January 23, 2009

LX11118

First transmission of twisted radio beams

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


***

LW11444

A new way to see molecules

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


***

LX10929

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

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

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


***

LU11652

Society benefits from behavioral diversity

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

***

BX10909

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

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

January 21, 2009

LK11128A

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

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

***

BX10585

The striped superconductor: (another) new state of matter

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

***

EX10489

Waving motion induces physical properties changes

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


***


LX11503

Cavity Tuning of Molecule Colors

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


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

***

AY10417

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

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


***

ET10538

Speedup through recursion

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

***

LY11801

A Newtonian System that Mimics the Baldness of Rotating Black Holes

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

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

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

***

LU10982

Hot Atoms can Freeze Images

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


***

AY10309

Entanglement Distillation with local common reservoirs

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

January 16, 2009

LK11566E

ARE JACKSON POLLOCK'S DRIP PAINTINGS FRACTAL?

Can fractal analysis be used to determine whether a drip painting is
an authentic Jackson Pollock? A highly publicized claim, originally
published in Nature, asserts that Pollock's drip paintings contain
fractal patterns so distinct they can be used to identify and date
his work. The issue is of more than just academic interest since fractal
analysis has recently been invoked to help settle a major debate in the
art world regarding the authenticity of a cache of drip paintings
discovered by Alex Matter.

In this paper we demonstrate conclusively that fractal analysis
provides no useful information about artistic authenticity. To
this end we have analyzed paintings known to be authentic works
by Pollock and drip paintings by local artists. According to fractal
analysis, known Pollocks fail to be authentic, while paintings done
by local artists in 2007 are authentic Pollocks done between 1948
and 1950. We also report the fractal analysis of two Matter
paintings.

In an earlier paper published in Nature we had demonstrated a number
of logical inconsistencies in the application of fractal analysis to
drip paintings. Foremost among these: a childish drawing made by one
of us, depicting about twenty hastily scrawled, five-pointed stars,
was shown to be an authentic Pollock according to fractal analysis.
Together with the painting analysis in the current paper, this
demonstrates the complete ineffectiveness of fractal analysis at
recognizing Pollock's hand.

The painting analysis also motivated us to uncover some new
fractal mathematics. We studied the intrinsic noise in box
counting staircases that are commonly used to determine the
fractal dimension. We found that the noise provides a novel
distinction between fractals and ideal Euclidean objects.

***

LV11223

Mechanical manipulation of single polymers proves classic theory of non-linear spring behavior

Polymers in solution adopt a characteristic ‘self-avoiding’ random walk structure; both Flory and de Gennes received Nobel Prizes, in part, for theoretical description of that structure. Thirty years ago, Pincus extended their work to predict that a self-avoiding polymer can behave as a new type of spring. Pincus showed that, when stretched with an applied force, a self-avoiding polymer will extend in a non-linear fashion (in contrast to standard ‘Hookean’ spring behavior, where length grows linearly with extension). In this paper, Saleh and collaborators report on experiments involving stretching and measuring the length of individual single-stranded DNA molecules that provide the first direct confirmation of the existence of the non-linear elastic regime. By analyzing this new elasticity regime, the authors determined that electrical interactions between charged DNA segments were critical to the non-linear behavior, and showed that it was possible to recover linear elasticity by using salt to turn off the electrical interactions. This work will be important to the theory of charged polymers, the design of polymeric materials, and the study of biological polymers such as DNA and proteins. In addition, the single-molecule stretching technique utilized provides an important new experimental tool in the study of self-avoiding polymers.


***

BXR1063

High-capacity hydrogen storage on the flakes of graphite layers

Hydrogen being lightest element in the universe does not allow an
efficient storage in pressured tanks. Recently, efforts have been devoted
to develop safe and efficent storage of H2 molecules on carbon based
nanostructures. Researchers from UNAM, Bilkent University predicted that
graphene flakes can be used for high-capacity hydrogen storage medium.

Ca atoms can be chemisorbed uniformly above the center of hexagons at both
faces of graphene flakes. Interestingly, each adsorbed Ca atom can hold up
to four H2 molecules attaining a gravimetric storage capacity of 8.4 wt %.
This value is above the value 6 wt % set by DOE for feasible hydrogen
storage. Charge transfer from partially occupied Ca-3d orbitals,
polarization of H2 molecule and van der Waals interaction with other H2
molecules are combined to form a mixed bond between Ca+graphene complex
and H2 molecule. The strength of this weak bond is only 0.4 eV and allows
H2 molecules to be discharged easily upon heating. Recycling of
Ca+graphene complex can be sustainable, since the repulsive Coulomb
interaction between positively charged Ca atoms hinders the formation of
Ca clusters on both faces of graphene.

The storage mechanism predicted through high performance computations
based on quantum mechanics appears to be feasible by using graphene
flakes. This work also reveals another interesting aspect of graphene, a
wonder material offering exceptional properties.

***

LU11425

A new test for quantumness in multi-level systems

We have discovered a surprising new behavior in a simple quantum random
walk that can be used as a definitive test to determine whether certain
multi-level systems behave according to quantum mechanical or classical
laws. Although isolated microscopic systems such as individual atoms
or molecules obey the simple and well-known laws of quantum mechanics, and
macroscopic bodies such as baseballs and rocket ships obey the laws of
classical mechanics, nanoscale systems in the "mesoscopic" regime between
these two extremes can display a wide variety of behaviors characterized
by quantum, classical, or a complicated mixture of the two sets of laws.
Understanding this regime is important both for practical applications,
such as nanoscale electronic devices used in a range of information
processing devices, and for improving our fundamental knowledge about
physics at the boundary between the quantum and classical worlds. In our
work, we find that the average displacement of a particle executing a
quantum random walk through a ladder-like network of decaying states is
quantized exactly as an integer. For systems described by this model,
quantization shows up as a discontinuous behavior of measurable
properties as parameters are varied; for a classical system, a smooth
dependence will be observed. This technique can be used as a test for
quantumness in a variety of technologically relevant systems such as
superconducting circuits and trapped atoms/ions.

***

LU11847

Unveiling a new elementary structural building block on silicon surfaces

For the construction of complex ordered structures at the atomic or molecular level, nature often employs a method called self-assembly. Self-assembly makes use of tiny structural building blocks to form organized structures of higher complexity without external direction. Examples in nature range from the formation of crystals to the folding of DNA. Due to its potential for nanotechnology to form small functional devices, physicists at the Université de Neuchâtel in Switzerland in collaboration with theoreticians from the Università di Milano in Italy explored the mechanism of self-assembly on silicon surfaces, which remains the standard substrate of the semiconductor industry. They were able to reveal the so-called silicon pentamer (see image), composed only of five silicon atoms, as a new elementary structural building block used by nature. Further investigations aim to understand the electronic properties of these atomic-scale building blocks in more detail.

***

BV10924


Exciting plasmons in double fishnet metamaterials

In 1998, surface plasmon polaritons (SPPs) were suggested to be responsible of the light transmission enhancement through metallic sheets drilled with subwavelenght holes. The particular wavelengths at which the phenomenon of extraordinary transmission occurs are related to SPPs running on the metal surface resonantly excited by the impinging light. In our paper, we study the connection between the extraordinary transmission phenomenon and SPPs in two drilled metallic layers separated a distance enough as to allow coupling of the SPPs bound to the inner metallic interfaces. In this situation, transmission peaks appear from the excitation of, not only, external SPPs, but also, internal ones at wavelengths predicted by our developed model. In addition, internal SPPs show certain unique properties different from the external ones: they can give rise to a magnetic response strong enough as to lead to a negative effective permeability, allowing the possibility to achieve negative index metamaterials with extraordinary transmission.

***

LW11442

The Makings of an Electron Crystal

In the 1930's, Wigner predicted that, at densities and temperatures
sufficiently low for Coulomb repulsion to dominate, an electron system
would crystallise into a periodic array, i.e., a lattice, to minimise
energy. This phenomenon aroused enormous interest in the physics community,
prompting many experimental investigations in the régime where strong
interactions might produce such a lattice. In this work, we study the
behaviour of a quantum wire (or line of electrons) formed from a
two-dimensional electron gas when the potential creating the
one-dimensional confinement is weakened. The electron topology is inferred
from the conductance plateaux, quantised in units of 2e^2/h for a ballistic
one-dimensional electron gas, which is well understood on the basis of
one-electron physics and a simple model of wave-function confinement.
Surprisingly, as the confinement is weakened, the electron system relaxes
to form two rows, whereupon the conductance jumps directly to 4e^2/h,
indicative of two parallel rows, both spin degenerate. Applying a parallel
magnetic field introduces mixing of spin-aligned levels when the two rows
are sufficiently close together. The breakdown of the simple "particle in a
box" model characterising one-dimensional wires, replaced instead by a
ground state of two degenerate rows, is an important step in the
exploration of the formation and properties of a two-dimensional Wigner
lattice.

***

LX11576



Coffee Inspired Nanostructures

Differential evaporation and convection rates in the drops of spilled coffee drive the granules to the rim to leave stain rings on the table. Substitute the coffee with a suspension of 20 nm gold particles and left to dry on a glass plate topped with a 2D layer of closely packed (50 – 100 µm) latex microspheres which act as geometric template and regulator of the evaporation rate, the end result can be remarkable networks of fully interconnected and conducting gold wires made up of the gold nanoparticles (see photo). Such networks, up to a few square centimetres dimension, are remnants of the meniscus network on the substrate that comprise of pendular rings around the base of the microspheres and interconnecting bridges. “A range of topologies of the wire network can be tuned by just the right combination of evaporation rate, microsphere disposition and surfactant concentration in gold suspension,” said Ivan Vakarelski, lead author of the paper in Physical Review Letters, “and we can even make networks on 3D substrates and hopefully 3D wire network with photonic applications, for instance.” With a little caffeine powered ingenuity this low cost, low environmental impact approach can replace expensive and complex fabrication facilities.

***

LA11447 and CU10073

The interference demolished with a Trojan Horse

Despite the Coulomb force is extremely weaker than the nuclear
one, its influence cannot be always disregarded. For example, it
takes considerable energy to force nuclei to fuse, even those of
the lightest element, hydrogen. This is because all nuclei have a
positive charge (due to their protons), and as like charges repel,
nuclei strongly resist being put too close together. This is a
critical point in nuclear astrophysics, that deals with explaining
the nucleosynthesis in the universe via nuclear reactions. At the
relevant temperatures all reactions are strongly inhibited because
of the Coulomb repulsion. An original technique, the Trojan Horse
Method, has been developed to overcome the Coulomb barrier. Its
name was inspired by the Homer Odyssey. There, since the Greeks
knew they could not win against Trojans by "force", they decided
to do this by "trickery": a few of the men hid themselves in the
Trojan Horse. Likewise here, the particle inducing the nuclear
reaction of interest is hidden inside another nucleus. This
technique was applied to a number of reactions providing for the
first time their measured rate. Recently the suppression of the
Coulomb force was checked also in scattering processes by looking
at the Coulomb plus nuclear interference. In the $p-p$ scattering,
such an interference causes a sudden and huge decrease of the
interaction probability. When one of the two interacting protons
is hidden inside a deuteron, the interference effect is missing
and the two protons behave like they would suffer only their
mutual nuclear fields. Astonishingly, we have found a universal
way to suppress the Coulomb interaction, valid for both binary
elastic and rearrangement processes.

***

LX10902

Interacting electrons in carbon-based electronics.

We have demonstrated the importance of the interactions between
electrons in bilayer graphene, and revealed the origin of a previously
unexplained experimental result.

Monolayer graphene is a one atom thick sheet of carbon, where electrons
behave like massless neutrinos in that they exhibit a gapless, linear
relationship between their momentum and energy. Bilayer graphene
consists of two stacked monolayer sheets and the electrons behave in a
hybrid way between traditional systems and monolayer graphene. Bilayer
graphene is a new and very exciting material, and may in time be used to
fabricate high speed (ballistic) transistors, and potentially a host of
other devices.

We have investigated how the interactions between electrons in bilayer
graphene manifest in its electronic properties when it is placed in a
strong magnetic field. We show that these interactions affect the low
energy electrons more strongly than those with higher energy, and that
this leaves a signature in the way that the material absorbs light, as
seen in a recent experiment. In this experiment, the frequency of
absorption was different for bilayer graphene with an excess or deficit
of electrons, in contrast to simple theoretical predictions. Our (more
complete) theory predicts this difference and therefore reveals
fundamentally important information about the electronic properties of
bilayer graphene which must be taken into account in the design of any
device which utilizes this material and so our work will contribute
significantly to the advancement of carbon-based electronics.


***

LP11321E

Biologically Inspired Flexibility

Biological systems work flexibly using mechanisms that are totally
different from those of artificial machines. Although artificial
neural networks have been studied with a view to mimicking brain
functions, they lack certain key features of biological systems
including adaptability and robustness against environmental change. In
this paper, we incorporated stochastic processes into artificial
networks in order to overcome these shortcomings. Using ring circuits
based on the principle of stochastic resonance and an excitable
threshold system, we created attractors that represent quasi-
equilibrium states into which a system settles until disrupted by
environmental change. Furthermore, noise-driven attractor
stabilization and switching were embodied by electronic circuits that
introduced a brain-mimicking inhibitory connection. Noise works as a
power source to stabilize and switch attractors, and endows the system
with hysteresis behavior that resembles that of stereopsis and
binocular rivalry in the human visual cortex.

***

LY11078A

Cloaking with optimized homogeneous anisotropic layers

We present a method to reduce the scattering from arbitrary objects by
surrounding them with shells composed of several layers of homogeneous
anisotropic materials. An optimization procedure is used to find the
material parameters for each layer, the starting point of which is a
discretized approximation of a coordinate transformation cloaking
shell. We show that an optimized, three layer shell can reduce the
maximum scattering of an object by as much as 15 dB more than a one
hundred layer realization of a coordinate transformation cloaking
shell. Moreover, using an optimization procedure can yield high
performance cloaking shell solutions that also meet external
constraints, such as the maximum value of permittivity or
permeability. This design approach can substantially simplify the
fabrication of moderate size cloaking shells.

***

LT11564

Real-time Observation of a Qubit

In recent experiments, the time evolution of solid-state qubits has
been observed with destructive measurements, where a time trace must be
reconstructed from many experimental runs. In our work we propose a less
invasive measurement scheme, which allows the time-resolved observation
of coherent qubit oscillations already in a single run.
The situation in quantum mechanics is quite different from the
classical world: Taking photographs of a moving classical particle,
for example, does not alter its motion. A quantum measurement,
however, acts back on the measured object. This leads to the collapse
of the wavefunction, and eventually the system is found in a
particular state with a given outcome probability. One has the average
over many runs in order to reconstruct the full information on the
system. A compromise are weak measurements, where one tries to extract
incomplete information on the system state while keeping the
backaction low.
The main idea of our work is to drive the qubit with a high-frequency
signal. The time-dependent phase of the outgoing macroscopic signal
contains information about the low frequency dynamics of the
qubit. This phase can be revealed readily in an experiment by lock-in
techniques. This enables time-resolved monitoring of coherent qubit
oscillations.

***

EW10456

A Model of Creativity and Innovation in Cities

In this paper, we develop a network model based on the
interactions between residents of a city to show how creative output can
occur in large urban settings. While it has been observed that cities obey a
fast-growing mathematical relationship between their populations and their
rates of productivity and innovation (the larger a city is, the greater the
increase in productivity per person), a mathematical model had not been
previously developed to explain this phenomenon.
Our model demonstrates that the larger the city is, the greater
the chance of there being socially distant ties between individuals, which
are the foundation for fruitful and productive interactions. In this way, we
demonstrate how large urban areas can have the advantage when it comes to
creative output, be it producing patents, generating economic growth, or
even employing more people in research and development fields.

***

LV11385

Changing the frequency of light in a similar way we do with a guitar allows us to manipulate photons just like electrons

If you have a guitar, you can easily demonstrate adiabatic frequency shifting in acoustic regime. You can change the tone of a guitar by modulating the tension of the string even after it has been plucked. This is what we call an adiabatic frequency shifting. Our careful investigation on a dynamic photonic crystal nanocavity directly showed that a same phenomenon is taking place in light frequency. The frequency of light changes when we modify the resonance of a nanocavity. This new type of controlling light allows us to manipulate photons just like electrons. Indeed, we showed that light can be trapped and released from a high-Q photonic crystal nanocavity when we use adiabatic frequency shifting. And the operation of this device is described by an analogy with a field-effect transistor. Such analogical view may open possibility for developing various novel photonic functional devices.

***

EU10429

Light Leads to Crystals

Researchers have now discovered studied how intense light can trigger the crystallization of small organic
molecules dissolved in water. They have built a strong case that the
electric field of light is aligning molecules, helping them to get
organized into a crystal. The phenomenon can be used to crystallize
pharmaceuticals and other industrially important substances.

In an effort to test the alignment hypothesis over a wider range of
materials, they set out to see if an analogous phenomenon
exists in liquid crystals, which are materials that flow like liquids
but are orientationally ordered, like crystals. When cooled, many
liquid crystals undergo a transition from a disordered "isotropic"
state to an ordered "nematic" state. The researchers found that, when
the liquid crystal was exposed to intense laser pulses as it was
cooled, the molecules in the resulting nematic state tended to be
oriented in the direction of the electric field of the light. This
observation provides evidence that the electric-field-induced
alignment of molecules plays a key role in the ordering process in
liquid crystals. The phenomenon thus encompasses a broader range of
materials and disorder-to-order transitions than was previously
realized.

***

BW10785


Engineering Magnetic Tornadoes at the Nanoscale

Magnetoresistive random access memories (MRAM) promise revolutionary changes in computer architecture. New research shows that memories using magnetic vortices, or nanoscale-size ‘tornadoes,’ can show improved efficiency and stability. MRAM can preserve stored information without consuming power, and is an attractive candidate for both storage media and magnetic logic. Unlike current MRAM technology, which can store only a single bit of information in a cell, vortex-based MRAM can store two bits of information while maintaining superior thermal stability. This paper points to ways of controlling vortex switching by designing the magnetic multilayered structure of the memories, so that they can be robustly used in a variety of applications. Just like a real tornado, the ‘magnetic tornado’ can have a clockwise or anticlockwise chirality, but its core (polarity) can point either up or down. While scientists can control the polarity of a magnetic tornado relatively easily with magnetic fields or pulses of current, manipulating the chirality in a controllable fashion has proved more difficult. Previous research used large field gradients or exploited asymmetries in the shape of the memory element. In our approach, we tailored magnetic interactions within the multilayer that enabled us to control the tornado’s chirality with an uniform field, which is easier to produce than large field gradients. The memory elements are disk-shaped with no asymmetries, which reduces memory losses caused by the bit-bit interaction. While being thermally stable, the written states are also resettable, a property which may prove useful in re-configuring magnetic logical circuits to adapt to different tasks.

***

LV11556BR

Gilded Metallicity

Though it is now well known that disorder can drive a thin
superconducting into a highly correlated insulating phase, the
precise nature of this superconductor-insulator transition remains
unknown. Recent speculation that the insulating phase is, in fact,
mediated by localized, incoherent, Cooper pairs has been supported by
the observation that metallicity can be reestablished in highly
disordered films by the application of a magnetic field. The high
field phase, termed the quantum metal state, has now been observed in
low atomic mass films such as Be and TiN. We show that by "dusting"
Be films with a sub-monolayer coating of gold, we can strengthen the
localized Cooper pairs against the applied field. This, in effect,
pushes the quantum metal phase to field scales well above that
attainable with typical laboratory solenoids, thus explaining why the
phase has not been generally observable in films comprised of heavier
elements.

***

LS11700E

CRUSHING VIRUSES WITH OSMOTIC PRESSURE

The protein coatings (or capsids) of bacteriophages, viruses that attack
bacteria,
withstand huge pressures from the DNA that they protect. The pressure is
of the order
of 50 atmospheres and seems to be an important prerequisite for the
injection of
the viral DNA in bacteria. In order to preserve functionality and
stability of
a virus, the protein-protein interactions in the viral coating must be strong
enough so that the capsid can resist the pressure. Little is known about the
resistance of capsids to pressure that acts not from the interior, but from
the exterior of a virus and that may induce collapse of capsids. The
experiments
that probe such response of viruses can be performed by dissolving empty
viruses
(without their DNA or RNA molecule) in a solution that contains molecules
that
cannot diffuse through the viral capsid (such as poly-ethylene-glycol),
creating
thus the osmotic pressure. We have performed the calculations for such
experiments.
We find that there are criticaly large pressures that induce the buckling
of the
capsids. We numerically demonstrate that there is a universality in the
buckling
so that the appropriately scaled critical pressures depend only on the
dimensionless
parameter related to the elastic properties of the capsids. This number is
known in the theory
of elastic deformation of plates as the Foppl-von Karman number and is
known to
also determine the details of the shape of non-pressurized viral capsids.
The scaling
properties of critical pressures can be nicely explained utilizing a large
body of
mathematical and physical studies on collapse of spherical and cylindrical
shells
under hydrostatic pressure. We also obtain the buckled shapes and propose
a ''buckling scenario''
visualizing deformations that transform the critically pressurized
icosahedral
shape into a buckled one that lacks the icosahedral symmetry. Our studies
predict
that the critical pressures that would have to be reached in order to
observe buckling
of viral shells should be of the order of 5 atmospheres which is easily
achievable
by present day osmotic pressure techniques.

***

LY11264

Roundabout for Vortices

When caught in the current, vortices usually flow happily down stream.
However, in this work we have discovered that quantum vortices can
behave very differently. When the number of particles and holes in a
lattice are roughly equal, as more and more particles are added, the
vortices exhibit an abrupt change in their behavior and flow vigorously
upstream. This jump in their behavior will happen as soon as there are
more particles then holes in the lattice. When the number of particles
and holes are exactly equal, the vortices don’t have a preference to
flow up or down the current, and another peculiarity happens. The
vortices acquire a spin half quantum number, whose state determines the
particle distribution at the heart of the vortex. Moreover, we have been
able to estimate their mass using an exact numerical study, and have
found it to be very light. The small mass opens the possibility for
observing a novel state of matter, a quantum liquid of spin half vortices.

***

January 13, 2009

LU11476

Metamaterials Put the Brakes on Light Signals

Light normally travels at a speed of about 300 thousand kilometers per second, or seven times around the Earth every second. A collaboration of physicists from the Vrije Universiteit Brussel, Belgium, and Iowa State University, Iowa, has now devised a way to slow down electromagnetic signals a hundred fold with metamaterials.

Light is the preferred carrier for transmitting large amounts of information around the globe, but its high speed makes it difficult to route this data to various destinations. When optical packets must be switched, most fiber-optic systems convert the signals to electrical pulses, so that the data can be temporarily stored, rerouted, and then reconverted to optical signals. This conversion, however, limits the data capacity of communication systems such as the internet significantly.

Avoiding the conversion from optical to electrical format requires slowing down the speed of light. Currently, the most successful way to do so is by the use of a quantum mechanical effect in metal vapors called electromagnetically induced transparency. Unfortunately, this technique requires complicated and expensive equipment to create the proper conditions, e.g., to cool the metal atoms to a temperature close to the absolute zero.

In Physical Review Letters, Philippe Tassin and coauthors describe a metamaterial design in which an effect very similar to electromagnetically induced transparency can be observed. Metamaterials are materials that contain—typically metallic—elements that are carefully designed to achieve materials with desired optical properties such as a slow speed of light.

The proposed “slow-light” metamaterial would work at room temperature and does not need additional equipment apart from the metamaterial itself. It can currently be manufactured to work with microwaves and terahertz waves; slowing down visible light with the proposed technique will first require more advances in the fabrication technology of metamaterials.

***

LS11445

Frequency beats -but only for a while

Frequency beating -the periodic increase and decrease of intensity
perceived when two sinusoidal signals are superimposed- is one of the
most fundamental and useful phenomena in physics. This yields the
familiar acoustic beats of airplanes in WWII movies and it is also the
basis of many ingenious detection techniques. In this paper, the authors
consider the situation where, rather than being externally imposed, the
periodic signals are internally generated. This happens in nonlinear
spatially extended systems undergoing a pattern-forming instability and
the author take an optical cavity containing a photonic-crystal fiber as
an example. In some circumstances, they find, beating lasts only for a
while, leaving a simple harmonic oscillation for the rest of the time.
The number of beats is arbitrary and depends only on the initial
conditions. The phenomenon is analogous to the appearance of localized
dissipative structures in other spatially extended systems encountered
in chemistry, optics and biology.

***

LY11386B

Sign change of equilibrium superconducting Hall coefficient due to gap
anisotropy

The magnetic Lorentz force is among the fundamental forces in physics
characterized by a unique property to deflect charged particles, causing
the Hall effect on dissipative currents in metals and semiconductors. In
this paper, we show theoretically that the force works even on
supercurrents without dissipation to induce finite charge distribution
and the resulting electric field in equilibrium superconductors. An
analytic expression is obtained for the corresponding Hall coefficient
of clean type-II superconductors with simultaneously incorporating the
Fermi-surface and gap anisotropies. It has the same sign and magnitude
at zero temperature as the normal state for an arbitrary pairing, having
no temperature dependence specifically for the isotropic s-wave pairing.
The gap anisotropy may bring a considerable temperature dependence in
the Hall coefficient and can lead to its sign change as a function of
temperature, as exemplified for a model d-wave pairing with a
two-dimensional Fermi surface. The sign change may be observed in some
high-Tc superconductors.

***

LV10908

A Laboratory Probe of the Particle Nature of Dark Energy

The GammeV collaboration presents the results from the second component of
their experimental suite; a test of chameleon dark energy. This is the
first dedicated laboratory test of a dark energy model. With chameleon
dark energy, the observed acceleration of the universe is caused by a
light scalar particle that has evaded other terrestrial experiments. This
is due to the fact that the properties of the chameleon particle---namely
its mass---depend upon the environment. We exploit this effect to trap
these chameleon particles in a jar. Chameleon particles that are
generated from the interaction of laser light and a magnetic field bounce
off of the walls of our vacuum chamber, including the optical windows at
each end. When the laser is turned off, the jar empties as the chameleons
reconvert to detectable photons. This afterglow is a telltale signature
of the chameleon particle. While no signal was found, the resulting
limits constrain the properties of the chameleon particle, including its
coupling to photons and its mass, for a range of chameleon dark energy
models.


***

LV11596

Ultracold neutrons reveal the robustness of Berry's geometric phase

Devising fault-tolerant methods for the manipulation of fragile quantum
states is a major challenges in the field of quantum information processing.
In this paper, we demonstrate that the Berry phase can potentially be used
for this purpose since it remains widely unaffected by fluctuations induced
by the environment. Berry's phase is based on the curvature of Hilbert
space, the space of quantum states, in close analogy to the precession of
Foucault's pendulum which is determined by the curved surface of earth. This
geometric origin leads to its interesting behavior with respect to noise.
We have built a setup to store ultracold neutrons and manipulate their spin
by controlled variations of magnetic fields. By generating additional
fluctuations on top of the control fields, we could measure the resulting
uncertainty in the Berry phase for state manipulations of different
duration. We have verified that the uncertainty in the Berry phase vanishes
as the manipulation time increases. Hence, for long operations Berry's phase
is not influenced by noise.

***

LP11321E

Biological systems work flexibly using mechanisms that are totally
different from those of artificial machines.
Although artificial neural networks have been studied with a view to
mimicking brain functions,
they lack certain key features of biological systems including
adaptability and robustness against environmental change.
In this paper, we incorporated stochastic processes into artificial networks

in order to overcome these shortcomings.
Using ring circuits based on the principle of stochastic resonance and an
excitable threshold system,
we created attractors that represent quasi-equilibrium states into which a
system settles
until disrupted by environmental change.
Furthermore, noise-driven attractor stabilization and switching were
embodied
by electronic circuits that introduced a brain-mimicking inhibitory
connection.
Noise works as a power source to stabilize and switch attractors, and
endows the system with hysteresis behavior that resembles that of stereopsis
and
binocular rivalry in the human visual cortex.

***

AY10391

Ring resonator goes quantum

Ring resonator is commonly used in many devices in optical
communications, as well as many frontier quantum optics experiments.
A ring resonator supports two counter-propagating modes, the
whispering gallery modes, that makes the transport properties of
light through such component an interesting and rich plethora.

Now the researchers at Stanford University provides a fully quantum
mechanical analytic results for single-photon transport through a
ring resonator. Among their results, they found there exists a
critical coupling condition for such system that an on-resonance
incoming single photon would be blocked completely, in spite of the
presence of dissipations and imperfections of the resonator. In view
of the increasing attentions on single-photon generation, controlling
and traffic regulating, their theory should be a timely work for
fields such as quantum information processing and quantum communication.

***

AU10216

*Towards water-soluble fullerenes: what happens next*

Highly hydroxylated fullerene C60 derivatives (fullerenols) are especially
important in medical sciences because they form water-soluble substances and
may be used in drug delivery applications. Despite the diverse therapeutic
use of fullerenols, little is known on the electron distribution and spatial
arrangement of their molecular structures. For this reason, we have employed
density-functional-theory methods to unveil the structural features and
electronic properties of fullerenols as a function of the hydroxylation. Our
recent published results showed that fullerenols with more numerous hydroxyl
groups (e.g., C60(OH)24) can exhibit a kind of electronic confinement effect
able to reduce the interaction of the carbon cage with the environment. In
other words, the great number of hydroxyl groups adsorbed on the fullerene
surface can protect it of immediate reaction as is it occurs for the
pristine fullerene C60 in aqueous solution.

***

LW11249

Nano Test Tube

As test tubes go, it doesn't get any smaller than a single-walled carbon
nanotube. The confinement offered by a nanotube constrains chemical
reactions. We show such nanochemistry enables electron doping through
the 1D van Hove singularity of single-walled carbon nanotubes. This
yields enhanced density of conduction states, providing a model base for
challenging fundamental physics. "Screening", in physics, is the damping
of electric fields caused
by the presence of mobile charge
carriers. Through
electronic excitations from the carbon 1s core level to 1D quantized
states of initial semiconducting tubes, we demonstrate that their
increased density of conduction states leads to enhanced core hole
screening. This fact illustrates the importance of many body effects in
understanding core level excitation process in carbon nanotubes. Using
1D quantum confinement of carbon nanotubes as a probe, our archetypical
nanochemistry addresses a fundamental physics issue and highlights how
such advances could determine future nanoscience.


***

LU11744

Digging for buried microstructures, or why a new liver cancer medicine works

Buried nanoscopic crystalline domains embedded in solid biodegradable
polymer microspheres, have been detected and characterized by
nonlinear light scattering spectroscopy, which is a combination of
light scattering and nonlinear optics. We find that inside the 20 -50
micron sized spheres, which are key to a promising new treatment for
an incurable form of liver cancer, there is a distribution of
crystalline domains with sizes below 600 nm. This finding explains
the apparent structural robustness of the microspheres that seems to
be crucial in the understanding of the working mechanism of the
treatment: In the proposed medicine the crystalline domains can form
a host matrix for the necessary medicinal complex. The ability to
look inside a solid matrix without cutting open the material opens up
new avenues for the study of heterogeneities in chemistry and
physics, such as the monitoring of crystal nucleation and growth, or
the detection of small amounts of biological crystals (such as
proteins and biopolymers).


***

BT10815

Engineering deterministic aperiodic structures for nanoplasmonics

The ability to control, enhance and extract
sub-wavelength (sub-_) optical radiation from
engineered nanostructures is at the core of
nanophotonics/nano-electronics integration.
An efficient scheme for local-field enhancement,
input/output coupling and precise addressing of
sub-_ radiation at the nanoscale is essential to
benefit from the extreme downscaling of state of
the art nano-optoelectronics components such as
light-emitting quantum structures, nano-tubes,
single molecule detectors and plasmonics devices.
Crucial to this picture is the development of a
novel engineering approach for the control of
electromagnetic sub-_ field enhancement at the
nanoscale. This has the potential to enable a
variety of novel nanoplasmonics active devices
offering large radiative-rate enhancements of
quantum dots, efficient photon injection and
extraction at the nanoscale, enhanced sensing
capabilities and unprecedented nonlinearities for
on-chip switching and frequency generation.
Crucial to this vision is the development of
novel rational approach for "engineering optical
fields at the nanoscale".
Presently, the best approaches to generate
nanoscale electromagnetic giant fields rely on
random "roughening" of metal surfaces by etching
or by colloidal synthesis of nanoparticles
resulting in aggregates statistically described
by non-periodic fractal objects and morphologies.
However, despite non-periodic statistical fractal
aggregates and rough metal surfaces led to the
successful generation of giant fields with
applications in single molecule spectroscopy,
they lack reproducibility and simple engineering
design rules for deterministic optimization.
The study of Deterministic Aperiodic Structures
of metal nanoparticles overcomes the fundamental
limitation of randomness by uniquely enabling
controllable spatial localization of
sub-wavelength plasmonic modes combined with wide
frequency spectra, which can be accurately
engineered. This approach is uniquely suited for
the establishment of predictive theories and
accurate design rules to understand and control
highly localized electromagnetic fields in
nanofabricated plasmonic-photonic devices such as
Fibonacci quasi-crystals [1-4] and more complex
deterministic structures with far richer spectral
properties [5,6].
Central to this novel approach is the development
of electromagnetic models for the accurate and
efficient solution of large-scale aperiodic
systems of interacting nanoparticles. This
provides significant challenges to classical
computational electromagnetics (CEM) techniques.
Conventional numerical approaches based on
discretized grids, such as Finite Difference Time
Domain (FDTD), Finite Difference (FD), and Finite
Elements (FE) becomes very inefficient and
inaccurate for the solution of large-scale
aperiodic systems with nanoscale features, which
require prohibitive computational power.
Despite the field of aperiodic deterministic
nanoplasmonics structures is still in its
infancy, our original modeling, computation
[1,2,4,7] and experimental work [3,5] has
demonstrated the onset of plasmonic band-gaps and
localized field states in metal nanoparticle
chains and arrays, unveiling the fundamental
connection between aperiodic morphologies and
eigenmode spectral and localization properties.
We envision that in the near-future the study of
deterministic aperiodic order will become a
primary paradigm in nanophotonics and
nanoplasmonics design technologies, where it
could open novel pathways for the demonstration
of nanoscale optical sensors, engineered SERS
substrates with single molecule sensitivity, and
a variety of active nanoplasmonics devices
enabled by the manipulation of localized
electromagnetic fields at the nanoscale.

***

LT11942A

Chaotic shock waves of a Bose-Einstein condensate

The harmonically confined Bose-Einstein condensate (BEC) has been studied widely, however, the exact solution of the system has not been reported yet. It is demonstrated that the well-known Smale-horseshoe chaos exists in the time evolution of the one-dimensional (1D) BEC driven by the time-periodic harmonic or inverted-harmonic potential. The first exact solution of the system is constructed , which describes the matter shock waves with chaotic or periodic amplitudes and phases. When the periodic driving is switched off and the number of condensed atoms is conserved, we obtained the exact stationary states and non-stationary states. The former contains the stable `non-propagated' shock wave, and in the latter the shock wave alternately collapses and grows for the harmonic trapping or propagates with exponentially increased shock-front speed for the antitrapping. It is revealed that existence of chaos play a role for suppressing the blast of matter wave. The results suggest a method for preparing the exponentially accelerated BEC shock waves or the stable stationary states. The chaotic shock wave as a new type of superfluid turbulence warrants further investigation.


***

LY11450

Origin of electromagnons in multiferroics

In most materials electricity and magnetism do not strongly interact so
that mixed magneto-electric devices are not yet a part of the
electronics industry. Similarly, the electromagnetic response (i.e., to
light) is generally separate – the magnetic field of light can excite
magnetic resonances and light’s electric field can excite lattice
vibrations. In multiferroic materials, where magnetism and
ferroelectricity coexist, it is possible to excite mixed spin and
lattice vibrations with electromagnetic waves. These excitations are
called electromagnons.
Based on a study of the absorption spectrum of the multiferroic compound
TbMnO_3 as a function of magnetic field, temperature, and polarization
of light we propose a theory for the origin of these electromagnon
excitations in the whole multiferroic family RMnO_3. Interestingly, we
find that the mechanism responsible for electromagnons is different from
the one that couples static magnetism and ferroelectricity. Our model
also explains the appearance of ferroelectricity in another family of
multiferroic materials with collinear magnetic structures - the so
called E-phase. Our results show how to strongly couple spin and lattice
excitations, and that this mechanism can exist in non-multiferroic
materials. Therefore, in principle, this effect could be useful for
spintronic applications that take advantage of this coupling even at
room temperature, something that is not possible in the current families
of multiferroic materials where these effects exist only at cryogenic
temperatures.


***

LN11455ER

Polymer Adsorbtion

A recent computational method developed by University of Minnesota
researchers may open the door to substantial improvements in a broad
range of technologies that depend on polymer adsorption.
The ability of polymers---molecules composed of hundreds of repeat
units---to stick to surfaces is critical to numerous processes such
as coating, lubrication, and drug delivery. When adsorbed to a surface,
polymers undergo random changes in their conformation and
position thanks to bombardment by much smaller solvent molecules,
a phenomenon known as Brownian motion. Describing these dynamics
in an accurate way has remained a long-standing challenge due to
their complexity. University of Minnesota researchers have developed
a method to exactly describe the most difficult
aspect of the random motion: hydrodynamic interactions---the
solvent-mediated interaction of one part of the polymer with another part
and with the surface. Their results make clear for the first time the
role of these interactions, and allow for systematic investigation of the
effects of polymer and solvent properties on this complex behavior.
The method is expected to ultimately enable the design of
surfaces to precisely control the dynamics of adsorbed polymers, which in
turn may lead to stronger coatings, better lubricants, and more
effective drug
delivery strategies.

January 13, 2009

January 5, 2009

LX11263

TWO AGING REGIMES MAKE RETHINK 1/F NOISE

We study a liquid crystal in a regime where ordered
"rolls" (similar to convection rolls) are present,
due to the application of an external voltage.
Defects show turbulent dynamics yielding 1/f
(to a power) noise.

Treating the birth/death process of defects as
memory-resetting "quakes" we provide a theory
correctly describing the crystal response to
external perturbations in all regimes.

In detail, the inverse-power-law Fourier spectrum has a
power index monotonically decreasing with
the applied potential. "Pure" 1/f noise corresponds
to a threshold voltage, below which non-ergodicity is
apparent. In other words, the system never reaches
equilibrium, while averages over different experimental
runs show that macroscopic light transmittivity decreases
with time with respect to the preparation of the system.

However, a very weak form of non-ergodicity is also
present
in the more turbulent phase (above the 1/f threshold),
making it impossible to predict the average response to
perturbation with the use of the conventional linear
response theory.

We imagine that our approach may be accurate
for all systems exhibiting 1/f noise.


***

LQ11230

Observing particles inside a pile of sand

Using a laser sheet scanning technique, researchers at the University of Maryland are now able to observe the motion of all particles inside a pile of sand. Additionally, they can determine how the grains rearrange relative to their neighbors to allow such a compaction.

Sandpiles are packed tightly with one grain of sand held in place by its neighbors. Despite their initial tight-packing, they can be forced to compact into an even denser configuration. To achieve this, a simple “tapping” method is applied in which researchers cyclically expand and contract the container size. This process, which is similar to tapping a cup full of flour to increase space, allows researchers to observe the grains as they settle.

To see inside the container, researchers immerse the grains in a fluid with a matching refractive index. The fluid is dyed so that it fluoresces, as a sheet of laser light passes through it; the beads appear as dark circles. This creates a cross-sectional image of the system. The sandpile is “scanned” by taking pictures while moving the sheet through the system. Through these images, they find the centers of individual grains of sand.

Through this process, UMD and NIST researchers have discovered that the direction of motion is correlated with the positions of neighboring particles. The same correlation had been found in cooled fluids, highlighting the similarity between fluids close to the glass transition and granular matter close to jamming.



***

BU10709

"Theory of direct scattering, trapping, and desorption in atom-surface
collisions"

The famous nineteenth century scientist, James Clerk Maxwell, is
perhaps best known for his work elucidating the theoretical underpinnings
of electricity and magnetism, but he actually spent most of his
intellectual effort trying to explain thermodynamics and the motions of
particles making up gases. One of the major problems that perplexed him
during a large part of his career was the fact that a seemingly realistic
mathematical description of a gas as made up of tiny hard spheres could not
be made to come to a state of equilibrium all by itself. In a paper
published the year of his death, Maxwell showed that one way to resolve
this problem was to assume that when the gas particles collided with the
surfaces of their container, a portion of them became temporarily trapped
by the binding force at the surface. Some time later these temporarily
trapped particles were assumed leave the surface and go back into the gas
phase, but with a distribution of energies that was in equilibrium with the
temperature of the container walls. This "Maxwell Assumption" has been
used ever since to analyze experimental data for the interactions of gas
particles with well-defined surfaces, although up to now nobody has been
able to verify it with actual theoretical calculations. In this paper,
using a simple model for the surface binding force combined with a correct
description of the collisions of the gas atoms with the atoms of the
container walls, we have been able to determine how some of the gas atoms
become trapped and how these later escape the surface and rejoin the
gas. These calculations allow us to determine the conditions under which
the "Maxwell Assumption" is valid and when it is not. Important and
necessary verification of our calculations comes from the fact that they
explain recent experimental data taken by the group of Steven Sibener at
the University of Chicago for the scattering of argon atoms from silver
surfaces that were covered with a single self-assembled layer of
1-decanethiol molecules.


***

LU12089

Geometric frustration leads to peculiar morphology and glassy dynamics
in buckled colloids

We use a system of colloidal spheres to study geometric frustration
similar to that found in anti-ferromagnetic compounds and thereby
provide an important connection between hard- and soft-condensed matter
physics. Frustration appears whenever a set of constraints may not be
simultaneously satisfied. One of the prototypical physical models
exhibiting such a situation is that of anti-ferromagnetic Ising spins on
a two-dimensional triangular lattice. In this model, the magnetic moment
of each particle may point in one of two directions, and energy is
minimized when two neighboring spins are anti-parallel. Frustration
arises because it is impossible to arrange any triplet of neighboring
particles such that all pairs of neighbors are in opposite states.
Motivated by recent experiments in diameter-tunable microgel spheres
[Nature 456, 898 (2008)], we consider monolayers of spheres confined
between parallel plates. We establish the analogy between our system and
the Ising model by showing how maximization of entropy in the hexagonal
sphere packing induces effective anti-ferromagnetic interactions between
neighboring spheres. We explain the glassy dynamics characterized by
logarithmically slow relaxation as well as jamming in metastable
configurations, and show how lattice deformations favor zigzag stripes,
thus elucidating the importance of elasticity as a mechanism for
frustration relief.


***

LV11694ER

Proper time for relativistic Brownian motion

Einstein's theory of relativity tells us that the progression of time
experienced by a physical object is tightly linked to its state of
motion. Our paper discusses implications of this phenomenon for
Brownian random motions.

The paradigm of Brownian motion has proven very useful for
understanding the behavior of particle-like quantities that interact
quasi-randomly with a complex environment. Modern applications cover a
wide range of different areas in biology, chemistry, finance, and
physics. Due to vast experimental progress in high energy physics and
astrophysics, Brownian motion concepts will play an increasingly
important role in these fields as well. It is, therefore, important to
understand how the underlying ideas can be consistently embedded into
the theories of special and general relativity.

Our paper explains how relativistic Brownian motions can be expressed
using different time parameters. We are able to show that two
controversially discussed relativistic generalizations of Maxwell's
velocity distribution are, in fact, closely related to each other --
they correspond to different time parameterizations of the same
underlying process.

These results will be useful for modeling complex high energy
processes, where one must distinguish between the proper (life) time
of a particle and the laboratory time measured by an observer. In
particular, our analysis allows one to extend Brownian motion concepts
to general relativity in a straightforward manner, e.g., in order to
describe quasi-random motions of relativistic particles around black
holes.


***

BX10501

Electrically controllable magnetic fields in semiconductor quantum dots

A key ingredient for semiconductor based future electronic devices is a quantum mechanical property of electrons, called the spin of electrons. Normally, this spin can only be controlled by applying external magnetic fields. However, magnetic fields can neither be switched rapidly nor can they easily be applied locally to small nanostructures. In this paper, we predict an alternative and surprisingly effective way to control this spin in particular nanostructures. The controlling "knob" in our case is an electric voltage that is applied across the nanostructure. Electric fields can easily be switched rapidly and can be applied individually to small nanostructures. Our result applies to particularly engineered nanostructures, so-called stacked quantum dot molecules that are commonly fabricated and studied in semiconductor labs. In these structures, the electric field effect is an order of magnitude larger than previously reported. Our finding opens up new promising perspectives for quantum logic gates which are the building blocks of quantum computers.

***

BV10714

Confining Dirac electrons in graphene: a challenging
task

Graphene is widely considered as a perpective base for a post-silicon
electronics. To this end,
the confinement of electrons in designed space regions is very important
for producing the
building blocks of electronic devices. For massless chiral Dirac
electrons in graphene, this is
however a challenging task due to the so-called Klein tunneling:
electrons can perfectly propagate
through an infinite steep potential. How can we learn whether electrons
are really confined ?
While confined, electron energy spectrum becomes quantized and electrons
localize in certain
levels. Besides the position, each level is characterized by a width,
which is inversely proportional
to the electron lifetime in the level. The state is called quasi-bound
(QBS) if the level width is
so small that electrons may remain for a long (but finite) time before
tunneling away. To identify
a QBS one has to determine both the level position and the level width.
In this work, we suggest
a simple (T-matrix) approach for studying QBSs induced by any smooth
1D-potentials in
graphene. Detailed calculations have been performed for double barrier
structures and
n-p-n junctions. The suggested approach is quite general and could be
also used for calculating
different fundamentail quantities.


***

LU11583B

The mechanism for the creation of slow waves near cut-off frequencies in periodic waveguides

Slow waves are of considerable topical interest; we show that they can be created within a simple waveguiding structure, a planar waveguide with periodic corrugations, and describe the physical mechanism responsible. We show this is a universal feature of this geometry in very different areas of physics such as water waves in rigid pipelines, transverse electromagnetic waves in infinite conducting waveguides, as well as in anti-plane shear acoustic and coupled in-plane shear and pressure elastic waves in freely vibrating and clamped waveguides. The presence of slow modes within elastic waveguides has remained unexplored and so potential applications have not been exploited, we demonstrate that the physics of slow light is responsible for the elastic slow modes with a subtle nuance connected to the presence of negative group velocity modes within the elastic system (see a chain-like mode in Fig. 1) . Many new applications are possible, particularly in elasticity, to generate analogies of optical delay lines, furthermore the guiding structure we describe is simple to construct.


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LN11295BR

Cryptoferromagnetism: a new monster or lovely child of intimate interplay?

Put together key words of superconductivity and magnetism: Cooper
pairing, London penetration depth, Meissner effect, coherence,
chirality, breaking of time reversal symmetry, cooperative phenomena,
crystallographic anisotropy, domain structure, magnetic moment, then
you will get impression what it is a
crytoferromagnetic state of superconductor. If you magnetise this
superconductor by applying magnetic field and measure it positive
magnetization, as we predict, you will be wondering, whether it is a
superconductor at all. But, a recovery of zero resistance will bring a
peace to you - you deal with unusual state of unconventional
superconductor, cryptoferromagnetic state. Then, it is up to you
decide, whether cryptoferromagnetism is a monster or lovely child of
intimate interplay between superconductivity and ferromagnetism.

***

BU11025

Carbon Nanoparticles as Closed Conductive Networks

In last two decades, discoveries of fullerenes, nanotubes, graphene and their fantastic properties successively instigated three booms in nanocarbon research. We attract attention to another fullerene-related material - large carbon nano-onions, the astralens, suitable for industrial-scale synthesis. High-Resolution Transmission Electron Microscopy, X-ray Diffraction and Raman spectroscopy revealed a polyhedral multi-shell structure of astralens. Astralens have average sizes of 40 nm, flat defect-free faces of ~15 nm and defects condensed at the polyhedral edges. The faces comprise stacking of 20-50 graphene sheets. Using electron and nuclear magnetic resonance techniques we demonstrated qualitative difference of the electronic properties of astralens compared to those of quasi-spherical and small polyhedral onions. The most unusual feature of the material is a temperature independent Pauli-type behavior of paramagnetic signals from room temperature to liquid helium. Such behavior, never reported for nanocarbon samples, is attributed to delocalized charge carriers whose quantity considerably exceeds that of spins localized in defects on polyhedra edges. We suggest that each astralen nanoparticle constitutes a closed network of delocalized electrons. Our findings may open a new avenue for further study of the electronic properties of such closed conductive networks and search for their device applications.

***


EU10295

Transition to the irreversibility: an explanation based on the second law

To understand how the behavior of many-particle systems may
become irreversible upon the action of an external driving force is one of
the fundamental problems of thermodynamics and statistical physics since their foundation. An analysis based on the second law of thermodynamics
reveals that this transition can be explained within the framework of
non-equilibrium thermodynamics.

The transition from a reversible (oscillatory) to an irreversible (chaotic) behavior of massive (non-Brownian) particles subjected to an oscillatory shear in a Taylor-Couette cell is described from the entropy production rate of the particles, derived from the second law, and the Onsager relations connecting the diffusion current to the driving force, the shear flow. The observed chaotic behavior of the trajectories of the particles whose origin is the presence of hydrodynamic interactions can be
interpreted macroscopically as a shear-induced diffusion effect. We have analyzed this diffusion process by means of non-equilibrium thermodynamics and computed the corresponding effective diffusion coefficient. Its value explains the behavior of the mean square displacement observed in the experiments. Our study shows that the irreversibility inherent to the chaotic behavior of the macroscopic motions of particles is perfectly compatible with the second law of thermodynamics.


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LW11234

Ion generator produces a very large ion current – greater than the discharge current

Researchers at Lawrence Berkeley National Laboratory have succeeded in producing an extraordinarily high flux of metal ions in vacuum by operating a conventional sputtering magnetron in an unusual, “abusive” way. By overpowering the device with pulses exceeding the manufacturer’s average power specification by a factor of over 100, they were able to ionize the sputtered atoms so that they entered the regime of self-sputtering, characterized by a return of sputtered atoms, now ionized, to the target. Although self-sputtering has been known before, the researchers were able to demonstrate both theoretically and experimentally that the usable ion flux scales exponentially with the “power distance” from the onset threshold of self-sputtering. In fact the usable ion current can exceed the discharge current. As a side benefit, no process gas is needed. For the case of copper, 250 amperes of ions can be produced with a discharge current of only 100 amperes. This research promises applications in the metallization of semiconductors and for producing coatings in the vacuum of space.


***

LN10909

"Electromagnetic pulses squeeze through metamaterial at a fraction of the speed of light"

Electromagnetically-induced transparency (EIT) is a famous quantum interference phenomenon appearing when a control beam of light makes atomic gas transparent for a signal beam of a different colour. It also leads to a dramatic reduction of the velocity of the signal pulses. EIT is intensively studied as a solution for delay lines and light ‘storage’ needed for all-optical information processing.

Here we show that slowing electromagnetic pulses can be achieved in a planar metamaterial, a metal film artificially patterned on a sub-wavelength scale. Our approach does not require the presence of the control beam and is possible in a very thin patterned layer. The pulses induce strong resonant current oscillations in the metamaterial grid that interfere in a similarly way as the quantum states of the gas medium: the pulses slowly squeeze their way through the thin metal grid with only five-thousandth of the speed of light in vacuum.

A successive stacking of several layers of the metamaterial increases transmission and improves operational bandwidth, thus offering a radically new solution for delaying optical pulses in data processing applications.

***

LW10897

A time-dependent black hole reveals natures of expanding plasma

We reveal natures of relativistically expanding plasma through a
time-dependent black hole. The Quark-Gluon Plasma (QGP) is a new state
of matter whose internal friction (viscosity-to-entropy ratio) has
recently been observed to be far smaller than any other known fluids.
Such a tiny internal friction indicates extremely strong interactions
among the constituents of the QGP, quarks and gluons. Since the
universe was once made of the strongly-interacting QGP, it is
important to build a theory that can account for its behaviors, in
order to deepen our knowledge on the Big Bang. Theoretical
(microscopic) description of the QGP is one of the biggest challenges
in physics due to its very strong interaction. A breakthrough came
from superstring theory which suggests that a certain
strongly-interacting system, similar to the QGP, is mathematically
equivalent to a five-dimensional black hole. However, previous
attempts on a time-dependent system have suffered from an appearance
of divergence (naked singularity). We have succeeded, for the first
time, to construct a consistent time-dependent black hole that can
completely account for the relativistic flow of the QGP-like fluid. We
have proven that all the possible divergences are safely hidden by a
time-dependent horizon (from which no signals can escape). This work
provides a basis to treat a time-dependent system from both the
microscopic and macroscopic points of view simultaneously.


***

LU11507BR

Spintronic Switch

To become a reliable substitute for Electronics, Spintronics
first has to get into a possession of an important elemental
unit - a "magnetic switch". This unit is supposed to be either
ferromagnetic or paramagnetic depending on the applied voltage.
In accordance with Maxwell's electromagnetics, the electric field
does not penetrate into a metal and the conventional wisdom suggests
that the external control of ferromagnetism in metals is not possible.
The possibility to control ferromagnetic order by voltage was known
for almost a decade in materials called low-temperature dilute magnetic
semiconductors (DMS), and the scientific community has been intensively
looking for the high-temperature DMS for the room temperature operation.
In our paper, we showed that contrarily to the widespread belief, the
control of the ferromagnetism is, in fact, more pronounced in metals
than in the DMS. Thus, the "magnetic switch" can be metallic. This
finding is especially promising in the light of the fact that because
of the high electron density, metals, as compared with
semiconductors (including DMS), can be scaled down to
much smaller feature sizes.


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LY11086B

One level to rule them all

We have experimentally shown that a single energy level determines the
behaviour of defects, impurities and surfaces of electronic materials.
This should allow scientists to predict the electrical characteristics
of all semiconductors, and even custom engineer specific materials with
desired properties for use in devices as diverse as light emitting
diodes, chemical sensors, and photoelectrochemical cells for hydrogen
production. The bulk electrical properties of semiconductors can differ
drastically from one material to another. They are known to be
controlled by a small concentration of imperfections such as missing
atoms or other defects, or foreign atomic species known as impurities,
which can add electrons to, or remove electrons from, the material.
Also, even for perfect crystals, surface properties can vary greatly,
supporting either a build up or a complete absence of charge,
significantly affecting electrical contacts to devices. Investigating
the material cadmium oxide, we have now shown that how these defects,
impurities and surfaces act electrically depends on the position of a
common energy level. From the position of this so-called charge
neutrality level, which can be estimated fairly straightforwardly, the
electrical behaviour of any semiconductor can instantly be understood
and even predicted. This will allow custom materials to be developed
with desirable electrical properties for high-performance current and
future device applications.