September 18, 2009

ED10563


Lava Lamp Physics

Lava lamps are widely known and beloved gadgets decorating living rooms
and offices. Besides the commercial products, hundreds of recipes can be
found in the web on how to tinker a working piece at home. It might be
surprising, but the public literature contains practically nothing about the
physics of lava lamp convection. This work reports on experiments with
a laboratory specimen designed to permit quantitative measurements, since
the main ingredients (silicone oil and ordinary salt solution) are chemically
stable, non-toxic, and heat resistant. The dynamics is found to be quite
regular in the given parameter range. The characteristic behavior is a single
blob exchange, where a superheated ball of silicone oil rises from the bottom,
sticks to the top, cools down and sinks back to the bottom. One period is
shown in the picture, the time stamp format is min:sec. This two-fluid system
seems to be very simple, however a precise physical understanding
represents a real challenge: all the material parameters (density,
viscosity, heat transfer properties, and interfacial tension) have nontrivial
temperature dependence and apparently essential role in the dynamics.

***

BE11433

Universal limit of below vacuum thermal conductance in multi-layer
photonic crystals

Vacuum is commonly thought to be the best thermal insulator. Can we
engineer the vacuum to achieve even lower thermal conductance?
Intuitively, since heat is entirely carried by photons in vacuum, the
vacuum thermal conductance can be suppressed by using photonic band gap
nanostructures. The simplest of such nanostructures is the multi-layer
photonic crystal, consisted of alternate layers of vacuum and dielectrics.
In general, the overall thermal-conducting behavior is determined by the
detail geometry of the crystal, and it would be interesting to find out
the extent of the best achievable thermal insulation, and the corresponding
structural design. In this paper, we derive the analytical expression for
the lower limit of normalized thermal conductance with respect to vacuum
for the multi-layer photonic crystal, where the best thermal insulation
occurs. This limit, however, is universal, since it is independent of the
relative thicknesses of the layers, and depends only on the choice of the
dielectric material. Strikingly for such crystals, while highly
thermally-insulating, are optically transparent for narrow bands of
light with spectrum outside the photonic band gaps. From theoretical
viewpoint, such geometric independence reveals the deep fact that the
distribution of the underlying photonic bands in frequency space is
ergodic.

Figure Caption:
Normalized thermal conductance of silicon-vacuum multi-layer photonic
crystal versus normalized temperature, for different relative layer
thicknesses of the silicon and vacuum layers. The lower limits of all
curves converge to the same value, where the best thermally-insulating
effect occurs. The inset is the geometry of the structure.

***

LG12505

Does the Universe change as fast as it can?

Our work supports the idea that the very early universe shows
behavior in periods of rapid change that can be mimicked in systems
undergoing rapid change in the laboratory, in that each change as
fast as they can.

When we change a system it cannot respond in its entirety
immediately. There is a maximum speed (e.g. speed of light) at which
information about one part of the system can reach another
part. This is particularly true in the first millionth of a second
of the universe, when rapid cooling turns its primordial constituents
into the ingredients of everyday matter. If the universe changes as
fast as it can, it will only be uniform in domains over which
information about its state can be communicated in the time it takes
to change, which can lead to the formation of intergalactic 'defects'
at the boundaries. We have looked at simpler changes in the
laboratory, cooling conductors into showing superconducting
behavior. By counting the defects (magnetic flux lines) we have
shown that our laboratory system does, indeed, change as fast as it can.

September 14, 2009

BG11360

First True Atom Images

Since the concept of the atoms as the fundamental unit of matter was introduced by Democritus and Leucippus nearly two and a half millennia ago, scientists have attempted to image individual atoms. The most significant microscopy milestone that was achieved in the last century was the imaging of individual atoms with field-ion microscopy (FIM) by Muller and Bahadur (1956). The FIM is an outgrowth of field emission electron microscope (FEEM) invented in 1936 by Erwin Muller in Gustav Hertz’s laboratory. Today, there are three atomic resolution microscopes – FIM, high resolution electron microscopy and scanning tunneling microscopy. However, images of the single atoms look like relatively wide structureless spots, and hence it is more argued to consider such a situation in recent atomic-resolution microscopy as detecting a single atom rather than obtaining its real image. To date, there have been no reported experimental observations of the spatial form of the atomic orbitals. The experimental evaluation of the electronic orbitals of atoms is one of the most long standing problems in quantum physics, material science, and nanotechnology. The paper presents, for the first time, the direct, sub-angstrom-scale, real-space experimental determination of atomic wavefunction. Here we show that a recently developed high-field technique has made it possible to attain the ultrahigh resolution FEEM, which can be used to direct imaging the electronic orbitals of single atoms, i.e. the internal structure of atom.

***

LE12679

ATOMIC CLOCKS SLOW DOWN NEAR WALLS

An Aussie-American team discovered that atomic clocks slow down near walls. For modern clocks the effect is appreciable. The researchers predict that in a very close proximity to surfaces, each second measured by the clock will be off by a 10 billionth fraction of a second. While seemingly small, this slowing down is gigantic in the precision world of modern timekeeping, as the best clocks boast fractional accuracies that are million times better. Slowing of atomic clocks depends on the distance of the clock atoms from the wall. As the separation is increased to about 10 micrometers, the effect becomes negligible. Yet, over this small distance, the feeble force of attraction between the atoms and the wall evolves through three distinct physics laws, spanning nearly 150 years of theoretical developments. Researchers find that atomic clocks may be able to map out these laws in a single experiment, the feat none of the previous attempts has been able to accomplish. Researchers hope that ultimately the clocks may help in discovering new physics laws, such as non-Newtonian gravity, predicted to dwell in close proximity to the walls.

***

LE12265

What does a kayak paddle and a bacterium have in common?

A great deal, it turns out. In this paper, we demonstrate that
non-flagellated /E. coli/ strains exhibit closed rotational orbits as
they drift near a surface in the presence of flow, replicating the
periodic motion of a kayak paddle. For the first time in this field, we
precisely characterize these orbital trajectories as a function of
bacterial length and height from the surface. The precision in the data
is made possible by a combination of microfluidics and sophisticated
computer vision algorithms developed in our laboratory, enabling
tracking, automatic analysis and averaging of tens of thousands of
bacterial trajectories. Interactions between the cell bodies and a
surface in the presence of flow is but one component of bacterial
motility, but our data will allow the formulation and calibration of
hydrodynamic models to better understand bacterial migration. For
instance, hydrodynamic surface effects provide a stabilizing influence
on the trajectories of motile bacteria, keeping them near the surface
and leading them to find quiescent routes to swim upstream under a wide
range of flow conditions. Understanding the physics behind these effects
could potentially lead to breakthroughs in the prevention of bacterial
migration and eventual pathogenesis without the need for antibiotics.

The attached picture shows a composite image of E. coli bacteria drifting at slightly different heights (and hence, different speeds) in shear flow near a surface. The periodic, kayak paddle-like motion is clearly visible. The orbital period depends on the length of each bacterium, as well as its height from the surface.


***

LD12112AR

Silicon photonic wires are shown to self-align by the forces of light they carry

Laser light may have significant mechanical effects on microscopic objects. In this paper, we theoretically demonstrate that a silicon photonic wire that is broken by a gap and an offset may tend to self-align by the forces generated by the very light it guides. The two parts of the silicon wire, each of cross-section dimensions of a few hundred nanometers, tend to bend to form a continuous wire. Conversely, depending on the geometrical parameters, the light flowing inside the wire may cause the two parts to deflect away from each other. These novel effects we present may be used for nanoscale machines on a silicon chip. The results suggest the guided light may hold a tiny silicon wire cantilever in stable equilibrium or vibrate it, with a promising application in sensing of nanoscale objects.

Sept. 10, 2009

LF12003

Single-Electronic Radio-Frequency Refrigerator

We demonstrate experimentally that a hybrid single-electron transistor with superconducting leads and a normal-metal island can be refrigerated by an alternating voltage applied to the gate electrode. The simultaneous measurement of the dc current induced by the rf gate through the device at a small bias voltage serves as an in situ thermometer.


***

Accepted PRL

Atomtronic circuits of diodes and transistors

We illustrate that open quantum systems composed of neutral, ultracold atoms in one dimensional optical lattices can exhibit behavior analogous to semiconductor electronic circuits. A correspondence is demonstrated for bosonic atoms, and the experimental requirements to realize these devices are established. The analysis follows from a derivation of a quantum master equation for this general class of open quantum systems.

9-9-09

LG12079

Random Walks in the Park

Habitat conservation areas must allow
enough room for animals to survive: one must ensure that their home range
is preserved. Relying on the analogy between the monitored positions of an
animal searching its habitat, and a path drawn at random, this paper
provides a formula to compute a mathematical estimate of the home range,
be it for individual animals or for herds. Most interestingly, this
general result reveals that, in the elementary model where individual
animals move around independently and food supplies have not reached their
limits, the size of the home range grows very slowly when the group
becomes larger and larger. Indeed, going from 10 to 100 individuals will
only double the home range of the herd - whatever the species. Such a
universal conclusion illustrates the power of the conceptual tools that
mathematicians and theoretical physicists have developed to treat random
processes. A seminal point in the development of this toolbox was
Einstein's 1905 description, through molecular collisions, of the anarchic
motion exhibited by pollen grains in water. This is now known as Brownian
motion, in memory of the British botanist who reported the observation.
From botany to habitat conservation - maybe not so much of a random walk
after all!


***

LZ11347

Tiny Black Holes may leave relic radiation behind them...


Some theories predict that tiny black holes -- weighing from less
than an ounce to millions of tons -- form immediately after the big
bang. In a recent paper, we showed that the tiniest of these black
holes would generate a distinctive background of gravitational
waves. All black holes leak energy via a process known as Hawking
radiation: the smallest black holes evaporate completely in less than
a second, black holes weighing as much as a star survive trillions of
times longer than the present age of the universe. Small "primordial"
black holes decay so fast that it was thought they left no relics
behind them. We showed that gravitational wave emitted as the black
holes evaporates would survive until the present day. These
gravitational waves have very high frequencies and could not be seen
with any detector currently on the drawing board. However, this is
the first time anyone has identified a "signal" generated by a
population of tiny black holes in the very early universe, and one day
it might be used to test models of the big bang which predict that
these black holes exist.

***

LF12857ER

The best place to survive

A prey, hunted by a population of "blind" predators moving randomly in a
complex labyrinth (network), has to choose a place to stay. Are there places
better than others to survive as long as possible? In this paper it is shown
that the answer depend on the large scale geometry of the labyrinth: if it
is such that a predator returns to its starting point with certainty
(recurrent network), then all sites, at large times, are equivalent, while,
in the opposite case (transient network) the survival probability can
strongly depend on the chosen site, and it is possible to study "geometric
strategies" in order to survive longer. This result applies to a large
variety of different problems and situations, ranging from chemical
reactions in disordered media to biological processes in human body and
information spreading in social systems.


***

LX11561

An entangled photon photonic machine gun

Photons are like teenagers - they are obsessed with the question of
whether they are the same or different as their peers. For this reason
individual photons of light are actually hard to create - the photons
from lights and lasers like to bunch together. However, because single
photons would be an important resource for quantum communication and
computation, there is a considerable worldwide effort to produce them.
In practise having these single photons would only be the first step -
there would then be a long and complicated process to manipulate their
quantum states so they become "entangled". Entangled quantum states are
the driving resource of quantum information theory. We outline a
technique which will enable certain single photons sources to produce
entangled states directly, at a high repetition rate, thereby
circumventing many of the obstacles on the road to building an optical
quantum computer.

***

LE12747


“PHOTONS REVEAL THEMSELVES”

What do photons look like? Researchers have provided insight into this question by studying the interaction between a pair of photons in a nonlinear crystal. By varying the degree of overlap of the photon pair members, while monitoring the strength of their interaction, it is possible to see behavior previously unobservable. The physical principle is identical to that used to characterize ultrafast laser pulses, but the novel aspect of this work is that it is conducted at the photon level. Pairs of photons are prepared in entangled quantum states using standard experimental techniques, and one photon of each pair is subjected to a controlled time delay. The pairs are then reunited in a nonlinear crystal, where the pairs are combined into single photons through upconversion. By varying the time delay while monitoring the upconversion rate, a 'temporal portrait' of the photon pairs is obtained. The results thus demonstrate an ultrafast correlation between
the members of each pair, with a temporal width of 28 femtoseconds, corresponding to a correlation length of only 8 microns.

***

EDJ1044

Percolating Cities

In this work we model the formation of a city. Based on empirical
findings, we show that the city emerges in a process that can be
successfully described as a two-dimensional bond percolation.
In the presented approach, the evolution of the urban system is driven
by the growth of the road/street network that brings the system to the
point (the percolation threshold) where it changes its structure and
transforms from the collection of separated settlements into a new
interconnected structure – the city. In this respect, the city formation
resembles physical phenomena, such as diffusion in disordered media,
polymerization, forest fires, or conduction, that are modeled within
the bond percolation theory.
The process leading to the city was deduced by analyzing the way people
divide their land into the smallest units – the land parcels.

***

LH12029

Might black holes reveal their inner secrets?

Black holes harbor a spacetime singularity of
infinite curvature, where classical spacetime
physics breaks down, and current theory cannot
predict what will happen. However, the singularity
is invisible from the outside because strong gravity
traps all signals, even light, behind an event horizon.
In this paper we show, using a simplifying approximation,
that it might be possible to destroy the horizon, if matter
is tossed into the black hole so as to make it spin faster
than a certain limit. One could thus expose a "naked" singularity
unless, as we suspect, effects beyond our approximation
intervene to protect the horizon.

September 3, 2009

EZ10465

When a little noise can be harmful to neurons - an effect opposite to stochastic resonance

Noise has often been found to increase activity in dynamical systems.
Sometimes a maximum effect is found at a particular noise amplitude, a
phenomenon called stochastic resonance.
We report the discovery of the opposite effect where noise inhibits
neuronal spiking activity and a minimum occurs at a particular noise
strength. Such a phenomenon can be
referred to as /inverse stochastic resonance./ We discovered this
while investigating the effects of noise on periodic firing in a
nonlinear system frequently used to model neuronal activity.
As the mean current strength (in microamperes per square cm) increases,
repetitive spiking occurs at a critical value.
The firing behavior was studied as a function of the mean and variance
of the input current, firstly with initial resting conditions. Noise of
a small amplitude can turn off the spiking for values of the mean
current close to the critical value, and the number of spikes undergoes
a minimum as a function of the noise level. The observed effects of
noise are expected to occur in diverse fields in systems with the same
underlying dynamical structure.

September 2, 2009

LB12021ER

Cells, cancer and rare events

Enormous strides have been made towards understanding the molecular and
genetic origins of cancers. At the same time something of a mystery
remains in the incidence rate of lung cancer in ex-smokers. Detailed
analysis of the dynamics suggests that part of the mechanism at least
may have nothing to do with genetic changes in DNA. In this paper a
possible mechanism is examined, which the author terms "homeostatic
metastability". The idea is that the clinical appearance of cancer may
be a random, rare event arising from the collective behaviour of the
cells, a bit like the way bubbles appear in a fizzy drink or ice
crystals nucleate in supercooled water. At the moment, homeostatic
metastability remains an intriguing possibilitity, put forward as a
hypothesis to be supported or knocked down by experimental evidence. If
it should prove to be a factor in cancer though, it opens up interesting
possibilities for novel treatment regimes.


***

LG12379

First Bose-Einstein condensate of an alkaline earth element - Matter wave
meets optical atom clock

Bose-Einstein condensates (BECs) as a source of coherent matter waves have
been used in the past years for a variety of measurements in fundamental
quantum mechanics as well as a model system for solid state physics and
for quantum information. Most BECs are made from alkaline atoms sharing
one disadvantage: For optical transitions they have a broad line width
i.e. they can be excited not only by a single frequency but by frequencies
in a range of several megahertz. The energy uncertainty related to this
line width is large compared to typical energy scales in a BEC as
temperature, photon recoil, chemical potential, or trap level spacing.

For the first time, a BEC of alkaline earth atoms has been produced. The super-narrow intercombination lines of this class of atoms allow optical excitation
with high precision and make them candidates for optical clocks. Combining
this feature with the coherent matter wave of a BEC does not only promise
new measurements on matter light interaction but can also be used for
precision spectroscopy of the properties of a condensate or for new
interferometric sensors for various kinds of forces, e.g. gravity.

LF12828


Broadband electromagnetic cloaking in the microwaves and visible

In our recent article “Broadband electromagnetic cloaking of long
cylindrical objects” (to appear in Physical Review Letters) we have
shown how very simple metallic parallel-plate structures can be used to
cloak or, in other words, to make “invisible” e.g. cylindrical metallic
objects. Two designs are presented: one operating in the microwave
region and the other in the visible part of the electromagnetic
spectrum. Operation of both structures is confirmed with numerical
simulations and the microwave device is also realized and measured. The
results confirm that this new cloaking phenomenon can be realized with
very simple structures and the designed devices are shown to operate in
relatively wide frequency bands.

***

BE11444

Scientists Solve Mystery of Glass Flow

A popular urban legend concerns the apparent flow of stained glass windows in medieval cathedrals. This problem is of critical importance for modern industrial glass, particularly the ultra-thin glass sheets used in liquid crystal displays (LCDs), where such flow can lead to unwanted dimensional changes during the LCD manufacturing process. In a newly published paper, “Nonequilibrium viscosity of glass” (Phys. Rev. B), a trio of scientists have conducted the first-ever thorough investigation of viscous flow at temperatures below the glass transition. They present major advances in the underlying theory of nonequilibrium viscosity, developing a new model which accounts for the full thermal history dependence of glass flow behavior. Using an internally designed beam bending apparatus capable of accurate viscosity measurements well below the glass transition, the authors present a detailed validation study of Corning’s EAGLE XG glass, the most popular glass used in today’s large-scale LCD televisions. The new theory unveils a striking relationship between the history-dependent viscosity of the glass at low temperatures and the high-temperature viscosity of the equilibrium melt.

***

BTR1060BJ

Coiled Nanotubes

Carbon nanotubes are promising materials for the future. It would be
interesting to study what would happen when it is curved or coiled. In this
paper, we report that ring shaped nanotubes would behave differently than
straight ones. In our experiment, we investigate many rings with different
diameters using laser spectrum. We found that rings would show more peaks in
their spectra. The smaller, the more peaks. Usually graphene sheet have only
one peak in its Raman G band, when it is rolled up and forms a tube, there
would be two peaks in the G band. In our case, the tube is further rolled up
to form a ring and the number of peaks increases to six. This interesting
phenomenon may promote a deeper understanding of the mechanism of the spectrum
of carbon nanotubes and related materials like graphene and graphite.
Currently, we attribute the increasing of peak number with additional
curvature to the changes in electronic structures resulted by the residual
strain during the formation process of ring structures.

August 24, 2009

EF10631


Effective bacterial micromixers

Scientists have discovered that commonly found bacteria such as Bacillus
Subtilis are in fact highly efficient mixers for the liquid they live
in. Thanks to original non-invasive optical coherence tomography (OCT)
developed by Imalux Corporation, OH, the scientists observed with
unprecedented precision the phenomena unfolding in the liquid containing
the bacteria. The measurements revealed up to 100-fold increase of
mixing and Oxygen intake rates due to coordinated swimming of the bacteria.

The study sheds a new light on possible survival mechanisms developed by
bacterial colonies under harsh conditions. In addition, the results are
important for fundamental and technological reasons, from understanding
collective motion in groups of interacting animals such as bird flocks
and fish schools to miniature bacteria-powered mixers and reactors.

Image illustrates three-dimensional distribution of bacteria obtained by
the OCT scan

***

LW10950AR

Space Time Sensors Juggling With Multiple Ultracold Atomic Waves


Thanks to their ability to measure time with an extreme accuracy, optical
atomic clocks are of great importance for modern physics [1]. The best
clocks to date control the atomic motion by trapping the sample within
optical lattices and then probe the atomic transition by shining on
these atoms a distinct laser of controlled frequency. In order to perform
both operations simultaneously and with the same laser field, we explore
in this paper a different strategy: using fine-tuned laser pulses, one can
perform a quantum juggling with a Bose-Einstein condensate and exploit the
resulting interferences to enhance the measurement sensitivity. The
condensate goes through an unusual levitation process: it is split into a
myriad of wave-packets exploring a network of paths, thereby experiencing
simultaneously a controlled diffusion in altitude and a localization in
momentum. Thanks to the chosen geometry, this proposal combines the best
aspects of optical clocks based on atom traps and on atom
interferometers. This system is also able to measure accelerations. It
represents an attractive alternative to current atom gravimeters and
atomic clocks.


***

LD12391

Quantum Limit for Probing Quantized Energy Levels of a Mechanical
Oscillator

Recent novel experiments with electromechanical and optomechanical
systems unveilpossibilities of exploring quantum behavior of a macroscopic mechanical oscillator. If energy levels of a mechanical oscillator were observed to be quantized, this will give us an unequivocal sign of quantumness of a macroscopic object. Motivated by the pioneering work of Thompson et al., we derive a standard quantum limit for observing energy quantization in systems with a mechanical oscillator coupled parametrically to external degrees of freedom. In order to successfully probe the quantized energy levels, the mechanical oscillator needs to strongly interact with the external degrees of freedom. In the case of optomechanical system, as intuitively expected, it requires zero-point motion of the oscillator to be comparable to the linear dynamical range of the optical system. This condition indicates the threshold when nonlinearity in the system plays a significant role. Interestingly, it is also the point where momentum kick by a single photon exceeds zero-point fluctuation of the oscillator momentum, allowing realization of macroscopic quantum supposition [3]. Therefore, if this condition is satisfied, many fascinating nonlinear and non-Gaussian properties of the optomechanical system will show up in the quantum regime.


***

LA12165Z

Making bright electron beams for compact x-ray lasers

A novel method to accomplish highly-brilliant electron beams required for coherent x-ray generation in a compact x-ray free-electron laser (XFEL) is presented. Since the nonlinearity of electron beam compression limits the attainable peak current, a correction cavity operated at a high-harmonic frequency of a main accelerator is conventionally used for nonlinearity compensation. However in a compact XFEL using a high-frequency main accelerator, a conventional scheme encounters a technological difficulty of an extremely high-frequency microwave system. In this paper, a novel nonlinearity correction scheme is proposed for a compact XFEL, in which an effective frequency up-conversion of a correction cavity obtained in the beam compression is directly used. As a result, the frequency of the correction cavity can be decreased to the same frequency as the main accelerator. Derived analytical formulae and simulations confirm the successful generation of highly-brilliant electron beams in a compact XFEL. This new scheme will become a key technique to downsize the scale of the facility, which is an essential issue for widespread application of coherent x-ray light sources.

***

LE12697

How perfect can graphene be?

We have identified the cyclotron resonance response of purest
graphene ever investigated, which can be found in nature on the surface of
bulk graphite, in form of decoupled layers from the substrate material.
Probing such flakes with Landau level spectroscopy in the THz range at
very low magnetic fields, we demonstrate a superior electronic quality of
these ultra-low density layers (close to 10^9 cm-2), expressed by the
carrier mobility in excess of 10^7 cm2/(V.s). These parameters set new and
surprisingly high limits for intrinsic properties of graphene and
represent an important challenge for further developments of current
graphene technologies. Graphene samples with mobilities comparable to the
nowadays highest-mobility semiconductor devices thus seem to be
achievable. Intriguingly, electronic states in such high-quality graphene
could be quantized into Landau levels by magnetic fields as low as the
field of the Earth.

August 18, 2009

LE11966

When atoms line up on a semiconductor surface

Starting with the pioneering research of Eigler et al. in the early
1990s, scanning tunneling microscopy (STM) at cryogenic temperatures
opened up the possibility to place single atoms at selected positions at
a surface. Since then, STM-based atom manipulation has been achieved
mainly to metal surfaces. In this paper, we show for the first time the
reversible repositioning of adatoms on a semiconductor surface by
vertical atom manipulation and study the elementary steps of the
process. Vertical manipulation allows us to assemble on-atom-wide chains
by adding one atom at a time and to follow the emergence of confined
electronic quantum states by scanning tunneling spectroscopy. Our
results demonstrate that the combination of atom manipulation and local
spectroscopy is capable to explore the effect of interatomic coupling in
atomic-scale quantum structures on semiconductor platform. This approach
appears as very promising to analyze model systems that aim at
individual dopant atoms as functional units -- such as, e.g., the
concept of quantum computation utilizing dopant-based coupling schemes.

***

LY117094B

High-capacity hydrogen storage in calcium-decorated carbon nanotubes

Hydrogen storage at appropriate density in solid-state materials can be an essential requirement for the development of hydrogen fuel-cell powered vehicles. However, feasible candidate materials are scarce. In recent years, theoretical studies of early transition metal (i.e., Sc, Ti, and V) decorated carbon nanotubes (CNTs) have attracted much attention as possible systems for hydrogen storage applications at room temperature and ambient pressure. However, based on energy consideration, transition metal atoms generally prefer being clustered to being individually dispersed on nanomaterials. This is a major problem when trying to produce metal-decorated hydrogen storage nanomaterials.
In this paper, using Ca atoms instead of transition metal atoms, we demonstrate that individually dispersed Ca-decorated boron-doped CNTs can serve as a high-capacity hydrogen storage medium that operates at room temperature and ambient pressure. Unlike transition metal atoms, calcium clustering is suppressed on B-doped CNTs, and individual Ca-decorated B-doped CNTs can reach the gravimetric capacity of ~5 wt % hydrogen, which is close to the U.S. Department of Energy (DOE) goal of 6 wt %.


***

LC12687

Supernovae shine longer

The structure of atomic nuclei is important for understanding
astrophysical phenomena like supernova explosions or the formation of
chemical elements in the Universe. Therefore, scientists explore rare
isotopes of an element: A chemical element can have several isotopes
differing in the number of neutrons in the nucleus. Exploring a
radioactive iron isotope with 34 (usually 30) neutrons, a group of
scientists found a surprise: It is much longer active than thought, its
half-life being 2.6 million years instead of previously measured 1.5
million years.

The iron isotope Fe-60 was present in the early solar system and acted
as a heat source in freshly formed planets. It has been found at the
ocean floor as a leftover from supernovae near the solar system a few
millions of years ago. Further, the radiation of Fe-60 originates from
stellar sources in our galaxies. For all these processes, the accurate
measurement of the radioisotopes activity is critical.

The method the scientists used to measure the activity of Fe-60 was
quite unusual: The sample was chemically extracted from the "garbage",
i.e. a beam dump at the Paul Scherrer Institute (PSI) in Switzerland,
which had been irradiated with high-energy protons for 12 years. At the
underground laboratory of the Technische Universität München scientists
looked for the slow increase of Co-60 (Cobalt) from the decay of Fe-60
almost for three years, at PSI they measured the number of Fe-60 atoms
with a special mass spectrometer - and thus deduced the new half-life
value of 2.6 million years.

***

LB12374


Revealing primordial magnetic fields through ripples in the relic radiation.

Magnetic fields are ubiquitous in the cosmos and
could arise in the very early universe.
We have developed a novel probe of primordial magnetic fields using
the statistics of the relic radiation from the early universe, known as
the Cosmic Microwave Background (CMB).
The CMB is nearly uniform in all directions, but has small fluctuations
at the level of about 10 parts in a million, some of which
may arise from primordial fields.
In most theories, to the leading order, the statistical distribution
of these CMB anisotropies obeys what is called a Gaussian
distribution. However, primordial magnetic fields intrinsically lead
to a statistically
non-Gaussian distribution, which
is sensitive to the field strength and nature.
We have shown that currently
observed limits on the CMB non-Gaussianity imply
an upper limit of about 35 nano Gauss for these fields.
Although this field is a hundred million
times smaller compared to that on the surface of the earth,
it can significantly influence cosmological evolution.
Our study demonstrates that future observations
will help place more stringent constraints on
these fields and can also lead to its detection.
Detecting such fields will provide a window
on the physics of the early universe.


***


LD12642

Scaling up microfluidic devices: a Lego obstacle course for steel balls.

One important challenge of microfluidic devices for “lab-on-a-chip”
applications is to design methods to sort accurately and rapidly
particles, cells, or molecules by size. A remarkably simple way to
achieve particle separation recently demonstrated is to flow a mixture
through a periodic array of obstacles, which surprisingly makes
particles of different sizes move in different directions. In this
paper, the governing mechanisms underlying the observed separation
were revealed using a scaled-up version of the microfluidic system in
which steel and plastic balls move through an obstacle array build out
of LEGO pegs. These macroscale experiments eliminate the random motion
present in colloids thus showcasing the deterministic (predictable)
nature of the method. Tracking the motion of individual particles
shows that separation is, counterintuitively, induced by small,
irreversible displacements that take place as the particle moves
around the obstacles, but which are amplified by the periodic nature
of the array. Many trajectories could easily be probed for different
angles of the driving force due to the simplicity of the experimental
system, which revealed that the observed dynamics was part of a large
class of physical systems that display phase-locking behavior.


***

EDR1039E

Branching Process in a Stochastic Extremal Model

How the extinction of one species disturbs the the whole ecology -
a question that is attempted to answer for a long time. Using the
spirit of Darwinian principle of survival of the fittest Bak and
Sneppen argued that repeated extinctions makes the ecological
evolution a Self-Organized Critical (SOC) process. In this paper we
show for the first time using numerical evidence that when an extincted
species triggers mutations stochastically to its neighboring species a
SOC process is guaranteed irrespective of the specific structure of the
underlying network of different species, whether it is scale-free,
small-world or has a regular geometry. We also argue that such a state
is possible as long as the branching factor of the triggering process
is larger than unity.

***

LG12384

Nothing is faster than optical precursors

In this paper, we report the observation of optical precursors, for the first time, clearly separated from a main pulse when it passes through a dense cold atomic ensemble. Optical precursors, the propagation of the transient wave front of a step-modulated light pulse, always travel with the speed of light in vacuum. Predicted by Sommerfeld and Brillouin about 100 years ago, nothing can travel faster than the optical precursors in a dispersive medium. However, all previous claims about experimental observation of optical precursors could not show clear separation between so called precursor and main pulse, thus provide no solid evidence for Sommerfeld and Brillouin's long-standing predictions [1, 2, 3, see also comments 4, 5, 6]. In this paper, using advanced cold atom technology and electromagnetically induced transparency, we successfully generated optical precursors without mixing with the unabsorbed main pulse. Our result supports that the information velocity does not violate the Einstein Causality and may be different from the group velocity.

LU11963E

Binary Droplet Collision at High Weber Number

Droplet collisions are relevant to miscellaneous nature and practical problems, of which splashing of multiple droplets may be one of the most astonishing outcomes impressing the public. While such phenomena have been studied extensively in droplet impact upon a surface, they have rarely been investigated in collisions between two droplets (as an elemental unit of related structure such as spray processes), partly due to the difficulty to achieve the desired experimental conditions. In this work, by using techniques for creating high-speed droplets, we found new regimes of binary droplet collision with relatively large impact inertia (head-on between two stable droplets at 23 m/s) and identified the transition boundaries for various break-up mechanisms that shed light on the fundamental structure of droplet collision dynamics and further application to industrial purposes. Various liquids were also used to explore the effects of viscosity and surface tension.

***

LE12526

Has PAMELA really detected dark matter?

"Scientists have detected particles that may come from invisible dark
matter" said the BBC on 1 April 2009. This was not an April Fool joke
but a reference to the observation by the PAMELA satellite of high
energy positrons in excess of what is expected from cosmic ray
collisions with interstellar matter. The annihilation of dark matter in
the Galaxy naturally generates such positrons so this result attracted
wide interest. It was noted however that nearby pulsars can also emit
such positrons. A third possibility is that the positrons are indeed
cosmic ray secondaries but accelerated by the same shock wave that
created their parent particles.
In this paper, we outline a test to discriminate between these models.
Apart from positrons, rare nuclei like boron are also created by cosmic
ray collisions. If PAMELA finds their flux to follow the relative
increase in the positron flux with energy then the third possibility
above is the correct one. Even if the dark matter explanation is ruled
out, there is the equally exciting prospect of establishing an
accelerator of cosmic rays nearby. This may soon be directly seen in TeV
gamma-rays or neutrinos by experiments such as HESS, MILAGRO and IceCube.

***

LE12232

SAILING BEFORE THE LIGHT

Just as the wind boosts a sail, the pressure of light may accelerate a
reflecting object. A few years after the invention of the laser, the
idea of a
rocket boosted by an Earth-based laser system was proposed for interstellar
travel. With the recent advent of ultrapowerful laser systems the "Light
Sail"
concept, scaled down to lengths of a few microns and times of one
millionth of
millionth of a second, may yield acceleration values similar to those
near the
surface of a Black Hole and provide compact sources of high-energy ions for
applications. In this paper it is shown that the relativity textbook
model of a
light-accelerated mirror is surprisingly accurate in predicting the final
velocity of a laser-driven ultrathin metallic foil although the underlying
dynamics is complex, involving self-organization of the system and
leading to
apparent paradoxes in its physical description due to the different way
ultraintense light acts on electrons and ions.


***

LE11948

Is it possible to rectify the acoustic energy flux?

By coupling a superlattice and a strongly-nonlinear medium, we present a simple but efficient "acoustic diode" model that first makes it possible to control the acoustic energy flux. Remarkable similarity is revealed between the behaviors of an acoustic diode and an electric diode. The acoustic energy flux is allowed to transport in one direction but cannot propagate as its incident direction is reversed. This is different from our traditional knowledge and may be identified as a significant rectifying effect on acoustic energy flux that could not be fulfilled by any existing acoustical system. By employing such an acoustic diode, it is promising to control the acoustic waves in an acoustical system effectively and easily just like controlling the electric currents in an electric circuit. Potential practical applications can therefore be expected in the future for a variety of important situations that require special manipulations of acoustic energy flux, such as unidirectional sonic barriers and controlled destruction of kidney stones via ultrasonic lithotripsy etc.

***

EF10540

Obstacles may speed Evacuation

We have discovered that we can evacuate faster when we put a certain
obstacle at a certain position in front of an exit both theoretically
and experimentally.
In recent years, evacuation dynamics are studied by many physicists
using physical models.
We have newly introduced the two new functions, which are the
frictional function and the turning function, to the model.
Frictional function represents the impact of conflicts among
pedestrians and the turning function represents the decrease in
walking speeds when pedestrians turn.
Frictional function tells us that when an obstacle is put in front of
an exit, it blocks the pedestrians moving to the exit at once and
weakens the impact of the conflict.
Thus, the total evacuation time theoretically decreases when an
obstacle is set up.
We have also verified this phenomenon by our experiments.
However, the obstacle does not always work well.
The turning function tells us that when an obstacle is put at the
center of the exit, pedestrians have to detour it, so that their
walking speeds decrease and the total evacuation time increases.
Therefore, the obstacle should be shifted from the center to decrease
the evacuation time according to our theory.



***

BD11158

Towards perfectly defective diamonds

Perfect diamonds are boring; it is the color-center defects that give
diamonds their special colors. Some defects can also be very useful allowing
researchers to create tiny, exquisitely sensitive magnetometers. In this
paper, a UC-Berkeley led collaboration of researchers from several
institutions in US and Europe describe their analysis, using a battery of
modern techniques, aimed at understanding and optimization of various
stages of the preparation of diamond samples with nitrogen-vacancy (NV)
color centers, with a combination of high density and long spin-relaxation
times optimal for ensemble magnetometry.

August 6, 2009

DGR1037

Q-balls, the Neutron Star Destroyers

I have shown that dark matter, in the form of supersymmetric Q-balls, may
rapidly destroy neutron stars. This both constrains the properties of
Q-ball dark matter and provides the possibility that such Q-balls may be
inducing anomalous gamma-ray bursts from the dying neutron stars. The
absorption of even one Q-ball is enough to destroy a neutron star. A
single Q-ball inside a neutron star grows by imbibing neutrons.
Eventually it reaches a certain critical size at which point it fragments
into two equally-sized daughter Q-balls. These daughter states can
likewise grow and subsequently fragment. This leads to the exponential
growth of the number of Q-balls. The new constraints on Q-ball dark
matter are found to be in a regime in which supersymmetry is otherwise
devoid of a dark matter candidate.

August 4, 2009

ZF10037

First LHC beam data help finding magnet powering problems

Despite the extremely fast and successful start-up of the CERN LHC,
the incident occurred on September 19th 2008 stopped the LHC beam
commissioning
at an early stage, when beam optics was not fully probed.
Nevertheless, a single beam trajectory that was recorded over 90 turns
around the machine circumference has been used to spot a critical magnet
powering problem.
In this paper we present state of the art signal analyzes applied to beam
trajectories aimed at probing the beam optics, see attached figure.
This, combined with the
development of new algorithms for precise uncovering of magnetic errors,
led to the finding of a cable swap between magnets of the two LHC rings.
This cable swap was also confirmed by hardware tests. The LHC
commissioning,
to resume by mid-November 2009, will certainly benefit from this
improvement
of the machine. The newly developed algorithms for the optics error
localization will be further challenged during this exciting phase of
the LHC.

Caption of attached Figure: Relative deviation between LHC measured
and design optics together with
tolerances. The powering problem was found at about 10km.


***

LD12594

A Black Hole on a Chip

Thirty five years ago, Stephen Hawking famously showed that black holes radiate energy according to a thermal spectrum. However, his calculations relied on assumptions as to the unknown physics of ultra-high energies and quantum gravity. Adding to this is the inability to measure and verify the exceedingly low radiation temperatures predicted for astronomical black holes. In this paper the authors show that a magnetic field-pulsed microwave transmission line comprising an array of superconducting quantum interference devices, or SQUID's, not only reproduces physics analogous to that of a radiating black hole, but does so in a system where the high-energy and quantum mechanical properties are well understood and can be directly manipulated in the laboratory. Furthermore, by tuning the strength of the applied magnetic field, the pulsed microwave SQUID array can be used to probe black hole radiation beyond the regime considered by Hawking, in particular where the quantum fluctuations in the analogue spacetime geometry are large. This may lead to experiments that help to shed light on the as yet unknown physics of quantum gravity.

August 3, 2009

LD12301

Ultrahigh energy photons as a probe of Universe's highest energy particles

The origin of ultrahigh energy cosmic rays (UHECRs) has been one of the biggest mysteries in astrophysics. There are two ways to identify the sources: examining the arrival directions of UHECRs, and hunting neutrinos and photons produced via hadronic interactions between cosmic rays and ambient photons and nucleons. The latter is more crucial when the sources are transient like gamma-ray bursts, because cosmic rays have significant time delays by cosmic magnetic fields while neutrinos and photons may not. In this letter, we have demonstrated that ultrahigh energy (above 10 EeV) photons produced in the source can be the most useful probe of nearby transient UHECR sources. Their detections directly suggest UHECR accelerators, and give us useful information on the uncertain cosmic radio background and cosmic magnetic fields. Furthermore, they may allow us a much more stringent test of possible Lorentz invariance violation that is often predicted in quantum gravity theories. Such ultrahigh energy photons could be detected by the Pierre Auger observatory and future larger detectors such as the planned JEM-EUSO can provide us with more chances to observe them.

***

LF12637

HOW VIBRATION REDUCES FRICTION

It is known empirically that friction between two bodies can be
reduced if the system is vibrated, but it turns out that this only
happens under well defined conditions.
Using molecular dynamics simulations of repulsive particles confined
between a horizontally driven top plate and a vertically oscillating
bottom plate, Capozza et al identify the range
of vibration frequencies for which friction is strongly suppressed.
This range depends on the vibrating amplitude, the normal applied
load, the system inertia and the damping constant. The result is
important because it could help improve frictional properties of nano-
devices. In addition, it could be used to explain earthquake
triggering by small dynamic perturbations: vibrations, for certain
frequencies, can induce a drastic reduction of the friction force
between fault planes, leading to an earthquake.

***

EB10523

Kramers-Moyal coefficients in the analysis and modeling of heart rate
variability

Modeling of recorded time series may be used as a method of analysis
for heart rate variability studies. In particular, the extraction of
the first two Kramers-Moyal coefficients has been used in this
context. Recently, the method was applied to a wide range of signal
analysis: from financial data to physiological and biological time
series. Modeling of the signal is important for the prediction and
interpretation of the dynamics underlying the process. The method
requires the determination of the Markov time. Obtaining the drift and
diffusion term of the Kramers-Moyal expansion is crucial for the
modeling of the original time series with the Langevin equation. Both
Tabar et al. and T. Kuusela et al. suggested that these terms may be
used to distinguish healthy subjects from those with heart failure.
The research groups applied a somewhat different methodology and
obtained substantially different ranges of the Markov time. We show
that the two studies may be considered consistent with each other as
Kuusela et al. analysed 24 h recordings while Tabar et al. analyzed
daytime and nighttime recordings, separately. However, both groups
suggested using the Langevin equation for modeling of time series
which requires the fluctuation force to be a Gaussian. We analyzed
heart rate variability recordings for ten male young (age 26-4 + 3 y)
healthy subjects. 24 h recordings were analyzed and 6 h long daytime
and nighttime fragments were selected. Similar properties of the data
were observed in all recordings but all the nighttime data and seven
of the ten 24 h series exhibited higher order, non-negligible
Kramers-Moyal coefficients. In such a case, the reconstruction of the
time series using the Langevin equation is impossible. The
non-negligible higher order coefficients are due to autocorrelation in
the data. This effect may be interpreted as a result of a
physiological phenomenon (especially occurring for nighttime data):
respiratory sinus arrhythmia. We detrended the nighttime recordings
for the healthy subjects and obtained an asymmetry in the dependence
of the diffusion term on the rescaled heart rate. This asymmetry seems
to be an effect of different time scales during the inspiration and
the expiration phase of breathing. The asymmetry was significantly
decreased in the diffusion term found for detrended nighttime
recordings obtained from 5 hypertrophic cardiomyopathy (HCM) patients.
We conclude that the effect of RSA is decreased in the heart rate
variability of HCM patients - a result which may contribute to a
better medical diagnosis by supplying a new quantitative measure of
RSA.

July 31, 2009

LE12150

Ghost Images Reveal Quantum Properties

Ghost imaging occurs when a image is obtained using light that never
passes through the object in question. Such systems measure the
correlation between two optical beams and there has been an ongoing
debate as to whether they can be explained by classical as opposed to
quantum physics. We make a new kind of ghost imaging system
incorporating a hologram which is also placed remotely from the object.
The "non-local" hologram gives edge enhancement of our images and in
doing so reveals a violation of a Bell inequality. Such violations are
a hallmark of quantum physics and hence our system shows that some ghost
imaging is indeed quantum imaging...

...and here is the image!

***

LF12649BR

There is order in the carbon cage

Endohedral fullerenes are scientifically intriguing and technologically relevant nano-objects where one or several atoms are enclosed in a cage of carbon atoms. In our work, we show that such endohedral atoms are ordered when a single layer of a particular multi-atom endohedral fullerene (Dy3N@C80) is adsorbed on a crystalline metal surface. By combining two experimental techniques - scanning tunneling microscopy and X-ray photoelectron diffraction - we are able to analyze both the ordering of the encaging carbon atoms as well as the ordering of the endohedral dysprosium and nitrogen atoms. With the large variety of currently available multi-atom endohedral fullerenes, the adsorption of such endohedral fullerenes on single crystal surfaces, as shown in our work, provides a means to create ordered arrays of endohedral, decoupled clusters in two dimensions. Apart from its scientific value, there is an inherent beauty associated with the photoelectron diffraction experiments: Analogous to the shadow produced by a lampshade where a light bulb is sitting inside, the scattering at encaging carbon atoms of the photoelectrons from the endohedral nitrogen atoms produces an inverted shadow of the cage - due to the so-called forward-focusing effect - hence producing a forward-projected picture of a fullerene as viewed from inside.

LC12636ER

Swimming in Sand

One of the properties of granular materials that makes them unique and leads to striking and unexpected behaviors is their ability to act either as a solid,
supporting a load like sand on a beach, or as a fluid, flowing freely as in avalanches. Nowhere is this behavior more remarkable than in the behavior of lizards known as Sand Swimmers. These creatures, being cold blooded, cannot survive on the surface of their desert habitats during the day, and have evolved the ability to submerge up to 10 cm beneath the sand to lower temperature regions, and even to travel within the sand bed. In this paper we present the first computational study of "sand swimming", using a simple model that has been recently proposed. It was found that optimal conditions that idealized swimmers must use to critically fluidize a sand bed so that it is rigid enough to support a load when needed, but fluid enough to permit motion with minimal resistance. In other words, the sand swimmers locally probe the fundamental time-scale in a granular packing.

***

LE12132

Stirring, not mixing: binary black holes action on electromagnetic fields.

Electromagnetic fields threading a spinning black hole have long
been key in models attempting to explain powerful emissions in
fascinating astrophysical systems such as active galactic nuclei, gamma
ray bursts, quasars, etc. This work studies the effects produced by a binary
black hole system on an encompassing electromagnetic field.
In particular, it highlights the possibility that the merger of the black holes
may amplify the electromagnetic field strength. In addition, a time varying
oscillation of the electromagnetic field induced by the shrinking orbit and
merger which may lead to observable emissions that cab be captured
by current and future power telescopes. These systems will also
emit copiously in gravitational waves as the black hole collide.
Consequently, merging black holes interacting with electromagnetic fields
would provide an unprecedented opportunity for studying such systems via
both electromagnetic and gravitational waves. Such studies will advance our
understanding of these systems, and gravity within general relativity and
beyond.

EB10605

PREDICTING WHERE AN EXTREME EVENT WILL TAKE PLACE?

There is growing interest in the study of extreme events, in view of
their paramount importance in such key areas as global environment,
sociology and finance. Ordinarily, the question asked is whether an
extreme will or will not occur globally. In this paper the issue of
prediction of extremes - and hence also of protection against them -
is put in a new perspective by inquiring on what is the particular
location in space that will witness at a given time period the
occurrence of such an event. For this purpose the dynamics of
extremes is analyzed in representative case studies and the
mechanisms by which extremes spread over space starting from an
initial location are identified. Of special interest is the
occurrence of long-range connections as opposed to step-by-step
propagation through successive neighbors.

***

AF10516

A single pulse suppressing undesired noise

A group of physicists of the University of Technology of Dortmund, Germany,
have succeeded in designing an optimum shape of a single electromagnetic pulse
for high-precision suppression of noise effects in nuclear magnetic resonance
and more generally in the manipulation of single quantum bits.

Nuclear magnetic resonance spectroscopy is a non-destructive, preeminent
technique for investigating matter and imaging of tissues. Lately, it has
also become a starting point for constructing a quantum computer.
Nuclei with a magnetic moment are coherently manipulated at a precise
resonance frequency. The aim is to reduce the noise due to the unavoidable
interaction with the environment which disturbs the detected signal.

The single application of the new proposed pulse is enough to average to zero
the disturbing noise, with an improvement of an order of magnitude compared to
previously known pulses. Thus less pulses are needed in total for an improved
precision which is among the fundamental prerequisites for the realization of a
quantum memory.

***

LC12202BR


First steps towards a Wigner lattice in a quantum wire

In the 1930's, Wigner predicted
that as electrons are charged particles and repel each other, so there must be
conditions where they will stop their continual motion and form a crystalline
lattice, or array, in order to minimise their mutual repulsion. This work generated
enormous interest in the physics community, with numerous papers being produced
on the theory of the "Wigner Lattice" and many experimental
investigations, although evidence of the phenomenon has been limited.

We have been studying the properties of electrons confined by an electric field
to a one-dimensional configuration, a "quantum wire". As the electric
field compressing the electrons is weakened, we have found that the electrons relax
and form two separate rows in order to minimise repulsion. In this paper, we
demonstrate that the two rows of electrons thus formed can be coupled together,
by means of hybridization, or a mixing of the wave-functions between the two
rows. The formation of two interacting rows is a first step towards the creation
of a two-dimensional Wigner lattice in a quantum wire.

LT11861

O fish, where art thou?
How fish follow the invisible trace generated by other fish

Fish can detect objects and other fish under water by their lateral-line system measuring the change of pressure or velocity due to water flow generated by these objects. Have you ever observed the swirls that are released by a canoe paddle and are left behind the canoe as relatively stable objects? Many fish and also seals track the underwater invisible swirls, or wakes, arising from the fins of other
fish either to stay behind them during schooling or to hunt them. In this paper, it is shown what the sensory input resulting from wakes looks like and how fish can determine the direction of a wake̓s vortex ring and thus can follow the track of other fish. To measure water flow around their bodies, fish use their lateral-line system consisting of narrow tubes directly underneath the scales and connected to the surrounding water through small pores. Between each two neighboring pores there is a cupula, a gelatinous body covering hair-cell receptors that respond to water flow in the canal and in this way generate neuronal signals. It is shown mathematically, and supported by experimental neuronal evidence, how fish can reconstruct the orientation of a vortex ring through the information they receive
and, thus, how they can track comrades or prey.

***

BZ10860

Observing the first step of electron-hole recombination

Unlike metals, semiconductors conduct electricity not just via electrons
but two kinds of charge carriers, electrons and holes. When electrons
and holes encounter each other, they can form pairs which can decay
under release of their energy either into heat or light. This is called
recombination, and recombination is sometimes detrimental to electronic
devices - for instance in solar cells where it reduces efficiency - but
sometimes it is beneficial for technical applications, for instance in
light emitting diodes where recombination is the very process that
converts electrical energy into light. Observing recombination and the
various quantum mechanical processes which contribute to recombination
is very difficult but crucial for the understanding of this
technologically important phenomenon.

We present here an experiment which allows the observation of
electron-hole pair formation which precedes recombination. The
experiment is based on a novel electrically detected magnetic resonance
pulse sequence which forces magnetic moments of electrons and holes to
dephase apart during the pair formation process before they are forced
to rephase later. The rephasing produces a current signal (an echo)
which is different for pairs which formed before and after the
dephasing. This allows to very accurately observe the formation of
electron hole pairs.


***

LE11889

Shaking of an optical lattice reveals fine features of the quantum many-body state

For more than two decades physicist have struggled to understand why
some ceramic alloys can conduct with zero resistance up to an unusual
high temperature. More or less during the same period, physicists working with cold
atoms have managed to control and manipulate atoms in such a way that
it is now conceivable to use them to simulate crystalline structures,
such as the above mentioned superconducting alloys, with high
flexibility. In our work we show by exact simulations in one dimension that modulation of an optical lattice - an artificial crystal - can reveal essential information about the many-body quantum state of the repulsively interacting fermionic atoms it is holding; even small energy gaps are accurately resolved. This study in one dimension has relevance to the three dimensional counterpart where
antiferromagnetic states are predicted to occur, suggesting the observation of the
antiferromagnetic gap by the lattice modulation. Understanding such systems is
important due the crucial role that the magnetic properties are believed to play in high temperature superconductivity.

***

LB11867

Dynamic chirp control of attosecond pulses demonstrated

High-order harmonic Generation (HHG) and the attosecond pulse emission based on HHG have been extensively investigated in recent years. The attainable shortest harmonic pulse duration is limited by the intrinsic chirp of the harmonic emission. Consequently, the phase control and chirp compensation of harmonic pulses is of vital importance for producing the transform-limited harmonic pulses. We demonstrate a novel scheme of chirp control by exploiting the dispersion characteristic controlled by the laser field, which is referred as the dynamic control of chirp, different from previous approaches based on the static dispersion of the materials. To control the material dispersion dynamically, we add a weak second harmonic laser pulse to the fundamental driving laser pulse for HHG. The demonstrated method, as opposed to other demonstrated methods, can provide positive chirp. Using this technique, we have compensated the negative chirp for the first time. Additionally it has the advantage that the chirp of the attosecond pulses can be varied easily and continuously by simply changing the time delay between the two color pulses, which could open new opportunities for attosecond coherent control.

July 20, 2009

LD12017

The Speed of Light Under Pressure

Measuring how fast light travels is not so easy, but inside
opaque materials, like clouds, bone, skin, or paint the problem is
particularly daunting. Faez et. al in a recent issue of Physicsl Review Letters have
teased out this rate of transport using a simple, yet novel, trick: changing the
ambient pressure. As the pressure is slowly tuned so is the so-called
effective refractive index, which determines the speed of light.
Even though the light is following an extremely complex path as it
bounces through the material, its speed can be characterized by
tracking the influence of pressure on the outgoing light intensity pattern.
So simple and direct is the technique that it offers an entirely new way for
probing inside important biological materials such as bone or wood as
well as complex photonic materials such as photonic crystals or
metamaterials.

***

LB11720

Molecules on silver waveguides: showing a way towards coherent nanophotonics

Strong coupling between light and matter is the key for all coherent
photonics. Cavities as well as emitters with a narrow spectrum are
usually required, but recent developments with surface plasmon
polaritons (SPP) allow reaching the strong coupling regime simply due
to the tight confinement of this ?nanoscale light?. In this work we
show that strong coupling between SPP and organic molecules with a
broad spectrum (Rhodamine 6G) is possible, and study for the first
time the strong coupling dynamics by controlling the interaction time,
in analogy to cavities with a tunable Q-factor. The interaction time,
and therefore the magnitude of the observed vacuum Rabi splitting, is
controlled by lithographically fabricating molecular interaction areas
of predetermined lengths on a silver plasmonic waveguide. The results
are an important step towards applications such as nanoscale plasmonic
amplifiers and lasers, and the system may be used also for studies of
coherent energy transfer, relevant for understanding light harvesting
systems.

***

LC11913

Tiny Forces Are Drastically Enhanced in Antihydrogen-Hydrogen collisions

Antihydrogen, the simplest atom of antimatter, is believed to
have exactly the same internal energy structure as hydrogen,
its matter counterpart. Any tiny difference in energy
structures of hydrogen and antihydrogen would mean violation of
fundamental symmetry between matter and antimatter, which is
one of the cornerstones of the Standard Model. The precision
tests of such symmetry are planned in ongoing experiments in
CERN. In such experiments the energy structure of antihydrogen
atom, isolated from any contact with matter, is planned to be
measured. In our research we found, however, that another
scenario for such a comparison is possible. When ultracold
antihydrogen and hydrogen atoms meet each other their
collisions may result, apart from annihilation, in energy
exchange between atom and antiatom. This process is known as
transition between hyperfine energy states. We found that any
difference in hyperfine structure of atom and antiatom would be
detected as a dramatic change in the hyperfine transition rate
behavior. Moreover, we found that the interaction which is
responsible for hydrogen-antihydrogen hyperfine transitions is
drastically, one billion times, enhanced by long-range
interatomic forces. This opens a perspective to observe tiny
and very short-range effects in hyperfine transitions
caused by hydrogen-antihydrogen collisions.

***

LF12348

Polarizing Electrons With Phantom Superconductivity

This manuscript reports a new technique for producing polarized electrons
from the incoherent remnants of superconductivity. We exploit a
little known resonance
that exists in the high magnetic field normal state of Al films to
produce spin-polarized tunneling currents.
The technique can also be used to measure the electron polarization
in thin magnetic films. Its advantage over other
polarization probes is that it can be implemented in very high
fields, well over 20 T!
The physics discussed in the paper is unique and forms the basis of
a counterintuitive mechanism for electron spin polarization.

***

LF12862

The dark side of molecules on surfaces revealed

This study introduces and demonstrates a new technique to probe
experimentally the ultra-fast dynamics of light emission by molecules on
metal surfaces. It makes it possible to indirectly measure processes
occurring in a single molecule on timescales in the range 10-100
femtoseconds, a feat unthinkable with current time-resolved spectroscopy
techniques.
Light emission of molecules in close proximity to metal surfaces forms
the basis for several emerging ultra-sensitive optical techniques with
wide-ranging applications in analytical chemistry, forensic science,
art, archeology, and biology (to name a few).
For molecules in solvent, where most processes occur on a
picosecond-to-nanosecond timescale, these processes are well understood.
However, everything speeds up dramatically for molecules sitting on a
metallic surface. The timescales become inaccessible to conventional
methods, a problem exacerbated by the fact that most of the emitted
light is directly absorbed in the metal, and therefore invisible.
Theoretical predictions in these extreme circumstances abound, but
experimental evidence has remained elusive.
The method developed in this work exploits subtle differences in how two
optical processes, fluorescence and Raman scattering, are modified by
the metal surface. By comparing their intensities for a single molecule,
ultra-fast decay rates become accessible to experiments.

July 16, 2009

LA12388

Gossip Tipping Point: the more popular a news item is, the faster it is passed around.

Information popularity and propagation speed go together. The paper "Impact of human activity patterns on information diffusion dynamics" analyzes the propagation dynamics of viral marketing campaigns run through social networks and proves that the speed in forwarding a viral message increases dramatically when its forwarding rate exceeds the spreading process tipping-point. Thus, popular messages will not only reach a majority of the public but will do so in a matter of hours compared to the days or months it would take a less interesting message to reach a much smaller fraction of the audience. The importance of the result is that the phenomenon is traced to the large heterogeneity of human behavior since, for widely forwarded messages, fast reacting individuals are more abundant than those with average response times. On the contrary, if information is not very appealing, its speed is controlled by those people who take a lot of time to respond, slowing down the propagation to the extend that it can take years to spread. This finding will have a great impact in the understanding and modeling of information diffusion in social processes like rumor spreading, opinion formation, diffusion of innovations, marketing tactics, business cooperation or cultural dynamics.

July15, 2009

LD12168

Fossil fuels? No thanks, I use water!

It is possible to generate electric power simply by using water.
This astonishing result can be reached exploiting the salinity difference
between water from the sea and water from a river, giving brackish water
as the only waste product. Some solutions of this kind have been already
proposed, but none of them is actually viable, due to high costs or
technical difficulties.
Here I describe a radically new technique, derived from supercapacitor
technology. Two porous electrodes, immersed in sea water, constitute a
capacitor, that is a device storing electric energy, somehow similar to
a battery. When the capacitor is charged, positive and negative salt ions
(red and green in the picture) migrate towards the oppositely charged
electrode, thus neutralizing them, and are kept in the electrode pores
like water in a sponge. After charging, the salt solution is brought
into contact with fresh water: the ions are washed away from the electrodes.
Since the electric charge of each electrode is no more counterbalanced
by salt ions, the electric potential of the capacitor becomes increased.
It is possible to extract the energy in excess in the form of
electric current, thus turning salinity difference into a
completely renewable energy source.

***

LC12635

PARTICLES AS TRACERS FOR THE MOST MASSIVE EXPLOSIONS IN THE MILKY WAY

At their deaths, the most massive stars in the Milky Way seem to leave
behind unambiguous signatures of particles. Recently, a mysterious
population of electrons and positrons was observed by several
experiments. Attempts were made to attribute such a signal to the decay of
dark matter, an unknown kind of matter in our universe. But nature may
provide an even simpler solution to the problem, as reported in an article
in Physical Review Letters, finds a natural
explanation in the explosions of giant stars that are more than 15 times
heavier than our sun. A dying star of very high mass ejects most of its
matter in a final explosion, which then ploughs its way through a massive
stellar wind. During this process, electrons and positrons escape from two
different regions: the lower energy signal comes from the entire surface
of the exploding star, while at higher energies, the regions around the
poles of the rotating star start to dominate. This naturally explains the
observed energy behavior of electrons and positrons.


***

LD12352E

Geometry of genetic logic gates

Genes integrate signals by transcription factors, often in a manner of combinatorial control in analogy with Boolean logic operations that basic electronic devices perform. Knowing which mode of combinatorial regulation (typically, AND or OR operation) that a gene employs is important for determining its function in regulatory networks. We propose an efficient approach to infer regulatory synergies, based on geometric characteristic of a physical quantity (i.e., so-called dynamic triple cross correlation) that can be computed directly from available experimental data, e.g., the cross correlation function with AND operation is convex and the one with OR operation is concave. Such an approach uses only the nature of noise inherent in gene regulation processes but does not need any external intervention, thus having potential application perspectives.

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LZ11379

Casimir force confronts a new challenge from good, old electrostatic
effects

When two conducting plates are facing each other in vacuum, a mutual
attraction takes action simply due to a presence of the so-called virtual
photons of zero point energy. This charming force, also known as the
Casimir effect, is a force of pure quantum fluctuation and has attracted
immense interest among physicists in the last decade, with the latest
experiments showing a satisfactory agreement with its theoretical
prediction. Now, we closely inspect the surfaces used in our measurements
and ask ourselves, "How clean, how perfect are these? Is there enough
experimental control to nullify the familiar force of residual electric
fields that are rampant in any real surface?" Our paper concerns the first
measurement of the Casimir force between a pair of Germanium plates in
which a force due to surface potential patches appears to coexist with the
Casimir force, posing a great, new challenge on this modern, precision
measurement- deja vu of those ongoing gravitational wave projects like
LIGO and LISA. Whether the experimental data are to be attributed to the
Casimir force of quantum vacuum fluctuation or electrostatic force due to
spatial fluctuation of surface patches will require a detailed knowledge
of surface profile of a given sample under study. Our results underscore
the inherent difficulty associated with distinction of forces of different
physical origins. Dr. Casimir is perhaps right, but that does not mean
that we can completely dismiss Mr. Coulomb.

July 14, 2009

LX11811

Local pulsar exposed as source of energetic electrons and positrons

The surprising excesses of positrons and electrons recently detected
by the PAMELA and Fermi space experiments suggest that a source of such
cosmic rays must reside within our neighborhood of the Milky Way. Many
possibilities, including the annihilation or decay of dark matter
particles, have been proposed, although without any conclusive evidence.
In this Letter, we discuss how the unexpected "halo" of very-high-energy
gamma rays recently observed to surround the Geminga pulsar (a
rapidly-rotating neutron star) may be essential to resolving this
puzzle. We conclude that these gamma rays imply that a wind of
highly-energetic cosmic rays is escaping from Geminga, confirming the
presence of a powerful particle accelerator near the Earth. Cosmic rays
produced in Geminga's active past are, after a circuitous journey
through the Galaxy's tangled magnetic fields, likely the mysterious
"Positron Excess." This invites the exciting prospect that the long
sought identity of the origin of the highest-energy electrons and
positrons yet seen is now known.


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Excitons Can Couple Strongly to Plasmons in Individual Carbon Nanotubes

New theory and calculations show that excitons can couple to surface plasmons in individual semiconducting carbon nanotubes. The exciton-plasmon coupling strength can be tuned using a perpendicular electrostatic field. This new effect opens up new paths for the development of tunable nanophotonics device applications with carbon nanotubes.

One straightforward application is the exciton emission control from the individual nanotube by means of electrostatically driven exciton-plasmon coupling. This offers the advantage of less stringent fabrication requirements over the microcavity-controlled exciton emission (commonly used approach) since a photonic crystal microcavity is no longer required. Electrostatically controlled coupling of two spatially separated excitons to the same nanotube's plasmon mode would result in their entanglement, the phenomenon that paves the way for solid-state quantum information processing with carbon nanotubes. The theory developed also lays the foundation for understanding inter-tube energy transfer mechanisms in nanotube bundles and films, helping to reveal their potential for the development of high-yield optoelectronics applications with carbon nanotubes.