8-5-08

LQ11723
Moving Quarks Help Solve Proton Spin Puzzle

The famous proton spin problem, which began with data from the European
Muon Collaboration 20 years ago, keeps producing new surprises. It was
thought that the spin of the proton would come from its quarks, but the
EMC found that the quarks' combined spin didn't account for all of the
proton's spin. Researchers began investigating other spin sources, such as
the gluons that hold the quarks together and spin generated by the
movement of the quarks, called orbital angular momentum. This Letter
explains that within the successful model recently proposed by Myhrer and
Thomas, more than half of the spin of the proton is actually carried as
orbital angular momentum by its quarks. Even more fascinating, it is
shown, as a model independent consequence of QCD evolution, that whereas
the orbital angular momentum of up quarks is much larger than that of down
quarks at low resolution, this reverses at higher resolution. The
resulting high-resolution values are in surprisingly good agreement with
state-of-the-art lattice QCD calculations, as well as with recent studies
of Generalized Parton Distributions conducted at Hermes and JLab.

***

LQ11129B
Charge and spin pairing instabilities in nanoclusters

Scientists have been trying to explain the unique superconducting, ferromagnetic, ferroelectric properties in inhomogeneous cuprates, manganites and multiferroics for many decades. However, not much progress has been reached based on studies of long range electron correlations appropriate for large homogeneous systems. In contrast, we propose exact microscopic analyses of local correlations in nanoclusters (triangles, tetrahedrons, octahedrons) that give important clues for understanding the origin of pseudogaps and inhomogeneities in respective frustrated (triangular, pyrochlore, perovskite) bulk structures. This bottom-up approach allows to unravel the details which depend on the local cluster geometry, electron interaction and temperature e.g. to investigate many body physics and spatial inhomogeneities in high Tc cuprates, manganites and multiferroics. For the first time we show that electrons' charge and spin in nanoscale undergo spin-charge separation instabilities and various types of pairings. The exact phase diagrams that we obtain provide novel insight into electron condensation, magnetism, ferroelectricity and display a number of inhomogeneous, coherent and incoherent nanophases seen recently by scanning tunneling microscopy in numerous nanomaterials, assembled nanoclusters, ultra-cold fermionic atoms.



***

BP10522
Observation of the Josephson effect in thin films YBa2Cu3O7 samples


We have observed for the the first time the Josephson modulation of the
maximum superconducting current flowing in YBa2Cu3O7 films with
non-nanometric dimensions. Josephson effect is the transport of
superconducting electrons from a superconductor to a neighboring one
across a non-superconducting barrier. A well-known manifestation of this
effect is the modulation of the maximum current flowing across the barrier
by a magnetic field. Since the discovery of the high temperature
superconductor YBa2Cu3O7, many authors have pointed out that this oxide
includes boundary planes between crystalline domains that could behave as
Josephson barriers. However, the magnetic modulation of the barrier
current had never been observed, except in samples with nanometric
dimensions and in films including artificial grain boundaries. In this
paper we establish the experimental conditions required for the
observation of the effect in thin film YBa2Cu3O7 strips with
non-nanometric dimensions and we report the modulation of the maximum
supercurrent by a low magnetic field observed in two samples.


***


LP11484
Alpha Particle Condensation in Nuclei

An atomic nucleus can have a gas-like structure composed of alpha particles.
The alpha particle, helium nucleus, is a tightly bound quartet containing two protons and two neutrons. A clump of alpha particles interacting weakly with each other is bound as a gaseous nuclear state.
The most amazing is that they can be condensed into a single quantum orbit,
reflecting Bose statistics of them with spin zero, as might be called
``Bose-Einstein condensation'', which is analogous to the ultra-cold atomic
gas condensation. A characteristic aspect inherent to the nuclear system is
that the phenomenon can only be observed in excited states not in the ground
states, because in the ground states with higher density all of the alpha
particles dissolve into nucleons in pieces, i.e. into liquid of nucleons. Ever since the possibility of occurrence of the novel structure was suggested by some of the present authors, a lot of theoretical and experimental efforts have been devoted to
revealing presence of the structure. The Hoyle state in 12C, which plays a special role in carbon production in stars, now becomes convincing to be of the dilute three alpha particle condensate.

We investigate quite a wide energy
region in 16O theoretically for finding the quantum condensate with a
gas-like structure of four alpha particles (see figure), and give strong evidence of the existence. This exotic state is obtained together with the
other well-known quantum states, and its candidate for experimental
observation is also mentioned. The present discovery is of a great
significance in convincing the existence in heavier nuclei and in
establishing the new aspect in nuclear physics.

8-1-08

LP11283
TO THE COLD OR TO THE WARM? A THERMOELECTRIC EFFECT IN COLLOIDAL SUSPENSIONS

When applying a temperature gradient to an aqueous colloidal suspension,
one observes a flow of its components, or "thermophoresis". Recent
experiments on solutions of lysozyme protein, polystyrene beads, micelles,
DNA, and Ludox particles revealed surprising dependencies on solvent
temperature, acidity, and salinity. In all cases, the solute diffuses to
the warm at low T and to the cold at higher T; a change of sign occurs at
some intermediate temperature. A similar behavior as a function of salt
content was observed for a suspension of charged latex spheres at low
acidity; the particles migrate to the cold at low salinity and to the warm
upon adding NaCl.

In the present paper, we explain these observations in terms of the
thermoelectric effect of the electrolyte: the temperature gradient induces
an electric field of the order of 100 V/m which, in turn, drives the
charged colloidal particles to the cold or to the warm, depending on the
sign of their charge.


***

LB11269
HUNTING QUANTUM BUTTERFLIES

There is a paradox at the heart of quantum mechanics, stemming from the
classical phenomenon of chaos. Large everyday (macroscopic) systems,
which are described by classical mechanics and are generally nonlinear,
can manifest chaos, including sensitive dependence on initial conditions
(the "butterfly effect"). Microscopic systems, e.g. atomic and nuclear
systems, are described by quantum mechanics which is a linear theory,
where chaos is not possible. But classical mechanics is supposed to
emerge from quantum mechanics! Recently, an understanding has evolved
that the theory of open quantum systems, which accounts for local and
random interactions with other nearby systems, is generally non-linear,
and can manifest chaos. However, it has been generally assumed that as
you go to smaller scales, chaos is suppressed. Kapulkin and Pattanayak
present theoretical evidence to the contrary: it is possible to take a
macroscopic system with regular (non chaotic) dynamics, scale it down,
and obtain a mesoscopic system which manifests chaos due to quantum
effects. Scaling the system down further results in a microscopic system
which is strongly quantum mechanical and the chaos manifested in the
transition regime gets washed away by the quantum fluctuations. Thus the
quantum to classical transition is, in general, qualitatively
non-monotonic, and quantum effects can induce chaos in a regular
classical system, contrary to folk wisdom. This should be generic and
beyond fundamental questions of theoretical principle has implications
for quantum control, quantum computing, and nanotechnology. The
transition from quantum to classical behavior continues to yield much
counterintuitive and beautiful physics.

***

LS11482
Oscillating reaction in advanced materials synthesis

The unusual phenomenon of a chemical reaction that oscillates in time has
been observed during synthesis of indium nitride (InN), an advanced
semiconductor material for optoelectronics. Previously, oscillating
chemistry had been found only in reactions of certain molecules in solution
or on surfaces, and in complex living systems such as microbe colonies
and heart muscle. This new oscillatory system is the first one discovered
that involves reactions and transformations between bulk condensed materials
(solids and liquids). When a GaN surface is exposed to a steady vapor flow
of ammonia and an indium compound at high temperature, we observe that a
film of particles forms and then repeatedly transforms back and forth
between crystalline InN and liquid elemental indium, alternately absorbing
and releasing nitrogen to and from the vapor. The oscillatory behavior
indicates that the production of active nitrogen by catalytic decomposition
of ammonia at the surface is key to forming InN. This may help solve the
longstanding problem of synthesis of this important material.

***

LT11043
NEW STRUCTURE(s) OR EFFECTS ON HADRON PRODUCTION

psi(3770) is a bound state of the c and anti-c quarks. It can be
produced in e+e- annihilation. It is believed to be the only observed
structure in the energy range from 3.700 to 3.872 GeV, and almost entirely
to decay to D and anti-D meson pair. However, the BES Collaboration found
that (15+-5)% of psi(3770) does not decay to D and anti-D pair in
assumption of that there is only one psi(3770) in the energy range.
Recently, the BES Collaboration report an anomalous line-shape of cross
sections for e+e- --> hadrons in the energy range, indicating that either
there is likely a new structure in addition to psi(3770) around 3.773 GeV,
or there are some new physics effects reflecting the D and anti-D
production dynamics. This information is important in the understanding of
the QCD and the potential models based on the QCD theory, and even more
important in guiding experimental physicists to search for new kind of
particles such as glueball, hybrid, multiquarks and molecule states
predicted by the QCD theory.



***

ES10437
Hydrostatic and frictionless behavior in loaded granular materials

Granular materials have a reputation for weird phenomena, acting out
somewhere in between solid-like and liquid-like behaviors and sometimes
elsewhere. How loose granular material such as well sorted sands, behave
under loading and unloading cycles reveals unexpected brands of behavior
such as friction free points and stress anisotropies perpendicular to
the loading direction. Hysteresis, consisting of different physical
paths for a system when you load it versus when you unload it, is a well
known phenomenon in rock and soil mechanics. Recent research results, in
two and three dimensional granular systems, have demonstrated that a
peculiar force network supports an externally applied stress. Such a
network consists of a backbone of highly strained grains, that
percolates parallel to the direction of applied stress (positive
anisotropy) and a weak network strained in the perpendicular direction
(negative anisotropy). In this work we find that the unloading stage is
considerably more complex as the two previous subnetworks release the
stress. The most unusual feature of the unloading path is that one
reaches a point, before complete unloading, where the subnetwork stress
anisotropies switch sign and the system is both hydrostatic, like a
liquid, and macroscopically frictionless as seen by an external agent
applying stress, even though the grains themselves interact through
frictional forces. An interplay between microscopic friction and the
stress pattern within the granular medium is proposed as the mechanism
for the emergence of a macroscopic friction.

***


LT11307
Short Ranged Electric Fields Emanating from Molecules: A Novel Approach for Controlling Current Flow in Nanoscale Circuitry at Extreme Packing Densities

We have demonstrated experimentally and theoretically a novel principle for controlling electric currents flowing in the smallest nanoelectronic circuits that can potentially be realized. Our new approach takes advantage of the very short range of the electric fields emanating from molecules. These fields are obvious and forceful to atoms and electrons positioned nearby (within a nanometer), but are effectively cloaked or invisible beyond that. This is not down-sizing of standard methods: The field patterns generated by molecules cannot be emulated by conventional electrodes, not even by the smallest solid state transistor nano-gates currently realized under ideal laboratory conditions. Nanomolecular circuits exploiting this principle should operate without interfering with each other even at the extraordinarily high packing densities achieved by molecular self-assembly. The short range of these electric fields also implies greatly reduced energy dissipation associated with switching of circuits and therefore much less undesirable heating of electronic devices. The extremely small sizes of molecular devices also imply extremely fast communication between active entities. This work opens the way to begin imagining new information processing architectures not previously accessible.

7-31-08

LS11058
Optical micro-structures on quantum chaos

In the last decade optical micro-structures, especially photonic
crystals, have been widely used to suppress spontaneous emissions, to
enhance the efficiency of Light Emitting Diodes (LED), and to make
high reflecting omni-directional mirrors as well as low loss
waveguides. Now optical micro-structures enter the community of
quantum chaos, where "scarred" states are very important for
controlling the electronic structure of atoms and the transport of
electrons in resonant tunneling diodes. In this paper, with a
two-dimensional photonic crystal micro-structure fabricated on the
surface of a Vertical Cavity Surface Emission Laser (VCSEL), direct
observations of different lasing modes, including whispering-gallery
and chaotic modes, are demonstrated by collecting the near field
radiation patterns. The use of optical micro-structures to tailor the
modes and resulting emission from scar-based lasers will be of great
interests to researchers using ideas from quantum chaos to develop
efficient and highly-directional microlasers. The experimental
observations and the simulation results here also provide an
alternative but effective approach to access chaotic modes in VCSELs
at room temperature.


***

LN11200
The wave-particle duality of single H-atoms and a molecular double-slit

We have performed a real double-slit experiment in which one proton at the
time passes close by either nucleus of a hydrogen molecule. Here it can
capture an electron and continue as a neutral hydrogen atom to a
position-sensitive detector, where it is registered as a single particle.
The underlying wave character is revealed in the spatial distribution of
many such hydrogen atoms. The orientation of the hydrogen molecule (the two
slits) is controlled in the experiment, but no measurement reveals which
'slit' the proton passed. Young introduced the double-slit experiment in the
early 19'th century to demonstrate the wave character of light. Much later,
the wave-particle duality became a key phenomenon for understanding the
concepts of modern quantum theory, and a clear demonstration was provided by
the double-slit experiment with electrons. Recently D. Akoury et al,
referred to the hydrogen molecule as 'the World's simplest Double-Slit'
[Science, 318, 949 (2007)]. In the present experiment this double-slit is
used to demonstrate quantum interference for an object with extremely short
wavelength (only 1/4000 of the diameter of a hydrogen atom) making it by far
the shortest wavelength object for which quantum interference has been
demonstrated in any double-slit experiment.

***

BR10843
Thermal spike model best describes the origin of ion tracks in
semiconductors

The use of semiconductor devices under extreme conditions, as, for
example, in space or in the vicinity of particle accelerators or nuclear
reactors leads to the formation of disordered tracks whose origin is not
yet fully understood. However, our recent experimental and theoretical
results on ion track formation in various semiconductors (InP, InSb,
GaAs, GaP, Ge, and Si) give support for the extended thermal spike
model, and at the same time contradict three other competing mechanisms,
such as the Coulomb explosion, the shock waves, and the athermal
melting. The radiation hardness of the studied materials depends
primarily on the target material, ion species and energy, and
irradiation temperature, but also on the ion flux and target doping.
Generally, visible amorphous or heavily disordered ion tracks occur, if
the electronic energy deposition per ion and unit length exceeds the
threshold value for melting of the corresponding material. This was
found to be possible, e.g., for elemental ion irradiation of InP and
InSb, but not in Ge and Si, which is in agreement with conclusions made
by other groups.

***


LL11635
Domino Day on a Nano Level: Magnetic Moments Topple Over in Rows

Storing increasing amounts of data on increasingly small spaces is one of
the prerequisites for further progress in information technology. A magnetic
effect, which makes magnetic switches and logic elements conceivable on an
atomic scale, has been discovered by theoretical scientists from the
German Research Center "Forschungszentrum Jülich". The results of
their computer simulations mean that nanowires made of transition metals
could be suitable for transporting and storing magnetic information.

Lounis, Dederichs and Blugel discovered a kind of domino effect in rows of
individual manganese atoms on a nickel surface. They noticed that the
magnetic configuration of these nanowires differed depending on their
length. Astonishingly, only one atom more or one less makes a drastic
difference. When the number of atoms is odd, the orientation of the magnetic
moments is antiparallel, and when the number is even, they line up in a
toppled compromise position between parallel and antiparallel. Adding an
atom at the end of the nanowire or taking one away simultaneously changes
the magnetic configuration of the entire cluster. Just like a row of
dominoes, the magnetic moments topple over. However, this is where the
analogy ends because the effect can be completely reversed in contrast to
dominoes that have toppled over.

This new quantum mechanical effect, which holds for long stretches
of at least 100 atoms, has been discovered using a multiscale approach,
mapping density functional results obtained by a refined KKR-Green function
method to a Heisenberg-type model hamiltonian. The authors hope that it
will be proven experimentally in the near future.


Caption for the included Picture:
The magnetic moments of a row of manganese atoms (red) organise themselves in an antiparallel fashion on a nickel surface (blue) when their number N is odd, e.g. 3 or 9. When their number is even, say 2 or 10, the magnetic moments take up a compromise position.

***

BQ10959
Atomic mechanism of self-diffusion in silicon

In silicon, self-diffusion, i.e. the thermally-induced mobility of the host atoms, is determined by the migration of vacancies and self-interstitials. We performed comprehensive atomistic simulations in order to study the correlation between self-diffusion and the migration of both point defects. The comparison with the recent analysis of experiments on wafer processing and on simultaneous self- and dopant diffusion in isotope heterostructures revealed that the migration of the self-interstitials should be governed by a dumbbell mechanism. A dumbbell consists of two atoms which are situated at symmetric positions relative to a lattice site. During self-interstitial migration the dumbbell moves from one lattice site to the other by exchanging an atom. Our result is in contrast to the widely-used statistical diffusion theory developed by Compaan and Haven fifty years ago. Their work based on the assumption that self-interstitial migration is characterized by the exchange of an atom between a tetrahedral interstitial site and a lattice site and vice versa. On the other hand, we found that vacancy migration is characterized by the transformation of the tetrahedral vacancy to the split vacancy and vice versa which is also assumed in the statistical diffusion theory.

APS Physics Tip Sheet: New colossal carbon tubes created; and flipping spins at the speed limit

Colossal Carbon Tubes leave Kevlar and Nanotubes in the Dust
H. Peng, D. Chen, J.-Y. Huang, S. B. Chikkannanavar, J. Hänisch,
M. Jain, D. E. Peterson, S. K. Doorn, Y. Lu, Y. T. Zhu, and Q. X. Jia
Physical Review Letters (LS11423)

A collaboration of Chinese and American physicists has discovered a way to make a new carbon structure that could lead to fabrics 30 times stronger than Kevlar and 224 times stronger than cotton. The group dubbed the structures colossal carbon tubes because they're thousands of times larger than carbon nanotubes. At 40-100 millionths of a meter across and centimeters long, they’re comparable in size to typical cotton fibers.

The structures consist of nested inner and outer tubes separated by hollow channels, making the tubes both light and strong. While they are nowhere near as strong as carbon nanotubes, the colossal tubes are much more ductile than the nanoscopic variety, making them more suited for spinning into threads and weaving into fabrics. The colossal tubes conduct electricity and show some of the properties of semiconductors, which means that they could lead to novel microelectronic components as well as super strong cloth.

The details regarding how the intricate structures form is still hazy, but the researchers propose that colossal carbon tubes could go into to improved body armor, stronger carbon fiber composites (which are often shaped into parts for high-performance and lightweight vehicles), or components in microelectronics and tiny machines. - JR

----------------------------------

Spin Flips Hit the Speed Limit
S. Serrano-Guisan, K. Rott, G. Reiss, J. Langer, B. Ocker, and H.W. Schumacher
Physical Review Letters (LS11409)

A team of physicists at Physikalisch-Technische Bundesanstalt in Germany has managed to flip a nanoscopic magnet as fast as the fundamental speed limit allows. Their experiment consisted of two stacked layers of tiny magnets separated by a thin barrier to form what is called a magnetic tunnel junction. Such magnetic tunneling junctions are promising candidates for future magnetic memory chips.

The researchers allowed electrons aligned in a special way to flow between the layers, developing a spin torque, or twisting force that is transferred from one layer of nanomagnet onto the other. This torque pumps enough energy to the nanomagnet to make it move faster and faster until it changes direction. Several measurements showed that the researchers were able to switch the direction of magnetization as fast as physically possible.

Their spin torque record is important for the next generation of low current, ultra fast magnetic memory chips and sensors. This new generation of electronics encodes information in an electronic spin, rather than in an electronic charge. The spin torque switching effect is a powerful new approach to controlling electronic spins. - NR

----------------------------------

50 Years of PRL
Martin Blume

Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal’s birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of Marty’s Milestone PRL project, visit http://prl.aps.org/50years/milestones

This week, Marty is taking a look at a milestone paper from 1982 that led to the 1998 Nobel Prize in Physics.

Two-Dimensional Magnetotransport in the Extreme Quantum Limit
D. C. Tsui, H. L. Stormer, and A. C. Gossard
Phys. Rev. Lett. 48, 1559 (1982)

Following on the discovery of the integer Quantized Hall Effect by von Klitzing and coworkers, Tsui, Stormer, and Gossard undertook studies of a two-dimensional electron fluid at higher magnetic fields and lower temperatures than had previously been done. In this Letter they presented surprising results showing a plateau of the Hall effect at 1/3 the von Klitzing conductance value, (1/3)(e2/h). Several explanations for the results were discussed in the Letter, but the authors finally concluded “At the present there is no satisfactory explanation for all of our observations”. Many possible explanations were put forward by others in a flurry of papers, but the conclusive explanation was given by Laughlin in 1983 (selected as a Milestone for that year). He showed that a new state of matter with many-particle interactions accounted for the experimental results.

The 1998 Nobel Prize in Physics was awarded to R. B. Laughlin, Horst Stormer, and Daniel Tsui “for their discovery of a new form of quantum fluid with fractionally charged excitations”. See Physical Review Focus 2, story 18 (http://focus.aps.org/story/v2/st18) for a readable description of this work.
---------------------------------------------------------

Nadia Ramlagan and James Riordon contributed to this Tip Sheet

Journal articles and preprints are available to journalists on request.

Contact: James Riordon
American Physical Society
riordon@aps.org
301-209-3238

7-28-08

AN10175
MICROCAVITIES PROBE FUNDAMENTAL OPTICAL PROCESS IN NANOMETER SIZED
SILICON

Silicon, the flesh and blood of the microprocessor, suffers from a
major drawback when it comes to photonic functionality: it cannot emit
light. Efficient light emission from silicon will enable functionality
hitherto unobtainable in semiconductor chips and will forever change
the face of electronic industry as we know it. In 1990, it was
observed in several laboratories across the world, that silicon does
emit light when it is diced up into nanometer sized chunks - popularly
known as nanocrystals. In spite of these advances, the dream of a
silicon laser has remained elusive for nearly two decades. Is it even
possible in theory? This is the question we seek to answer in our paper.

In doing so, we have demonstrated the use of optical microcavities to
probe fundamental optical losses in silicon nanocrystal doped devices.
Microcavities are not only an essential component of any laser but can
also serve as excellent diagnostic tools for optical processes. This
analysis has enabled us to fabricate state-of-the-art resonators
having quality factors exceeding 1,400 -- nearly 4-5 times larger than
reported previously. More importantly, we have been able to estimate
the limiting performance silicon nanocrystal-doped cavities. Our study
indicates that, even at low temperatures, it will be extremely
difficult to overcome the optical losses in a continuous-wave
operation. Owing to the prevalence of fast optical loss processes, our
only hope may be to resort to pulsed operation.

Over the past few years, microcavities have been at the center of
attention in the photonics community. Our work has highlighted their
use as extremely sensitive diagnostic tools for exploring fundamental
optical processes at the materials and device levels.


***

LL11012ER
Hot Transformers: Chemical Oscillations Found in Collapsing Bubbles

The mysterious slow modulations of an air bubble in water
oscillating nonlinearly by high frequency ultrasound driving
are shown to have their reason in chemical oscillations.
Collapsing bubbles are known for a phenomenon called sonoluminescence:
A very short light flash is emitted at the time when its volume is
compressed a million times from its biggest size.
And this process was thought to be very stable. Until the team of
Thomas and Holt of Boston University measured
large periodical variations.
Nobody found a definite explanation by that time.
In this paper it has been shown for the first time
that a bubble may transform slowly
from a big and relatively cold nitrogen bubble
to a smaller but hotter argon bubble.
But this process may get unstable - the bubble gets too small
and colder and transforms back into a nitrogen containing sphere.
One single chemical oscillation usually takes 2 - 5 seconds to complete
until it repeats all over. And these oscillations are stable,
they only change when an external parameter is varied.
The chemistry happening during the brief moments of collapse
is normally of interest only for rocket scientists:
At 10,000 deg Kelvin under pressures
exceeding 10,000 bars the gases in the bubble are compressed nearly to
liquid density. The calculations in this paper help to explore
reactions in the same extreme environmental conditions,
only for a much smaller approx. 10 micrometer in diameter sized bubble.


***

ER10316
Hierarchical Structure in Branching Patterns

Branching objects appearing in nature have hierarchical structures.
Regarding a river network, for example, merging of two streams forms a broader stream and all streams finally flow into the main stream.
To measure the hierarchical structures quantitatively, a method of branch-numbering, named Horton-Strahler ordering, was introduced.
A stream originating from a source has order 1, joining of two streams of order 1 arises a stream of order 2, and so on.
And a simple scaling law (called Horton's law of stream numbers) has been confirmed empirically and analytically.
In this paper, we consider the simplest case in which every type of branching patterns is appeared randomly.
We introduce "hierarchical hanging method" and calculate the average of the number of streams of the same order.
This is expressed in the form of a recursive equation about the order of streams.
It is noted that for the streams of order 2, the exact solution is derived.
By using this recursive equation, Horton's law is proved numerically and analytically.
Furthermore, we also derive a relation for the k-th moment, which is regarded as the extended form of Horton's law.

***

BS10524

Exciton magnetic properties very strongly dependent on the kinetic energy

Excitons are quanta of electronic excitation traveling through condensed
matter and are central to a huge range of optical and energy transfer
phenomena in the physical and life sciences. In semiconductors, excitons
are of the Wannier type, in which an electron is bound to a hole by the
Coulomb interaction, the resulting energy states being analogous to those
of a hydrogen atom. Motion of excitons in bulk material can therefore be
thought of in terms of hydrogen-like particles or, alternatively, in terms
of waves. However, little has been known about how excitons behave when
they are actually moving, largely because there are very few experiments
which enable them to be ‘caught’ whilst under motion. In the present work,
we investigated how the magnetic properties of excitons depend on kinetic
energy in quantum wells which are very wide, but in which the exciton
energies (and hence those of the optical transitions) are nevertheless
quantized. By studying each transition individually, we find the magnetic
properties to be very strongly dependent on the kinetic energy. This
unusual behavior is ascribed to mixing between the 1S ground state and
higher-lying states. The mixing is induced by the exciton’s motion and
appears to be a universal feature for semiconductors.


***

AM10316
Interference of Subwavelength Light Beams Enhances the Light Energy

The wave nature of light causes the light waves passing through slits in
screen to interfere in the observation plane, creating an interference pattern
of bright and dark bands on the screen. Normally, the interference (addition)
of two or more waves causes enhancement or suppression of the light intensity,
but not the light energy. We show that the waves (beams) generated by
multiple, subwavelength-wide slits can have similar phases and can enhance the
light energy. If the spacing of the slits is smaller than the optical
wavelength, then the phases of the beams at the detector are nearly the same
and beams add coherently (the light energy increases as the number of
light-sources squared, regardless of periodicity). If the spacing is larger,
then the addition is not so efficient, but still leads to enhancements and
resonances versus wavelength in the total energy transmitted. The anomalies in
transmission spectra of gratings, a long standing problem in optics, follow
naturally from the interference properties of the model. The mechanism could
be interpreted as a non-quantum analog of the superradiance emission of a
subwavelength ensemble of atoms (the light energy scales as the number of
light-sources squared) predicted by the Dicke quantum model.

7-25-08

LQ11930
Cold atoms are lasing.

A new laser has been developed at the Institut Non
Linéaire de Nice (France). This laser uses a cloud of laser-cooled rubidium
atoms (~100 µK) as the gain medium, placed inside a cavity. As there is no
stationary population inversion in such a medium, gain is obtained with
trickier mechanisms, corresponding to multi-photons transitions. The team
led by Robin Kaiser has demonstrated lasing action with three different
mechanisms, namely Mollow gain, Raman gain, and four-wave-mixing. Up to 300
µW laser power have been produced. The gain mechanism that produces the
laser depends on the pumping parameters (detuning, intensity,
polarization…). The laser can thus be tuned continuously from one regime to
another. This laser could find applications in quantum optics or in the
study of laser dynamics, but the primary goal of the Nice team is to
combine one of these gain mechanisms with multiple scattering in the atom
cloud. This should lead to the first realization of a random laser (when
the feedback is due to multiple scattering in the gain medium itself) with
cold atoms. The present result, published in Phys. Rev. Lett. (W. Guerin,
F. Michaud and R. Kaiser, …) is the first step towards this goal. The next
step is thus to preserve the laser... without the cavity !


***

BSR1107

Spin-polarized current plays an important role in
nano spin-electronics because it can change the direction of
magnetizations or move magnetic walls in magnetic nano structures.
However, the ratio between the coefficients of adiabatic and
non-adiabatic components of the torque exerted by the spin-polarized
current on magnetizations has been a controversial issue for many years.
In this paper, the authors derived the equation for the magnetization
dynamics in the presence of a spin current based on the non-equilibrium
thermodynamics. They showed that the coefficient of non-adiabatic
torque is not equal to the damping coefficient called the Gilbert
damping constant, in general. The equality holds when the relaxation
time of the fluctuating magnetic field is very short compared to the
time scale of magnetization dynamics. They applied the theory to
current-induced magnetization reversal in magnetic multi-layers and
showed that the switching time is a decreasing function of the
relaxation time.



***

EQ10326
Tracing hidden signal inside plasma

First time, we have shown that the plasma medium can be useful to detect
untraceable subthreshold signal applied externally in the plasma medium,
by applying external noise, whereas, previously only autonomous dynamics
of the plasma was detectable using noise. Usually noises are disturbance
for the modern communication devices. In this experiment it is shown that
for suitable parametric regions and configurations of the system, untraceable
signal can be traced using noise and inherent nonlinearity of the plasma.
This technique may be used in plasma antenna to detect hidden signal.

We have also devised very simple statistical tool (AMD) to quantify stochastic
resonance that is free from computational difficulties of usual statistical
tools like Signal to noise ration (SNR) or cross-correlation techniques.

Vedio 1: Oscilloscopic view of stochastic resonance in plasma. Lower trace
is the applied subthreshold signal plus noise and upper trace is the output
signal. The video shows that initially for low amplitude of noise applied
signal was not traceable (noise has been increased continuously) and with
increase in the noise system detects the hidden signal. Maximum transmission
is at optimum noise level and for high level noise, transmission becomes noisy.

APS Physics Tip Sheet: Superfluid insight into rogue waves; and lumpy life

---------------------------

Origin of Monster Ocean Waves
A.N. Ganshin, V.B. Efimov, G.V. Kolmakov, L.P. Mezhov-Deglin, and P.V.E. McClintock
Physical Review Letters (LS11531)

Physicists at the University of Lancaster, UK and Institute of Solid State Physics, Russia have for the first time discovered that wave energy can rapidly concentrate to create monster waves under some conditions, in contrast to previous work suggesting that it requires hundreds of miles of open sea for the waves to appear and disappear.

By using superfluid helium to model complex wave processes, researchers observed that monster waves in the laboratory developed by what is called an inverse wave energy cascade, whereby energy is forced to flow in a way that drives the rapid formation of abnormally large waves. Their results confirm theoretical predictions by the Russian mathematician Vladimir Zakharov of the University of Arizona and Landau Institute Russia.

For centuries monster waves (also known as rogue waves) have been blamed for occasional mysterious disappearances of ships and sailors. These giant, violent waves are believed to reach 100 feet or more in height, and are often described as a towering "wall of water." What makes them so dangerous is their tendency to emerge unexpectedly from relatively tranquil seas. It is the unpredictable nature of rogue waves that has prompted many to investigate their origins.

There is much interest in the origin of rogue waves, as they have huge adverse affects on maritime commerce and industry. If this new understanding can be exploited to explain how rogue waves arise on the ocean, it may be possible to predict them. -NR

----------------------------------

Lumpy Life
B. Houchmandzadeh
Physical Review Letters (LQ11828)

Even in a perfect world life would be imperfectly distributed, according to a new experiment performed at the Centre National de la Recherche Scientifique (CNRS) and Grenoble University in France.

Plants and animals tend to live near their relatives, rather than spreading out uniformly across the planet. Some of the clustering is due to variations in the environment, because species are generally better adapted for certain types of conditions than for others. But physicist Bahram Houchmandzadeh discovered that even in a nearly perfect experimental environment, such as a petri dish with nutrients and living conditions that are uniform from place to place, amoeba clump together.

Houchmandzadeh attributes some of the clumping in his amoeba colony to the fact that newborn creatures start life close to their siblings. Although the microbes move about randomly, which smoothes out some of the lumpy distribution, it's not enough to erase the uneven distributions entirely. The experiment followed robust amoeba colonies as they grew, and did not address the effects of death or population fluctuations that occur in the real world. Houchmandzadeh, however, argues that such things would amplify the tendency of populations to cluster in groups.

The natural inclination toward clustering of plants and animals only partly explains population distributions in nature, but the new experiment shows that it is likely to be much more important than many researchers had previously suspected. -JR

----------------------------------

50 Years of PRL
Martin Blume

Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal's birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of Marty's Milestone PRL project, visit http://prl.aps.org/50years/milestones

This week, Marty is taking a look at milestone papers from 1981 and 1982 showing that no hidden variables can be responsible for weird quantum effects.

Experimental Tests of Realistic Local Theories via Bell's Theorem
Alain Aspect, Philippe Grangier, and Gérard Roger
Phys. Rev. Lett. 47, 460 (1981)

Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell's Inequalities
Alain Aspect, Philippe Grangier, and Gérard Roger
Phys. Rev. Lett. 49, 91 (1982)

Experimental Test of Bell's Inequalities Using Time-Varying Analyzers
Alain Aspect, Jean Dalibard, and Gérard Roger
Phys. Rev. Lett. 49, 1804 (1982)

In these Letters, Aspect and collaborators experimentally tested the spacetime behavior of an entangled system. (While two of the Letters were published in 1982, they are included here as part of the 1981 Milestone selection.) According to quantum mechanics, strong correlations are to be expected between measurements performed on systems that have interacted, even though they are separated at the time of measurement. Consideration of these predictions had led A. Einstein, B. Podolsky, and N. Rosen [Phys. Rev. 47, 777 (1935); see also Physical Review Focus 16, story 10] to argue that quantum mechanics cannot be a complete description of reality. Nearly 30 years later, John S. Bell proved that certain inequalities must hold among polarization measurements performed on two separated particles which had previously interacted if, as Einstein and collaborators felt, quantum mechanics is underlain by local "hidden variables"; these inequalities can be violated in a purely quantum-mechanical system.

Aspect and collaborators tested Bell's inequalities as generalized by J.F. Clauser, M.A. Horne, A.Shimony, and R.A. Holt [Phys. Rev. Lett. 23, 880 (1969)], in a series of experiments which approached ever closer to the ideal experiment first envisioned by David Bohm, based on the concerns of Einstein, Podolsky, and Rosen. These experiments used pairs of correlated photons produced by laser excitations of an atomic radiative cascade. In the first Letter, the use of single-channel analyzers did not allow the direct measurement of all polarization states; two-channel analyzers were used in the second Letter to overcome this limitation. In the third Letter, variable polarizers were used to eliminate the possibility that unknown interactions among the measuring instruments could evade the inequalities. In these Letters, and in subsequent work by a number of groups, the results showed violations of the generalized Bell inequalities, in accordance with quantum mechanics and in disagreement with local hidden-variable theories. Thus, more than 45 years after the paper of Einstein, Podolsky, and Rosen, the gedankenexperiment suggested by their work was performed, with results that would have disappointed them. Further research on entanglement has led to the new field of quantum information.
---------------------------------------------------------

Nadia Ramlagan and James Riordon contributed to this Tip Sheet

Journal articles and preprints are available to journalists on request.

Contact: James Riordon
American Physical Society
riordon@aps.org
301-209-3238

7-24-08

LH11797
Inside Aerogels in Nanoscale 3-D

Until now, no one has ever seen inside an aerogel to determine its
structure on the nanoscale. By performing high-resolution diffraction
imaging of an aerogel with laser-like soft xrays, a research team has
revealed its three-dimensional bulk lattice structure down to features
measured in mere billionths of a meter.

The x-ray beam passed through the aerogel sample and was diffracted onto
a CCD detector screen; some 150 diffraction patterns were stored as the
sample moved and rotated. Fast computers and improved algorithms allowed
millions of calculations to position each measured photon precisely in
three-dimensional space, a key advance that made the 3-D nanoscale
imaging possible.

¿Seeing inside bulk porous materials has never been done before at this
resolution,¿ says Stefano Marchesini, who led the research at beamline
9.0.1 of the Department of Energy¿s Advanced Light Source at Lawrence
Berkeley National Laboratory. ¿This one of the first applications of
x-ray diffractive microscopy to a real problem.¿

The strength of foam-like structures typically scales with their
density, but aerogels, less than two-percent dense, are orders of
magnitude weaker than expected. The structure revealed in the new 3-D
images reveals why, and suggests changes in aerogel preparation that
might improve their strength.


***

LU11168
The lowest energy state of the "bottomonium"


Collaborators working on the BaBar experiment at the Stanford Linear
Accelerator Center
have discovered the lowest energy state of the "bottomonium" family
whose members are
bound states of a bottom quark with an anti-bottom quark held
together in different angular
momentum configurations. This spin-0 particle, named eta_b, was
observed for the first time
some thirty years after the discovery of the spin-1 tower of states
of the bottomonium spectrum
called Upsilon(nS). The precise determination of the hyperfine mass
splitting between the eta_b
and the Upsilon(1S) plays a key role in understanding the effect of
spin-spin interactions in these
systems as well as fundamental properties of the strong force. In the
BaBar detector, the
highly-energetic collision of an electron and a positron can result
in the production of an
Upsilon (3S) particle, which in turn can decay to an eta_b by
emitting a gamma ray. This eta_b
production mechanism occurs just once for every two to three thousand
Upsilon(3S) decays.
To identify a significant sample of eta_b's in the presence of
background noise it was necessary
to record more than 100 million of such Upsilon(3S) events. This
achievement was made possible
by the excellent luminosity of the PEP-II accelerator and the
advanced technology of the BaBar detector.

***

BPJ1079BR
Size-specific cracking shakes out at the nanoscale

As the size of a structure is reduced towards the nanoscale, atomic
vibrations (phonons) begin to feel its size and shape, an effect
called phonon confinement. While these size effects on phonons are
known to play an important role in thermal transport, electronic
processes, and thermodynamic stability, little is known about their
role in fracture. In this article, we report that for plates of
cerium hydride there is a characteristic thickness at which the
entropy of these confined vibrations leads to a minimum in the
fracture energy, resulting size specific fracture. While this idea
was developed to explain the behavior of cerium hydride, it is much
more general and has important ramifications for the design of
nanostructures. For example, it implies that certain sizes may be
more susceptible to failure by fracture than others. It may also
prove useful in the deliberate creation of large quantities of stable
nanostructures by fracture.


***

LN11150
Tuning in on ultracold molecules

Ultra-cold molecular gases (at a temperature typically below one
micro-Kelvin) are the coolest molecular system in the universe.
Nowadays, the most successful route to form them is to associate
previously cooled atoms by using either laser or magnetic
fields. Unfortunately, only highly excited molecules can be
efficiently populated leading to unstable and short-lived samples.
In our article, we proposed a novel method to efficiently produce
ultra-cold stable molecules in their lowest vibrational levels.
This work solves many of the challenges to produce such molecules by
using giant formation rates obtained with a tunable magnetic field
and a laser field. Consequently, it opens the door to a broad variety of
applications ranging from quantum computing to high
resolution spectroscopy at a frontier between physics and chemistry.


***

LN11381BR
Oscillatory Hall effect in high-mobility two dimensional electron
gases

Summary: The formation of magnetically ordered states in initially
non-magnetic materials makes research on spontaneous spin polarization
exciting. An excellent tool to investigate such spin phenomena is the
measurement of the zero-field Hall coefficient: In the presence of
localized spins, electron scattering is spin-selective and leads to an
anomalous contribution to the Hall voltage. This manuscript reports an
unexpected anomaly in the zero-field Hall coefficient of two dimensional
electron systems (2DES). At very low temperature, both positive and
negative deviations from the non-interacting Hall coefficient
$\gamma_{\rm H}^0$ are observed, which can be twice as large as
$\gamma_{\rm H}^0$ itself. A distinct regularity in the deviations and
their temperature dependence are interpreted as the spontaneous
formation of localized spins and their mutual indirect interaction,
indicating predominantly anti-ferromagnetic spontaneous magnetic
interactions in 2DES at low temperature.



***

LN11510

Nanoearthquakes and MicroTornados Form and Wipe Out Colloidal Islands

Just like earthquakes potentially lead to the generation of tsunami waves, tiny vibrations on a plate on which a small water drop sits can create large waves at the drop surface---with peculiar consequences. If the drop were to contain nanoparticles, Australian scientists Drs Haiyan Li, James Friend and Leslie Yeo at Monash University's Micro/Nanophysics Research Laboratory have discovered a peculiar phenomenon in which these waves could result in the formation, evolution and destruction of island-like clusters of these particles on the water surface.

The tiny vibrations are surface acoustic waves or SAWs, which are megahertz frequency sound waves about ten nanometers in amplitude. A nanometer is one billionth of a meter---the width of a human hair is therefore about one hundred thousand nanometers. SAWs have, for decades, been employed in the telecommunications industry for signal processing. A typical mobile phone would therefore have several SAW devices.

Though they may be small, SAWs have large surface accelerations---typically, millions of g's. This is the reason why they can create large waves on the surface of a water drop measuring several millimeters in diameter sitting above them. These waves, in turn, bounce the nanoparticles around on the drop surface to form island clusters in a manner similar to the patterns formed when sand is sprinkled onto the surface of a vibrating metal plate.

As the islands form at nodal positions of the standing wave, i.e., points where the waves cancel each other due to destructive interference, which depend on the size of the drop, the number of islands decrease successively as the drop evaporates and hence reduces in size. In their work, the researchers show how the number of islands and their positions on the drop can be predicted.

Curiously though, the islands are wiped out intermittently when the SAWs trigger an instability which causes the liquid in the drop to recirculate rapidly like miniature tornados. This anomalous behavior, attributed to chaotic flow, only lasts a few seconds. When the recirculation stops, the islands reform and the cycle is repeated.

This research, which is to be published in Physical Review Letters, has important implications in microfluidics, which is the science of how fluids and particles can be manipulated at tiny scales. Already, the researchers are using SAWs to make tiny pumps, reactors, centrifuges and separators, as part of a larger effort to develop portable medical diagnostic kits and drug delivery systems.


***

LL11129
Subtleties in the Quantum Coupling of Light and Matter in a Semiconductor

Cavity Quantum Electrodynamics harnesses the quantum theory of
light an matter in the confine of a cavity. For decades, optical
macrocavities have been used to trap light, into which Rydberg atoms
were sent as probes of "the best of our theories". The quantum (or
"strong") coupling (SC) of a photon with an atom results in the notion
of light and matter to vanish away. New quantum states substitute
them, combining their properties. With atoms, the signature of SC is
the observation of anticrossing of the photon and atom energies: when
they are brought together, the emergence of new quantum objects give
rise to two new energies. This is seen as two peaks in, e.g., an
emission spectrum. This strong qualitative effect has been regarded as
the landmark of SC, but is not. We found that in a semiconductor, the
situation is more subtle: the physics remains the same, but can reveal
differently. Anticrossing can be overlooked, or spurious interferences
can mimic it. By considering self-consistently pumping and
decoherence, we obtained excellent agreement with the experimental
data, that so far were content to observe the qualitative effect of
anticrossing. We confirmed quantitatively the reports of SC in
semiconductors, but found that it manifests here with unexpected
subtleties.

7-22-08

LM11397
Why spark branches sometimes reconnect








When sparks and lightning strokes pave their way, they frequently form
characteristically branched trees, but sometimes branches also
reconnect. This was observed between tens of kilometers huge sprite
discharges in the thin air high above thunderclouds, and also in
much smaller discharges in normal room air in the lab. In our
Physical Review Letter, we explain how this can
happen. Generically, one expects and observes that the branches repel
each other. This is because the discharge channels penetrate into the
non-ionized air through a strong local enhancement of the electric
field at their heads which is created through local electric
charges. However, we show that this electrostatic repulsion can be
overcome by a nonlocal ionization reaction that lets the channel heads
merge; the same reaction is also responsible for the fact that
positive sparks in air can extend with approximately the electron
drift velocity (free electrons are the fastest particles in the
discharge), but against the direction of electron drift. Whether the
discharge channels attract or repel each other depends actually on the
gas density as well as on the oxygen-nitrogen ratio. Discharge merging
therefore gives us a new handle to investigate this little studied,
but important nonlocal interaction of discharges in our atmosphere.

Discharge branching and reconnection are intrinsically
three-dimensional phenomena while pictures show only two-dimensional
projections. We now can study the full three dimensional structure of
the discharge trees in experiments with stereo-photography, and in
simulations with the computational technique developed in.

***

LQ11425

A passive quantum error correction strategy is presented that can be used
to design intrinsically-robust Bell inequalities. Most quantum error
correction schemes use active quantum information processing to remove
deleterious errors due to the inevitable coupling to the environment. In
this paper we show that quantum error correction may be achieved in a
completely passive manner, without quantum processing or feed-forward, for
at least one application. Our passive quantum error correction strategy
is almost as efficient as the optimal combination of active quantum error
correction and a Bell inequality test.

***

EP10347

Manipulation of nano/micro magnetic/nonmagnetic inorganic/organic materials

An economical, efficient yet environmentally friendly manipulation method was developed utilising tumbling motions of particles. Nano/micro objects can be precisely positioned anywhere on a 3-D substrate. The manipulation of nano- and micro-scale objects is an important task across the board in science and technology, and various manipulation techniques have already been developed. The dynamic control and precise positioning of nanomaterials such as nanoparticles, nanowires, fullerenes and carbon nanotubes are key technologies in nanoelectronics, and the development of efficient methods of transporting and sorting biological cells is urgently required for the design of biological and biomedical devices such as micro-total analysis systems. In this paper, a new method of manipulating both magnetic and nonmagnetic particles is shown using a rotational magnetic field. The method proposed is simple, low-cost and kind to the environment. Indeed, no extreme conditions are required for the operation. The necessary intensity and frequency of external magnetic fields are relatively low, that is, ~ 10 mT (100 Gauss) and several Hz, respectively. Furthermore, there is no need for either nano/micro fabrications or any other complex devices. A variety of new manipulation methodologies of particles, fullerenes, nanotubes, biomolecules and cells can easily be developed utilising the present principle. These could be used instead of or in combination with the conventional methods.

***

LS11491

Liquid anti-diamond: onset of core chemistry in compressed Lithium

New computer simulations of liquid Lithium predict that at high
pressure and
temperature, the behavior of the lightest metal will be much closer to
that of
molten Carbon than an ordinary liquid metal. These new results
describe a
unique state of matter where an element's chemical and physical
properties are
determined by interactions between both core and valence electrons. At
ambient
pressure, only the outermost, valence electrons are responsible for
chemistry. It
is for this reason that the Periodic Table is made up of families of
atoms –
members have the same number of valence electrons. Under high pressure,
however, this paradigm breaks down, and core electrons begin to
overlap and
interact with one another. In molten Lithium, such interactions result
in the
formation of short-lived tetrahedral Li5 clusters. These clusters can
be found at
temperatures as high as 1000 K – a phenomenon that has been previously
observed only in strongly bonded materials such as molten diamond.
However,
unlike diamond where the valence electrons localize between adjacent
atoms to
form attractive bonds, in Lithium they occupy the voids of a
tetrahedral network
while the core electrons reside between neighboring atoms. The
resulting “anti-
diamond” structure is thus stabilized by predominantly repulsive
interactions.

7-21-08

ER10469

Complex Networks Made Simpler

A novel elegant way to construct connected graphs from a limited set
of types and a set of connection rules is offering new insights into
complex networks. Complex networks, such as the Internet and the WWW,
networks of flight connections, or networks of social contacts have
been the subject of much recent scrutiny, due to their ubiquity in
everyday life and in numerous science disciplines. However, as their
name implies, the structure and topology of complex nets is intricate
and difficult to analyze, hindering their understanding. In this
paper, "sequence" graphs, or networks, are constructed from ordered
sequences of just a few types of nodes and a simple set of connection
rules between the various types. The mechanistic rules allow for
rigorous analysis of sequence nets. Despite their ostensible
simplicity, sequence nets serve as suitable models of everyday life
complex nets, exhibiting many of their hallmark features .

***

LP11098ER

A set of equations which describes "liquid rope coiling" effect ---
When we pour honey over toast, we can see that the thread of honey
folds to make a coil.
This phenomenon is called "liquid rope coiling" and have been studied
several decades.
In this paper, we found a set of differential equations, which is
very simple and successfully
describe the coiling behavior. We theoretically obtained a state
diagram for coiling and uncoiling
for the first time, and predicted there may be a strong hysteresis
effect. Experiments have been
revealed three distinct scaling laws for the frequency of coiling.
Our mathematical model
reproduces all the scaling laws. Thus, we conclude the rich and
complex dynamics of
"liquid rope coiling" comes out only due to the compressional viscous
stress in the liquid thread.

Liquid rope coiling simulation movies

7-18-08

EN10253
How rigid are viruses?

Viruses have traditionally been considered dangerous entities capable of infecting and mutating plant and animal genetic codes. In recent years, however, viruses have been explored for applications ranging from drug delivery and gene therapy to nano-technology. Monodispersity, surface chemistry, temperature and pH stability make viruses versatile tools for photonic and electronic templates. Although structures of many viruses are known, their properties remain largely unexplored. In this paper we employ light scattering to analyze the mechanical rigidity and inter-virion coupling of Wiseana iridovirus (WIV). The measurements indicate unexpectedly high Young's modulus ~7 GPa, a surprising value for a biological objects that we traditionally consider in the class of 'Soft Materials'. This modulus is higher than the modulus of usual hard plastics. We show that the hard DNA core is the reason for so high WIV rigidity. The results also indicate a strong mechanical coupling between individual virion particles. The obtained results and the developed technique for analysis of virus rigidity might be very helpful for various bio- and nano- technologies and for studies of other nanoscale objects.

***

BR10615

Microstructure and oxygen concentration of YBCO

The microstructure and oxygen concentration profoundly affect the
electronic properties of the high-temperature superconductor
YBa2Cu3Odelta-7 (YBCO). Using quantitative high-resolution electron
microscopy, we measured the local atomic structure associated with
twinning in several YBCO samples. We observed inhomogeneities in the
lattice parameter ratios on the scale of the twin domains that are
ubiquitous in YBCO. This result was surprising because YBCO is
generally considered to have well defined lattice parameters. Our
measurements reveal that features commonly considered to be twin
boundaries often do not have a true twinning symmetry and are more
accurately described as phase boundaries. Furthermore, the
lattice-parameter ratios in a single sample tend to oscillate back and
forth between two different values from domain to domain, apparently
due to the local variation of oxygen concentration. As a result, we
expect these samples to have locally variable superconducting
properties. Furthermore, one can easily imagine a material with a
network of superconducting channels dispersed within an insulating
matrix. Such a network could have profound implications for theories of
flux pinning and superconductivity in YBCO.

***

LS11726

Fluctuations of a liquid surface: from the molecular scale to
the scale of capillary waves

The fluctuations in the local density of molecules in a liquid
depend sensitively on whether the density is measured close to the
liquid's surface. Far from the surface the density fluctuations are
mild and molecular in range but close to it they are dominated by
long-wavelength "capillary" waves along the surface, with the
interfacial tension and gravity acting as the dominant restoring
forces.

In this paper we show that to describe the full spectrum of
density fluctuations from the molecular scale up to the scale
of capillary waves, one must take account of the energy associated
with *bending* the surface as described by the interface's
bending rigidity. Two of the important results are (1) that the
bending rigidity for the interface between phase-separated
colloid-polymer mixtures is *negative*, with the result that
capillary waves are "more violent", less restricted by interfacial
tension, at smaller wavelengths, and (2) that on approach to the
critical point it vanishes proportionally to the interfacial tension
rather than, as had often been supposed, varying proportionally to
the product of the tension and the square of the correlation length,
thereby approaching a finite, non-zero limit. Both features are shown
to be in accord with what is found in computer simulations.


***

BM10751

Stone Wales defects are unstable in planar graphene

Defects in graphene are extremely important because they alter its
electronic and magnetic properties. We have predicted that Stone
Wales defects in planar graphene are unstable [1]. This is surprising
because the opposite was believed based on Molecular Dynamics
simulations and experiments in carbon nanotubes [2]. Despite this
previous belief, our prediction is natural. Stone Wales defects
comprise two pentagon-heptagon defects whose heptagons share one
side. Each pentagon-heptagon defect is the core of an edge
dislocation and the Stone Wales defect is a dipole formed by two edge
dislocations that have opposite Burgers vectors and share the same
gliding line. In an unstressed sample, planar linear elasticity
implies that the two dislocations glide towards each other and
annihilate. To show this, we have numerically solved the dynamics of
a novel periodized discrete elasticity model on the hexagonal lattice
and obtained the stable cores of different edge dislocations and
dipoles. Recent experimental results by Meyer et al at Lawrence
Berkeley National Laboratories confirm our prediction. They use a
novel imaging technique based on a TEM microscope to observe the
evolution of Stone Wales and other defects created by irradiation of
a graphene membrane [3]. These defects annihilate within a few
seconds thereby leaving a perfect lattice. These time scales are
several orders of magnitude longer than typical ones in Molecular
Dynamics simulations (pico to nanoseconds).


***

LQ11682A

Antimatter is seen to bounce from a matter wall

If you were to ask scientists what antimatter does
when it comes into contact with ordinary matter, all of them
will reply: "annihilation".
This is actually also the layman's general idea of the
matter-antimatter interaction: they are expected to cancel out
each other immediately in a burst of energy.
However, what we report in this paper is that this is only true
half the time. Our measurements show that, when a bunch of low energy
antiprotons hits an aluminum wall, around half of them are reflected and,
in our experimental conditions, can still travel tens of centimeters
from the reflecting wall.
What's the trick? Actually, none. What we observe is connected to
an effect known since the century-old Rutherford-Geiger-Marsden
experiments: when a charged particle approaches an atom,
and steals into its electron cloud, the Coulomb force exerted by
the nucleus is able to deflect its trajectory.
Through interactions with many atoms, the incident (anti)particle
can eventually be pushed backwards.
So why was this not seen before with antimatter? It's because most studies
were done at relatively high energies, where the annihilation probability
is much higher than the reflection probability.
But this is not true at very low energies, where reflection is
greatly enhanced.
The principal difficulty was to find out a set of data taken at very, very
low energy, in the heaps of data taken by the OBELIX experiment, around
a decade ago, at CERN.
While the antiprotons with "normal" energies annihilated soon after
impacting on the wall, the ones with 1000 times less energy showed
this unexpected behaviour.



***

LS11568

Does the expansion of spacetime do the same thing to physical
theories that scaling up lengths and times does in flat space?
If so, we could understand some issues in cosmology the same
way we do when the temperature and pressure of water is tuned
to eliminate the distinction between liquid and vapor. This
idea would effect such a vast simplification that it has found
many adherants. On the other hand, there has never been a clear
demonstration that the formalism --- known as the Renormaliztion
Group --- either should or should not apply to cosmology. In
this paper I analyze the model that is the paradigm for all
systems with a one dimensional order parameter and I show that
its cosmological evolution is not correctly described by the
Renormalization Group. My analysis makes use of a nonperturbative
resummation technique due to the Russian cosmologist Alexei
Starobinsky. Although the Renormalization Group does not describe
the effects of cosmological expansion, the good news is that
Starobinsky's technique does, and it is even simpler to use.

7-15-08

LQ11550
Zero Sum Game

Scattering of energetic electrons from neutrons reveals a series of
excited states of the neutron, providing clear evidence of the neutron's
quark-gluon substructure. Calculating the individual response of each
resonant state is presently beyond the reach of theoretical calculations
in all but the most simplified circumstances. However, when the spin of
the neutron is polarized perpendicular to the spin of the incident
electron, a relation known as the Burkhardt--Cottingham sum rule
predicts that regardless of the complexity of the individual responses,
the sum total of all possible states of the neutron vanishes. This
relation was strongly supported by experimental evidence presented in
2004 by the Jefferson Lab E94-010 collaboration.

The collaboration has now tested this same relation for the first time
with a nuclear target : He-3. Since this nucleus consists of two protons
and a neutron, its excitation response is more complicated and includes
several reaction states not present in the single neutron case. But even
when these additional states are included, the total sum still vanishes,
indicating an intriguing level of correlation among the separate states.
In this sense, the He-3 nucleus behaves very similarly to the neutron
located within it.



***

LQ11834
Brain Dynamics Motivate New Mathematics

To understand the brain and its functions, researchers often focus on
the individual activity of specific neurons. We have done so here and
discovered that models of the brain's electrical activity motivate a new
type of mathematics. Abrupt transitions (or bifurcations) typically
characterize two obviously different states (for example, the
bifurcation in behavior many of us experience each evening from "wake"
to "sleep"). In the particular neurons we studied (Purkinje cells ---
some of the largest neurons in the brain) we discovered a novel
bifurcation between two distinct types of behavior. In one type, the
neuron exhibited rapid, incessant activity. In the other, extended
quiescent intervals separated the active states. We found that, to
understand the transition between these behaviors required a new type of
mathematics that we labeled a ¿torus canard¿. A mathematical canard
exhibits quite surprising behavior. Like a tightrope walker, the
dynamics follow temporarily one narrow, unstable region of space before
falling to another, more stable region. For the Purkinje cell model,
this space is five-dimensional, and the tightrope walker navigates
complicated, donut-shaped surfaces that change under his or her feet.
Understanding this complex, torus canard, behavior --- perhaps
fundamental to Purkinje cell activity --- motivates new types of
mathematics.

7-14-08

LR11808
Connecting Qubits to Physics

Quantum Communication is the research area that exploits quantum mechanic to complete tasks not acessible in a standard classical communication world. The most prominent example is quantum cryptography. While the design of such quantum communication protocols is usually done efficiently in the abstract language of qubits, any physical realisation will resort to physical systems that are not directly qubits. For optical communication, light pulses have a much richer structure than qubits and also photo-detectors do not operate on a qubit level. We have been able to develop a powerful tool which helps us to connect the qubit language with the language of optical implementations. This way, one can design protocols in the qubit language and be assured that they fit the physical optical implementation.

***

LS11513
MANIPULATING SPINS WITH A LASER, IN SUPERFLUID HELIUM NANODROPLETS

In our research group we investigate, via their electron spin (in the
future nuclear spin too), atoms and molecules assembled on helium
droplets. Helium droplets consist of only a few thousand He atoms,
have a diameter of some ten nanometers, and a temperature of 0.4 Kelvin;
we dope them with exactly one rubidium atom each, which contributes
an electron spin of 1/2.

Electron spins are the source of magnetism in virtually every
material, thus their study has very important practical applications
(most notably magnetic storage). Electron and nuclear spins can also
be very sensitive to their environment. Nuclear spins are thus a
routine diagnostic tool in medicine (MRI); both nuclear and electron
spin are of common use in research laboratories to learn about the
structure of molecules and materials. Electron spins are also
candidate Qubits for quantum computers.

In all these applications, the ability to probe and control the spin
alignment is crucial; this is often accomplished via optical excitation.
Superfluid helium is a great environment: being cold, weakly interacting,
and nonmagnetic, it preserves the spin polarization of the dopant under
study for a long time. We excite rubidium atoms with a polarized laser in
a strong magnetic field, and are able to find a "sweet spot" where the the
strong laser does not destroy these fragile systems but simply flips one
electron spin. This is the first time that electron spin manipulation
in helium droplets is ever achieved; the way to spin resonance spectroscopy
(in progress in our group) is now open.


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BR10825

The structure of a large family of TS lattices is equivalent

In (J. Appl. Phys. 102, 093511 ), Torquato and Stillinger explain the most
dilute known way to pack spheres in a crystal lattice.
The result is a family of heretofore unknown lattices (which we call TS
lattices) whose physical properties present an interesting case-study.
Our work elucidates the magnetic properties of a large family of TS
lattices. These are geometrically frustrated: there is no way to
arrange microscopic magnetic moments (or spins) on all of the lattice
sites so that each is anti-parallel to all of its neighbors. In such
cases, a rich variety of phenomena can occur at very low temperatures.
We find two general possibilities, on the lattices we study: if each spin is free to
orient itself within a plane, there is a unique optimal arrangement, in
which all spins point along one of three directions. If the spin can
choose between only two directions, however, there are infinitely many
configurations which have the same energy. It is thus not obvious
which configuration(s) will 'win' at very low temperatures. We
explain, using a combination of mathematical and numerical arguments,
the expected behavior in this case. The significance of these results extends
beyond the expected material properties of the compounds in question: we show that,
at least from the point of view of simple magnetic models, the structure of a large family
of TS lattices is equivalent, and moreover very similar to a well-studied lattice structure.
This perspective greatly simplifies our analysis, and gives a powerful tool for understanding
and classifying the structures of the TS lattices.

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LF11807
Electrical control of nano-pendulum motions

The motion of a movable superconducting island coupled to
superconducting contacts, can be controlled by electric currents. By
application of specific voltages between the contacts, the island can
be forced to oscillate between the contact with predetermined
frequencies. Scientists in Los Alamos, NM, and Uppsala, Sweden,
provides a theoretical prediction of the influence from the electric
current on the motion of a movable superconducting island, when the
island is coupled to superconducting leads. In absence of the electric
current, the motion of the island is determined by its mass and spring
constant (Newtonian mechanics). Turning on the current, modifies the
island's motion, and this modification is controllable by adjusting
the size of the current. Signatures of the island motions are fed back
into the current, which would make it possible to read-out the
specific frequency of the island's oscillations.

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LP11401 (Embargoed until August 5, 2008)

Chaotic Dance of Nuclear Spins

An experimental study of atomic nuclei in a substance used widely for
medical imaging of human lungs has revealed a new fundamental property of
interacting nuclear spins in solids. Radically different signals measured
by nuclear magnetic resonance (NMR) exhibit identical long-time behavior.
It has been proposed that this universality is related to the chaotic
motion of the nuclear spins, which erases the memory of the initial spin
state. Such universal behavior is extremely challenging both to establish
experimentally and to understand theoretically and had remained
undiscovered in the 60 years since the advent of NMR. In the experiment,
nuclei of xenon were "hyperpolarized" with a laser in the gas phase,
liquefied and then solidified. The resulting enormous nuclear polarization
made it possible to track the spin signal with great sensitivity. The
experiment focuses attention on an unsolved 20th-Century problem--the role
and the implications of chaos in the behavior of large ensembles of
quantum particles. The observed universal behavior indicates that,
contrary to conventional wisdom, collective quantum dynamics exhibits
extreme randomness even when the individual behavior of quantum particles
is not yet randomized.


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LG11893 (Embargoed until August 6, 2008)

Shaping up graphite using light

Can we make diamond from graphite without resorting to extreme heat and
pressure as diamond was made in the Earth in geological time? The group
of scientist in Michigan State University raises such a possibility by
shining a short burst of intense femtosecond laser pulse on graphite and
watching using ultrafast diffraction the movements of some loosely
separated carbon atoms in graphene layers undergoing rehybridization,
forming more tightly bound diamond-like bonds. Whereas this intriguing
intermediate structure is short-lived, approximately for 30 picoseconds,
this experiment shows the possibility of a structural transformation
purely induced by light in this very versatile class of material.
Assisted by density functional theory calculation, the cause for such a
transformation is attributed to the electronic structure changes and the
Coulomb field buildup following the photoexcitation.




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LS11409
Spin torque breaks the speed limit
New spin torque speed record

A collaboration of researchers has realized spin torque switching of a
nanomagnet as fast as the fundamental speed limit allows [1]. These
findings are important for a new generation of ultra fast magnetic memory
chips.
Spin torque is an effect that allows programming of a magnetic memory cell
simply by application of a current pulse. It can be used for a novel high
density, non-volatile magnetic memory chip. Several major semiconductor
producers have demonstrated spin torque memory prototypes and market
introduction is expected, soon.
In a spin torque memory cell the current pulse excites a rotational motion
of the magnetization ? the so-called precession. Normally, the
magnetization has to undergo several precessional turns before
magnetization reversal takes place. Reliable programming of a memory cell
can therefore only be achieved by rather long current pulses of several
nanoseconds duration which limits the speed of the memory chip. In an
experiment carried out at PTB Braunschweig spin torque magnetization
reversal has now been realized by a single precessional turn, only. This
so called ?ballistic? spin torque magnetization reversal corresponds to
the ultra short physical limit of magnetization reversal time. It was
achieved by precise tailoring of the spin torque pulse parameters in
combination with a small magnetic field. Ballistic spin torque reversal
could allow future non-volatile magnetic memories operating with GHz clock
rates and thus faster than the fastest volatile computer memories
available.


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ZJ10029

Efficient acceleration of electrons by launching two weakly relativistic laser pulses, one behind the other, in plasma


If one asks a strong, well built individual to lift a heavy carton and then ask two moderately built persons to perform the same task, which of the two processes would be performed in a more stable and smooth manner? The answer is obvious - two men (though moderately built) working together would give a better performance. This is exactly the idea projected in the present paper, wherein a novel concept of accelerating fundamental particles, such as electrons, to high energies (multi MeV and) has been proposed, using two mildly intense laser beams propagating in plasma. Earlier studies and experiments have shown that an ultraintense laser beam propagating in plasma leads to the generation of large amplitude wakefields which are used to accelerate electrons to ultrahigh energies. However, in the process, many laser-plasma instabilities arise and lead to depletion in efficiency of the acceleration process. Plasma based laser wakefield accelerators (LWFA) are important because they can accelerate particles to energies that cannot be attained by conventional accelerators such as rf linacs which can produce electrons of only a few MeV. These accelerated electrons have important applications in chemical and biological spectroscopy, medical imaging and radiation therapy.

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BM10877

We study the phase diagram of ferri-feroelectric mixed crystals
by broadband dielectric spectroscopy. The phase diagram of investigated crystals
is strongly asymmetric - the decreasing of ferroelectric phase transition
temperatures by doping is much more flat that the corresponding decreasing of
ferrielectric phase transition temperatures. In the middle part of the phase diagram
the dipolar glass phase has been observed. In boundary region between ferroelectric
order and dipolar glass phases at low temperatures the nonergodic relaxor phase appears.

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Lq11479
Polariton Light-Matter Particles Moving in Step

It is shown that condensed light-matter particles in a solid-state resonator remain locked together over times much longer than their lifetime. In such a Bose-Einstein Condensate (BEC) a large number of particles accumulate in a single state. A fundamental property of these condensed particles is that they move in phase, forming a single coherent whole. In this work it is demonstrated that polariton condensates exhibit long phase memory times, two order of magnitude longer than the particle lifetime. Such long times permit the fundamental mechanisms determining the phase memory times to be revealed.
A key characteristic of the polariton particles, which arise in semiconductor solids due to coupling between electronic excitations and light, is that they can be manipulated by light beams on length scales of hundredths of millimetres and exhibit condensation at high temperatures (~20 K). In contrast to atomic condensates the polariton system is non-equilibrium: it loses particles as fast as they fall into the condensate. Nevertheless, we are able to show that polariton condensates exhibit properties expected for an equilibrium system, and like atomic condensates, have potential for use in quantum information processing experiments.