6-21-07

NEW CLUES ON THE TRACK OF LORENTZ VIOLATION

Violations of Lorentz invariance are allowed in many theories of
quantum gravity, but have never been detected. Now a collaboration of
researchers at Stanford University, University of Western Australia
(Perth), Observatoire de Paris, and Humboldt-University (Berlin,
Germany) has followed a new lead:

The team combined two Michelson-Morley (MM) experiments to
simultaneously observe the times-of-flight of light propagating back
and forth in dissimilar interferometers. Berlin used optical
Fabry-Perot cavities made of quartz, whereas Perth used sapphire
whispering-gallery resonators, which enclose microwaves into a
refractive medium.

The individual experiments are sensitive to Lorentz violation in the
physics of light (the photon sector) as well as the physics of the
material of the cavity walls, which acts through distorting the
cavities (the matter sector). Since the effects could cancel, this
leaves a loophole for Lorentz violation.

Holger Müller (holgerm@stanford.edu) of Stanford explored how these
influences depend on the geometry, material, and index of refraction
of the cavities. He calculated them in a Lorentz-violating extension
of the standard model and disentangled them in the data analysis. The
result: Einstein?s relativity triumphs, in spite of a new lead and the
very best sensitivity ever achieved in MM experiments, a part in 10^16.

***

On making viscous “veins” and “swirls” in microchannels

Microfluidic devices are used to create novel and well-controlled
environments to study the behavior of viscous stratifications. In these
systems, due to the importance of confinement, a viscous liquid swept
along in the flow of a less viscous liquid can form viscous “veins” or
threads. By taking advantage of the laminar flow conditions, the
position and the size of the threads can be readily manipulated. This
Letter presents an experimental study of two miscible viscous threads
that flow off-center in microchannels. In particular, the shear-induced
buckling of thin threads near the channel walls is investigated.
Subjected to this instability, the viscous columns develop into
swirling patterns that lead to their eventual rupture into array of
viscous swirls, the miscible counterpart of droplets. The swirling
instability provides a means for passively producing discrete diffusive
microstructures in a wide range of fluid environments.


***

Micro Swimmer with a ``Diffusive Jet Engine''

An artificial micro-swimmer that propels itself by asymmetric distribution of
reaction products around it is made and fully studied experimentally. The
observed properties of the swimmer are in perfect agreement with the
theoretical predictions [PRL, 94, 220801 (2005)].

The directed propulsion of micro- and nano- scale objects in water is
problematic
because of the combination of low Reynolds number and Brownian motion on
these length scales. In order to achieve an artificial micro- or
nano- scale swimmer
that is able to propel itself in a purposeful way, one needs both a
swimming strategy
that works in the environment of low Reynolds number, and a strategy
for steering
and directing the motion that can overcome the ubiquity of Brownian motion.
Common bacteria, such as E. Coli, achieve propulsion by non-time-reversible
motion of long flagella, and employ a "run and tumble" strategy to be able to
swim towards or away from environmental stimuli. Here we fully characterize
the motion of an artificial micro-scale swimmer, which uses diffusio-phoretic
effects for autonomous propulsion. We show that at short times, the motion
has a substantial component of directed motion, with a velocity that depends
on the concentration of fuel molecules. At longer times, the motion reverts
to a random walk, in which runs of directed motion are interrupted by random
changes of direction. Our results suggest strategies for designing artificial
chemotactic systems.


***

Zigzag coherent structures produce rogue waves.

A specific, genuinely three-dimensional mechanism of rogue wave
formation has been recognized through numerical experiments.
As the present simulations show, freak waves, one of the most
striking phenomenon of the ocean, can appear through interaction of
coherent structures, consisting of obliquely oriented stripes
with increased wave amplitude. Spontaneous formation of zigzag
patterns takes place in a nonlinear stage of the so called
Benjamin-Feir (or modulational) instability from a planar wave.
If initial wave steepness is sufficiently high, these coherent
structures produce giant waves. The most tall waves appear in
``turns'' of the zigzags. A simple explanation for this mechanism
has been suggested. This work is a first-time systematic study of a
late stage of the Benjamin-Feir instability, based on fully nonlinear
explicit equations of motion for long-crested water waves.
Unlike previously used weakly nonlinear models, the present model
is sufficiently accurate to simulate very steep waves.


***


Ultra-high energy density capacitors

Capacitors in current use have low energy storage capacity as the
polarization saturates at low electric field. This limits the
application of such materials as energy storage device. We uncovered
a new mechanism for storing unprecedented amount of energy in
capacitors, i.e. through reversible transformation between non-polar
and polar phases in nanoscale materials.

Like batteries, capacitors are energy storage devices. Capacitors
provide quick bursts of energy, necessary for power conditioning,
high-density electronics, or quick acceleration. Unlike batteries, no
chemical reactions are involved in their energy storage mechanism,
rather polarization, the electric field equivalent of "magnetization",
is involved. Our supercomputer simulations explain how certain polymer
mixtures store energy at very high densities and tailor it to specific
applications. These mechanisms are universal and can be exploited in
other systems through, for example, controlled deposition of
nanodomains that undergo a non-polar to polar transition at different
values of the electric field. In general, our work provides a
systematic route, not limited to polymers, for obtaining
nanostructured materials with very high energy density. Polymers are
also nontoxic and environment friendly. In contrast to batteries,
capacitors can be charged quickly, charged many more times, and energy
can be extracted much faster.


Figure Caption (figure attached with the mail):
The left side of the diagram shows the nonpolar alpha phase and the
polar beta phase of PVDF. In the nonpolar phase the polarization on
the alternate polymer chains within the unit cell cancel the total
polarization, whereas in the polar phase there is a non zero
polarization within the unit cell. In the right hand side, the red
shaded area is the energy density for a single phase material (here
for a polar beta phase). The blue shaded area is the energy density
when a non polar alpha phase transforms to polar beta phase under the
application of electric field. A single phase material has a given
polarization at zero electric field and the polarization saturates
(Dsat) at very low electric field. In our calculations we show that it
is possible to convert an impurity modified polymer from non polar
alpha phase to polar beta phase under electric field. Polarization
saturates at large electric field and leads to higher energy storage
in the mechanism proposed by us.


***

The glass transition in tight situations

Are glasses merely extremely slow liquids? Liquids take on the
shape of their container. In this paper, we show that unlike
liquids, glasses aren't comfortable in confined spaces.
We study pastes composed of tiny plastic particles in a
liquid, a model system which acts like a glass when the
particle concentration is increased. Using an optical
confocal microscope, we directly view particle motions in
three dimensions. When the sample is confined between
two parallel plates, particles move slowly and the
sample becomes even glassier. As the sample particle
concentration is increased toward the glass transition
point, we observe confinement-induced slowing at larger plate
separations. Previous research has shown groups of particles
in dense suspensions move cooperatively. Our work suggests
glasses are solid-like because these groups can't move when
the sample chamber is thinner than the typical size of these
groups. These experiments help us understand earlier work done
with thin polymer films and other glassy materials, but as we
use particles rather than atoms, we get to directly see how
confinement influences the glass transition.


***

Nonlinearities predicted to be strengthened by photonic crystals

Typically, photons can pass by one another unchanged; however, a
number of important scientific and technological applications can be
enabled by using matter as a medium for photons to talk with one
another. These interactions are generally weak; however, in this
paper, we discuss a scheme that can strongly enhance the strength of
these interactions in a completely new way. It consists of placing a
nonlinear material inside a photonic crystal. The latter is
characterized by its photonic bandgap, a range of frequencies for
which photons are almost perfectly reflected. Its presence allows
more time for nonlinear processes to take place. However, the
nonlinear material is probed at a frequency just below the photonic
bandgap. For certain special materials, such as single nanocrystals
of cadmium selenide, the degree to which this lifetime can be
increased may be as much as a factor of forty at room temperature.
For other materials, an enhancement of at least a factor of two is
expected. Enhanced optical nonlinearities should allow much lower
powers and volumes to be used in nonlinear devices, which will have
implications for telecommunications, optical computing, and
ultimately, quantum computing.


***

Electrons in ferromagnets invited to the quantum dance!

Condensed matter physicists have for a long time been intrigued by the
manner in which electrons in metals assume a quantum wave-like character
as the temperature is lowered. This character trait becomes manifest when
electrons remember their travels and interfere with themselves, much like
water waves building up in amplitude when suitably channeled. The
resulting tendency of the electrons to stay in one place, or equivalently
to be "localized", causes an increase in electrical resistance that has a
non-classical quantum origin. This signature quantum resistance is easily
observed in ordinary metals but is not expected to be present in magnetic
metals (i.e., ferromagnets) such as iron, since intrinsic magnetic fields
severely compromise the ability of the electrons to self
interfere. Contrary to these expectations, this paper shows that electrons
in ferromagnets can behave much like their brethren in ordinary metals and
execute a dance in which they revisit their former coordinates and self
interfere. In the experiment, atomically thin films of iron are fabricated
and then measured at low temperature and high magnetic field without being
exposed to harmful air oxidation. Theoretical analysis takes into account
electron-electron interactions and explains the observed localization
behavior.



***

Luttinger liquid in the core of a screw dislocation in Helium-4

On the basis of first-principle Monte Carlo simulations we discover that the screw dislocation in He4 crystal features a superfluid at its core. This is the first example of a regular quasi-one-dimensional supersolid -- the phase of matter featuring both crystalline and superfluid orders.

The supersolid state of matter in ideal quantum crystals was hypothesized about by Yang, and later by Andreev, Lifshitz and Chester about half a century ago. In this regard He4 crystals were considered to be the best candidates due to their extreme quantum properties. It appears, however, that this idea does not materialize, as recent numerical studies have shown. Supersolid state of He4 is anticipated now to emerge from extended structural defects, i.e., contrary to the common wisdom, disorder promotes superfluidity. Previous studies found that grain boundaries with high degree of frustration between the crystallites and fully disordered “glassy” states in He4 have superfluid properties. In this respect, dislocations constitute a special system which is characterized by translational order along their cores, as perfect crystals do. Here we find that in the case of screw dislocations the resulting state is a superfluid in the core. This is the first example found in Nature of a regular supersolid structure formed in continuous space.



***

Complex Yet Similar Networks

Imagine that suddenly all the molecules of air in your office move to
one corner of the room. It is interesting that this possibility is
actually not prohibited by any law of physics but instead by statistics:
the probability of observing this, or even any noticeable fluctuation in
the pressure, is just too small to scare any statistical physics
practitioner. In this paper we demonstrate that a similar effect is at
work in a class of much more complex systems, namely those modeled by
complex networks. Previous studies had difficulty in defining
unambiguously the thresholds of dynamical processes such as epidemic
spreading and synchronization, because of the huge number of possible
configurations of the underlying networks. We have solved this problem
by showing that the network spectra, which encapsulates the relevant
information about the thresholds, are remarkably similar in ensembles of
large complex networks. Therefore, within these ensembles, the
probability of strong deviations in the network dynamics is negligibly
small, much in the same way as the probability of having all the
molecules at the corner of the room.



***

NONABELIONS APLENTY

The so-called "NonAbelian
matter", a.k.a. the stuff of quantum computers,
may be commonplace in semiconductor devices. Condensed
matter physicists have
studied since two decades the behavior of electrons under intense magnetic fields
when they are confined in an atomically thin interface between semiconductors.
Tuning the applied magnetic
field leads to various states of matter for the electrons
that are understandable only by quantum mechanics. The
systematics of these phases
has been obtained by various theoretical arguments in the past. Recently in 2003
a team of experimentalists lead by Wei Pan from Sandia national Lab has observed
a new series of states that
needed contrived but plausible explanations to be
accomodated in the established theoretical framework. However
the present theoretical work
shows that, contrary to previous belief, all these new states may be of the
"non-Abelian" kind,
a very delicate state of matter - the Holy Grail of topological quantum
computation.
Such states support several kinds of particles replacing the ususal electron as an elementary
particle and braiding these particles by electrostatic means can be used
to perform all the operations needed
in a quantum computer based on topology.
So we may not have to wait for complex atomic physics devices to create
these
much awaited states, they may be already in abundance in (some) semiconductor devices
when operated in
the proper regime.




RELATED RECENT PAPERS ADVERTISING SAME TOPIC:

"devices
based on the fractional quantum Hall effect may fulfill the promise
of quantum computing"
charles Day
in Physics Today, 58, 21 (2005).

"Anyon There ?"
David Lindley, Physical Review Focus, 16,
story 14, Nov 2005

"Computing with quantum Knots"
Scientific American, p.57 (2006).


***

Acceleration of nuclear spin polarization is found possible by a short
laser pulse!

Regardless of the extreme difficulty to polarize nuclei, spin polarized
nuclei are highly needed for the efficient utilization of radioactive
isotope (RI). Use of laser beams to polarize nuclei is one of the efficient
methods, but the efficiency is completely hampered if the nuclei need to be
polarized within a few microsecond, which is a typical lifetimes of unstable
isotopes. We have theoretically found that the combined use of short laser
pulses and a DC field results in the acceleration of nuclear spin
polarization, and the polarization can be completed within a few to tens of
ns. This is about 3 orders of magnitude shorter than the time needed by any
known optical methods to polarize nuclei and the achieved polarization turns
out to be surprisingly high.


***

Diving below the surface: Dynamic force microscopy reveals ordered arrays of sub-surface vacancies in ceria

Ceria is a most important material in oxidation catalysis promoting and regulating chemical reactions by taking up and releasing oxygen and is used, for instance, in automotive catalytic converters. Oxygen transport is facilitated by vacancies, i.e. single ions missing in the atomic lattice, that may diffuse from the bulk to the surface of the oxide or vice-versa. Dynamic scanning force microscopy (DFM) now allows to unambiguously identify sub-surface oxygen vacancies buried in the third surface atomic layer of a well prepared CeO2(111) surface. In this work we present evidence for subtle attractive and repulsive interactions between these sub-surface vacancies forcing them into well ordered arrays having an open structure. Such an arrangement reflects the topology of highest symmetry at high vacancy densities. This letter demonstrates by means of an impressive example the capability of advanced force microscopy to characterize atomic details not only at the surface of an insulator but also to reveal and identify complex sub-surface structures that may have a major impact on surface chemistry.

***

The nature of the mysterious dark matter may very soon be revealed through its
emission of gamma-rays. A very striking smoking gun signal is robustly
predicted for one of the simplest models for dark matter. This feature could be
detected with the GLAST satellite telescope to be launched later this year.

Astronomical observations have established that ordinary visible matter only
makes up a minor part of our Universe. Instead the mass of the Universe is
dominated by so called dark matter, believed to be made up by unknown exotic
particles. The precise nature of these particles remains one of the great
unsolved questions in cosmology. A recently proposed solution comes through a
new type of particle, called the inert Higgs. Although normally not emitting
any light at all, such particles traveling through space will occasionally
annihilate each other and produce high energy gamma-rays. We have shown that
this dark matter candidate is capable of providing an extremely characteristic
gamma-ray spectrum with good prospects for detection with the space-borne GLAST
telescope to be launched in December 2007. Such detection would unambiguously
confirm the existence of dark matter particles in our Universe.



***

All optical magnetic recording

We have experimentally demonstrated controlled magnetization reversal
induced by a single 40 femtosecond circularly polarized laser pulse in
the magnetic alloy GdFeCo, a material relevant for data storage. No
external magnetic field is required for this opto-magnetic switching,
and the stable final state of the magnetization is unambiguously
determined by the helicity of the laser pulse. This finding,
previously believed to be fundamentally impossible, reveals an
ultrafast and efficient pathway for writing magnetic bits at
record-breaking speeds. With the recent development of compact
ultrafast laser systems and the successful incorporation of lasers in
magnetic storage devices, the present demonstration of ultrafast
all-optical magnetization reversal might spur the realization of a new
generation of ultrafast magnetic recording devices.

Figure caption:
Demonstration of compact all-optical recording of magnetic bits,
achieved by scanning a circularly polarized laser beam across the
sample and simultaneously modulating the polarization of the beam
between left- and right circular.

***

Dissipation in Shock Waves in the Solar Wind
In this paper we describe the first statistical results of electric field waveform captures taken in the ramp
region (transition region) of interplanetary (IP) shocks. Near 1 AU IP shocks result mainly from explosive
eruptions of massive amounts of plasma ( 1011–1013 kg) known as Coronal Mass Ejections (CMEs). CMEs
propagate out into the solar wind at velocities in excess of 1000 km/s dragging the sun’s magnetic field with them.
Their large velocities can result in shock wave formation. Unlike a regular fluid, IP shocks cannot rely on particle
collisions to prevent the wave from breaking by dissipating energy. Our paper investigates a possible method
for such dissipation, wave–particle interactions (i.e. electric field oscillations scattering particles). Because
these shock waves can accelerate particles to high energies that can damage space systems, understanding the
underlying mechanisms which govern their evolution and propagation from the sun to the Earth is important.
In addition, they provide an accessible laboratory for studying collisionless shocks.

***

Black hole radiation may reveal hidden symmetries

Black holes are hot: as Hawking showed 35 years ago, they emit
radiation at a temperature governed by their size. Just as air
temperature is a manifestation of the motion of molecules, the
Hawking temperature may reveal the underlying microscopic states
of the black hole. Until recently, the nature of these states
was completely unknown. Today, we have too many answers: at least
ten distinct theoretical descriptions are known, portraying very
different microscopic physics but all giving the same temperature.
In this paper, I explore the possibility that a hidden symmetry
of the event horizon could explain the thermal properties of black
holes, independent of other details. I show that such a symmetry
exists and yields the correct value for the black hole entropy.
I offer evidence that it may provide a "universal" explanation:
at least one string theory approach to black hole thermodynamics
occurs as a special case, and there are tantalizing hints of a
connection to a calculation from loop quantum gravity. Black
hole radiation may thus reveal hidden symmetries of the horizon.



***

Nonlocal Properties of Dynamical Three-Body
Casimir-Polder Forces

The existence of quantum nonlocal correlations between distant microscopic
objects is one of the most striking features of quantum mechanics. Also
quantum fields, such as the electromagnetic fields in quantum
electrodynamics, are predicted to possess nonlocal spatial correlations.

Our paper has shown that in a system of three electrically neutral atoms at
different locations the long-range interatomic forces, known as
Casimir-Polder forces, bear an imprint of the nonlocal correlations of the
electromagnetic field, although these correlations cannot be used for
transmitting information at superluminal speed. The importance of these
results is that it is shown for the first time that the dynamical
Casimir-Polder interaction between atoms can be used to detect nonlocal
properties of field correlations.

In our paper the three neutral atoms play the role of probes for detecting
field correlations, and an expression for the time-dependent three-body
Casimir-Polder interaction is obtained in two different ways: i) from the
interaction between two atoms in the field created by the third, and ii)
from the interaction of one atom with the field created by the other two. In
a stationary situation both ways yield the same result. In a time-dependent
situation, however, the results are different, since in case i) the
interaction energy displays nonlocal features, related to the nonlocal
properties of the field created by the third atom; in contrast, nonlocal
features are absent in case ii).

The different results obtained for the time-dependent situation can be
explained in terms of the three-body forces stemming from the interaction
energy, since the two different ways of calculation seem to correspond to
two different ways of measuring these forces. Case i) in fact involves a
correlated measurement of the forces acting between two atoms in the
presence of the third. In contrast, case ii) involves the measurement of the
force acting on a single atom arising from the other two. In other words,
correlated measurements on two atoms are necessary to show nonlocal
properties of the electromagnetic field, whereas single force measurements
are not able to reveal field nonlocality.

This conclusion seems to raise question about the physical meaning of
three-body forces (which are inherently nonlocal quantities) and how these
three-body forces should be measured.

6-11-07

Minimum-risk path finding by an adaptive amoebal network


The plasmodium of the primitive organism, the giant amoeba Physarum,
is able to find the minimum-risk path in a spatially inhomogeneous
field of risk.
Thus the amoeba has the capacity for information processing in
optimizing its physiological requirements.
When two food-sources are presented to the plasmodium in the dark,
a thick tube for absorbing nutrients is formed that connects the food-
sources through the shortest route.
When the light-avoiding organism is partially illuminated,
however, the tube connecting the food-sources follows a different route.
Defining risk as the experimentally measurable rate of light-avoiding
movement,
the minimum-risk path is exhibited by the organism, determined by
integrating along the path.
We note that the evolution of the tube network shares features in
common with Hebbian learning
found in neuronal networks, as the tubes grow or shrink and disappear
based on their level of activity.


***


A BIOPHYSICAL THEORY FOR ACTION-AT-A-DISTANCE IN THE BRAIN

Brain-imaging studies have shown that---during directed mental tasks
and also during resting wakefulness---separated regions of the cortex
"light up" to generate waves of synchronized electrical activity.
But what causes these different brain centres to become active
simultaneously?

In this paper [1], we propose that this "action at a distance" co-
ordination of neuron behavior arises naturally from the gap-junction
connections that couple the neurons and glia (neuron support cells)
within the central nervous system. These electrical synapses are
found in all mammalian cells. In the brain, they provide a direct
electrical connection allowing fast inter-neuron communications that
supplement the well-studied primary communication channel involving
chemical synapses driven by action potential "spikes."

Our theory suggests that---provided electrical diffusion via gap-
junctions is sufficiently strong---the brain will spontaneously
organize into distinct regions of high- and low-firing activity that
will exchange contrast on slow time-scales. These spatial patterns
are called Turing structures, named for mathematician Alan Turing,
who, in 1952, first demonstrated that competing diffusion reactions
could explain pattern-formation in biology. The figure below shows a
sample of the range of Turing patterns generated by our cortical model.

***

Colloidal traffic-control on a magnetic lab-on-a-chip

If you have ever worried about times to commute from one place to another
in a big city you will appreciate a smart traffic system. Pietro Tierno,
Tom Johansen and Thomas Fischer from the Florida State University and the
University of Oslo have designed a smart magnetic traffic control for small
particles on a lab on the chip that might be used as fast drug delivery
system. A magnetic field rotating with a frequency slightly faster than the
particles on the lab-onthe-chip can follow significantly reduces their time
of commuting through array of magnetic domains.
49eaefd9.jpg

***

With a second dimension of time, 2T-physics appears to give a better
description of the natural world.

Evidence has been accumulating that the ordinary formulation of physics
(1T physics) is insufficient to describe certain aspects of our world,
just like shadows on walls alone are insufficient to capture the true
essence of an object in a three dimensional room. According to Two-Time
Physics (2T physics), there is more to space-time than what can be
garnered with 1T physics.

The Standard Model of Particles and Forces (SM), which captures all the
physical realities that are experimentally confirmed up to now, happens
to be a particular 3+1 dimensional shadow of a 2T-physics field theory
in 4+2 dimensions, as shown last year. Among the successes of the
2T-physics approach to the Standard Model is the resolution of the
strong CP problem of QCD without an elusive axion.

To help grasp the relation between 1T-physics and 2T physics, consider
the many possible shadows of a 3 dimensional object projected from
different perspectives on the surrounding walls of a 3-dimensional room.
Just like a flatlander, that can crawl and measure only on the surface
of the wall, would think that the shadows of different shapes are
different "beasts" and move differently, so does 1T-physics presents
different dynamical systems in terms of different Hamiltonians, although
according to 2T-physics these apparently different systems are clearly
related to each other since there is a unique dynamical system in 4+2
dimensions that generates all of the 1-time "shadows". Thus 2T-physics
provides a new kind of unification.

So 2T-physics claims that there are other "shadows" of the 4+2 version
of the Standard Model that have different interpretations from the point
of view of 1T-physics in 3+1 dimensions. A program for studying these
"duals" of the Standard Model has been launched recently in a separate
paper (arXiv:0705.2834, accepted for publication in Phys. Rev.)

In this letter the formulation of the general supersymmetric version of
2T-physics field theory in 4 + 2 dimensions is outlined, for fields of
spins 0, 1/2 , 1, with N = 1 supersymmetry (SUSY). This will be a
starting point for physical applications in the form of the
supersymmetric version of the SM in 4 + 2 dimensions. Due to symmetry
constraints that arise through the 4+2 higher spacetime, it is
conceivable that there will be measurable consequences that can
distinguish 2T-physics from other approaches in experiments at the Large
Hadron Collider at CERN that will begin to produce new data in 2008.

For more information see links at my homepage:
http://physics1.usc.edu/~bars/


***

A twist in the search for the Higgs boson


The emergence of the Higgs boson decay mode into two pseudoscalar
bosons, which can relieve the so-called little hierarchy problem and
reduce the LEP2 Higgs boson mass bound, may affect the golden search
modes (h -> gamma gamma, b b-bar) of the Higgs boson significantly.
The LHC may not be able to find the Higgs boson if the
two-pseudoscalar decay mode dominates. Our Letter explicitly shows
that the associated production of the Higgs boson with a W or Z boson
can recover the loss of sensitivity in the golden modes. With the
Higgs boson decaying into two pseudoscalar bosons, which further decay
into 4 b jets, together with at least a charged lepton from the W or Z
boson decay, a significant Higgs boson signal is observable at the
LHC. The ultimate goal of the LHC is hunt for the Higgs boson, which
is responsible for electroweak symmetry breaking and mass generation.



***

The new structure of mother-of-pearl


Biominerals are of interest to material scientists, physicists,
chemists and engineers because of their remarkable mechanical
properties and their fascinatingly complex, self-assembled architecture.



Biominerals, including mollusk and crustacean shells, bone, teeth and
eggshell, are composites of micro-crystals and organics. Metzler et
al. (PRL, accepted for publication) discovered that the orientation
of each micro-crystal in the nacre (or mother-of-pearl) layer inside
red abalone shells can be revealed by imaging nacre with synchrotron
spectromicroscopy. With this high-resolution and sensitivity
microscopy approach the micro-crystals composing nacre stand out, and
their relative orientations are revealed by different gray-levels.
The contrast arises from a mechanism called x-ray linear dichroism,
discovered by Joachim Stöhr (J. Stöhr et al., Phys. Rev. Lett. 47,
381 (1981)), now director of the Stanford Synchrotron Radiation
Laboratory. Linear dichroism has been widely used to study man-made
structures, including superconductors and magnetic materials, but
this is the first discovery of such mechanism in a natural, biogenic
material.

The experiments were conducted at the University of Wisconsin
Synchrotron Radiation Center, where the principal investigator in
this work, Prof. Pupa Gilbert, of UW-Physics, led a group since she
joined UW in 1999. She also served as Scientific Director at the SRC
from 2002 to 2006.

The new dichroic contrast mechanism revealed individual stacks of co-
oriented tablets, and how these are packed in situ, in pristine
nacre. Comparing previous data and their new data, Gilbert and
physics graduate student Rebecca A. Metzler, in collaboration with
theorist Prof. Susan N. Coppersmith, also at UW-Physics, found that
the data are most consistent with a specific model for nacre
formation: randomly distributed nucleation sites are pre-formed in
organic matrix layers, and tablet n+1 nucleation and growth only
starts when the underlying tablet n has reached its final height.
More experiments, currently underway, will make all observations
quantitative. More theoretical models, in three dimensions, will
further refine the nacre formation mechanism, which has puzzled the
scientific community for decades.

6-6-07

An improved 'improbability drive' to calculate reaction rates

An accurate calculation of atomistic dynamics requires extensive
computer power. To produce a millisecond movie of moving molecules
literally takes ages even by the fastest supercomputer. Chemical
reactions require even longer timescales as they involve rare
collisions between molecules with a very specific orientation and
momentum. Luckily, intelligent algorithms prevent us for having to
wait that long. The recently developed path sampling methods can be
best described as a kind of "Improbability Drive" from the
"Hitchhiker's Guide to the Galaxy"[1]. Instead of performing a single
calculation, a series of simulations is released. The probability of
being reactive is artificially enhanced at each step. Still, reaction
rate evaluations can last for several months. One of the problems is
that present methods have much difficulty to find competing mechanisms
once it is trapped in a specific reaction channel. This letter [2]
shows how the efficiency can be improved considerably when the
separate simulations are allowed to exchange information during the
run. A test on the opening transition of DNA showed that the
efficiency increased by a factor 20. This opens up the possibility to
study many interesting problems which were hitherto impossible.

[1] Douglas Adams, "The Hitchhiker's Guide to the Galaxy", 1979
[2] Titus S. van Erp, "Reaction rate calculation by parallel path
swapping", Phys. Rev. Lett. accepted for publication (Article is
expected to be published in the July 6, 2007 issue).


***

Atomic Force Spectroscopy Captures the Fundamental Forces Stabilizing DNA Structure

Piotr Marszalek and colleagues at Duke University's Pratt School of Engineering have reported the first direct measurements of the forces that contribute to the stability of DNA structure. DNA molecules consist of two intertwined chains, and it is the precise sequence of stacked nucleotides-the four different varieties of 'links' in each chain--that encodes the genetic blueprint of all living organisms. The DNA structure is primarily stabilized by two types of interactions: hydrogen-bonding between complementary nucleotide bases in opposite chains (Watson-Crick pairing) and base stacking between nucleotides that are positioned on top of each other along the DNA axis. The respective contribution of these interactions to DNA's integrity has been difficult to establish because these interactions cannot be isolated in the complete double helix. Now, the research team has captured the "mechanical fingerprint" of a single-stranded DNA (ssDNA) chain by atomic force microscopy, a method that allows force measurements at the level of individual molecules. They attached ssDNA molecules consisting solely of adenines (one of the four nucleotide bases) to a gold substrate and grabbed the end of individual molecules with the microscope's cantilever tip. They then repeatedly stretched and relaxed the DNA and measured its elasticity profiles. Unexpectedly, they found two pronounced plateau features in the force-extension relationships (at ~23 pN and ~113 pN respectively) that indicate some force-induced structural rearrangements within the ssDNA. Because the stacking of adenines forms a spiral structure within ssDNA, Marszalek's team suggests that the two plateaus represent the unraveling of the helix as the base-stacking interactions sever. Thus, the measurements directly captured, for the first time, the strength of base stacking interactions in DNA in the absence of base-pairing, a significant contribution to understanding the forces that stabilize DNA structure.



***

Non-Abelian Josephson Currents

Quantum mechanics is most commonly used to describe physics on microscopic
scales, those of atoms and nuclei. Superconductivity, on the other hand, may
be thought of as a macroscopic manifestation of quantum physics for a
collection of particles where electric currents flow without resistance. In
the absence of coupling between the two superconducting regions, there is a
separate symmetry for the each of the regions. The Josephson effect occurs
when electrical currents spontaneously flow through a normal barrier
separating two superconducting regions. The wave functions describing the
states of the separate superconducting regions are coupled across the
barrier and the current depends on the phase difference of the two wave
functions. This causes a loss of symmetry from the uncoupled system of
superconductors. This paper explores the possibility of similar junctions in
more complex systems, possibly including high-density quark matter present
in neutron stars and some forms of high-temperature superconductors studied
in recent times. Such systems have multi-component wave functions,
describing many possible phases of the system. The corresponding
transformations that leave the dynamical equations describing the physics of
the system invariant in this case are non-abelian. The transformations
describing the symmetry of the system depend on the order in which they are
done. Here there is a much richer set of circumstances, depending on the
more complicated order parameter-wave function and on the non-abelian
counterpart of the electric and magnetic fields in the ordinary Josephson
effect. The coupling again reduces the symmetry and the consequences of the
reduced symmetry are the subject of this paper. There are different fashions
in which the Josephson currents could flow depending on the way the two
sides of the junction are coupled. So it appears that the same quantum
mechanical phenomena that are observed in ordinary Josephson junctions
produced as sandwiches between superconducting and normal regions in the
laboratory can be observed in other far more complicated systems, some
possibly as large as neutron stars and perhaps the very early universe.


***

New laws for self-organized island growth on surfaces

How many islands form when atoms are deposited randomly onto a smooth
surface and find themselves via a complex interplay of diffusion,
nucleation and aggregation processes? This question has been answered a
long time ago for deposition of one type of atom. An extended theory
developed in the recent issue ... of Physical Review Letters now
allows one to predict the island density when different types of atoms
are co-deposited with varying stoichiometries. In this multi-component
case the situation becomes more diverse due to the fact that islands
of the same size exhibit different stabilities in dependence of their
specific composition. The extended theory can be applied to adjust the
number of islands in self-organized nanostructuring. It furthermore
gives access to numerous growth parameters. In particular, binding
energies between different species can be determined by evaluation of
measured island densities.


***

Force microscopy overtones for nanoimaging

Our paper describes a new atomic force microscopy (AFM) method for achieving subnm
resolution on native membranes by exploiting higher harmonics in the oscillating
movement of a cantilever. The mechanism behind this method can be better understood by
drawing an analogy to music:
A driven non-linear oscillator, such as the human voice, a blown wind instrument, or a
bowed violin string and also an AFM cantilever tapping on a sample, will oscillate in a
periodic, non-sinusoidal manner. This generates the impression of sound at integer multiple
frequencies of the fundamental known as harmonics. The overall combination of the
instrument's specific harmonics is what determines the timbre ("flavour of sound") of that
instrument. Timbre is what gives the listener the ability to distinguish different instruments
that play the same note at the same volume in a band or orchestra, and what gives the scientist
the ability to distinguish different sample elasticity.
In our study a point mass model was introduced, to qualitatively explain the origin of
these harmonic contributions, and this model was compared to experimental findings. In order
to demonstrate the great potential of this novel method, the 2nd harmonic amplitude in the
movement of an AFM cantilever was recorded during scanning a bacterial surface proteinlayer
and a layer of human rhinovirus under physiological conditions. A lateral resolution of
~0.5 nm was obtained, one order of magnitude higher than the resolution in the
simultaneously recorded topography image.

***


Radioactive decay of beryllium(Be)-7 speeds up at 5 Kelvin by almost 1.5%
when the Be-7 stays inside C60 cage. This is the largest shift yet seen
in a chemically and/or metallically induced modification of the Be-7 lifetime.
In this case, one of the orbital electrons, usually from the K or L shell,
is captured by a proton in the Be nucleus, forming a neutron and a neutrino
(EC decay). The 1.5% difference between the EC decay rate
inside C60 (52.4 days) and inside other materials (53.2 days) represents
a strong environmental effect on the Be-7 EC decay rate, caused by
the different electronic wave-functions. This finding may suggest
that the Be-7 atoms stay at the center (potential minimum)
of the C60 at 5 Kelvin and the L shell electrons can be fully restricted to
the Be
nucleus as if they were isolated without chemical bond in vacuum.
The magnitude may be the achievable largest value in laboratories on earth.
A schematic view of the closed K and L electron shells when the Be-7 is
inside C60 is shown in the figure.

6-4-07

Efficient and broadband photon funneling at optical frequencies

In optoelectronic devices, spontaneous emission has to be directed toward a
single mode channel, for the sake of efficiency, noise reduction or speed
enhancement. However in general, this is not achieved and a large portion g
of the radiation escapes out of this mode, so that only a small fraction b
is effectively coupled. To overcome this difficulty, one may use 3D
photonic-crystal materials, with theoretically vanishing ã’s in the
bandgap, but they are difficult to manufacture. Alternatively, one may use
microresonators to dynamically boost the photons (Purcell effect) into the
cavity mode, but the restrictive matching between the quantum-dot source
and cavity energies severely reduces the device yield.
A new Bloch-mode formalism has been used to predict that single-row-defect
waveguides in photonic-crystal membranes offer large b-factors in excess
of 95%. In this 2D system, it is the photon emission into the spurious
modes that is naturally lowered, like in 3D bandgap materials, while the
emission into the desired channel is comparable to that in the bulk
material. Since no Purcell effect is involved, the efficient funnelling is
achieved over a remarkably large spectral interval of 40-nm, a value
comparable to the inhomogeneous dot linewidths at l=950nm. This prediction
may impact the design of future single mode LEDs or low threshold lasers.


***


Nanoscale Machines Controlled by Light Get a Leg Up

We present experiments revealing the first detailed images of individual
molecules reversibly changing their shape in response to light. We were
successful because we added tiny ¿legs¿ to molecules that allowed them
to stand up above the fray of their clinging nanoscale environment.
These molecules (azobenzenes) are very special because they perform
mechanical work when they change shape. They are exciting for
nanotechnology applications as they could perform as the smallest
imaginable remote-controlled mechanical pistons or electrical switches
(i.e. nanomachines). While previously these molecules have been studied
tumbling in solution or organic polymers, nanotechnology applications
require them to be mounted very closely on a surface. But mounting
molecules on a surface generally destroys their mechanical properties:
nanomachines interact with their nano bench tops. The legs on our
molecules surmounted this challenge by causing the molecular engine to
stand up away from the surface just enough (fractions of a nanometer) to
escape surface interference, while still being firmly mounted to the
nano bench top. Our images make clear the previously murky but essential
dependence of nanomachines on their detailed nanoscale environments, and
pave the way towards future nanotechnology applications.

6-1-07

Coherence peak; a universal feature in high-Tc superconductors

The BCS (Bardeen-Cooper-Schriefer) theory tells us that two electrons are condensed to make a pair (Cooper pair) in the superconducting state, producing a superconducting coherence peak (SCP) just below the Fermi level. However, it has been a long-standing puzzle why single-layered high-Tc superconductors (HTSCs) such as La2-xSrxCuO4 (LSCO) do not exhibit the SCP in contract to multi-layered HTSCs such as Bi2Sr2CaCa2O8. This inconsistency has provoked a lot of debates in modeling the high-Tc mechanism, for example, an essential difference between single- and multi-layered HTSCs. This PRL paper reports the first direct observation of a SCP in LSCO, together with the precise momentum dependence by angle-resolved photoemission spectroscopy (ARPES). They found that the SCP is confined in a surprisingly narrow momentum region between the node and the antinode in sharp contrast to multi-layered HTSCs. The experimental results suggest that a pseudogap dominant around the antinode affects this anomalous momentum dependence of SCP in LSCO. Thus, the present high-resolution ARPES study has established the universality of a coherence peak in HTSCs and at the same time provided a key for understanding the difference in the superconducting transition temperature between single- and multi-layered HTSCs.

5-31-07

Quantum phase transition in ferroelectric oxides

A new quantum phase transition (QPT) has been interestingly observed in
ferroelectric oxides. In this case, the coulomb interaction, which
creates the long-range order of dipole moments, competes with the
quantum mechanical fluctuation that destroys the order.

Usually, the phase transition, for example the freezing of water, is
governed by the thermal fluctuation; however, if the temperature
approaches the absolute zero temperature, the quantum mechanical effect
emerges in the macroscopic scale. In this study, we have clearly
observed the crossover of the critical behavior of the 腧soft mode腨 from
the classical to the quantum regime, where the soft mode is the
anharmonic lattice vibration that triggers the phase transition. This is
the first QPT brought about by the phonon (the quanta of the lattice
vibration), in contrast to those induced by electronic interactions. In
particular, the novel ferroelectric-paraelectric phase coexistence state
has been revealed in the vicinity of the so-called quantum critical
point for the first time. These findings have been achieved in the solid
solution of SrTi^16 O_3 and SrTi^18 O_3 by carefully controlling the
amplitude of the quantum fluctuation without modulating the chemical
natures of the material by a proper selection on the ^18 O-concentration.

***

Bose-Einstein condensation in the presence of an attractive interaction and its relation to the cosmological Q-balls

(1) It is well known that stable atomic Bose-Einstein condensates can
be formed when the interaction between the atoms is repulsive. An
attractive interaction, unless it is extremely small, leads to the
collapse of the Bose gas. The Bose-Einstein condensate of spin waves
(magnons) with a repulsive interaction was experimentally stabilized
in 1984 in superfluid 3He-B. In this paper it is shown that a
specific feature of this Bose condensate is that it survives the
attractive interaction between the magnons. The magnon condensate
with attractive interaction is formed in the potential well produced
by the texture of the order parameter of 3He, and is observed in
nuclear magnetic resonance experiments as a long-lived coherent
precession of magnetization, which can persist for an hour without
external pumping of energy.
(2) The structure and stability of the magnon condensate with an
attractive interaction are similar to those of certain objects
discussed in particle physics. These are the so-called Q-balls,
which are stabilized due to the conservation of the global charge Q,
typically the baryon or lepton number. In the magnon condensate the
role of the charge Q is played by the projection of the total spin of
the liquid in the direction of the magnetic field. At the quantum
level, Q-balls are formed due to a suitable attractive interaction
that binds the quanta of the scalar field into a large compact
object. Q-balls may contribute significantly to the dark matter and
baryon contents of the Universe. Observation of Q-ball objects in
3He-B confirms the possibility that stable cosmological Q-balls might
exist and should be searched experimentally.


***


Origin of Antiferroelectricity Explained from First Principles

The phenomenon of antiferroelectriciy wherin a compound develops two (or more) oppositely polarized sublattices was first discovered in 1938 in ammonium dihydrogen phosphate, commonly called ADP. ADP and its ferroelectric analogs have been used extensively in electro-optical applications. However, the atomistic details of why ADP becomes antiferroelectric has not been fully understood. A detailed understanding of such properties is needed in order too be able to hone their desirable characteristics. Now efficient algorithms for /ab-initio/ electronic structure calculations have enabled a group of solid state theorists at the Rosario National University in Argentina, working with chemists at Florida State University, to show that ADP's antiferroelectricity is a result of a delicate balance between the energetics of the two different types of hydrogen bonds in the lattice : O-H···O and N-H···O bonds. In a small range of atomic configurations, the optimal N-H···O bonding is enabled by the antiparallel alignment of molecular dipole moments in the unit cell and the crystal thus exhibits antiferroelectricity. The calculations show also that a small change in the atomic displacements could also render ferroelectricity to ADP, thus providing a theoretical basis of a long-standing observation that ADP appears to possess coexisting ferroeelectric and antiferroelectric regions whose relative population changes with temperature. (J. Lasave et al., Phys. Rev. Lett. (2007, in press)).

***

Potassium Bose-Einstein condensate: last but not least

A recent revolution in modern physics has been the achievement of Bose Einstein condensation, i.e. the formation of macroscopic quantum objects offering unprecedented possibilities for the study of the quantum world. This phenomenon has been demonstrated for a number of atomic species including all the stable elements in the first column of the periodic table, except one isotope of potassium. An experimental team at LENS, University of Florence, has now succeeded in creating a Bose-Einstein condensate of such last isotope, potassium-39, and found that, surprisingly, it is one of the most interesting among those studied so far. The team has overcome what appeared to be a fundamental obstacle to condensation of this species, i.e. the presence of an attractive interaction between pairs of atoms. The researchers have found that with the application of appropriate magnetic fields such interaction can be changed to repulsive and a condensate can be produced. Moreover, they have found that in a potassium-39 condensate the interaction can also be nulled with high accuracy, opening a route to the study of an ideal, i.e. non-interacting, quantum object. This novel Bose-Einstein condensate is expected to allow the study of some aspects of the quantum world that have not yet been explored.

***

Surveying with Electrons: Putting Lloyd's Mirror on the Map

A major puzzle for many researchers working in the field of materials
research is how to obtain 3D movies of surface dynamic events, often
during thin film growth or annealing. In a new approach, Monash
Physicists have shown how in situ electron microscopy can be used to
obtain surface height resolution in real-time. Just as the contour lines
on a topographic map provide mountaineers with the capacity to plan
their assault on a mountain, Lloyd's mirror combined with electron
microscopy can provide a relief map of surfaces in which the spacing
between fringes is directly related to height. In the classic 19th
century Lloyd's mirror experiment, light reflected off a mirror
interferes with light coming directly from the source. PRL XX reports a
novel 21st century version of the famous Lloyd's mirror, in which
ultraviolet light is used to illuminate gallium droplets on a gallium
arsenide substrate. The bright interference fringes result in the
emission of electrons, which are imaged in a surface electron
microscope. The fringes are sensitive to the 3D shape of the gallium
droplets, but are also distorted by the electric field due to the
topographic features. However, these distortions can be corrected using
image processing, to provide a real-time relief map showing surface
dynamics of the evolving metallic droplets. Lloyd's mirror electron
microscopy using a synchrotron light source will allow the technique to
image surface features with nanoscale resolution, opening up new
possibilities to make 3D movies of dynamic events during thin film
growth and processing.

***

Towards ultrafast x-ray physics: a novel method to manipulate x-ray
pulses

Theoretical calculations predict a novel physical effect for x rays that
may facilitate the generation of ultrashort x-ray pulses with tailored
shape. This prediction opens new vistas in science, in connection with
both existing x-ray facilities such as Argonne's Advanced Photon Source
and future x-ray free electron lasers. The effect is called
electromagnetically induced transparency (EIT) for x rays. A gas that
would otherwise strongly absorb, and thus be opaque to, x rays of a
given wavelength can be rendered transparent to the x rays by
simultaneously shining an intense laser into the gas. EIT for visible
light is well known and represents a unique tool for manipulating light.
Inner-shell vacancies produced by x-ray absorption have very short
lifetimes of the order of femtoseconds or shorter. Therefore, the laser
intensity required to induce EIT is so high that the standard
description of EIT becomes invalid. Before this study, it was
questionable whether EIT may exist at all in the x-ray regime. The
theoretical analysis carried out in this paper demonstrates that
existing intense-laser technology may be harnessed to imprint the
temporal shape of optical laser pulses onto x-ray pulses.


***

Unique due to relativity

Heavy element polonium enters harmfully our lives via air, soil and
plants such as tobacco or tea, but very little is known about its
physical and chemical properties. It has not been clear why polonium, as
the only element from the Periodic Table, crystallizes in the simple
cubic structure, one of the simplest structures ever. Our paper shows,
on the basis of the fundamental quantum mechanics, that this
unique structure of polonium is, similarly as the yellow luster of
gold, due to relativistic effects. We also draw the attention to
polonium strong elastic anisotropy, which has no equal in other solids,
and demonstrate that this is an inherent peculiarity of its crystal
structure. Finally, as a challenge for future experimental studies, we
predict a mechanical instability of simple cubic polonium at relatively
low pressures resulting in a mixture of two lower-symmetry structures.


***

The quantum nature of an atom enables its simultaneous propagation in two different regions of space: this property has been used by the research group of J. Vigué (Université P. Sabatier and CNRS, Toulouse) to make an absolute measurement of the refraction index of gases for lithium atomic waves. This experiment is based on an atom interferometer, in which each atom follows two paths. Their maximum distance, equal to 100 micrometers, is sufficient to introduce a material object between them so as to apply a weak gas pressure on one of them.

The only comparable experiments have been done on sodium by the research group of D. Pritchard at MIT but they have not achieved an absolute measurement.

This index describes the attenuation and the delay or advance of the atomic wave induced by collisions with the atoms of the gas. The comparison of the present attenuation measurements with previous values is excellent. The delay or advance of the atomic wave can be accessed only by the present atom interferometry method and the measured values are in agreement with theoretical estimates.

Finally, this type of experiments changes our representation of atoms, from point-like particles to strongly delocalised quantum waves!



***


Plasma Density Fluctuations Are Universal, But This Was Hidden Beyond A
Picturesque Smile

A unique parabolic relation links two statistical quantities related to
plasma density fluctuations. As in many other physical systems, a
significant effort is dedicated to finding universal aspects in a
statistical description of plasma turbulence. The probability of
occurence of any event can be given by its probability density function
(PDF hereafter). For a PDF, the degree of asymmetry and the weight of
its wings are contained in two parameters called skewness and kurtosis,
respectively. In this paper, we compute these two parameters for around
ten thousands signals, measured over the whole cross-section of a
toroidal magnetised plasma for a broad range of experimental conditions.
For the first time in plasma physics, universality has been sought by
plotting the kurtosis against the skewness, revealing two proofs for
universality. Firstly, all the experimental points are not randomly
distributed but form a picturesque smile. Secondly, all the PDF of the
measured signals are universally described by one and only one
analytical distribution. Fluctuations in the frequency range of the
paradigmatic drift-interchange instability are necessary and sufficient
to assure that PDF can be described by this specific distribution.

Phys Rev Hot Paper: 5-21-07

WHY IS WATER SO GOOD AT NEUTRALIZING ELECTRIC FIELDS

Compared to other liquids made of polar molecules water has a far
greater power
to dissolve ionic substances. This is a consequence of the network of
hydrogen bond links
between pairs of neighboring molecules, which greatly enhances
the ability of water to neutralize electric fields. This is because
(a) the hydrogen bonds
effectively increase the average dipole moment of each molecule in
the liquid with respect
to the vapor phase, and (b) they enable the molecules to react in
unison to an applied field.
The actual magnitude of these two effects has been an open and
frequently debated question
for more than 60 years. This paper provides a precise and
indisputable answer from a computer
simulation based on fundamental quantum theory, i.e. a simulation in
which the molecular
dynamics derives from the dynamics of the nuclei and the electrons
with no need for empirical input.
The results are in good agreement with experiment not only for liquid
water but also for solid ice.
In the simulation the effects that are by far the most important
occur at short range,
between a molecule and its first shell of neighbors. Thus, even the
minuscule quantities of water
that can be found in protein pockets should be capable to
substantially shield electrostatic
interactions which play an essential role in protein structure and
function.

***

New Superstring Predictions for Gravitational Scattering

It has been known for over 20 years that superstring theory predicts
deviations from Einstein's general relativity for the scattering of
gravitational waves at high-energies.
In this letter, these high-energy deviations are determined with much more
accuracy using a new more powerful formalism for computing superstring
amplitudes.


***

Drunk cars

If you drive a car in a huge empty park at constant speed while randomly
turning the wheel, you perform a well-known type of random motion called
persistent Brownian motion. If now instead of keeping the speed constant
you randomly press the gas and break pedals while still randomly turning
the direction wheel, the resulting movement is no longer well described
as plain persistent Brownian motion, and the statistics of the movement
change. In this work the new features introduced by the fluctuations in
the speed are exposed. In particular, it is shown that there is a complex
transient, together with changes in the asymptotic properties of the
movement due to the speed fluctuations. Our results could be relevant to
understand the motion of small self-propelled particles subject to large
fluctuations in their motion. It is suggested that some crawling cells,
which exhibited fluctuations in the speed and the direction of motion,
could be described as such "drunk cars".

Phys Rev Hot Papers: 5-17-07

Manipulating the mechanisms of crystallization in simple systems

Controlling the structure (or polymorph) in which a molecule
crystallizes is a long-standing issue. Since polymorphs have
different physical properties, it is crucial for many applications
(e.g. in the making of pharmaceuticals) to understand and control
this phenomenon. In this paper, we use molecular simulations to shed
light on the molecular mechanisms underlying the choice of a specific
polymorph during the crystallization of spherical particles. We show
how, by modifying the temperature and pressure of crystallization, we
succeed in manipulating the mechanisms of crystal growth. We not only
determine the conditions enabling us to control the overall structure
of the crystallite but we also control its purity. Furthermore, we
provide new insight on how one structure can form on the surface of
another and on how we can prevent this phenomenon known
experimentally as cross-nucleation. In (a), we show a cross-
nucleation event as large domains of a structure (yellow) grow on top
of another (grey) while cross-nucleation is greatly reduced in (b).

***

The nuclear trigger for X-ray bursts

X-ray bursts belong to the most fascinating of astrophysical phenomena.
They are explained as thermonuclear explosions in the outer atmosphere
of accreting neutron stars. The thermonuclear explosion is triggered by
a single reaction, 15O(alpha,gamma)19Ne which has been experimentally
determined for the first time after two decades of failed attempts. The
paper not only reports on the successful experiment but demonstrates the
impact of the results on X-ray burst ignition and X-ray burst
periodicity in the framework of a thermonuclear explosion model. The new
results define stringent boundaries for the actual accretion rate in
observed X-ray bursters.

***


Lightspeed? Not so fast

What would happen if light were slowed to the speed of sound?
Tasgal, Band, and Malomed looked at pulses of light (solitons)
in an optical fiber with a Bragg grating, i.e., a periodic modulation
of the index of refraction of the fiber. Bragg reflection off the
periodic modulation can act as a uniformly distributed mirror that
couples forward- and backward-moving photons. This kind of fiber
allows light pulses as slow as zero velocity, though the current
experimental velocities are a sixth the speed of light (but dropping).
Interactions with sound had not been included in studies of this system.
Near the speed of sound, the electrostrictive coupling of light and sound
waves can result in braking, bringing the light pulse abruptly from
supersonic to subsonic. For reasons not yet completely understood,
this sonic braking can decelerate the light pulses sharply down to almost
zero velocity. (See the figure.) This form of slowed or stopped light
can exist at room temperature, in relatively unexotic materials
(as opposed to in Bose-Einstein condensates, which exist near absolute
zero temperature). The coupling of the light to sound provides extra
complications for theorists to grapple with, but it may simplify the work
of experimentalists in bringing light to a stop.


Figure caption: A slightly supersonic soliton spins off sound waves
and thereby slows to a velocity well below the speed of sound. The
top panel shows the light intensity versus time and position in the
fiber, and the bottom panel shows the sound wave amplitude, with the
newly generated sound pulses propagating away from the surviving
subsonic soliton.

***

Low energy interaction of the ozone cation with electrons

Ozone is one of the most important triatomic molecules, since it absorbs harmful
solar ultraviolet radiation and therefore plays a large role in protecting life
on Earth. We have studied the low energy interaction between an electron and
the ozone cation. Such an interaction leads to the almost exclusive rupture of
all the covalent bonds in the ozone molecule. Due to the relatively high
exothermicity of the reaction, oxygen fragments are predominantly formed in
{^3}P and {^1}D states, though formation in the {^1}S state is highly
unfavourable. Oxygen in the {^1}D state is responsible for the red aurora and
red airglow.

***


Equilibrium Flux Patterns in Type-I Superconductors

Patterns play an important role in strongly correlated systems with long
range interactions where different phases can co-exist. In type-I
superconductors, topologically rich physics is observed in so-called
intermediate state that appears due to finite size of the system. For about
80 years a model first suggested by Lev Landau was used in textbooks on
superconductivity. In this model stripy (lamellar or labyrinth) pattern was
suggested. Any hysteresis in macroscopic response (such as magnetization)
was considered due to various imperfections or particular edge structure.
In this work we show that 1) topology of the intermediate state depends on
macroscopic sample shape. We show experiments in spheres, hemispheres and
cones where so-called geometric barrier is absent; 2) equilibrium pattern is
represented by flux tubes, not stripes; 3) the difference in pattern
geometry gives raise to topological hysteresis even in perfect samples.
We conclude that it is impossible to deduce equilibrium topology from energy
minimization arguments. We hope it will prompt development of new methods
in the physics of complex systems.

Real time visualization of magnetic fields is available at:
http://www.cmpgroup.ameslab.gov/supermaglab/video/Pb.html


***

A handle on chaos in electron billiards

Regular or chaotic motion of balls on billiard tables is “win” or “lose”. This paper investigates how electrons, restricted in their motion to nanometer-sized billiards, can win or lose, in technical words can increase or decrease the billiard’s electrical resistance. This “game” is controlled by the application of a variable magnetic field, switching the electron motion between predominantly regular and predominantly chaotic. The detailed interpretation of the experimentally observed quantity – the electrical resistance – involves classical and quantum-mechanical theoretical concepts and relates prominent structure in the resistance to regular or chaotic behaviour. Also, an intriguing question since the advent of quantum theory - the correspondence of these two concepts – is addressed and shown to be applicable to the electron billiards. Regarding technological aspects, the results are of great relevance for the development of nano-electronics within the next decade, when devices will become electron-billiard-like due to further miniaturization.


***

Giving Faraday's law a new spin

Faraday's law of induction is a basic principle of physics dating from 1831 and
which explains the electromotive force that drives electrical currents in
generators and transformers. In Physical Review Letters [date], Barnes and
Maekawa show that, for magnetic materials, there is a correction to this law.
Electrical currents usually correspond to a flow of electrons, elementary
particles that carry a tiny electrical charge. In 1930, Dirac showed that, in
order to be consistent with Einstein's special relativity and quantum mechanics,
an electron must also have "spin", in effect, be a tiny magnet. The above
modifications to Faraday¿s law occur when this spin is accounted for.
"Spintronics" is an emerging electronics technology in which the electron spin
is used in an essential manner. While the Barnes and Maekawa corrections are
small for transformers or generators, they embody the requirements of energy
conservation for the spintronic devices being developed by the likes of IBM,
Freescale and Hitachi. Barnes and Maekawa show that the corrected Faraday's law
can be elegantly restated in terms of the time rate of change of the so called
¿Berry phase¿.


***

Quantum Repeater

For long-distance quantum communication one must realize quantum
network via quantum repeater protocol, a combination of entanglement
swapping, entanglement purification and quantum memory. In a seminar paper,
Duan et al. proposed a promising quantum repeater protocol (DLCZ) with
atomic ensembles and linear optics. However, in DLCZ protocol entanglement
generation and entanglement swapping rely on single-photon Mach-Zehnder-type
interference, which is sensitive to phase instabilities. This drawback is
severe enough to make long-distance quantum communication impossible. In
this paper, we present a robust quantum repeater architecture building on
DLCZ protocol. The architecture is based on two-photon Hong-Ou-Mandel-type
interference which relaxes the long distance stability requirements by about
7 orders of magnitude, from sub wavelength for the single photon
interference required by DLCZ to the coherence length of the photons. The
prize which has to be paid for this benefit are spurious contributions of
states with higher number of excitations. We subtly design the protocol so
that the spurious contributions can be automatically eliminated during the
entanglement connection process. Our protocol provides an exciting
possibility for robust and realistic long-distance quantum communication.


***

Gating electrons at a free silicon surface

Gating electrons at a free silicon surface terminated with a monolayer
of hydrogen enclosed in a vacuum cavity provides a revolutionary
technique in which to create a high quality two-dimensional electron
system (2DES) in silicon. It has been known for some time that a
silicon surface terminated with hydrogen is an ideal electronic surface
with its atomic flatness and low number of defects. In this paper we
present the first systematic low temperature electron transport
measurements on a hydrogen terminated silicon surface whereby the
quality of our 2DES can be attested by the first observation of the
integer quantum Hall effect on a (111) orientated silicon surface.
Unlike 2DES created by conventional methods at an interface between
silicon and an amorphous glass (SiO2), the hydrogen terminated
silicon-vacuum interface results in an order of magnitude less disorder.
As a result we observe the ground state in silicon (111) surfaces to be
sensitive to the wafer orientation and in-plane magnetic fields. We
believe these results are seminal for two reasons: new ground states may
occur in the quantized Hall regime in a multi-component system such as
Si (111). Secondly because the 2DES is directly below the hydrogen
terminated silicon surface, a new breed of atomic-scale quantum devices
may arise whereby electrons can couple to molecules or atoms
specifically grafted on to the surface.

***

Polytetrahedral nature of the dense disordered packings of hard spheres
by A.V. Anikeenko, N.N. Medvedev


The paper is a new examination of the structure of non-crystalline
and partly-crystalline dense packings of hard spheres. It throws some light
on the unanswered question concerning the disordered dense packing is why
its maximum density is ~0.64. This problem is a "non-crystalline
equivalent" of the long-standing Kepler conjecture about maximal density
(~0.74) for crystalline packing of identical spheres which was solved only
recently. The performed analysis of computer models justifies the
polytetrahedral nature of the dense disordered packings of spheres, i.e.
the packings contain an appreciable fraction of tetrahedral configurations
of spheres that prefer to coalesce via their faces to form a variety of
locally dense aggregates which are inconsistent with the formation of a
lattice. Such tetrahedra are not perfect, the gaps between the neighboring
spheres may be as large as 25% of the diameter. These tetrahedra coincide
with the class of quasi-regular tetrahedra introduced by Hales in his proof
of the Kepler conjecture. The paper is of intense interest to both
mathematicians and condensed matter physicists in general. The model of
hard spheres is very successfully in trying to understand the structures of
liquids, glasses, colloids and granular matter.

***

Novel schemes of measurement-based quantum computation

Imagine one has a quantum system with many constituents prepared in a
certain entangled state in a laboratory. This could be a state of a
many-body system like of cold atoms in optical lattices or of atoms in an
array of cavities, entangled via a light bus. Could this state be used
for quantum computing, based merely on local measurements, but abandoning
the need for any unitary control to realize quantum gates?

In our work [1], we show that with a great deal of flexibility, many-body
states allow for quantum computation in such a fashion that fully
abandons the need for unitary control during the computation. The
celebrated result by Briegel and Raussendorf showed that local
measurements on a single very specific state, the so-called cluster state
[2], gives rise to universal quantum computation. This cluster state has
a number of interesting, but also rare properties. In our work, we hence
address the question: what if the state is simply not a cluster state,
but just some other state?

Our work opens up an avenue to follow such a line of thought: We find
many new models for quantum computing, and also many resource states that
have radically different correlation and entanglement properties. For
example, do we really need to have no long-range correlations in such a
resource state? This would be bad, ruling out many ground states from
scratch. But, fortunately, one can overcome this limitation. This
suggests that to some extent, the theorist does not necessarily have to
approach the experimentalist, asking for the preparation of a particular
state that may possibly be fragile with respect to decoherence effects.
But that the theorist may construct a computational model based on the
very state that can relatively feasibly be prepared.



***

How to make negative charge positive

In this paper we show that some negatively charged particles
may produce a positive charge density. An everyday experience tells us
that it is impossible. Indeed, ordinary particles like negatively charged
electrons produce negative charge density everywhere, any time.
However, the situation is different for W-bosons, fundamental particles
which mediate the weak interaction (that is responsible for
beta-decay of unstable nuclear isotopes).
The reason why W-bosons demonstrate this misterious behavior stems from
the fact that they are described by the relativistic quantum mechanics.
The spin S of W-bosons is large, S=1
(electrons have spin 1/2). In naive terms the W-boson rotates vigorously
around its axes. Precisely this strong rotation, combined with effects
of special relativily, leads to the puzzling phenomenon of the
"wrong sign" of the charge distribution. This effect does not contradict
the charge conservation, i.e. no charge disappered or created.
For example, W-bosons at rest have a given negative charge.
For fast, relativistic W-bosons there are some areas of space with
positive charge and some areas with negative charge, but the total charge
does not change.



***

Are protons getting lighter with time?

It is common knowledge that protons are 1837 times heavier than electrons.
Recent astronomical observations [Reinhold and others, PRL, 96, 151101 (2006)]
suggest that this ratio was even bigger in the past. If proven, that
would require reconsideration of the most fundamental physical theories,
including Einstein's general relativity.

In this paper we have shown that the proton-to-electron mass ratio has not
changed by more than 2.5 parts per million during the last 6 billion years.
We used microwave spectra from the galaxy B0218+357, which is
approximately 6 billion light years away from the Milky Way. These spectra
include famous ammonia transitions, which were used by Townes to build the
first maser [Nobel Prize 1964]. The frequency of these transitions appears to
be extremely sensitive to the mass ratio in question. Comparison of
ammonia lines with microwave lines of other molecules revealed no
relative frequency shift. This allowed us to place the most
stringent limit on the space-time variation of the proton-to-electron
mass ratio.

Phys Rev Hot Papers: 5-10-07

Photons help to confine electrons in graphene.

One of the most unusual quantum-mechanical phenomena that can be
realized in a graphene is the reflectionless transmission of electrons
through a potential barrier of arbitrary strength. This effect was
predicted in Ref. [1] and recently coined as the "Klein paradox" . The perfect
transmission of quasi-particles might lead to difficulties in confining electrons in
future graphene based electronic devices. In our paper [2] we have demonstrated that the resonant interaction of propagating quasi-particles with the radiation of a moderate intensity applied in the longitudinal (with respect to the interface) direction, leads to a pronounced suppression of the quasi-particle transmission through the n-p junction formed in a graphene. This effect occurs due to formation of a non-equilibrium dynamic gap between electron and hole bands in the quasi-particle spectrum. The value of the gap can be controlled by variation of the intensity of an external radiation. Propagation of quasi-particles is possible due to the non-equilibrium interband tunnelling. This specific type of the tunnelling results also in an N-type of current-voltage characteristics (CVC). The suppression of the quasi-particle transmission allows one to control confinement of electrons in diverse structures fabricated in graphene like, e.g., n-p-n transistors, single electron transistors, quantum dots, by variation of the intensity and
frequency of the external radiation.


***

What do advanced lightweight materials and the cytoskeleton have in
common?

Imagine a game of Jacks Straws in which the first player, having dropped the
straws, decides to glue them to each other where they touch, whilst
the other players have lunch. Later, the first of the young players
to try to remove a straw quickly realizes it is impossible without
moving any of the others. Being of a curious nature, the young
scientist decides to investigate the behavior of the clump of straws.
Are they all connected to each other, or can some straws be
separated? How much can the clumps be bent or stretched? In
this paper, we use simulations to answer these questions, and find a
transition from networks which are bendy to networks which are quite
stiff based entirely on the internal structure. Deciphering the role
of geometry in the mechanics of such networks can help us learn how
nature designs lightweight load bearing structures, such as spongy
bone or the cytoskeleton in cells. This, in turn, will ultimately
help us to design lightweight, yet sturdy, materials for a wide range
of applications.



***

Did the Universe undergo the stage of catalyzed nuclear fusion?

Three minutes after the Big Bang most of the light elements such as
hydrogen, helium, lithium (and their isotopes) were formed as the Universe
continued to expand and cool. This process, known as Big Bang
Nucleosynthesis, is well understood and has been used as a sensitive probe
of particle physics in the early Universe. In my paper, I show that in
many extensions of the Standard Model of particles and fields, a new
spectacular phenomenon will take place: the catalysis of nuclear reactions
by metastable heavy particles (e.g. charged supersymmetric particles) that
can interact via the electromagnetic or strong force. The Catalyzed Big
Bang Nucleosynthesis is triggered by the binding of nuclei with exotic
particles, and in particular leads to the explosive production of Li6
isotope at about 5 hours after the Big Bang. The reaction rate for the
synthesis of Li6 receives the catalytic enhancement of up to eight orders
of magnitude. Confronting the predictions of lithium abundance with
observations leads to the conclusion that even one such exotic particle
per 10^16 photons may significantly alter the nucleo-chemical history of
the Universe.



***

Reduction of quantum information loss due to strong driving


Are quantum computers a reachable vision? An essential requirement is that
one can identify suitable computational units, so called "Q-bits", which
keep their quantum state for a long period of time and that can be
externally manipulated as desired for computation. Unfortunately quantum
states change chaotically with time due to noise in the external
environment, which acts analogously to a friction force in the classical
world. This friction quickly destroys the information stored in a Q-bit. In
this paper it is demonstrated that for an important class of quantum
dissipative systems this loss can be significantly reduced. A prototype for
such a system is a semiconductor nano-sized dot where the friction results
from the interaction of the Q-bit with the surrounding crystal lattice. The
trick is to control the strength of the driving force. In contrast to common
expectations it is shown that in the class of quantum systems considered
here the friction is strongly reduced when the field is larger than a
threshold set by the properties of the environment. This result reveals
fascinating dynamical properties of an important prototype quantum system
and opens new perspectives to combat information loss in these systems.



***

Dynamics in Terms of Geometrically Defined Motions

Many mechanical systems occurring in nature follow equations of motion
that are nonlinear, often resulting in very complex patterns of motion
that are called chaotic. For this type of motion, nearby orbits,
corresponding to the setting of slightly different initial conditions for
the evolution, separate exponentially in time. The complex orbits of such
systems often appear to be a flow on geometrically defined surfaces.
Mathematical models of such systems are often given in the literature for
which dynamical flow follows geometrically determined orbits (geodesics)
in a very similar way to the motions of particles in a gravitational
field. The curved surfaces correspond to a geometry determined by a metric
function and a connection form, specified a priori in the model, fixing
the angles between lines and distances between points. Actual dynamical
systems are, however, not usually described in such terms. It is shown in
this paper that the standard form of the equations governing mechanical
motion can, quite generally, be cast into this type of model by
introducing a metric which induces appropriate scaling (conformal) on the
space of free particle motions. The resulting geodesics can be put into
correspondence with the actual motions of the particles by transforming to
a new connection form, a procedure which preserves the geometrical
interpretation of the theory. The computation of deviation of nearby
orbits, based entirely on the geometry of the space, is shown to provide
new useful criteria for instability which are very sensitive and widely
applicable.



***

Conduction Electrons Sense Devil's Staircase

A coupling between spins and electrons allows us to control resistivity by
magnetic fields. Indeed, applied magnetic fields can change perovskite
manganites from an antiferromagnetic insulator to a ferromagnetic metal. On
the other hand, frustrated Ising spin systems have been studied for decades
because of their interesting magnetism typified by the devil's staircase;
Magnetization, M, changes stepwise with having a value of MS/n as a function
of a magnetic field (MS and n denote a saturated magnetization and an
integer, respectively). Then, what happens if frustrated Ising spins coexist
and couple with conduction electrons? In this paper, we show one of the
clear-cut answers for this query, that is, giant two-staged
magnetoresistance with uniaxial anisotropy in SrCo6O11. In this material,
spin-polarized conduction electrons do sense the devil腦s staircase thanks
to a unique layered structure consisting of alternatively stacking
Ising-spin layers and conductive layers, which was confirmed by NMR studies
and band structure calculations. Our work demonstrates a prospect for novel
spintronic functions arising from a coupling between frustrated Ising spins
and spin-polarized conduction electrons.

Phys Rev Hot Papers: 5-07-07

How "relativistic" electrons in graphene avoid localization

Recent breakthrough in fabrication of graphene (monoatomic graphite
layer) and subsequent transport measurements revealed remarkable
electronic properties of this material. One of the most striking
observations is the minimal conductivity of undoped samples, which is
of the order of conductance quantum and stays constant from room
temperature down to 1K, showing no trace of quantum localization by
disorder. In this paper, we develop a theory of electron transport in
graphene with impurities that explain the experimental findings. The
following properties of graphene are crucial for our theory. First,
its low-energy states are split into two "valleys" in the
energy-momentum landscape. Second, the electrons in each valley
behave as massless relativistic particles. Third, experimentally
relevant scatterers in graphene -- charged impurities or so-called
ripples -- are of long-range character. As a result, the dominant
scattering cannot transfer the particle from one valley into the
other. We derive a quantum field theory describing electrons in this
situation and show that, due to the valley decoupling, the electrons
are characterized by very peculiar topological properties. As a result
the theory is at a novel quantum critical point, implying, in
partiuclar, that the conductivity is temperature-independent. This
criticality bears similarity with the famous quantum Hall transition
but occurs without any magnetic field! The non-trivial topology of
our theory explains also another striking experimental observation --
the anomalous, half-integer, quantum Hall effect in graphene in strong
magnetic field.

***

Enabling a Mother Qubit to deliver three lovely qubits

In quantum computers, interacting qubits perform computations.
In topological quantum computers, on the other hand, their offspirings
called topological qubits perform computations merrily. Their topological
character make them ghost like and oblivious to the material environment.
Quantum computation for them becomes a `merry go round', getting entangled
and picking up non-Abelian Berry phases, without ever losing their
coherence, even in an unfriendly and decohering environment.

Kitaev model, a remarkable, non trivial and exactly solvable 2 dimensional
quantum spin model exemplifies key ides of this topological quantum
computation. In our theoretical work on Kitaev model, where we present
certain exact results for dynamical properties, birth of a
Topological Qubit, from a `mother qubit' is made visible. We show that
changing the quantum state of one qubit results in the birth of a
triplet of topological qubits: an immobile `Siamese twin' of
`pi-fluxes' and a very dynamic `Majorana fermion' which frees itself
away from the mother qubit and is ever ready for quantum computation,
from the time of its birth.

Our finding will have a role in creating and organizing quantum computations
in the much awaited future quantum computers.

***

A Paul trap for neutral atoms

This paper presents the first trapping of ground-state neutral atoms in a macroscopic AC electric trap. Similarly to ions in a Paul trap (Nobel Prize, 1989), trapping is achieved here by alternating between two electric fields that result in a swinging motion of the atoms. For the first time, this motion in the trap is directly visualized using 2D images of the atom cloud.

The special feature of an AC electric trap is the ability to trap atoms or molecules in their ground state. The ground state is always attracted towards high electric fields, but it cannot be trapped in a static field, as static fields cannot possess a maximum in free space. Trapping in the ground state is therefore only possible when AC fields are used.

In any AC trap, an electric field is created with a saddle point at the trap center, resulting in attractive forces in one direction and repulsive forces along the other two directions. The electric field configuration is then switched to a second configuration in which the roles of the forces are reversed. Cycling between these two configurations leads to stable dynamic confinement of the particles.

***

Speeding up microfluidics

The term lab-on-a-chip summarizes the effort to miniaturize chemical
production lines into millimeter-sized devices. Due to the down scaling
of the dimensions, the fluid dynamics is altered and viscous forces now
take over inertia. This circumstance hampers rapid mixing in the
microchannels of lab-on-a-chip devices because now diffusion is the
dominant mechanism. A second challenge in microfluidic application is
the actuation of liquids. Currently the lab-on-a-chip devices rely on
additional fabrication steps and an elaborate integration of connections
and wiring, e.g. for pneumatic or electrical controls. An
interdisciplinary research team from the University of Twente (The
Netherlands) and Shimadzu Corporation (Germany) came up with a
revolutionary approach to overcome both limits. The team lead by
Claus-Dieter Ohl sped up microfluidics with single vapor bubbles thus
they surpassed the viscous constrains inherently found in lab-on-a-chip
devices. These short lived vapor bubbles are created with a focused
laser pulse of a few nanosecond duration. The miniature vapor explosions
accelerate the flow to 20m/s and more. Additionally, when the bubble
grows close to a channel wall liquid flow becomes focused and
accelerated to even higher speeds. This allows liquid pumping at fast
pace. And third advantage of the new technique implementing lasers
pulses is the easy applicability: on transparent lab-on-a-chip systems
any spot can be addressed just by positioning the laser spot thus
external connection are obsolete.


***

Emergence of collectivity in a few-electron system

The existence and nature of end and central plasmon resonances in a linear
atomic chain, the 1-dimensional analog to ¿surface¿ and ¿bulk¿ plasmons in
2-dimensional metals, has first been predicted by time-dependent density
functional theory. It is well known that electronic energies and
wavefunctions of such atomic chains are quantized, and can be understood
by the ¿particle-in-a-box¿ model. Whether these quantized states can
support collective electron oscillations has, however, not been
investigated. The calculation reported here shows the emergence and
development of collective excitations, as the chain length increases atom
by atom. In the long-chain limit, these collective modes converge to a
single longitudinal resonance and two transverse ones, which are localized
at the ends and center of the chains. These collective modes bridge the
gaps, in concept and scale, between the collective excitation in atomic
physics and nanoplasmonics. It also outlines a route to atomic-scale
engineering of collective excitations via atomic manipulation.

***

Dancing With the Spins

Using a novel device called the spin dynamo, researchers demonstrated that the
electron spins dance in a small stage with more than 20 different steps, rather
than the few steps we know before.

Watching the magnificent ball dancing in the fabulous Imperial Hofburg Palace in
Vienna, Austrian, is a very different experience as watching the ABC¿s TV show
of ¿Dancing with the Stars¿, where pairs dance in a compact studio stage. So is
for scientists studying the dance of electron spins in samples with different
dimensions. The dance of spins plays the most important role in operating your
computer hard disks and memory cells. Whenever you click on the mouse to save
your data, millions of electron spins in your computer disk dance collectively
to change their patterns, and thereby save the information.

Because everyone wants to have a computer that stores more data and faster,
scientists are given the task to study how electron spins dance in
nanostructures that are invisible to the naked eye. The fundamental questions
are how do electron spins influence each other (the collective phenomenon), and
how are they influenced by the finite stage where they are dancing (the boundary
condition problem). Find the answer is the major challenge of two emerging
disciplines known as the ¿spintronics¿ and ¿nanomagnetism¿.

Now Can-Ming Hu and his colleagues at the University of Manitoba in Canada has
created a novel device called ¿spin dynamo¿ [1], which generates current from
the dance of electron spins. Using this device, they have made a break through
in studying how electron spins dance in a small piece of magnet, where they
have observed more than 20 different ¿steps¿ of dancing for the electron spins
[2]! Previously, only a few steps of them have been seen. The observed new
phenomena allow the team to precisely explain how the size of a finite stage
influences the dance of spins, which is a long standing problem in the field of
magnetism, and it¿s solution is pivotal for the architecture of the next
generation computer memories.