rugged free energy landscape. However, the experimental
characterization of the physical laws that underlie this analogy remains
an open question in soft matter physics and cell biology. In our paper,
we report that the remodeling dynamics of the cytoskeleton is governed
by thermally activated forces that generate a free energy landscape. The
value of the characteristic energy of this landscape is measurable and
in the order of ~40 kT. Soft matter physics interprets cytoskeleton
dynamics in terms of coarse-grained descriptions underscoring molecular
independent details. By contrast, cell biology focuses on the molecular
details of the cytoskeleton and emphasizes molecular specificity. Here
we show that both conceptions are intimately linked through thermal
activation and that general physical laws are capable of describing
numerous observations concerning the dynamics of the cytoskeleton. To
the best of our knowledge, this result together with the value of the
activation energy reported in our manuscript provides the first direct
evidence that cytoskeleton dynamics can be described by structural
rearrangements over free energy barriers.


***

R05920PRB (published)


New nanostructured materials for photovoltaic applications

"Recent development in nanotechnology have witnessed the synthesis of
one-dimensional nanostructures, such as nanowires, with diameters of few
nanometers. The understanding of electron-hole recombination mechanisms in
these newly developed nanomaterials will enable the design of new solar
cell structures that help toovercome the efficiency limits of conventional
solar cell concepts. In particular silicon based nanomaterials where
electrons and holes are spatially separated are particularly promising and
offer the potential to combine the advantages of high efficiencies and low
production costs. Using ab-initio many-body simulations we have
investigated the structural, electronic and optical properties of SiGe
nanowires. The wires, which exhibit a clear interface between Si and Ge
regions, form the most stable structures, show a particular quantum
confinement effect and reveal, under optical excitation, a clear ability
to quantum confine holes and electrons in different spatial regions. Thus
these properties can have relevant technological applications and become
suitable for developing new nanodevices, such as Si-based solar cells and
nanoelectronic power sources.

Caption of the figure:
Figure Top (left) and side (right) view of the electron distribution
probability, the hole position is fixed on the top of the central Ge atom.
Yellow spheres represent Si atoms, magenta spheres Ge atoms, while the
small white spheres are H atoms used to saturate dangling bonds on
surface. The isosurface of the e-h distribution probability is shown in
celestial."

***

LZ11542A

Ghost imaging with a single detector

Ghost imaging is a curious phenomenon that puzzled physicists in recent
years, trying to understand whether it is a quantum or a classical
effect. In this work we carried out an experiment which aimed at
resolving this matter. In ghost imaging an object is imaged by a camera
which does not see the object itself. The image is formed by correlating
the measurement of a resolution-less detector looking at the object,
with images taken by a far away camera, looking at a different
direction. When it was first demonstrated in 1995 with entangled
photons, everybody was amazed by the strange power of quantum
entanglement. But later, various groups showed that entangled beams
weren’t necessary, and that two classically correlated beams would do
the job just as well. While interesting, that doesn’t rule out the
possibility that the two classical beams may be correlated in some
entangled-like quantum way.

In our experiment we imaged an object without even using a camera.
Instead, we correlated the data collected by the resolution-less
detector, with a "virtual image" which was calculated theoretically.
Since we used only a single detector, it is an experimental proof that
ghost imaging cannot rely on nonlocal quantum correlations.

***

LB12540ER

Physicists dropped the ball

Surprisingly, physicists like to study balls that do not bounce. These very forgiving balls become amnesic when hitting a wall. That is, whatever the initial conditions, they end up with zero speed after the impact. Such behavior may seem dull and trivial, but not for physicists! For a start, it makes the situation easier to describe. Now, to stir things a bit, in this work, the ball was dropped on a plate that is vertically shaken. The ball naturally takes off when the plate acceleration exceeds the 9.81m/s² of gravity. Then, it experiences a series of bounces before sticking on the plate. It takes off again when the acceleration is sufficient and the whole sequence is repeated indefinitely. The magic point is that this sequence may be as long as desired, though it never becomes chaotic. This strange property makes the system even more complex than the well-known low-dimensional chaos.

***

LV11769

A Bacterium's Fuel Mileage: Tuned by Evolution?

"Fuel mileage" is a term more often heard on the lots of car dealerships than in Microbiology departments. However just as you'd like to maximize your car's miles per gallon, a microorganism might want to maximize the distance traveled per energy expended, nanometers per ATPs (nm/ATP) consumed, let's say. We argue that, for the small swimming bacterium called Spiroplasma, evolution has done just that.

Spiroplasma, is a helix-shaped bacterium, small even by bacterial standards, which lacks any external means of propulsion. Its exact method of locomotion was a mystery until 2005 when researchers looking at high-speed videos discovered that it contorts its shape by flipping the handedness of its helical body, and then flipping it back again. The whole process looks like a pair of kinks traveling down the length of the cell. Figure 1a shows a set of simulated time traces of Spiroplasma's shifting and rotating helical axes (represented as blue line segments), as the distance between the two kinks is increased from left to right. (The far left trace superposes the cell body at three different times.) How efficient is this kinky means of mobility? We estimate it could be as high as 26 nm/ATP. For comparison, the fuel mileage of a single kinesin molecule is 8 nm/ATP. [For comparison with your gas guzzler, 1 nm/ATP is very nearly 1 mpfg (= miles per femtogallon of gasoline).]

By optimizing over many of the variables describing Spiroplasma's swimming, we have shown that the observed shape and kinematics of Spiroplasma are very near to what maximizes its overall swimming velocity and its fuel mileage. Figure 1b is a magnified view of the time trace that nearly maximizes the fuel mileage. We also show how the notion of optimized fuel mileage can be used to constrain the energy required by the still unknown, underlying mechanism of Spiroplasma's kink-generated shape change. It is possible that insights gained from this tiny bacterium could be used to design and engineer swimming micro-robots.

***

BD11043

OPTICAL NANO-TRANSMISSION LINES

Chains of nanoparticles may be able to guide light more efficiently when
operating near their quadrupolar resonance. Theoretical findings from
Alu and Engheta have proven that the proper design of collections of
nanoparticles may support a novel guidance mechanism similar to the one
used successfully at radio frequencies in common transmission lines.
Silver nanoparticles covered by silicon-based materials are proposed as
a possible venue for the realization of these novel optical waveguides.
Applications for low-loss optical communications at the nanoscale and
leaky-wave nanoantennas are envisioned.

***

BDR1155

Direct imaging of the structural domains in iron pnictides

Parent compounds of recently discovered iron pnictide high temperature
superconductors exhibit tetragonal-to-orthorhombic structural transition
upon cooling. In some compounds this structural transition co-exists with
superconductivity. For a crystal, the only way to accommodate such
transition is to create structural domains - twins. It is therefore very
important to know the actual structure of this twinned phase. The twin
morphology affects all properties of the crystals and is likely to be a
critical factor in the electrical current transport in the novel family of
high temperature superconductors, a key property for their application in
future technology.

In our paper we report on direct optical and synchrotron diffraction imaging
of the structural domains in a range of pnictide phases. Morphology and
structural characteristics of the twin boundaries is discussed.

***

LA12480AR

Continuous-variable quantum entanglement as a daily tool

Quantum entanglement lies at the heart of quantum physics and draw a clear
boundary between classical and quantum world.
Also, entanglement is a critical resource for realizing quantum
communication and quantum information processing that enable classically
impossible tasks.
The realization of robust source for continuous-variable entangled pulses at
telecommunication wavelength is reported here.
We have proposed and demonstrated a novel ring-interferometer configuration
that enables auto-compensation of phase drift between optical beams.
The robust sources will be inevitably required in an increasingly complex
quantum communication and quantum information processing.
In order to implement the practical communication, it is necessary to encode
the relevant information, and access it individually.
From such viewpoints, pulsed light is powerfully useful.
Further, telecommunication wavelengths are desirable for long-distance
communication, because optical loss in an optical fiber is minimum at this
wavelength.
So the development of the robust source of continuous-variable entangled
pulses at telecommunication wavelength will be a essential building block
for further quantum communication and quantum information processing.

***

LB11701B

Lattice-defect dynamics of DVD material

Ge2Sb2Te5 (GST) is the most popular material used as an optical
recording media (DVD), in which phase change between crystalline and
amorphous phases serve rewritable recording more than 100,000 times.
To date understanding the mechanism of the fast phase change (< 1 ns)
is strongly demanded to achieve higher speed of memory switching.
Recently, First-principles calculations predict that Ge2Sb2Te5 is
considered as superlattice (SL), which consists of two units of Ge2Te2
and Sb2Te3 layers, while molecular dynamics simulations uncovered the
formation of large voids in amorphous GST films.

The information on the existence of vacancies or voids in GST,
however, has not yet been explored from the lattice dynamical point of
view. In this paper we present ultrafast relaxation dynamics of
coherent lattice vibrations observed in atomically controlled GeTe/
Sb2Te3 SLs by using an optical pump-probe technique. Our experiments
revealed that the damping of the coherent A1 mode in crystalline SLs
is strongly temperature dependent, while that in amorphous SLs is
significantly temperature independent. This result indicates that the
damping of optical phonons in amorphous SLs is governed by the elastic
scattering due to phonon-vacancy interaction, and the finding signify
the existence of randomly distributed vacancies or voids in amorphous
phase of GST.


***

BYR1079

A battery for spins

In the vision of "spintronics", devices will use and manipulate the spin of electrons in a similar way to what transistors do for the electron charges in current electronics. This requires an ability to manipulate only the electron spins (but not their charge) and to generate a "spin current" without a "charge current", a task which turns out to be quite elusive. In this paper, we show that a nano-scale device, composed of two magnetic leads which bridge a molecule, may serve as a "spin-battery" if the leads are held at different temperatures, in an analogy to thermo-electric batteries (where a temperature difference is used to generate an electrical current, used e.g. to power satellites). With a temperature gradient being the force pushing the spins, this unique setup allows (with a correct tuning of parameters) for such a device to generate a pure spin current but without any charge current, a task which cannot be achieved by trying to push the spins with, for instance, an electric field. A detailed analysis reveals that the efficiency of such a device may exceed that of its "charge" counterpart, making it of potentially useful for future novel device applications.

LX11425B

Needle crystals eat square crystals

What happens to organic thin film devices as they age? Will the device stop working? Will the optoelectronic properties change? Can I prevent this by keeping it in the fridge?

In this paper, we observe the evolution of a thin film of PTCDI molecules which initially contains two coexisting species: needle crystal islands and square crystal islands. As time goes on, the needle crystals grow by consuming the square crystals, which shrink, eventually to extinction. We attempt to describe this evolution mathematically, look at why it happens, and consider what we can do to change how quickly it takes place.


***

LC12113

What's inside a black hole
--- an answer from superstring simulations

How can one see what's inside a black hole?
A possible theoretical answer has been given recently
by computer simulations.
In fact, inside any black hole there exists a singular point,
at which Einstein's general relativity breaks down and
a quantum theory of gravity becomes necessary.
Superstring theory is one such candidate, which is most promising
and attractive, and it provides a quantum mechanical model
which was conjectured more than 10 years ago to describe the
interior of a black hole.
In this paper we succeeded in simulating the model by a supercomputer
at the difficult low temperature regime, and obtained a relationship
between the energy and the temperature.
Surprisingly the results agreed well with Hawking's theory,
which deduces the relationship only from the exterior of the black hole.
Actually a small discrepancy grows with increasing temperature, but this
behavior has also been explained as corrections to Hawking's theory
due to thermal oscillations of strings.
This work not only enables us to understand microscopically the
mysterious thermodynamical properties of black holes that Hawking
discovered, but also suggests how they should be modified if
superstring theory is "the" quantum theory of gravity that describes
our real world. The next ambitious goal of superstring simulations
would be to understand yet another singular point that appears in
cosmology, namely the big bang.


***

LZ11642

Structural Memory of Glass

Recently amorphous materials have received an increased interest in their structure, dynamics, and mechanical properties, which remain ill-understood despite a considerable research effort. Unlike the plasticity of crystalline materials, the plasticity of glasses cannot be defined unless a completely novel description is introduced. This is what we propose in our accepted PRL paper reference number LZ11642 Rountree. Our work shows how the structure of silica can be irreversibly modified by the application of an external shear stress: under shear plasticity a non-reversible anisotropy sets in and appears stable. This provides a new interpretation of the small scale plasticity of glass; it is well revealed by the fabric tensor. The fabric tensor is a commonly used tool in granular materials/soil mechanics, more recently it has been applied to foams. To my knowledge our study is the first one to propose a "fabric tensor" at the atomic scale and to make evidence for plasticity induced anisotropy for a cohesive amorphous material. This is why we believe that it deserves additional coverage.

April 28, 2009

LW11727BR


Are ultrathin films always transparent?
In our paper we predict strong suppression of light transmission through ultrathin metal films when the light is mixed up with collective surface charge oscillations that “live” on the film’s faces. Under normal conditions ultrathin films are transparent and allow light to freely pass through them. Although one can naively expect that notching transparent slits in the film naturally admits passing more light through it, the fact is that it is not the case. It is rather unexpected that periodic cutting of optically thin metal film results in a strong opacity within some wavelength interval. It happens when the incoming light interacts via the slits with surface charges resulting in surface electromagnetic wave (Surface Plasmon-Polariton) excitation. The nature of this phenomenon is analogous to that of now classical extraordinary optical transmission through optically thick metal layers [T. Ebbesen et.al in 1998], but the effect is opposite. The discovery of extraordinary suppressed transmission by ultrathin metal films is of high importance not only due to a new insight into the light interaction with different media, but it promises a number of challenging applications in nanooptics and nanoelectronics. Specifically, it could be implemented into light emitting diodes design in order to enhance light extraction from them, and in other light emitting devices.

***

LZ10940

Competing mechanisms on ultrashort timescale in high temperature superconductors?

High temperature superconductivity is only partly understood so far. A material becomes superconducting below a characteristic temperature, where the electrical resistivity is zero due to the formation of electron pairs ('Cooper-pairs'). An inelastic light (Raman) scattering experiment, where the low-energy elementary excitations are probed and information about transferred energies are included, breaks up these Cooper pairs. In a novel two-color time-resolved Raman experiment on the high-temperature superconductor Bi_2Sr_2CaCu_2O_{8+\delta} one is able to study the transferred energy (Raman shift) as a function of time in the picosecond range. A picosecond is an ultrashort timescale corresponding to a billions of the time between a wing beat of a typical midge. We destroy these Cooper pairs with an initial light beam and track their reformation afterwards with a second light beam. One finds that a modern high temperature superconductor exhibits two different, probably competing mechanisms leading to the formation of the superconducting state on these ultrashort timescales. Since the information about the Cooper-pair formation is encoded in these timescales, this experiment allows to test several predictions for the still unknown pairing mechanisms in high temperature superconductors.

***

LB12406


Warm it up and you will grab particles

Accumulation of molecules by a thermal gradient has attracted interest recently from an application viewpoint as well as a candidate for prebiotic phenomena. In general, particles move along the thermal gradient due to so-called thermophoresis. However, most materials move to a colder place when its surrounding is locally heated, and the speed of the migration is highly material dependent. Besides these, nano-scale objects are notoriously difficult to trap without being tethered to larger objects by conventional optical tweezers. We just found a new method for their micromanipulation. A slight increase of local temperature by laser focusing with a little addition of polymers enables us to trap particles in the focus. The method developed allows us to invert and amplify the movement of colloid particles in the thermal gradient due to a thermally induced concentration gradient of polymers. This nonequilibrium effect does not rely on a specific character of materials but relies on a generic property of entropic force, and thus provides further applications for manipulating a diverse range of colloidal particles including biological cells and biomolecules such as DNA molecules. This poses a possible mechanism for the accumulation of macro-molecules in the mixture of prebiotic soup in thermal gradient.


***

LU12062

2D or not 2D? That's the question (for soap films)

Soap films are extremely thin, but our work shows that in some cases
three dimensional effects due to their finite thickness are important.
Soap films consist of a layer of fluid stabilized by soap molecules;
because of the thinness of the fluid layer (microns or less) they are
often thought to behave as 2D liquids with a 2D viscosity. However,
clearly this will not be true if the layer of liquid is extremely thick;
in such a case you'd have to see 3D liquid behavior. In this paper, we
identify precisely where the transition from 2D to 3D behavior occurs as
the thickness of the soap film is gradually increased. We do so by
putting spherical particles of size d in soap films of thickness h, and
observing the thermal Brownian motion of these particles with a
microscope. This is done for different values of h/d ranging from
0.5-15. We use the particles' motion to measure a 2D viscosity of the
soap films. Unphysical values of the 2D viscosity are found for soap
films with h/d > 7, leading us to propose that soap films transition
from 2D to 3D fluid-like behavior at a critical ratio h/d = 7.

April 23, 2009

LV11193B

Memories that Consume No Power

Single molecular magnets have long been regarded as one of the
promising candidates for next generation of magnetic random access
memories, because of their bi-stable magnetic orientation as well as
their nanometer size. A problem is that when using a current to
individually access such small molecules integrated at large scale,
the total current density could be enormously large and may lead to
many spurious effects. In this paper, we propose that the
magnetization of a magnetic molecule can be switched reversibly,
without the help of a magnetic field or a spin-polarized electric
current. Instead we suggest employing the so-called spin bias, which
can drive the same amount of spin-up and spin-down electrons
travelling in and out of a molecule simultaneously. While angular
momenta can be transferred to the molecule, much like a
current-induced spin torque, the two opposite spin flows cancel with
each other, yielding no net electric current. This idea can be applied
to any nanoscale magnetic storage units, to fabricate memories that
consume no power.


***

AC10669

A Universal Quantum Computer Exists

In 1936, Alan Turing formulated the model of classical programmable
computers, the Universal Turing Machine (UTM). Essentially all modern
day computers are based on the UTM. Less than a century later,
classical computers have become ubiquitous in modern life and
virtually all technology is ultimately dependent upon them. In 1985,
David Deutsch formulated the model of a quantum version of the UTM,
the Universal Quantum Turing Machine (UQTM). Since then, researchers
have shown that quantum computers can solve certain problems
significantly faster than classical computers ever will. However,
researchers also raised questions about the validity of some aspects
of the UQTM. Thus, the existence of a programmable quantum computer
became an open question. In this paper, we present an explicit
theoretical construction of such a computer, the Universal Quantum
Computer (UQC), addressing the open questions that were raised about
the UQTM. Our machine can emulate a UTM and can execute any quantum
computing algorithm. In addition, the UQC supports conditional
branching and execution, a feature that greatly aids computer
programming but that has not been directly possible in previous
quantum computing frameworks. We thereby show that a universal
quantum computer exists, settling the question about the validity of
the UQTM. The UQC serves as a prototypical model for general-purpose
programmable quantum computation in much the same way that the UTM
serves for classical computation. The UQC should find uses in the
development and analysis of quantum algorithms and complexity and
paves the way for the physical construction of a programmable quantum
computer some day.

***

LA12116

World’s smallest incandescent lamp

In this paper we report imaging the smallest incandescent lamp ever
constructed. The filament, visible to the unaided eye when lit,
appears as a tiny point of light since its length is comparable to the
wavelength of the incandescent light it emits. Even our modern
optical microscope barely resolves the filament’s non-zero length,
since the light’s wavelength places a fundamental limit on the
achievable optical resolution. To “see” the filament, a single carbon
nanotube, with atomic resolution we use an electron microscope (as
opposed to a light microscope). In this way we can correlate the
properties of the light emitted with the detailed molecular structure
of the filament, which is only about 100 atoms wide. Our goal is to
understand how Planck’s blackbody radiation law gets modified at small
length scales. Planck’s law dates from 1900 and describes radiation
from large, hot objects, such as a toaster, the Sun, or a light bulb.
Some such objects are of fundamental and current scientific interest;
for instance, the thermal radiation left over from the Big Bang
(called the cosmic microwave background) is described by Planck’s law.
The carbon nanotube makes an ideal filament for this experiment,
since it has both the requisite smallness, and the extraordinary
temperature stability of carbon. In fact, Thomas Edison’s original
light bulbs had carbon filaments. Our light bulb is very similar,
except that the filament is 100,000 times narrower and 10,000 times
shorter, for a total volume only one one-hundred-trillionth of that of
Edison’s version.

***

LB12061BR

Approaching reality by ab-initio calculations: symmetry is lost!

Symmetry is the invariance of an object to a set of changes, i.e. transformations.
and it is central to our understanding and description of natural phenomena.
However, the world we observe is often unsymmetrical, from daily life to
microscopic word, like atoms governed by quantum mechanics. The consequences
of broken symmetry can be dramatic!
It's a great challenge to describe the microscopic world by using ab-initio calculations,
i.e. just define your atomic systems and put everything in a computer, as much as close
to reality. We have achieved this in our work, and surprisingly we lost some symmetry,
as Nature requires for our system.
Example: imagine to insert a magnetic atom, like Manganese,
in a non magnetic semiconductor, like GaN. The magnetic atom carries a spin density,
which is an unbalance of spin-up and spin-down charge density. Remember that the spin is a
quantum mechanical property of the electron, like an up- or down-arrow carried by
the electrons in Mn. So far, approximate calculations describe this spin density as a
symmetric object: in the figure, left part, if you rotate by 120 degree, you end up with the same
situation before rotation. Our calculations, which describe better the interactions among
the electrons, clearly show that this is not the case: in the figure, right part, you lost
a 120 rotation symmetry, and your object (spin density) is no longer symmetric.
Experiments confirm our calculations: Nature prefers an unsymmetrical description, although
you start from symmetric quantum laws (spontaneous symmetric breaking)!

***

LX11701

A VOID PHASE IN DIPOLAR COLLOIDS

The spontaneous self assembly of nanoparticles into linear aggregates such
as nanowires is a process of much interest in applied physics and material
science, but one that is not well understood. Dipole-dipole interactions
are thought to be the driving force for this process.

In the current work, a Brownian colloidal system with a controllable
dipolar interaction is seen to produce linear aggregates (chains) that
further aggregate to form an very low density network structure at
particle packing fractions between 0.07% and 4%. We follow the real-space
structure and the long-time dynamics of the structure formation by
fluorescence confocal microscopy, and find that this structure is a an
equilibrium network-forming phase characterized by cellular particulate
structures surrounding large particle-free voids. We call this a void
phase.

Dipolar interactions are attractive at short range and repulsive at large
distances. We observe that the aggregates at network walls are not
compact. This might result from a shallow attractive interaction, possibly
as a result of a combination of dipolar and van der Waals interactions.

Ultra-low density equilibrium cellular structures also have potential
applications as structured macroporous materials. The void phase reported
here is 10-100 times emptier than typical low-density cellular materials.


***

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.


***

LC11988

Polarons under the microscope: electron meets lattice vibration

When an electron is injected into a nanometer-sized semiconductor
crystal - so-called quantum dot (QD) - the charge of the electron causes
a distortion of the crystal lattice. The combination of the electron and
the surrounding lattice deformation is known as a polaron in the physics
jargon. This process is the one of the most important factors governing
electron transport and energy relaxation in nanostructured materials and
has not been previously quantitatively investigated. We present the
first direct, clear-cut, and quantitative experimental determination of
the coupling between injected electrons and lattice vibrations (phonons)
in semiconductor QDs.

Many of the proposed optoelectronic applications of colloidal QDs, such
as infra-red lasers and solar cells, require that energy relaxation of
excited electrons can be slowed down. The electron-phonon coupling is
the fundamental process that ultimately determines the rate of energy
relaxation in colloidal QDs once competing relaxation processes (e.g.
coupling of the excited electron to vibrations of the ligand molecules)
have been minimized by nanocrystal engineering.

We measure the electron-phonon coupling strength in a single-electron
transport experiment by low-temperature scanning tunneling microscopy
(STM) and spectroscopy (STS). The quantum mechanically coupled
electron-phonon states are directly observed by STS and we explore how
the coupling strength depends on the nanocrystal size and shape and the
symmetry of the electronic orbital.

***


AB10501

Sub-Planckian physics in molecular system

The Heisenberg's uncertainty principle is a fundamental in quantum physics. This implies that the product of uncertainties of
position and momentum must always be greater than Planck constant. The sub-Planck scale structures have phase space dimension smaller
than Planck constant. Hence, one might be tempted to dismiss its existence even thought it appears in Wigner quasi-probability
distribution. These do not violate the uncertainty principle, since it only tells about the phase space area, not the quantum
uncertainty product of the system. These structures are first reported by Zurek (Nature 412, 712 (2001)) and showed that in a
chaotic system they are most sensitive against decoherence.

At present, no doubt about the existence of these structures in phase space. Importantly, sub-Planck scale structures can be found in a realistic system of vibrating diatomic molecule (Phys. Rev. A 73, 013411 (2006)). Superposition of few vibrational energy levels creates a wave
packet and its temporal dynamics reveal the signature of this smallest structure in phase space. In reality, every system is
coupled with its surrounding, called environment, which results in the decoherence. Now the question is whether these structures are
really sensitive to decoherence in the quantum system of diatomic molecule. In this work, for the first time, we show the
decoherence-sensitivity of these structures in a diatomic molecular system. The time evolution of a vibrational wave packet (coherent state)
is investigated under the influence of a bosonic environment describing either photonic or phononic excitations. These
structures, being smallest in quantum regime, are found to be most sensitive against decoherence.

Moreover, it is important to verify the decoherence-sensitivity of the sub-Planck scale structures due to the variation of
physical parameters like environment coupling, temperature and the system evolution time. Our quantitative analysis provides an
exponential decay of the amplitude of the quantum interference structures as a function of the coupling with the environment, in
agreement with usual predictions. We find that the sub-Planck structures decay fastest among the other interference structures.
Influence of the environment temperature on the decoherence is also investigated. This obeys the well known Bose-distribution
law, where we predict the value of the critical temperature for this realistic system. Sensitivity study upto a longer time again
shows that these structures are most sensitive compare to their original counterparts. In a nutshell, if a very small disturbance
acts on a diatomic molecule, these structures are the most convenient to be used as a quantum switch for detection.

Figure Caption:
The sub-Planck scale structures appear in the central region of the phase space as localized maxima (red spot) and minima (black spot). These are the result of superposition of two diagonal cat-state interferences. Due to the environment coupling, they are most sensitive against decoherence and disappear much faster compare to their counterparts (four coherent states).



***

ER10385

Can information travel faster than light?

This is an old, delicate, and as yet unanswered question. Previous
studies claimed that, under certain circumstances, a signal can cover
a distance in less time than that required for the same distance in a
vacuum. Electromagnetic tunneling is one way to observe this kind of
phenomenon. In return, just as many studies criticized this claim as
an unexpected possibility. Detractors argue that it is difficult to
locate a point of the signal as a reference for a correct measurement
of its velocity, the signal does not travel but simply sojourns in
the distance and is subjected to deformation, narrowing, reshaping.
In other words, the "faster than light" behavior should be considered
only apparent.
By using a set-up based on waveguide, that detects only the
transmitted components of an electromagnetic signal, the present
study provides a novel, unambiguous method for measuring the
tunneling time of subsequent wave packets. It shows that the signal
is not degraded and is delivered in a short, superluminal time, that
is independent of the tunneled distance.

(Figure caption) A tunneled, evanescent wave packet appears at the
end of the barrier in less time than that required for traversing the
same distance in a vacuum.


***

BB11176

Positrons as probes of free volumes in materials with complex defect
structure

A model is presented which paths the way to apply the powerful
defect-sensitive method of positron annihilation even in such complex
situations of fine-grained materials where point defects and interfaces
coexist. Positrons, the antiparticles of electrons, are versatile probes
for the study of atom-scale sized free volumes in condensed matter -- a
class of important defects which hardly can be detected by other
microscopic techniques. A focus of application is on the study of point
defects (such as single missing atoms in a perfect lattice), but there
is an increasing demand to employ this method in the study of interfaces
of polycrystalline materials and nanophase materials. The paper
addresses theoretically a physical situation where the positron probes
see both the point defects in the crystallites and the interfaces
between the the crystallites. Closed-form solutions are obtained which
can be conveniently applied for the analysis of experimental data. The
model is not only essential for studies which aim at issues of interface
physics or nanoscaled material, but is also of relevance for studies of
point defects in polycrystalline materials when grain sizes are in the
micrometer range.


***

LC11962

Breaking of an emulsion under ac electric field

An emulsion is a widespread liquid system used in many industrial
areas like food-processing (ice-cream, mayonnaise, …), cosmetics,
paints, ... The coalescence of emulsion droplets induced by the
presence of an ac electric field can be a beneficial phenomenon used
for enhancing the destruction of water-in-oil emulsions, as for
example in oil recovery technologies, or more recently for
controlling the fusion of individual droplets in digital microfluidic
applications.
In this letter, by using microfluidic technology, we investigate the
stability regarding coalescence of droplet pairs under electric field
as a function of drop separation and ac field intensity. Three
different regimes are found: stable, coalescence and partial merging.
From this, we identify the two breaking scenarii of a one
dimensional train of droplets: in one case the coalescence front
propagates, in the other case, which for pairs corresponds to the
partial merging regime, the coalescence front can become
heterogeneous. From these findings, we can propose a destruction
mechanism for a macroscopic emulsion, which includes the packing
condition for which total and immediate destruction is effective.

***

LX11244

From JET new experimental findings on ion heat transport in tokamaks

On the JET tokamak novel experiments using radio-frequency ion
heating and a powerful ion temperaure diagnostic have allowed the
determination of the threshold for on-set of ion turbulence and of
the level of rigidity of ion temperature profiles. Both are basic
ingredients of theoretical models of turbulent core ion heat
transport, which had so far escaped detailed experimental validation.
They play a key role in determining the core ion temperature value
achievable in a fusion device for a given (first wall compatible)
edge temperature. and influence as a consequence the fusion power
production. The non-linear threshold predictions have been found
optimistic with respect to this experiment, which is rather close to
the lower linear predictions. The major observation is that the
rigidity of ion temperature profiles in the core is very high in low
rotation plasmas, in accordance with theory, but decreases
significantly at high rotation, the latter result presently not
predicted by theory. This means that rotation is highly beneficial in
a fusion device, allowing a steeper ion temperature profile and
therefore high core ion temperature in the presence of a lower edge
temperature. It also implies though that rotationless devices would
be constrained by turbulence to ion temperature profiles close to
marginal stability, no matter the heating power level. This
observation further motivates studies of novel means of inducing
rotation in future fusion devices.

***

LU11039B

Quantum hurricane forecast for a nanoscopic globe

Sometimes physics at large and small scales unexpectedly meet,
demonstrating remarkable universalities. In a recent paper in Physical
Review B, a striking similarity is revealed between the behavior of
hurricanes in the atmosphere of a planet, like the earth, and the behavior
of electric currents on a spherical shell of superconducting material just
a few hundred nanometers accross. In this "superconducting nanoshell",
hurricane-like patterns of current, called "vortices", are present and
their dynamics under the influence of magnetic fields is elucidated. The
curvature of the superconducting nanoshell leads to a vortex-free zone
(the "Meissner belt") at the equator of the nanoparticle; it is also here
at the equator where the vortices originate and migrate towards the poles,
where they aggregate in regular lattices. Also in a 3 degree band near the
earth's equator, hurricanes are rare; they originate only at the edge of
this safe zone and move towards the poles. On earth, these "classical",
large-scale vortices don't reach the poles since they dissipate over
unfortunate lands and cold water, but on planets like Saturn vortex-like
patterns were recently observed at the poles by the NASA & ESA Cassini
mission. In the nanoscopic counterpart, this mechanism can actually be of
technical use since it allows superconductivity to survive in stronger
magnetic fields.

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.