10-31-07

LG11619,

Using rings to increase the current in quantum electronics.


Many of the electronic properties of today circuits depend on the
ability to induce and control the currents along its basic components.
In this work we show that at the nanoscale and when the electronic transport
is ruled by the laws of quantum mechanics, the circulating currents along
wires are ruled not only by the material's properties but also by the
mechanism employed to induce the transport.

The conductance of a one-dimensional wire connected to source
and drain electrodes can not overcome the fundamental quantum
(G=2e²/h). However, we show that when the wire is bended into a ring
and the transport is promoted by time-dependent magnetic fluxes,
additional effective transport channels are generated
that allow for a spectacular increment of the induced dc current and
to achieve conductances much greater than the fundamental quantum.

***


LF11779
First evidence for truly out-of-equilibrium phase transition found in
liquid crystal topological turbulence

Spreading phenomena can be seen everywhere, e.g. in epidemics, forest
fires, rumors, calcium signaling in cells, and even galactic evolution.
Over decades physicists have modeled them and found that, in spite of
the apparently different processes at play, they often obey the same
universal laws prescribing how activity grows, how it is distributed,
how it fluctuates, etc., in the critical region marking the onset of
unbounded spreading. Known under the name of directed percolation
universality class, this out-of-equilibrium universality is now well-
established like universality in equilibrium, except in one essential
aspect: no experiment so far has been able to convincingly show evidence
for the directed percolation universality class despite substantial
efforts. This long-standing puzzle is solved here, in the context of
topological turbulence in driven liquid crystals. This opens the door
towards establishing experimentally out-of-equilibrium universality
classes in natural phenomena.

***

LH11645

Increased Fusion Efficiency with Particle Beams

A theoretical approach to fusion that was dismissed by physicists 50 years ago as impossible has been revived by in-depth simulations. So called beam fusion works by firing beams of particles known as ions into ion targets at just the right energy so that ions in the target “prefer” to undergo a fusion reaction with them, rather than simply deflect them away as would normally happen. Fusion reactions are a CO2-neutral route to energy production and they occur when two ions are given enough energy that they can fuse together. The new simulations show that the beam ions bounce around inside the target allowing them to have multiple chances of fusing with target ions. The result is that more energy can be released than was put in and this may help pave the way to more efficient routes to fusion energy production using lasers.

***

LF11842
Microscopic Saturn Rings and Satellites

When studying tiny oil droplets and air bubbles rising in a liquid
crystal, we observe defect rings and points that closely resemble their
celestial namesakes. But the droplets and bubbles are merely several
hundredths of a millimeter in diameter, and the cause of the defect
rings and points are molecular misalignment in the liquid crystal. We
also discover that subject to disturbances in the ambience, the
satellite point can spontaneously open up into a Saturn ring, and a
Saturn ring can be swept by flow of the liquid crystal into a satellite
point defect. Using computer simulation, we have reproduced these
interesting transitions, and demonstrated that they are due to molecular
rotation driven by the flow on the one hand, and alignment among
neighboring molecules on the other. These findings open the possibility
of using liquid crystals to control the assembly of microscopic
particles into regular crystalline structures, with potential
applications as photonic crystals.

***

Small Particles Big Motions

The movement of small particles like bacteria, red blood cells and environmental contaminants are important
in sensor design for health care, understanding human physiology and cleaning up the environment. Recent
research by the Magnetic Resonance Microscopy Lab at Montana State University indicates that these motions
are more complex than previously thought and can be modeled with ideas from nonlinear dynamics or chaos theory.
In particular their research shows that motions in dilute suspensions of these particles flowing in small
tubes are irreversible, an effect thought to be dominant only in concentrated particle systems. When only
two particles interact through a fluid the motion can usually be reversed by reversing the fluid flow. However
when 3 or more particles interact the system behavior is more complex and the motions less predictable.
Understanding these complex motions has the potential the enhance design of microfluidic biosensors for blood
and bacteria as well as model contaminant motion in the earth’s subsurface.

***

LE10878

What makes a rod stiff?

Can surfaces modify the mechanical properties of nanoscale devices?
Measurements say yes, but theoretical models proposed over the past 30
years are found to violate Newton's third law. A rethink is required.

Tunability of the mechanical properties of nanoscale devices lies at the
core of potential applications in nanoelectromechanical systems. By tuning
stress at the surface of micro-cantilevers, researchers have observed that
they are able to modify cantilever stiffness. This observation has been
purportedly explained using theoretical models that describe measurements
remarkably well.

Surface stress is like adding cling wrap to a surface and shrinking it.
Current theoretical models describe surface stress as if one is pulling
the end of the cantilever, like stretching a rubber band. However, there
is nothing there to pull the cantilever so the rubber band stretches
itself! As Newton pointed out, every action has an equal and opposite
reaction. As such, all these models are in violation of Newtons third law
and unphysical.

John Sader and Michael Lachut from the University of Melbourne examined
this surface effect, by taking into account Newton's laws, and discovered
a new surface stress law that is particularly relevant to
nano-cantilevers. Their research will be published in Physical Review
Letters.

The outcome is that this controversial physical phenomenon defies
explanation despite more than 30 years of research. It remains to be seen
whether the problem lies in the measurements themselves, their
interpretation or in an alternative mechanical process. Further
miniaturization to the nanoscale and use of this new law are expected to
shed light on this intriguing effect.

***

LF11629
Solving the Schroedinger equation of atoms and molecules without
analytical integration based on the free iterative-complement-interaction
wave function

Schroedinger equation has now become soluble for general molecules,
which is a firm basis for building up an accurate predictive science.
The Schroedinger equation governs chemistry, biology and physics of matter. However, for 80 years after its birth, exact solution of the Schroedinger equation has been thought to be impossible, except for some very special simple cases. Recently, we have developed a general theory to construct an analytical wave function having exact structure, so that by applying variation principle, we have been able to calculate very accurate solutions of the Schroedinger equation of small atoms and molecules. However, a problem still remained since integrations over the complement analytic functions necessary in the variation calculations are extremely difficult for general atoms and molecules, which hindered the solution of the Schroedinger equation for general systems. In this paper, we have proposed a new simple general method of solving the Schroedinger equation analytically without doing integrations. This method has been applied to several atoms and molecules, giving very accurate energies and wave functions. This has opened a road leading to accurate predictive methodology in chemical, physical and biological sciences.

***


LE11181

Microscopic bodies that swim in the computer

Microrobots that swim through the bloodstream to deliver their medicinal
payloads to errant cells are still a futuristic application, but given the
rate at which nanotechnology is progressing the time is ripe to begin
studying microswimmer behavior in order to learn what makes a good design.
The present study of these minuscule swimmers employs molecular dynamics
simulation, a method in which the individual molecules of both the swimmer
and the fluid through which it swims are represented by interacting
particles; the motion of these particles is followed by solving Newton's
equations on the computer. Swimmers propel themselves using a variety of
mechanisms familiar from nature, such as rotating limbs, changing body
shapes or fluid jets. The power expenditure of different swimmer designs
can be compared to determine their relative efficiencies, and the wakes
generated by the swimmers in the fluid can be visualized. Owing to the
extremely small sizes involved, inertial effects are very small, so that
the efficiency of the swimmer designs can be very different from the more
familiar macroscopic world. This initial study treats the problem in two
dimensions, but extension to the more realistic three-dimensional case is
underway.

[The figure shows a three-dimensional swimmer with a rotating tail, also
simulated using molecular dynamics; the fluid particles are rendered
partially transparently.]


***

LG10977
A new link found between biological and computational descriptions of the
brain


Brains are often thought of as being analogous to computers in that
they process information and perform calculations.� Much of our
understanding of how the brain works, is however in terms of
biological elements such as channels and chemical transmitters.� The
relationship between these computational processes and the biological
elements is very unclear. We have given a mathematical proof that
provides a specific link between the biological and computational
descriptions of the brain.� We also have shown using computer
simulations and recordings from real neurons that this relationship
can be applied to understand how these biological elements implement
computational processes. �Such work is essential if we are to relate
our increasing knowledge of molecular aspects of the brain's workings
to its functional properties.�

10-24-07

LG12004

Pulsed Laser Deposition" proved to be the best deposition technique for the preparation of ultra-thin film-based devices

Forthcoming generations of data storage devices based on magnetic tunneling junctions MTJs (e.g., hard-disk read heads) designed to operate in the not-so-distant Tbits/inch^2 technology (current density limit ~100 Gbits/inch^2) demand tunneling junctions optimized in regard to: (i) sensitivity to the magnetic field as high as TMR=200% (described as the relative change of the tunneling resistance with the magnetic field) and (ii) low resistance-area product for the magnetic state of highest conductance (<1 Ohm-micron^2). These requirements are only available using tunneling junctions based on single-crystal barrier layers with homogeneous thicknesses where the coherent tunneling becomes feasible. Recent advances in the fabrication by sputtering of CoFeB/single-crystal MgO/ CoFeB MTJs exhibiting large-area TMRs so high as 400% have been achieved. Despite these progresses, current thin-film technologies based on physical vapor-phase deposition (PVD) techniques present severe limitations for the preparation of layers with homogeneous thicknesses of only a few atomic monolayers. At this thickness scale (<2 nm), the layer roughness, which is mostly determined by the growth kinetics of the PVD technique used, is of the same order of magnitude as the layer thickness. This produces a large dispersion of behaviors for the thickness-dependent properties (as large as 1000% for the tunneling current through a 2 nm-thick MgO layer with a standard roughness of 0.3 nm) that compromises the tunneling device reliability. In this work, the growth kinetics of the mostly used PVD techniques -molecular beam epitaxy, sputtering, flash evaporation and pulsed laser deposition- is theoretically investigated with the aim of testing the suitability of such techniques for preparing ultra-flat ultra-thin layers required for reliable tunneling devices. We demonstrate that Pulsed Laser Deposition is the best technique for preparing the flattest layers due to a combination of two unique key features [use of (i) a supersaturated pulsed flux of (ii) hyperthermal species] that promote the coarsening of the surface species by a distinctive mechanism, baptized as kinetically limited Ostwald ripening.

***

LD11076

We provide important insight on a very well-known problem, namely the relation between gravitational dynamics and thermodynamics of horizons. The thermodynamics of black holes suggests a deep connection between gravitation and thermodynamics. Then the connection between Einstein gravity and thermodynamics of horizons was proved in 1995. We showed that the cosmological equation in other theories of gravity can be thought as a thermodynamic equation at the apparent horizon. Our work has important consequences in cosmology because gravitational dynamics and thermodynamics are totally different problems in physics. The connection between the two sheds new light on gravity.


***

LE11491
Electrons suffer from the heat in an ion track

A team of physicists from Germany and Brazil found that
the absorption of fast electrons escaping from hot regions
inside solids is significantly influenced by the electronic heat
- similar to long-distance runners who are slowing down or
may even stop as a result of excessive heat. In their
experiments, they irradiated aluminum and beryllium
samples with fast ions and observed the resulting angular
intensity-distribution of the ejected Auger electrons.
Such angular distributions are known to be largely
independent of the type of excitation, following roughly a
cosine law at low ejected-electron energies. A very specific
distribution of excitations, however, is produced by a highly
charged fast ion at around 10% the speed of light. Such an
ion is fast enough to favor high charge states inside solids
and slow enough to interact strongly with the electrons of
the penetrated material. Thus, it leads to extremely strong
inner-shell ionization and electronic heat of some 10000 K
inside a nanometer-sized cylinder surrounding each
individual ion path. The authors of this work have shown
that Auger intensity ratios for different degrees of inner-shell
ionization vs. angle are sensitive to the high energy-deposition
density, consistent with enhanced inelastic electron-energy
losses or electron absorption, respectively. This proves that
the transport of fast electrons is significantly influenced by
the spatial electronic excitation distribution.

***


LF11017

Imagine a device that is half submarine and half weather balloon, but shrunk down to the size of a marble. We report novel measurements of thermally driven fluid flows using just such a device: a miniature, submersible, mobile, wireless temperature sensor. This "smart particle" is carried along with the fluid, reporting temperature with a tiny radio transmitter as it goes. In this way, we obtain the first quantitative and direct measurements of how the flow transports heat from point A to point B, i.e. of the dynamics of "thermal plumes". Our results impact understanding of thermally driven fluid motions as found in the atmosphere, the oceans, the molten metal core of the Earth , or the interior of stars. We anticipate that similar devices, outfitted with capabilities for measuring other quantities (e.g. acceleration, pressure, concentration) will open new doors for investigating the physics of mixing and transport by fluid flows.



***

LK10890.
Foaming of magnetic alloy boosts shape-memory effect


Researchers at Boise State University and Northwestern University have
boosted the magnetic shape-memory strain of polycrystalline Ni-Mn-Ga
sixty-fold (from 0.002 to 0.12 %) by casting the material into a foam.
This strain is developed as a magnetic field moves crystal defects
(known as twin boundaries) in a fully reversible manner over millions of
magnetic cycles. This shape-memory strain in the foam is comparable, in
terms of magnitude and response time, to the elastic strain exhibited by
the best commercial magnetostrictive material, Terfenol D, when exposed
to a magnetic field. However, Ni-Mn-Ga foams have lower density and
contain less expensive metals than Terfenol D. Their open porosity may
lead, beyond light-weight actuators, to micro-pumps without moving parts
or magnetic refrigeration near room temperature.

The researchers believe that the strain improvement in the foams is due
to their struts which are spanned by entire grains, thus reducing the
internal constraints present between adjacent grains in non-foamed bulk
polycrystalline Ni-Mn-Ga. Thus, the foam struts are similar to single
crystals, but they are constrained at their nodes, so they exhibit lower
strains than single crystals with achieve up to 10%. Foamed
polycrystalline Ni-Mn-Ga can be easily cast by conventional methods,
unlike Ni-Mn-Ga single crystals.

***

LG12016

Liquid-liquid phase transition in supercooled silicon.

Despite the occurrence of advanced materials in recent years, silicon
still keeps the leadership in semiconductor technology. Most
technological applications begin with crystalline silicon wafers
elaborated from the melt, and amorphous silicon made from slightly
supercooled melts. The properties of normal and supercooled liquid
silicon are therefore of importance for the manufacturing process.
Unusual behavior of the density in the supercooled liquid states, as
deep as 200 K below the melting point, was observed leading researchers
to consider plausible the existence of a liquid-liquid transition at
even lower temperature, unfortunately out of reach to state-of-the-art
experimental facilities. Therefore, the absence of direct evidence from
experiments has prompted to look for it using numerical simulations. We
have proposed a new mixed approach by combining efficiently classical
and first-principles molecular dynamics, giving a strong support to the
existence of a transition between a high density liquid to a low density
liquid near 1050 K, regardless of a specific empirical interaction
model. We were able to clarify the nature of atomic and electronic
structures of both the high and low density phase, which were
incorrectly described from early classical simulations. Our finding
indicates the liquid-liquid transition is accompanied by an enhancement
of the local tetrahedral structure and is not characterized by a
semimetal to semiconductor transition.

***


LG12007

“Extinction of the Giants”

The birth secret of the carbon buckyball has now been caught on tape. The nanometer-sized soccer ball was discovered and christened Buckminsterfullerene C60 two decades ago, but the intimate details of its formation remained a mystery. One “hot giant” hypothesis was that the atoms assemble first into graphitic planes, crumpled into large distorted polyhedral boxes, but then shed the loosely bound threads and chains. Eventually only the spherical buckyballs survive, thanks to their high symmetry and stability. This hot evolution is so rapid that no one could either prove or disprove it by observation. Now it has been documented for the first time, captured on high-resolution TEM video, matched by the atomic simulations, in a new joint study by scientists at Sandia Laboratory and Rice University. Thanks to a controllable heat bath inside the 10-nm-wide nanotube, they see how initial angular “giant fullerenes”, comprised of thousands of atoms, gradually shrink and perfect their shape. Honed and polished by great heat, like a raw diamond in the hands of a jeweler, they turn into the gem-quality symmetric buckyballs. If heat is sustained, the fullerenes undergo a further shrinking, ingeniously conjectured by the C60 discoverers, and vanish entirely, giving in to the entropy paramount at these conditions. The researchers believe this reveals the fundamental stage in fullerene formation, and also suggests a way to possibly engineer the nanocages for a variety of applications.

10-15-07

LE11531
We demonstrate an on-chip, frequency-tunable single photon detector operating in
the microwave range. The device consists of a quantum point contact and a double
quantum dot, which can be considered as an artificial version of a diatomic
molecule such as oxygen or nitrogen. Just like a normal molecule it can absorb
light by an electronic transition. The main difference is that the artificial
molecule is engineered in a semiconductor material, and can easily be integrated
into a complex circuitry.

Here we combine the artificial molecule with an ultra-sensitive charge detector
based on a quantum point contact. The charge sensor will detect the
single-electron transition that follow the absorption of a photon. The electronic
properties of the artificial molecule are controlled with gate voltages, making it
possible to tune the frequency of the absorbed radiation. The device may thus be
viewed as a frequency-selective single-photon to single-electron converter
operating at microwave frequencies.

The sub-micrometer size of the detector allows it to be mounted on-chip next to
another device, which opens up the possibility to investigate radiation emitted
from other nanoscale structures. We use the technique to study photons created
when electrons scatter in a nearby conductor.

***

LG11369
Atomic Landau-Zener tunneling in Fourier-synthesized optical lattices

Interfering laser beams can hold atoms in a precise array, forming a
so-called optical lattice. The atoms here are like marbles trapped in
the potential dimples of an egg carton. Whether the atoms now can move
through such a periodic array, following the theory of quantum
mechanics, depends on the atom¿s band structure. So far, researchers
have investigated the movement of atoms only in sinusoidal shaped
lattices, as can be created by placing atoms in an optical standing
wave. On the other hand, nature provides us with quite different
potential forms for electrons in solid state crystals and it is well
known that the electron transport properties, as one observes by
measuring the electrical conductivity, here much depend on both the
spatial symmetry and the form of the crystal potential. We have
developed a technique for the Fourier-synthesis of optical potentials
for atoms, aiming at a rebuilding of the variety of potential forms
experienced by electrons in natural crystals. Variable egg cartons for
atoms are formed by combining lattice potentials of different spatial
periodicities, similarily as in the Fourier-synthesis of electrical
engineering waveforms. Our fundamental spatial frequency of half the
wavelength of the used laser light is created with a usual optical
standing wave lattice potential. The higher spatial harmonics are formed
by so-called multiphoton processes. Effectively, each atom absorbs
several photons as a whole. For the first higher harmonic, two photons
act together as a single effective ¿giant¿ photon of double spatial
frequency. The spatial periodicity of the formed egg-cartons have half
the size of a usual lattice, i.e. a quarter of the used laser
wavelength. This technique can be extended to even smaller egg-cartons
for the atoms when working with more photons in whole. By combining
lattice potentials of different periodicities, now e.g. asymmetric,
saw-tooth-like egg-cartons can be formed! When slightly shifting the
position of the different lattice harmonics against each other, also
lattices consisting of a periodic sequence of dimples, or an egg-carton
with a periodic sequence of hills can be formed. We have studied the way
atoms move through such egg-cartons of variable spatial symmetry and
shape. It turns out that the atoms move best in the
¿hill-configuration¿, while being most hindered in their motion in
dimple-type egg-cartons. The saw-tooth-like geometry results in an
intermediate behaviour. The results allow to determine the
quantum-mechanical band-structure of the atoms in the egg-cartons of
variable spatial symmetry, and are in accordance with theoretical
calculations. More in the future, tailoring of lattices should allow for
the development of novel forms of quantum matter with novel physical
properties.

***

LC11219
Light diffuses through Mie resonances with different speeds.

Propagation of light through complex media often becomes diffusive much
like the transport of heat or the process by which two gases mix. This
is a phenomenon that very often escapes our understanding but not our
perception: light scatters by fat in a glass of milk or sunlight is
dispersed through dust in air. The past decade has seen great strides
toward fabrication of artificial materials to control the propagation of
light, of which periodic iridescent structures (photonic crystals) are
the most beautiful example.

In this work we have achieved a strong control of light transport even
in presence of strong diffusion and we have observed oscillations of the
speed of the diffused light as the result of microscopic ¿Mie¿
resonances which trap and delay the light at each scattering event. At
variance with chalk our new material, we have dubbed /photonic glass,/
is composed of identical micrometric spheres all having the same
spectral features, therefore certain frequencies of the light are slowed
down while others are accelerated relative to each other. The result is
that white light diffusing through this photonic glass is transmitted in
a spread of colour.

The news is that the microscopic resonances of the single spheres
survive even if the billions of spheres are tightly packed in the
photonic glass. Disorder, which is usually regarded as an uncontrolled
weakening factor in photonic devices plays here a central role and
allows engineering of the flow of the diffused light. This experiment
can open the way to obtain novel devices based on /perfect/ /disorder/
rather than careful order.

***

LE10997
Title: Can Dark Matter be Decaying Now?

Dark matter, the so far unidentified matter that binds galaxies
together, has long been believed to be comprised of stable elementary
particles that interact very weakly with ordinary matter. However, we
have shown that the stability of dark matter is not a requirement, and
in fact cosmological observations may demand that it is not stable. In
this scenario, the dark matter in the Universe is composed of two
types of particles. The heavier of these particles is unstable and
decays into high energy radiation and a ``daughter'' dark matter
particle, which is absolutely stable. The average lifetime of the
``mother'' particle is about 10 trillion years, or about a thousand
times the present age of the Universe. Although this lifetime is long,
a small fraction of these particles will be decaying now, and the
radiation produced in the decays would leave an imprint in the diffuse
radiation background at high energies. The predicted energy of the
radiation turns out to exactly match the excess of radiation energy
seen in previous experiments. Future experiments are being developed
and will be able to definitively confirm or refute the hypothesis that
dark matter is decaying now.

***


LG11588
Biodiversity and Complex Spatio-temporal Patterns in Ecosystems

Identifying the mechanisms allowing to maintain the earth's biodiversity is
a major challenge in theoretical biology and ecology. Recent experiments on
microbial populations have shown that the existence of local and cyclic
interactions promotes the long-term coexistence of different species
through the formation of spatial patterns. Motivated by these observations,
we have recently demonstrated [Nature 448, 1046 (2007)] a crucial influence
of individuals' mobility: There exists a critical value of the mobility
below which all subpopulations coexist and arrange in evolving structures,
while for higher mobility there is loss of biodiversity. In this Letter, we
investigate the influence of mobility and noise on the spatio-temporal
patterns. We show that individuals' movement leads to a fascinating
self-organization of subpopulations in entangled, rotating, spiral waves
(see figure). The emergence of these kaleidoscopic structures stems from a
subtle interplay between the deterministic dynamics and noise. We
quantitatively characterize the spiral patterns by devising an analytical
description taking stochasticity into account. In particular, we obtain
expressions for the velocity and wavelength of the propagating spirals. Our
methods can be broadly applied, e.g. to chemical reactions, epidemic
outbreaks, or in behavioral sciences.

***

LF11705
Reversing time changes right into left.

Chirality—or “handedness’—is quite common in nature. It is a geometric property of many molecules, in particular of complex biomolecules. Chiral molecules exist in a right-handed and a left-handed form (enantiomers), which are related by reflection on a plane. For example, all terrestrial life uses only right-handed sugars and left-handed amino acids. More than 50 percent of the world's top 100 drugs are chiral, including familiar brand names such as Lipitor, Paxil, Zoloft and Nexium.

Particle physics is another domain of chirality, where it describes a kinematical feature of massless particles: the spin of a fermion can either be in the same or in the opposite direction to that of its momentum. Both in chemistry and particle physics space inversion changes left-handed into right-handed systems.

Nuclei have been considered as achiral, because their shapes are, generally, too simple. However, Frauendorf [1,2] pointed out recently that a triaxial nucleus becomes chiral if it rotates about an axis that lies outside the three planes spanned by the principal axes of its triaxial ellipsoidal shape. The short, intermediate and long axes form a screw with respect to the angular momentum vector. The left-handed configuration is converted into the right-handed one by the time reversal operation, which changes the sign of all linear and angular momenta. In contrast to molecules and massless particles, space inversion leaves the nuclear chirality unchanged. Since both chiral structures have the same energy, one expects to observe two identical rotational bands (sequences of quantum levels) of the same parity. A number of such pairs of bands have been identified which were suggested as candidates for chiral partners [3,4,5]. A small observed energy difference between the states of the same angular momentum I in the chiral partners indicates rapid conversion between the left- and right-handed configurations (chiral vibration). With decreasing energy split between the partner bands, the left-right mode changes from soft chiral vibration to tunneling between well-established chiral configurations (static chirality).

In a new experiment at Gammaphere, led by researchers at the University of Notre Dame, electromagnetic transition probabilities have been measured for the transitions in the “chiral” bands in the nucleus 135Nd. These measurements affirm the chiral character of these bands (there were some doubts expressed recently [6] about the chiral nature of the observed bands in the nucleus 134Pr because the electromagnetic transitions did not conform to expectations from chiral bands). Further, the authors report novel calculations, combining the tilted-axis cranking model (previously used to describe chiral behavior) with the well-established technique of random phase approximation (RPA), to establish that this nucleus exhibits a transition from chiral vibration to static chirality with increasing angular momentum.

***

LH10936
Quasi-bound States in Continuum in a Two Channel
Quantum Wire with an Adatom

We have discovered an unexpected behavior for electrons (which we
call the quasi-bound state in continuum (QBIC)) in the conduction
band of a nano-scale one-dimensional quantum wire constructed by
alternating layers of metal alloys attached with an impurity atom
(called an adatom). The one-dimensional nature of the device results
in an anomaly called the van Hove singularity at the edges of the
electron energy band. We have found that by attaching two quantum
wires together along their length in a ladder shape, there appears a
quasi-bound state (QBIC) with very large lifetime due to this
singularity, even though the electron is inside the conduction band.
In other words, an electron can be trapped by an impurity for a very
long time, although electrons with almost the same energy can move
freely. Moreover, the singularity allows the QBIC to exist over a
wide range of values for the impurity electron energy. This effect is
in contrast to another well-known effect called the bound state in
continuum (BIC) that has an infinitely long lifetime. Contrary to
the QBIC, the BIC exists only at special values of the electron
energy; this is because the mechanism of the BIC effect is due to the
geometry of the devise and requires the adatom to be placed at an
extremely precise location in the wire. Therefore, it may be much
easier to detect the QBIC effect in an experiment. The QBIC effect
may also be useful in the construction of a laser that operates at a
moderately large energy scale.

***

LD11015
Novel superconducting phase in a model for quasi one dimensional strongly correlated systems

The search for a mechanism of superconductivity in systems with only repulsive interactions is a central subject for the physics community since the discovery of cuprate superconductors in 1986. One dimensional interacting electron systems at low temperatures are ideal laboratories because very powerful techniques are available to deal with them. Unfortunately the simplest example, the repulsive Hubbard model at half filling, is an antiferromagnetic insulator and the superconductivity is then ruled out. In this paper we show that when the effect of charge density on the hopping of electrons along the chain (bond-charge interaction) is added to the latter model superconductivity becomes possible; interestingly, the period of the charge modulation is not a multiple of the lattice parameter. The model has a rich phase diagram which also includes two insulating phases, one with breakdown of the translational symmetry and the other with dominant antiferromagnetic correlations. The above results were obtained by means of a plethora of cutting edge analytical and numerical techniques.

***



LG11196
Ultracold Onion Rings and the Coldest Bull's-eye Patterns in the Universe
>
> We normally think of everything being absolutely frozen and
> rather uninteresting at temperatures as low as -273C. Yet,
> Bose-Einstein Condensates defy the conventional wisdom and
> produce beautiful waves and patterns at nano-Kelvin temperatures,
> usually referred to as the lowest temperatures in the Universe.
> In our work, we illustrate the robust formation of ring-like
> 2D bull's-eye patterns and 3D onion-ring like dynamics
> for a two-component system of dilute alkali gases at such
> temperatures.
>
> The principal reason for this remarkable pattern formation (see the
> attached JPEG figures and link to movies) is that the inter-particle
> interaction introduces an effective repulsive nonlinearity in the system.
> In our system of two gases (two spin states of Rb 87), these
> repulsive interactions are such that these two species "dislike
> each other more than they dislike themselves". As a result, they
> induce phase separation and form remarkably robust ultracold
> rings (in 2D projections) which mirror the 3D onion-ring structures
> of the two condensates. In our work, we present novel experiments
> in this system and develop a remarkably accurate theoretical
> model describing, in excellent agreement with the experiments,
> the dynamics and unraveling the fundamental physics of this system.
>
>
> Figures:
> --------
>
> The attached figure contains 3D renderings of the density distributions
> in a binary Bose-Einstein condensate of 350,000 atoms of Rb 87 computed
> from the model at 54, 72, 144 and 151 ms. Each component is depicted by
> a contour slice at about half of its corresponding maximal density. Red
> and green surfaces correspond to components |1> and |2> respectively.
> The bottom (side) projection corresponds to, as it is observed in the
> laboratory, the z- (x-) integrated density for the |1> component in our
> model.
>
> Movies:
> -------
>
> Movies of the interpenetrating dynamics for the binary condensate can be
> found at:
> http://www-rohan.sdsu.edu/~rcarrete/noticeboard/DH/movie_paper_iso_60_30_long.mpeg
> http://www-rohan.sdsu.edu/~rcarrete/noticeboard/DH/movie_paper_iso_50_55_long.mpeg
> The movies depict the same information as in the figures mentioned above.
>

***


LG11477

Digital photography reveals secrets of rare nuclear decays

Photographic techniques have played a key role in many
triumphs of early subatomic physics. The discovery of the
positron is among well known examples. With time, optical
methods were superseded by electronic recording and processing.
However, modern technology has enabled the marriage of electronic
and optical techniques. In this work we introduce digital photography
to nuclear physics and we study the most exotic nuclear decay mode
to date: the emission of two constituent protons from the
ground state of an atomic nucleus. Employing a new type of
gaseous detector (Optical Time Projection Chamber) we recorded
convincing and vivid images (see an attached figure) proving
that a very exotic, artificially synthesized isotope 45Fe
disintegrates by the two-proton radioactivity.
Moreover, the analysis of many such decay images revealed for
the first time the mechanism of this rare nuclear process and
shed light on the structure of the 45Fe nucleus.
This technique opens a new field in nuclear spectroscopy
but it may also find applications in other branches of science
and in education. A message conveyed by an image can be
often understood without any special technical knowledge or
scientific background.

Figure caption:
An example CCD image of a two-proton decay event of 45Fe.
A track of a 45Fe ion, entering the chamber from the left, is seen.
The two, bright, short tracks are protons which were emitted
0.6 ms after implantation of the ion.

***

LF11542
Dancing at the nano-scale

or

Teaching tiny dance steps to magnetic flux quanta


In the 1950's a Latin American dance, called the "Cha Cha Cha", became a
very popular new dance craze around the world. It originated from the
sensual and older "Mambo", which eventually gave rise to the faster "Salsa"
in the 1970's. The beat in a Cha Cha Cha song is hard to miss: You hear two
slow beats and three quick beats. These three quick beats gave the dance
its name Cha Cha Cha.

New experiments [1] have been able to get quanta of magnetic flux to follow
a variety of dance steps, including the "Cha cha cha".

In these experiments [1], an externally applied current pushes quanta of
magnetic flux (also known as vortices) inside high-temperature
superconductors. When the applied current oscillates symmetrically back and
forth (like an "ac" current, with one harmonic or one oscillating
frequency), the vortices obediently follow the imposed rhythm, and also
oscillate symmetrically back and forth. The applied current acts as the
leading dance partner, and the vortices follow the steps imposed by the
current.

Now let us consider more complicated rhythms. When the applied current (the
imposed musical rhythm) has two harmonics (i.e., two superimposed
sinusoidals), it can get vortices to produce far more interesting dance
steps [1]. Moving, on average, in any desired part of the sample, their
microscopic "dance floor".

For instance, in one case, vortices can follow these dance steps: a slow one
to the left and three fast steps to the right (i.e., acting like some sort
of diodes). See, for instance, the animation
http://dml.riken.go.jp/fluxtronics/NL-rectifier.

More interestingly, these experiments [1] have produced the first very
nonlinear and efficient rectifiers (producing a net motion in one direction
even though the force applied by the leading dancing partner is symmetric).
In one set of experiments [1], the quanta of magnetic flux follow the Cha
Cha Cha steps.

Animations illustrating these effects are available by clicking
http://dml.riken.go.jp/fluxtronics/NL-rectifier.
Many research groups are currently exploring novel ways to control the
motion of flux quanta in superconductors, because their uncontrolled motion
can create noise in superconducting devices. Their controlled motion can be
used to sculpt microscopic magnetic profiles in small devices, to manipulate
spins in nearby magnets, and to produce nano-scale step-motors. These new
methods are also applicable for controlling the motion of particles,
interacting with optical tweezers, in colloidal suspensions, ions in ion
traps, electrons in Wigner crystals, and for the separation of different
types of very tiny particles (e.g., electrophoresis) based on their response
to applied forces.

***

LG11532
Extraction of weak transition strengths via the
(3He,t) reaction at 420 MeV

By studying the (3He,t) nuclear charge-exchange reaction at 420 MeV on
target nuclei over a wide mass range, an international collaboration of
researchers from the U.S., Europe and Japan have found that very simple
relationships exist between the strength of Gamow-Teller and Fermi
excitations and the cross section data obtained via this reaction as a
function of mass number. The results fill a gap in our understanding of how
to use the (3He,t) reaction to extract Gamow-Teller strengths and put it on
equal footing as (p,n) reaction, with the additional benefit that much
better resolutions, and hence more detailed information, can be obtained
with the (3He,t) reaction.
Besides providing detailed information to understand the structure of
nuclei, the Gamow-Teller strength distributions are important in
astrophysical studies of stellar processes (in particular supernovae) and
neutrino nucleosynthesis and for fundamental neutrino studies, such as
neutrinoless double beta decay. In addition, the improved understanding of
the (3He,t) reaction are very beneficial for studies with the inverse
charge-exchange reaction (t,3He), which is used for similar purposes
The experiments were carried out at the Research Center for Nuclear Physics
in Osaka, Japan and were led by Dr. R. Zegers from NSCL and Michigan State
University.

***

LG11011
Excited ions in intense femtosecond laser pulses: Laser-induced
recombination

Electron-ion recombination in a femtosecond laser field, the final step
in the
process proposed by Corkum over a decade ago to explain the production
of
attosecond pulses, has been finally isolated and studied. In this
‘3-step’
model electrons liberated from atoms during an intense laser pulse (step
1) are
energetically driven back towards their parent atoms by the electric
field of
the laser (step 2) before recombining with the simultaneous emission of
a very
short duration X-ray pulse (step 3). The duration is so short (less
than one
million billionth of a second) that the pulses can be used to study the
motion
of electrons in an atom. This has led to a massive worldwide investment
in
attosecond pulse technology, with attosecond pulse experiments expected
to lead
to many exciting discoveries and a quantum leap forward in our
understanding of
atomic physics in the next decade. The isolation of step 3 in the
present
experiment, using an excited ion beam target and charged particle
detection
techniques, is not only an important step in this quest but also a
pioneering
development in the study of fundamental electron-ion collisions.



**


LF11310

Polymers in a Vacuum

A polymer chain such as a protein molecule in a vacuum might
seem like an unlikely idea. How could it get vaporized in the first
place without having its structure completely obliterated? But the 2002
Nobel prize in Chemistry was awarded, in part, for the discovery of how to
do this. This was a key element in doing mass spectrometry on biological
samples which has had an enormous impact on biology and medicine.

But getting the mass of the molecule may not be the only interesting thing
one can do with this technique. In this work, I found that there is a
lot of new phenomena related to the internal dynamics and structure of
polymers flying around in empty space. It turns out that they behave
very differently from polymers in solution. Instead of being heavily
damped, in many situations they oscillate as they damp out. The size of
them is substantially altered by the conservation laws that don't exist
in solution.

This work has the potential for profound impact. Probing protein's
internal dynamics, by radio waves, should give a lot more information than
just the mass, allowing for a better identification of the actual molecule.
Other potential applications include examining a DNA molecule suspended
over a lithographed trench, and polymers in interstellar space.

***

LG11442


Is the Bragg peak flat?

In cancer treatment, the clinical aim of maximizing damage in the
tumor sites while minimizing damages to healthy surrounding tissues
has lead to the use of protons and, more recently, carbon ions to
provide high spatial specific damage profiles. In particle therapy,
it is widely assumed that the radiation-driven production of highly
reactive OH radicals from water molecules is largely responsible for
the main cell damage. The planning of this type of therapy is based
on the premise that tumor death follows closely the energy loss
profiles of C-ions along their whole trajectories including the Bragg
peak, the region where the major damage is supposed to occur. The
results reported by "LG11442" challenge the assumption of placing
entire reliance on the sharpness of the Bragg peak. It is shown that
ion production from water is not as sharply localized in energy as
suggested by Bragg peak energy loss profiles. In fact, primary ion
production from water is far from being localized and is actually
relatively uniform at a region where the C-ions come to rest. This
finding could have important implications in treatment planning of
C-ion therapy, mainly when tumors are located close to critical
organs such as optical nerves.

10-03-07

LB11409
Segregation, or un-mixing, of particles has been a topic of intense
research and industrial frustration for many decades, causing dramatic
revenue loss and product failure in a variety of industries. In this
paper we outline an elegant and robust method for eliminating
segregation that is generic for a huge class of particle flows,
specifically free-surface flows (one of the most popularly studied in
the literature). This paper will not only impact industrial practice in
fields as varied as ceramics, pharmaceuticals, mining, and agriculture,
but also change the way that academics think of attacking segregation
problems.

The crux of the technique relies on our identification of two critical
features of segregation: 1) that it has a preferred direction and 2) it
takes a finite amount of time. In order to exploit these two
observations we perturb a flow faster than a (theoretically identified)
critical frequency, essentially making segregation act as if it were "in
a hamster wheel", accomplishing nothing. Interestingly, mixing -- being
primarily random -- is not affected by these perturbations.

The attached figure shows experiments and simulations of unbaffled
mixers and mixers with baffles that perturb the flow as suggest in our
paper. Both density and size segregation are eliminated.

***

LZ10210
Vortex core spontaneously deforms

Usually vortices in gases or fluids (e.g. air or water) are rotationally
symmetric. However, we observe a deformation of a density defect, which is a
signature of the vortex core, into a planer shape in a Bose-Einstein condensate
(BEC) of atomic gas. This deformation becomes possible because the original
vortex we create is "quadruply" charged. In other words, our vortex has four
quanta of rotational motion. The quadruply charged vortex splits into four
singly charged vortices, and they take linear alignment. As a result, the
original vortex seemingly deforms into a linear shape. The reason why they
choose the linear alignment is theoretically explained by excitation spectrum
analysis. Using the analysis, we predict that other kinds of vortex alignment
are also possible by controlling the number of atoms. The linear defect we
observe moves in a counter-intuitive manner. The motion is simply explained as a
combined motion of "rotation" and "precession" using the velocity field model.
This work shows both experimentally and theoretically that BEC of dilute atomic
gas is a highly suitable system to study the dynamics of multiply charged
vortices such as splitting and deformation.

***

LF11419

Half-quantum vortices: Can the dream come true?

We have analyzed the possibility of finding half-quantum magnetic
vortices in Strontium Ruthenate (Sr2Ru04), an exotic
superconductor. Our theoretical work
can be considered as a first step towards an experimental observation
of half-quantum vortices.
In Strontium Ruthenate, a ceramic whose superconducting phase shares
many properties with
superfluidity in 3He, half-quantum vortices are expected to contain so
called Majorana-Fermion core states whose non-Abelian statistics would
protect them against environmental noise, but allow one to manipulate
them by moving one vortex around another. If half-quantum vortices
could be created and manipulated, they could potentially be used for
topologically protected quantum computation. Their properties would
mitigate the fast loss of information which so much troubles most
approaches to quantum information processing.
These unusual vortices, which carry only half as much
magnetic flux as ordinary vortices, are energetically forbidden to
appear in isolation in large samples. However, we have shown that
tightly
bound pairs of fractional vortices with a finite separation may be
stable or metastable. Furthermore, we find that it might be possible
to isolate them with present experimental techniques in
submicron-sized samples. Such an experiment which would be of great
fundamental and potential practical interest.

***

LA11605
Physics of eukaryotic chemotaxis

Multicellular organisms would not exist if eukaryotic cells could not
move in a coordinated way following chemical signals. But which physics
allows the cell to chose the right direction? A puzzling aspect here is
that cells must be highly sensitive to small gradients of soluble
chemical attractants, while being indifferent to high, uniform levels of
the same substance. Physical modeling shows that a selforganized phase
ordering mechanism is at the heart of this biological function. Under
uniform stimulation, clusters of signaling molecules on the cell
membrane grow according to universal scaling laws. When the cell is
exposed to a slight stimulation gradient, an initial tuning regime
characterized by these same laws is followed by a faster growth
characterized by a higher scaling exponent. The crossover between the
two regimes takes place at a time $t_\epsilon$ which is inversely
proportional to the applied gradient. This simple physical picture
explains most of the observed phenomenology, in particular the
observation of a size-dependent threshold of detectable gradients. This
may provide the reason why spatial directional sensing is absent in
small bacteria but present in large eukaryotic cells: there was not
enough space for it.

***

LD11133

Shepherding drops on patterned surfaces

Studying and controlling wetting of surfaces is important in applications ranging from drug discovery to ink jet printing. These engineered surfaces are typically composed of patterns of two or more surfaces with different properties. On well-designed surfaces liquid drops march precisely along specific paths -- even uphill -- when subjected to random vibrations enabling controlled sequential chemical reactions required in many applications. In this paper we developed a model of wetting based on a theory of superconducting phase transitions which can help design such surfaces.

The theory of superconductors first proposed by Vitaly Ginzburg and Lev Landau in 1950 (leading to a Nobel Prize in Physics in 2003 for Ginzburg) is widely used to study transformations between solid phases. By viewing wetted and non-wetted areas of solid surfaces as distinct phases, we described the drop motion using the Ginzburg-Landau equation. This allows a comprehensive description of wetting and enables design of preferentially wettable surfaces without recourse to expensive experimental trials.

This model has been used to study the range of applicability of a simple theory due to A. B. D Cassie proposed in 1947. Cassie theory, currently being used by the scientific community to design such composite surfaces, suggests that the cosine of the contact angle of a drop on a composite surface is given by an area average of the cosine of the contact angles of the component surfaces. Phase field theory was used to study the contact angle on such composite surfaces and it was found that the drop gets pinned to a different contact angle than predicted by the Cassie theory. Wide deviations from Cassie theory were found when the difference in properties of the component surfaces became significant.

***

LE11395
Balanced protein attractions help keep the eye lens clear

We have discovered that a fine balance of attractions between eye lens
proteins is needed to prevent clouding that could contribute to
cataract, the leading cause of blindness. Just as clouds in the sky
reflect water molecule attractions, forces between proteins cloud the
eye lens in cataract. Compared to the sky, though, the eye lens has a
richer set of ways to tilt the balance towards cataract, ways that are
still being discovered. Lens gamma crystallin proteins can gather
into droplets that cloud the lens. Alpha crystallins usually repel
each other, but can also form aggregates in cataract. Now, by
combining experiment and simulation, we find that mixtures of alpha
and gamma at high, realistic concentrations need finely tuned
interactions to avoid opacity. With too little alpha-gamma attraction
each protein can segregate with its own kind into domains that scatter
light (left panel). Too much attraction and the gammas glue the
alphas into clumps that scatter light (right). With the right
balance, however (center), either extreme is avoided. This new
potential reason for cataract adds to the framework for discovery of
specific molecular properties of alpha and gamma crystallin that
affect cataract, and possible ways to prevent the disease.

***

LF11424

Heating up ice

Various materials have very different capabilities to store heat. In
fact, ordinary water is a very effective heat storage. This property
plays an important role in diverse natural phenomena even for global
warming but is also utilized for various everyday technical
applications. Recent novel experiments utilizing x rays have now
revealed how thermal energy is stored in solid water, i.e. in ice, on a
microscopic scale. The technique utilized is called Compton scattering,
in which very intensive x rays are shined on a small volume of the
sample and the backscattered radiation is measured. Although it has been
known for over 30 years that the technique in principle could provide
such information on the energy,
experimental limitations have so far hindered these studies. In the
present work it is shown that accurate experiments at synchrotron
radiation facilities, which can provide unforeseen intensive x-ray
beams, have now made these investigations feasible. The study has
revealed that in ice the bonds between the molecules change gradually to
accommodate part of the heat transferred to the sample, the rest of the
heat being stored in the vibrations of the molecules. This work opens up
new possibilities to study fundamental heat storage properties of
various materials.

9-27-07

LF11407
Novel Macroscopic Force Mediated by Unparticles

An object called `unparticle¡¯ could exert a macroscopic force between
ordinary matter. It differs sharply from the only known macroscopic forces
in Nature, electromagnetic and gravitational: It does not follow the Inverse
Square Law; it is even a non-integral power of distance! The known
macroscopic forces are mediated by certain force carrier, a particle of zero
mass, which results in the Inverse Square Law in our physical space. The
notion of unparticle, as suggested recently by the particle theorist
H. Georgi, is something different from a particle. Mass is no more a
property that characterizes it; instead, a number called scaling dimension
dictates how it behaves when time and space shrink or enlarge proportionally.
We show in our paper how this new property leads to the unusual force between
matter when unparticles interact with particles. Since such a force cannot
appear in a theory of particles, if discovered, it would be an importance
advance in physics and would modify our conceptual framework based on
particles. Conversely, null observation of it would restrict unparticle-
particle interactions, if existing at all, to be very weak.

***

lf11644

Quantum-Chromo-Dynamics is an accepted theory of strong interactions. The
elementary objects of the theory are almost massless U and D quarks and
massless gluons. Ultimately it must explain masses and other properties
of all strongly interacting particles such as proton, neutron and their
excitations. The theory has a symmetry called chiral symmetry. It is
firmly established that this symmetry is spontaneously broken in the
vacuum. It was believed that for the mass generation mechanism of hadrons
consisting of U and D quarks spontaneous breaking of chiral symmetry in
the vacuum is crucially important. In the present as well as in the
previous papers of the author it is suggested that generally it is not
the case: in the (highly) excited hadrons the mass generation mechanism is
not related with the spontaneous breaking of chiral symmetry in the vacuum
and the chiral symmetry can be approximately restored in these excited hadrons.
The chiral symmetry restoration in the given hadron requires this hadron to
decouple from the Goldstone bosons (particles which necessarily accompany
spontaneous breaking of the symmetry). It is shown in the present paper that
existing for many years experimental data on strong decays of excited protons and
neutrons do support chiral symmetry restoration in excited hadrons.

***

LF11481
Surprise for Bose-Einstein condensation in semiconductors: Excitons condense dark

Excitons are semiconductor excitations made of conduction electrons and valence holes
bound in hydrogenlike bosonic atoms. They should thus display Bose-Einstein condensation
(BEC). However, for tens of years, claims of observation have been followed by denials,
probably because of the condensate unexpected nature.

Excitons resulting from photon absorption are created « bright » (coupled to light)
However, as fermion exchanges scatter bright excitons with opposite spins into « dark
states, photoexcited semiconductors ultimately contain bright and dark excitons.
Since dark excitons have the lowest energy - for they do not have interband (repulsive)
Coulomb processes due to spin incompatibility - the exciton condensate, made of the
lowest energy state, has thus to be dark. Through the Shiva diagram representation of
the new composite boson many-body theory, it is then easy to show that the coupling
between dark and bright excitons forces the condensed state to be linearly polarized
- to minimize the energy.

This new light on exciton BEC came because of rejecting the well accepted idea that
excitons behave as elementary bosons interacting through effective scatterings, idea
the new theory shows incorrect for many-body effects - essentially driven by fermion
exchanges without fermion interaction.

***

LF11505
CONDUCTION ELECTRONS IN GRAPHITE FLOW ALMOST WITHOUT RESISTANCE


Graphite - the well-known stack of fairly uncoupled graphene planes - shows amazing transport properties, which are being revealed nowadays after decades of experimental and theoretical studies. Similar to light diffraction the bending of conduction electrons by scattering obstacles that define the electronic mean free path in a metal, is effective only if the obstacles size is comparable to the electron de Broglie wavelength. The conduction electrons in graphite have such a large wavelength that usual scattering centres are rather ineffective to affect their transport properties providing a mean free path of several micrometers, even at room temperature. A direct way to “see” this is presented in this letter where it shows that the ordinary change of resistance with magnetic field is affected by macroscopically large sample sizes. The presented evidence suggests that graphite may provide the unusual possibility to study electron optics in a solid but also that superconducting-like paths even at room temperature may exist. These are the good news. And the bad news? Well, it depends on the reader point of view. Solid state physicists may realize now that the semiclassical transport models used in graphite in the last 50 years and also recently in graphene are actually not applicable and a general revision of the experimental and theoretical work is required.

***

LF11659
Diffusion Towards a Fractal Absorber

We expect the result of a simple medical test after a day or so, and wait for a few more days if the tests are complicated. Future tests involving personalized medicine ( e.g., sequencing the genome of a person) might take considerably longer, making them expensive and -- if the delay is too long – the results are practically useless. To speed things up, one could put many sensors in parallel and/or make each sensor more responsive. Unfortunately, despite considerable effort, our understanding of the physical limits of response time of a bio sensor is still evolving.

In a soon-to-be-published article in PRL, we show that it is the shape of a sensor that dictates the "geometry of diffusion" of the target particles around it and in the process, self-establishes its minimum response time. Indeed, the shape of a sensor and its response time are related by a simple scaling relationship that holds true even for complex surfaces defined only by its fractal dimension and the corresponding 'dimensionally frustrated' diffusion (of target particles) characterized by periodic flipping of between 1D and 2D diffusion profiles.



The concept that 'form dictates function' is hardly new: In 1960s, Mark Kac asked "Can one hear the shape of a drum?" exploring relationship between shape an object and the acoustic wave created by it. The PRL article provides a new example of such relationship that relates the shape of an object to the diffusion field towards it – and in doing so broadens the range of problems accessible through the so-called "diffusion-limited aggression".

***LH10911
Measuring complex flows by Nuclear Magnetic Resonance

When blood is pumped through the brain, the tortuous capillary pathways result in a complex, fluctuating flow. A new Nuclear Magnetic Resonance (NMR) method has just been developed which may unravel some of that complexity. And given that NMR underpins the medical imaging technology, MRI, the potential for human application is significant.

Beyond blood perfusion in the brain, complex flow lies at the heart of other medical processes such as respiration or human cell division, industrial processes like oil recovery or the behavior of packed bed chemical reactors, and environmental processes such as ground water remediation and purification by filtration. Any tool which assists better understanding of such flow is therefore of interest.

At the heart of that understanding is being able to connect fluctuations in fluid velocities displaced in position and time. The new NMR method measures, for the first time, the fundamental quantity in the mathematical description, namely the “non-local dispersion tensor”. First proposed by Caltech scientists Donald Koch and John Brady in 1987, this tensor, which contains the vital information about fluid space-time correlations, has until now, been unmeasurable. This new breakthrough involving NMR not only gives a new boost to the mathematical theory of complex flow, but also holds promise for use as a contrast in medical imaging.


***

LG11648
Quantum Nonlocality of the Original EPR State via Spatial Parity
Entanglement

For the first time since it was proposed more than 70 years ago, the
nonlocal character of the original Einstein--Podolsky--Rosen (EPR) state
has been exposed via an experiment on two photons that are perfectly
entangled in their positions (implying that measuring the position of
one particle reveals the position of the other). The experiment reveals
a definitive violation of Bell⿿s inequality, the hallmark of
nonlocality. While entangled states have been used to show the violation
using other degrees of freedom, the spatial degree of freedom has
remained elusive even though it was the parameter that EPR used to cast
their thought experiment. Most prior work focused on measuring the
positions of each particle, but the key variable turns out to be their
spatial parities, a binary property that is mathematically analogous to
polarization. Spatial parity rotation, the operation analogous to
polarization rotation, is achieved by the introduction of a relative
phase between positive and negative positions. Einstein and his
colleagues might have been disappointed to learn that quantum mechanics
does indeed allow for nonlocality, which they called ⿿spooky action at a
distance,⿝ but they might also have taken pleasure in the discovery of
parity entanglement.


***

LE11549
Suppression of the Anderson localization by metamaterials A theory of diluted white paint?

Localization, one of most fundamental characteristics of disordered materials, refers to the extinguishment of propagation due to strong scattering that is induced by disorder. Indeed, this the mechanism that makes white paint actually “white”, as discussed in P. W. Anderson’s famous article "The question of classical localization A theory of white paint? in Philosophical Magazine B, 52, 505 (1985). In our Letter, we show that the introduction of metamaterials—novel, manufactured materials having the property of negative refraction—when mixed with normal materials, are able to substantially suppress the localization of photons. In particular, the localization length (the characteristic distance of the exponent decay of the field) exhibits a markedly different behavior in the long wavelength spectrum behaving as the sixth power of the wavelength, rather than the square of the wavelength for regular, right-handed materials. This demonstrates a substantial suppression of loc
alization, and hence the disordered material will appear less “white” and more transparent at these wavelengths. Furthermore, these mixed structures, which incorporate both normal and metamaterials show much weaker transmission resonances at long wavelengths, due to the inclusion of negative phase materials which weaken scattering. Such properties are quite surprising and deserve further investigation that can lead to new insights into localization, even for normal materials.

***

LE10899
Nonlocality of a single particle

This Letter presents an exciting new scheme that could resolve the long-running debate over the fundamental issue of whether a single particle can exhibit nonlocality. This is a very important issue in science since quantum field theory is the most fundamental description of nature and, in this theory, excitations rather than particles are the most fundamental entities. If nonlocality only existed when we had two or more particles, this would present a serious problem, since there would suddenly be something peculiar about two excitations of the field that would not exist when we had only one.

Single-photon nonlocality has been hotly-debated ever since it was first suggested in 1991. So far, the schemes proposed to test this idea have been criticised as not representing real experiments. This Letter resolves the issue by presenting a simple scheme that is achievable in the laboratory with current technology. This scheme is particularly exciting because it applies to atoms as well as photons and could overturn the widespread view that superselection rules prevent us from observing the nonlocality of a single massive particle.

***

LD11381

Photonic crystal fiber enhances the efficiency and directionality of random laser action in highly scattering medium



Directional and efficient random laser emission was obtained by placing a highly scattering gain medium in the hollow core of a photonic crystal fiber (PCF). In conventional lasers, photons (i.e. light particles) are initially emitted at random within an amplifying medium. Mirrors placed at the medium’s edges make a few of these bounce back, inducing the emission of new photons in an avalanche-like process. In random lasers, which have been investigated for over a decade, the amplifying medium is highly scattering and emitted photons randomly bounce several times at the scatterers before leaving the medium. This random movement also generates amplification and laser-like emission. Because all photons, and not just those collected by external mirrors, are amplified, the threshold behavior in random lasers is much smoother, a characteristic that leans toward the long sought ideal of a thresholdless laser. However, the emission shows no directionality, which hinders some applications of random lasers as practical sources. In this paper, by inserting the random laser medium in the PCF core, a reasonable fraction of emitted and randomly scattered photons experience total internal reflection on the core boundaries and become guided, leading to a directional laser-like source. In addition, these reflections make photons spend more time in the gain medium, resulting in a device efficiency that is at least 100 times higher than those observed in similar systems in bulk format.

***

LE11563
A relativistic thermometer for Einstein

The unification of thermodynamics and special relativity
poses a long-standing, fundamental problem which continues
to haunt the physics literature. In this paper, we present
novel fully relativistic molecular dynamics simulations that
shed light on this challenging topic. Our numerical results
illustrate that a statistical thermometer can be devised
which is able to measure the temperature of relativistic
many-particle systems in a Lorentz-invariant way.
This implies important practical consequences:
A moving observer, who passes by a resting gas container,
measures the same temperature as a resting observer.
Thus, in contrast to previous, repeated claims,
moving bodies appear neither hotter nor colder.

9-18-07

A "nu" angle on fractional statistics

The effective particles inhabiting the two-dimensional fractional quantum Hall
world are said to have "quantum statistics" fundamentally different from those
of the fermions and bosons which constitute our three-dimensional world. In
this paper, we pinpoint the signatures of these particles which have
'fractional statistics' and compare them to their bosonic and fermionic
counterparts. While fermions tend to have an extreme exclusion behavior, not
allowing two identical fermions to be at the same point in space and time,
and bosons tend to bunch, we show that these fractional particles exhibit an
intermediate exclusion that depends on the fraction "nu" characterizing the
quantum Hall state. Another instance where statistics comes into play is in
the angular dependence of scattered particles - it is known that two fermions
approaching one another have zero chance of scattering at ninety
degrees. We show that fractional particles in the quantum Hall system
also reveal dramatic angular dependent correlation effects due to their
statistics. The prospect of detecting these and related effects in
experiments promises to be truly exciting as it would establish the existence of
fundamentally different quantum behavior. LE11009

***

Chaos control in the heart lead to less shocking defibrillators.
Physics for EP room : cardiac chaos can be terminated with hundreds times less shock energy


During the past few years, the energy needed to terminate arrhythmias
with
an external defibrillator (4,000- 5,000 Volts, 15-20 A) was decreased
by defibrillators manufacturers by no more than 40%. We found that a
better understanding of
the physics of the heart could permit to decrease it several orders of
magnitude (hundreds and thousands times).

Earlier results in this direction showed how to terminate one
rotating wave
anchored to an anatomical obstacle, by a small amplitude electric shock
(PRL2004 and AFS focus "Physics for EP room") and Nature news
("Cardiac Defibrillators becoming less shocking") were devoted.
Experiments
on rabbit heart preparations confirmed that termination of a single
vortex
could be achieved with an energy 20 times smaller than defibrillation
shocks.

A far more difficult and important task is to suppress fully chaotic
regimes
by using reduced electric field. Defibrillators suppress all existing
waves
by exciting the tissue, using the heterogeneities of the cardiac
tissue as
'virtual electrodes'. Indeed, the heart is filled with many
heterogeneities,
spanning a wide range of spatial scales. With a given geometry,
diminishing the
amplitude of the electric field simply reduces the number of 'virtual
electrodes' that act as a wave source. The strategy proposed in the new
article (PRL 2007) consists in using fewer electrodes, but pacing the
tissue
periodically. The results of a numerical study, as well as of
experimental
study using cell cultures, suggest that very significant gains in
energy could
be achieved. Theoretical arguments suggest gains in energy of several
orders
of magnitude.

This study, based on simple physical principles, shoud be
complemented by
experiments with real hearts. It may open the way to a completely new
strategy
with heart defibrillators. LC11418


***

Physical modeling explains statistical fluctuations in genetically
homogeneous biological populations

We present and study a model describing the nonequilibrium statistical
mechanics of protein distributions in a proliferating cell population.
Our model describes how total protein variation in the population is
composed of a stochastic source internal to the cells and variation in
division and inheritance at the population level. It enables us to
assess the contribution and character of each of these components of
variation separately. We find that, even if production is deterministic,
cell division can generate a large variation in protein distribution. We
draw an analogy between the dynamics of protein distributions along cell
generations and that of stress in layers of granular material. In both
cases, a model with deterministic production and uniform division can be
solved exactly. However, in contrast with the model for stress
distribution in granular packings, where a universal tail had been
found, here we find sensitivity to the division function due to the
inheritance structure of the biological population. At the other limit
of extremely noisy protein production internal to the cells, the details
of division do not affect the tail of the distribution. LD11645

***

A 2-PHOTON IS NOT "2 PHOTONS + SPOOKY ACTION"

What is the essence of quantum physics? The best way of sorting the many candidate features (indeterminism, contextuality, non-locality...) consists in proposing and testing alternative models, which look "reasonable" but deviate from the quantum predictions. Local models were falsified by Aspect in 1982 and by many subsequent experiments. In 2002, some non-local models were tested in Geneva: a bound was put on the speed of quantum information in a "quantum ether", and temporal order in quantum correlations was falsified. In this work, we study another non-local model, proposed in 2003 by Nobel Laureate Anthony Leggett. We provide its conclusive experimental falsification, after deriving a new criterion, which (contrary to the one used by Zeilinger's group for an experiment earlier this year) can be tested without invoking additional assumptions. Leggett's model formalizes the idea that each photon of a pair is in a well-defined local quantum state, the non-locality being due to some additional "spooky action-at-a-distance". Quantum entanglement, on the contrary, postulates no such action but denies individual properties to each photon. The falsification of the Leggett model strongly vindicates the counter-intuitive quantum description. This is a step further in the still on-going quest for the essence of quantum physics. LH11270

***

May quantum mechanics drive black holes into naked singularities?

Summary: According to general relativity space and time loose their meaning
and the present laws of physics become useless at the singularities hidden
inside black holes. Despite it, they are harmless because the event
horizons of the holes, which "dress" them, keep the rest of the universe
protected. On the other hand, "undressed" (i.e., naked) singularities can
influence a whole region of the universe in an unpredictable way. This
"immoral behavior" led R. Penrose to conjectured the existence of
some "cosmic censorship" to preclude the formation of naked singularities.
Today we ignore whether (I) "physical initial conditions evolved through
Einstein equations could generate naked singularities". This led
S. Hawking, J. Preskill and K. Thorne to run a celebrated bet, where
Preskill and Thorne favor (I) in contrast to Hawking. In our paper, we
discuss the formation of naked singularities from a quantum rather than
classical perspective. We show that the quantum tunneling of a particle
with large angular momentum into a charged hole could overspin
it leading the event horizon to possibly disappear. A final veredictum
whether any stable naked singularity eventually appears may depend
on spacetime backreaction effects to be computed with some full quantum
gravity theory. LE11746

***

Universality and the critical Casimir effect

The critical Casimir force found experimentally in Helium 4 at the
superfluid transition [1] has been calculated theoretically for the
first time. The excellent agreement of theory and experiment are an
impressive example of the concept of universality at phase transitions,
because the considered systems are very different at the first sight:
While Helium 4 is liquid at low temperatures and becomes superfluid at
the transition temperature 2.1768 Kelvin, the theoretical calculations
were performed using computer simulations of a classical magnetic spin
system on a lattice. Nevertheless, the results coincide within the error
bars without any free parameters. These results may lead to a better
understanding of the Casimir effect, which presently is an active
research area from cosmology to nanotechnology. LF11623

***

Metal oxides exhibiting high dielectric constants recently attracted tremendous interest due to their application in microelectronics as novel materials for the gate dielectrics (the so-called high-k dielectrics) in the latest generation devices. One of the major obstacles for practical introduction of these materials is their ability to capture electrons and holes that may result in the instability of the device performance. It has long been believed that these charge trapping properties originate from the structural imperfections in the high-k dielectrics. Based on this understanding, significant efforts were devoted to improving material stoichiometry. However, as is theoretically demonstrated in this publication, both electrons and holes may experience self-trapping by forming polarons in the structurally perfect highly polarizable high-k dielectric, such as HfO2. In this case the interaction of an electron or hole with the perfect lattice creates a potential energy well that traps the electron (hole), as a deformation of a thin rubber film traps a billiard ball. The prediction that at low temperatures electrons and holes in these materials can move by hopping between trapping sites rather than propagate as a wave can have important practical implications for their electrical properties. This new understanding of the polaron-formation properties of the transition metal oxides opens interesting possibilities for suppressing undesirable material characteristics and will stimulate further research on the polaronic features in high-k dielectrics. The obtained results also suggest that materials where both hole and electron polarons co-exist may be more common than is currently assumed. For the first time, theoretical modelling provided a direct look inside polaron structure in a transition metal oxide material, indicating that electron and hole localization as polarons can be a defining characteristic of these materials. LG11473

***

Cardiac Defibrillators becoming less shocking - for saving lives.

Physics for EP room : cardiac chaos can be terminated with hundreds
times less shock energy


During last 5 years, a huge electric shock delivered by an automatic
cardiac defibrillator ( 4,000- 5,000 Volts, 15-20 A) was decreased only
by 40% of its energy. We found physics permitting to decrease it
several orders of magnitude (hundreds and thousands times) for
terminating cardiac chaos.
To our first result in this direction: physics of termination of only
one rotating vortex in the heart by an electric field shock (PRL 2004),
AFS focus ("Physics for EP room") and Nature news ("Cardiac Defibrillators
becoming less shocking" ) were devoted. Later, experiments on rabbit heart
preparations confirmed energy decrease 20 times for termination a single
vortex. Now, we created a method to terminate developed cardiac chaos
(consisting of a large number of rotating vortices). An important
difficulty to decrease the energy was: it is well established that an
electric field shock with an energy smaller than that used by
conventional defibrillation, not only remove vortices but also creates
new vortices and fibrillation. We found that this happens only if a shock
amplitude is above the Maxwell level (in conceptional physical models),
corresponding to the Lower Level of Vulnerability (LLV) in cardiac
experiments. We applied these concepts to create a method for cardiac
chaos termination that requires shocks energy below LLV thus does not
create new vortices and fibrillation. It can significantly advance
implanted and clinical devices used to terminate cardiac chaos (250,000
implanted cardiac defibrillators and many clinical devices used for
several million patients with atrial fibrillation and ventricular
disturbances). LC11418

***

How to follow a jumping surface atom

Pulsed synchrotron x-radiation can probe diffusion jumps of single atoms
directly in time domain in the range of nanoseconds and on the space
scale of Angstroms. Diffusion in and on surfaces is an important process
for producing nanostructures and has recently been followed
experimentally by scanning tunneling microscopy. The interaction of the
microscope tip with the surface atoms can be exploited for manipulating
the surface, but at the same time strongly influences the diffusion
process, and even with the fastest microscopes only the result of
diffusion events can be seen, the elementary diffusion jump happening on
a time scales a thousand times shorter. To probe this time scale becomes
possible with synchrotron radiation. We develop the theory for jump
diffusion in two-dimensional systems and refer to a first experiment
which has proven the feasibility of such investigations. This is to our
knowledge the first time that jumps of single atoms on and at surfaces
can be followed and opens the way for detailed studies of the atomistics
of surface diffusion. LE10964

***

Tunneling between a tremble and a swing

New research from Canada and Germany is challenging the notion that
quantum mechanics is the science of the small and the static. Research
published in the Physical Review Letters suggests that quantum
tunneling, one of several phenomena associated exclusively with the
quantum level, may also occur with larger and dynamic systems.

In quantum physics, quantum tunneling draws on micro and nanoscopic
phenomenon in order to allow a particle to pass through a barrier that
is too high to overcome by classical physical events. It has been widely
assumed that the larger a macroscopic system becomes, the less likely it
is for the quantum physics effects, such as tunneling, to occur.

New results from Ioana Serban, of the University of Munich, and Frank
Wilhelm, of the University of Waterloo, suggest that quantum tunneling
may be more common than expected and can occur in macroscopic quantum
mechanical systems. They suggest that tunneling can occur not only
between two places, but between two patterns of motion. In particular,
it may be possible for a nanomechanical clapper to generate both a
pendulum swing and a tiny tremor at the same time. The discovery will
advance the development of detectors to be used in quantum computing. LF11625


***


Quantum information can’t travel down a rocky road



Suppose a quantum particle like an electron is flying towards a material with a random distribution of matter, like a rocky road. Then, even though the particle is moving so fast that it should be able to zoom through the material, it bounces back. It’s like saying a bullet shot at a soft mattress will bounce back! This is the celebrated Anderson localisation phenomenon first explained in 1958, and is one of the many intriguing peculiarities of quantum mechanics. Another oddity of quantum mechanics, understood only recently, is that by exploiting the ability of quantum objects to be in two places at once, two parties should be able to communicate information very quickly via processes like “quantum teleportation”. Thus the question arises: could one take advantage of tricky quantum effects like teleportation to overcome the Anderson localisation phenomenon? In our paper we study this possibility and we show that this is impossible: Anderson localisation is such a powerful effect that not only single particles bounce back, but all possible quantum ways of secreting information amongst collections of such particles will bounce back as well. Anderson localisation trumps quantum communication: quantum information cannot travel down a rocky road. LE10961

***

Exotic nuclear excitation mechanism is most efficient in isomer triggering

Atomic nuclei can exist in so-called isomeric states -
long-lived excited states which are much more energetic
than the lowest ground state. We show that interactions
with electrons in a process called nuclear excitation by
electron capture can be a surprisingly efficient way of
releasing this energy on demand. The search for practical
methods to trigger isomeric states has been the subject
of a number of sometimes controversial investigation in the
last decades. Major motivations for this are fundamental such
as the challenge of understanding the formation and the role
of nuclear isomers in the creation of the elements in the
universe, but also a number of fascinating potential applications
have been suggested. These applications usually rely on the
controlled release of energy, e.g., in isomeric nuclear
batteries which operate without fusion or fission. An
experimental verification of our findings at the borderline
of atomic and nuclear physics may be provided by upcoming
ion storage ring facilities and ion beam traps which will
commence operation in the near future. LG11685

***


Atomic correlations of tellurium nanoparticles

This article presents nature of atomic correlations in nanoparticles of
tellurium (Te) which has a highly anisotropic structure. It is known that
trigonal Te has hierarchical structure, that is, primary structure is covalently
bound chains, and secondary structure is formed by binding together of the
chains. Our results reveal that in spite of microparticulation the primary
structure is preserved in contrast with reduction of the secondary structure.
The paper is a first step to demonstrate our model “Yarn model” (see attached
figure) that long chains compared to size of the nanoparticles are folded and
tangled like yarn. In other words, surface effects may be non-existent or
negligible in the Te nanoparticles. This kind of uniformity of the nanoparticles
can be expected for the exotic structure of tellurium but not isotropic elements. LE11298

***

Explaining vortex nucleation in a rotating Bose-Einstein condensate is
a recurrent and difficult problem. In this Letter, we propose a resonant
mechanism, based on a formal analogy with the Zeeman effect. This
mechanism, suggested by the Larmor theorem, yields the only existing
theory in the literature that explains all experimental results which
have been obtained by the three leading groups in the field, namely JILA
(Boulder, CO), MIT and ENS (Paris, France). A Bose-Einstein condensate
is the coldest, purely quantum-mechanical, state of the matter
(temperature about a few nanokelvins) and displays strong analogies with
superfluidity. Immediately following its experimental discovery in 1995
on atomic vapors, the creation (or "nucleation") of a few vortices
resembling those previously obtained in superfluids was achieved by
rotating (actually by laser-stirring) the condensate. But the
experimental value of the condensate rotation that yields the first
vortex has long remained a theoretical puzzle. LA11484

***


Hot heavy nuclei behave like drops of syrup

In heavy-ion induced fission an atomic nucleus is hit by an energetic projectile nucleus. The two nuclei fuse into one united nucleus, which after a short delay time splits into two fission fragments. We have determined the delay time for 32S, 48Ti, and 58Ni ions bombarding a tungsten crystal, using the spacing of atoms in the crystal as a yardstick to measure the small recoil of the united nucleus before fission. The measured delay times are of order 10-18 s, a billionth of a billionth of a second. Although this is a very short time it is very long on the nuclear time scale, and it is orders of magnitude longer than derived from previous, more indirect experiments. Our observations indicate a complete break-down of the standard model for nuclear fission, developed by Bohr and Wheeler shortly after the discovery of fission. The delay is here determined mainly by an energy barrier towards fission, and this barrier is very small for the united nuclei in our experiments. The fission delay may instead be due to very viscous flow of hot nuclear matter: the united nucleus behaves like a drop of syrup rather than like a drop of water. LC11842


***

Taming heat for information processing

Heat has always been considered as useless and harmful for information processing. However, the latest work by Wang and Li [1] from National University of Singapore, may overturn this long perception. According to their work, heat can be used to carry and process information.

Wang and Li present thermal logic gates, which are made of properly combined thermal transistors [2], can do all the logic operations: REPEATE, NOT, AND and OR. Therefore, the basic components of a computer -logic gates- are realized for phonons. The thermal (phonon) computer might not challenge the existing electronic computer, however provides a smart and efficient way to control and manipulate heat in molecular level, and to process thermal signals locally without convert them into electronic signals and processed by electronic devices, unless necessarily.



Given the fact that the solid state thermal rectifier has been realized experimentally in 2006 [3], only a few years after the theoretical models, the authors believe that the thermal logic gates can be also realized experimentally in a foreseeable future. LG11007

***

LIMIT OF THE LHC GLIMPSED AT RHIC

The Relativistic Heavy Ion Collider (RHIC) at
Brookhaven Laboratory, near New York, has provided a glimpse of a
long-anticipated physical process that may eventually limit the
performance of the Large Hadron Collider, soon to be turned on at CERN,
near Geneva. RHIC typically collides gold nuclei ("heavy ions") of
energy 19.7 TeV (a TeV is a trillion electron volts) and the LHC will
collide lead nuclei of energy 574 TeV (both machines also collide proton
beams).
Nuclei are collided at these energies primarily in
order to "melt" their constituent protons and neutrons into a plasma of
quarks and gluons interacting via the strong nuclear force, a state of
matter that existed in the first microseconds of cosmic history. But
heavy-ion collisions also provide access to electric forces of
phenomenal intensity. Relativistic length contraction dramatically
squashes the electric field lines emerging from each highly-charged
nucleus into a flat pancake. Sparks fly, so to speak, when these
pancakes interact and large numbers of electron-positron (anti-electron)
pairs are ripped out of the vacuum. In some cases, the electron of the
pair is attached to one or other nucleus, converting a small fraction of
the beam to one-electron ions. These soon stray from the path of the
main beam and are lost in a patch of the surface of the beam pipe.
In this paper, we successfully tested our ability to
predict the details of this phenomenon for the LHC by measuring it for
the first time with 6.3 TeV copper nuclei at RHIC. The heat generated by
the lost ion impacts inside the magnets of RHIC was quite harmless,
similar to the peak output of a firefly (the losses were only just
detectable with instruments outside the massive magnets). At the LHC the
heating will be comparable to that of a reading lamp, taking the
ultra-cold magnets close to the brink of "quenching" out of their
superconducting state. This experiment gives us some confidence that our
calculations are correct and will help avoid unscheduled and costly
halts of the huge machine. LF11298

***

Molecular unbinding in cytoskeletal networks

Cells make use of various proteins to interconnect cytoskeletal polymers into complex networks. In contrast to a covalent chemical bond these interconnections are transient. This allows for continuous remodelling of the cytoskeleton, a hallmark of adaptability in cells. In this paper we show that under mechanical load the bond between distinct actin bundles can be forced to unbind in vitro – even in a complex network of bundles: the higher is the bundle interconnectivity, the larger stresses the bundle network can endure before single bonds are forced to unbind. We observe a logarithmic dependence of this yield stress on the force loading rate as also reported for single molecular bonds.

We introduce a novel principle of ‘polymer interconnectivity/time superposition’ which combines these molecular unbinding events with the self-similar network structure. This principle of superposition can be employed to rationalize both the mechanical behavior of interconnected biopolymer networks under high forces and the shape of the frequency response. Our results have broad implications for the understanding of adaptable biomaterials. The observed forced unbinding may even turn out to be an important mechanism cells employ for mechanosensing tasks. LE11747



***

Fat freezes water on a molecular scale

Fat people are not very mobile. We now show that fat also has an
immobilizing effect on a molecular scale. Fats and oil are so-called
hydrophobic, i.e. water-fearing compounds. We have used ultrafast laser
techniques to study the motion of water molecules around hydrophobic
molecules. We have discovered that these molecules lead to a very strong
immobilization of their immediately surrounding water molecules. These
water molecules show a reorientation that is >4 times slower than that of
"normal" water molecules. With this finding we solved a long-lasting
controversy: thermodynamic studies indicated that water surrounding
hydrophobes behaves ice-like, but spectroscopic studies did not find any
evidence for the presence of ice-like structures. We now find that the
dynamics of water surrounding hydrophobes are ice-like, while the structure
remains similar to that of liquid water. The interaction between water and
hydrophobic solutes plays an important role in biological processes, such
as the self-assembly of cell membranes, the folding of proteins, and the
binding of drugs to proteins. Our results shed light on the role of water
in these processes and can aid in their understanding. LE10862

***

New optical gain mechanism in quantum wires discovered

Scientists from Japan (University of Tokyo) and from two US organizations (Arizona State University and Bell Laboratories) have discovered a new gain mechanism in a one dimensional semiconductor. This new mechanism involves splitting of a 4-particle complex (bi-exciton) into a two-particle complex (exciton) and a photon, instead of involving high-density plasma.

The scientists measured both light emission and absorption simultaneously over a wide range of electron density in super-clean T-shaped GaAs quantum wires formed by advanced molecular-beam epitaxy called cleaved-edge overgrowth. They observed that optical gain in this system appeared at a much lower electron density than people normally expect. Furthermore, the appearance of the gain does not require ionization of bound complexes (excitons, bi-excitons etc).

This discovery allows scientists to reduce the threshold (the minimum energy input required to make a laser) of a semiconductor lasers dramatically. It also sheds light on a long-standing controversial issue in condensed matter physics: the Mott transition, which was predicted more than five decades ago. This discovery shows that optical gain is not necessarily indicative of the Mott transition and that there is likely an intermediate stage where multi-particle complexes co-exist before excitons fully ionize into plasmas. LX10043


***

String theory describes the spontaneous collapse of spatial dimensions

In their recent Letter, ``Charting the landscape of supercritical
string theory,'' S. Hellerman and I. Swanson describe a new set
of solutions to string theory, in which spatial dimensions
can spontaneously vanish from the universe. This process
of ``dimension quenching'' connects theories dynamically
in any number of spacetime dimensions. The so-called second
string theory revolution was sparked in 1995 by the discovery
that five known versions of ten-dimensional superstring theory
actually emerge as different versions of the same underlying
theory (known as M theory). These five theories, however,
constitute just a small slice of a much larger space of
consistent string theories. This larger space includes, for
example, an infinite number of theories living in any number of
spacetime dimensions. The results described in this Letter
bring this infinite tower of string models into contact
with the so-called ``duality web'' of theories that launched
the second string revolution. Within the same class of solutions
the authors have also found a transition that connects the
superstring to an entirely distinct class of theory, known as
bosonic string theory (which is the first string theory ever
discovered). These transitions have radically extended the
science of quantum gravity. LE11624


***


The Core Matters

During writing a bit in a so-called vortex-memory cell a vortex/antivortex pair is created and again annihilated
in less than 50 ps (see Hertel PRL 98, 117201 (2007)), because particles and anti-particles cannot coexist.
Both mentioned quasiparticles can exist in two different states, which are characterized by the core polarization.
One fascinating fact of micromagnetism stems from the fact that vortices and antivortices can be found in a stable
configuration, so called cross-tie walls.
In this paper we have studied the dynamic vortex-antivortex interactions. We employed an advanced microscopy technique
for observing the vortex-antivortex dynamics involving a stroboscopic imaging allowing a very high time resolution.
Furthermore we performed accompanying simulations. Depending on the respective core polarizations the dynamic
interactions between the cores are completely different, what can lead to some unexpected effects like the “quenching”
of the antivortex movement. We demonstrate that the core polarization configuration can be determined from the dynamic
interactions even though the vortex core size is below the lateral resolution limit of our microscope. LC10878


Caption Fig. 1: Micromagnetic simulation showing the magnetization pattern of a single cross-tie in the ground
state (top) and after a field pulse excitation (bottom).

***

Physics of Laser Microsurgery in vivo

Sweeping away extraneous details is generally a strength of physicists;
however, those working at the interface with biology are often criticized
by our biological colleagues for broadly extrapolating from measurements
in systems that are just too simple. A perfect example is the physics of
pulsed laser microsurgery. This technique is widely used to probe
biological systems through the targeted disruption of cellular and
sub-cellular structures. The underlying physics ¿ including laser-induced
plasma formation and cavitation bubble dynamics ¿ has been previously
studied in distilled water; however, biological systems obviously differ
from water in important ways. In this paper, we describe the plasma and
cavitation dynamics observed during pulsed laser microsurgery in a model
biological system (fruit fly embryos). We find important differences in
the plasma and cavitation dynamics observed in water and those observed in
the fly embryos (i.e. in vivo). One of these - physical constraint of
cavitation bubbles by the biological matrix - was anticipated in the
earlier studies and mentioned as a potential complication in vivo. The
other - a reduction in the plasma threshold due to biological molecules
that could serve as sources of seed electrons at low intensities - was
not. The fully complex in vivo system has revealed new and relevant
physics ¿ an important lesson for any physicist who works across
disciplinary borders. LE11486


***

Distinguishing noise from chaos

Chaotic systems share with stochastic processes (noise) several properties that make them almost undistinguishable: a wide-band power spectrum, a delta-like autocorrelation function, an irregular behavior of the measured signals, etc. This similitude makes it possible to replace stochastic series by chaotic ones in many practical applications. In this paper a method is devised that easily identifies the underlying nature of the process and allows one to discriminate between them by recourse to an appropriate graphic representation, whose starring role is played by a statistical quantifier called "complexity measure". Several well known, model-generated time series (the Logistic Map, the Henon's Map, the Lorenz Map of Rossler's oscillator, Schuster Maps, Noises with f ¡ k power spectrum, fractional Brownian motion and fractional Gaussian noise), usually regarded as being of either stochastic or chaotic nature, are used as examples that illustrate on the approach. The main achievement is thus to open the possibility of clearly distinguishing between stochacity and chaoticity in our representation space, something that is rather difficult otherwise. LF11384

***

A Perfect Spin Sieve

We predict that an interface between graphite or graphene and a
suitably oriented ferromagnetic metal will filter electron spins
in the same way as a polaroid film polarizes light - except that
in our case the polarization is complete.

Electrons have an intrinsic spin which comes in two flavours, up and
down. Most materials, including graphite and graphene, contain equal
numbers of each spin flavour and are not magnetic. Ferromagnets are
magnetic because they have a partial spin imbalance. Our calculation
shows that a graphene or graphite layer on top of nickel or cobalt
acts as a perfect sieve because it allows only electrons with one
spin flavour to pass.

The origin of the effect lies in the very special electronic structure
of graphite and graphene: it only matches that of nickel or cobalt for
one spin direction. This can happen because there is a near perfect
matching of the lattice constant of graphite with those of nickel or
cobalt when these are oriented in the (111) direction.

By introducing a sandwich structure with two interfaces, it should be
possible to construct an improved magnetic field sensor of the type
used in hard disk drives and numerous automotive applications. LF11085