December 15, 2009

LF12555E


If the leaf can do it, can we do it even better?

Photosynthesis is the process by which light energy is converted to chemical energy. It is one of the most important biological processes as it provides energy, food, and oxygen for all organisms living on earth. Artificial photosynthesis is a novel strategy with the aim to improve the underlying mechanism for producing sustainable fuels. The question is: Can we beat nature at her own game?
Plants and many species of bacteria are photosynthetic. The light energy is trapped by the chlorophyll molecules which are arranged as regular arrays within membranes. When a chlorophyll molecule absorbs light, it is electronically excited. It may either realize a photochemical energy transfer or return to its ground state through relaxation mechanisms. A high efficiency of photosynthesis requires a minimization of the relaxation mechanisms.
In this paper, we introduced neutron spectroscopy to measure the light-induced relaxation effects on a photosynthetic membrane system. We detected the existence of photo-excited lattice modes which tells us where, and how much, energy is lost in the initial steps of photosynthesis. In addition, we found evidence for „freezing“ of lattice modes under light irradiation which results in an optimum orientation of the chlorophylls. These exploratory results are essential towards the engineering of new photosynthetic materials to improve the efficiency of the photosynthetic process beyond the limits given by nature.

***

LH12180

Cloudy skies above a metal surface

According to quantum mechanics electrons that carry electrical currents in metals do not appear as particles with a well defined position, but rather as fuzzy moving objects, i.e. electron clouds. While it has been known for a long time that electron clouds can escape from the metal surface and move freely above it, we demonstrate in our paper that the clouds can get trapped above a small nanometer size metal island.

The electrons escaping from the surface still interact with the surface because they create positively charged holes that attract the negatively charged electrons. This way a series of “image-potential” states with quantized separation emerges that can be occupied by the electrons. The occupied states are detected with the atomically sharp needle of a tunneling microscope that probes the shape of electron clouds by measuring the electrical current flowing between the needle and the clouds. Our results reveal the triangular symmetry of the clouds that are trapped by the edges of a triangular cobalt island on a flat gold surface. The fuzzy edges of the clouds reflect the “uncertainty” that is imposed by quantum mechanics, implying that the clouds can leak out from the triangular confinement.

***

LL11820
Atomic-scale Distribution of Water Molecules at Mica/Water Interface Visualized by Three-dimensional Scanning Force Microscopy

We have developed a method referred to as three-dimensional scanning force microscopy (3D-SFM), which enables to visualize water distribution at solid/water interface with atomic-scale resolution in less than 1 min. Interfacial phenomena at solid/water interface play important roles in the industrial technologies as well as in the biological processes. However, the lack of a method able to visualize 3D distribution of water molecules has impeded progresses in molecular-scale understanding of such interfacial phenomena. 3D-SFM sheds light on this issue. As the first application of 3D-SFM, we investigated mica/water interface. This interface has been intensively studied due to the importance in ecological and geological sciences as well as in engineering and physics. The atomically-resolved 3D-SFM image visualizes 3D distributions of adsorbed water molecules as well as a hydration layer. In addition, the image also reveals the detailed atomic-scale structure of a cleaved mica surface next to an aqueous environment, indicating the existence of surface relaxation. So far, such real-space information has been discussed based on indirect spectroscopic data obtained by beam technologies. The direct real-space 3D images obtained by 3D-SFM should provide important information that has not been accessible by conventional technologies.

December 3, 2009

LG13021

Dense Magmas Deep within the Earth

The nature of silicate glasses and liquids under high-pressure
conditions are of key importance for deducing the formation and
differentiation of the terrestrial planets, especially through massive
primordial melting events of the protoplanets. Extensive melting of
the primordial terrestrial planets and the formation of a deep “magma
ocean”. Because silicate melts are so compressible, melts become
denser than their crystalline counterparts under high-pressure. This
characteristic is crucially important to consider in the evolution of
terrestrial planets. Murakami and Bass (PRL in press.) experimentally
examined the densification mechanism of SiO2 glass, the simplest
analogue for silicate melt, up to pressures equivalent to ~3500 km
depth in the Earth’s interior by the acoustic wave velocity
measurements using Brillouin spectroscopy. They discovered that the
SiO2 glass becomes super-dense above 140 GPa owing to a
pressure-induced structural change. This finding strongly suggests the
possible presence of gravitationally stable “super-dense magmas” deep
within the giant terrestrial planets through solidification from a
primordial deep magma ocean.

December 1, 2009

LJ12113

Stopping light for more than one second

Over the past decade, studies of ultra-slow and stopped light have
represented an extremely active area of research. Here we demonstrate that
a light pulse can be stopped and stored for 1.5 seconds in a Bose-Einstein
condensate of laser cooled atoms. During this time, light could – under
normal circumstances - travel roughly 10 times around the earth. In our
experiments, a light pulse, injected into the condensate, is dramatically
slowed and compressed, then extinguished, and in the process converted to
a matter imprint that is a perfect copy of the extinguished light pulse.
We manipulate atom interactions with use of magnetic fields, and the
system enters a phase separating regime: Much like oil and water separate,
the created matter imprint digs a hole for itself in the host condensate.
The imprint is nestled in this void and avoids collisions with condensate
atoms thereby minimizing losses. As a result, the matter imprint can be
stored for extended time scales. The imprint can further be moved to the
outer tip of the condensate where it can be converted back to light, and
from this location, the light pulse can exit the condensate with minimal
loss. The long storage times and the achieved control over the matter
imprint have important applications for creation of entangled states of
light and matter and for creation of long-distance quantum networks for
quantum computing, teleportation, and cryptography for secure data
transfer.

***

LJ12151

A stone hitting water creates supersonic air jet

A stone which lands on quiescent water triggers an impressive scene.
After the appearance of the familiar “crown” splash, a cavity forms
below the surface, whose collapse ends with the ejection of a thin
liquid jet. Hidden to the unarmed eye of a human observer is the equally
rich behavior of the surrounding air which culminates, as we show in
this work, in an air jet issuing from the cavity at /supersonic/ speeds.

Combining our visualization experiments with computer simulations we
show that the mechanism which produces this fast air jet is very similar
to that used in air plane jet engines where pressurized air is squeezed
through a narrow constriction. The unique feature of our situation is
that the confining geometry is a /liquid/ cavity which is rapidly
changing in time.

To study the impact process in a very controlled fashion, we use a
circular disc which is pulled through the liquid surface by a linear
motor. We visualize the air flow with small smoke particles produced by
a commercially available smoke machine commonly used for light effects
in theatres and discotheques. The motion of the smoke particles is
illuminated with a strong laser sheet and recorded with a high-speed
camera at up to 30,000 frames per second.

Figure caption:
The impact of a disc (a) creates a liquid cavity (b) which collapses (c)
and forms a very fast air jet being squeezed through the neck of the
shrinking cavity (d). The addition of smoke particles into the air (here
artificially colored in orange) allows us to visualize this air flow.

***

LF12383

Entangle global, act local: enhancing network connectivity with quantum operations

Complex networks describe many natural and socio-economical systems
such as protein-protein interactions, the brain, the internet, the
power-grid, and friendship networks. Structural properties of complex
networks have a crucial effect on their functionality, determining for
example the spread of information or deseases, the resilence of
internet, or the small world effect in human relations.

In this letter we show that new phenomena appear if networks are
governed by the laws of quantum mechanics. The transmission of quantum
information has already been studied in detail in 1-D chains and some
regular lattices. Here we study how to establish entanglement between
two arbitrary parties in a quantum complex network by means of local
quantum operations that change drastically the network's topology. We
thereby shift the attention from regular lattices to the feature-full
complex networks, opening the door to new ways of distributing,
protecting and manipulating quantum information as well as to the
study of new network models and dynamics generated by quantum
mechanisms.

With the promise of quantum computation and quantum cryptography, the
field of Quantum Information has had its role in the information
age, phenomena like entanglement percolation indicate that Quantum
information has also something to say in the forthcoming "connected
age".



***

LJ12114

Splitting Cooper pairs on demand

Superconductors are a potential source to produce entangled electron
pairs in a solid state device. The implementation of a superconducting
Y-junction in which the Cooper pairs injected from a superconductor
split into two electrons at different normal electrodes provides an
important step in this direction. Although there have been many
theoretical proposals in the last ten years it is the first time that a
tunable Cooper pair splitter is experimentally implemented. Two teams
have reached this goal independently. In the work, the Y-junction is implemented using a single wall carbon nanotube with a central superconducting electrode deposited on top and contacted at each side by two normal electrodes. The system thus fabricated behaves as two coupled quantum dots whose resonant levels can be
controlled by two independent side gates. The Cooper pair splitting
action of the device is demonstrated by comparing the conductance of
each arm in the superconducting and in the normal state, which is
reached by applying a magnetic field. The theoretical estimates
indicate that up to 50% of the injected Cooper pairs can be splitted at
the junction when the two quantum dots are at resonance. The findings
presented in this paper bring on very exciting perspectives and are
very likely to generate a renewed and intense experimental and
theoretical activity.

***

EG10639EJ

Ubiquitous chaos in the modern technological world

We found that a kind of chaos, i.e., chaotic magnetic-field lines
(CMFLs) exist everywhere and are ubiquitous in the modern
technological world. Although it is commonly believed that magnetic-
field lines generated by a wire carrying electric current always have
simple closed-loop structures, this belief is wrong, because CMFLs
can be generated by simple three-dimensionally crossed-wires. We
investigated the existence of CMFLs in realistic systems, and
revealed that this chaos exists widely in conventional electrical and
electronic products in our daily life, for example, in mobile phones,
personal computers, power wires in house etc., although we are
unaware of this fact.


***

LK12343

Understanding Explosive Percolation via Powder Kegs

Networks of various types (e.g., social networks, economic networks, computer networks, etc.)play a central role in modern society and have numerous applications to many fields of study, including sociology, epidemiology, and physics, to name just a few.

Recently, computer simulations have revealed that networks can sometimes undergo a very sudden change in character, dubbed an "explosive transition" (see in particular Achlioptas et al., Science 323, 5920, (2009)). A new paper in PRL describes the mechanism responsible for explosive behavior in some of these networks. It shows that, somewhat surprisingly, the key to explosive transitions is not the details of the edge-addition rules at work during the actual "explosion," but rather lies in the period preceding the explosion when a "powder keg" develops. In effect, the importance of the rules is to create an explosive situation, which can be detonated with almost any rule. The paper discusses the origin and nature of the powder keg, along with a heuristic criterion for determining whether a given network will be explosion or not. It illustrates these ideas in the context of social networks.

November 23, 2009

LH12027

Cloaking Device for Electromagnetic Surface Waves

A 'cloaking device' renders an object undetectable for electromagnetic radiation so that an outside observer cannot register any scattering or reflection effects from that object. Recent approaches commonly consider electromagnetic waves in three dimensions, i.e. light, as a probe. In this paper we demonstrate how it is possible to hide an object located on a metal surface from interaction with surface plasmon polaritons, a collective excitation of light and free electrons in only two dimensions. Our cloaking device consists of point scatterers distributed on two concentric rings around the object to be cloaked. By computer simulations we determine the physical properties of these point scatterers required to acheive the cloaking effect. As it turns out, the cloaking geometry efficently reduces scattering from the enclosed object to a minimum. We conclude that such point scatterers can be formed by surface defects that seem to be easy to create by the photon scanning tunneling microscope/direct-write lithography technique.

November 19, 2009

LE12253

So you think your theory is better than Einstein's theory of General Relativity?

In our paper, we calculated how much information about gravity the next generation of cosmology surveys will yield, and what types of theories will be subject to the most scrutiny. If you think that your theory is better than Einstein's theory of General Relativity (GR), you might soon know just how right (or wrong) you are.


GR has been well tested in dense local regions, like our solar system. The next generation of surveys will open the way to precision tests of GR on larger, cosmological scales. They will allow us to trace the evolution of galaxy clustering and gravitational potentials through multiple epochs -- the cosmological equivalent of tomography. This will offer an exciting opportunity to test the validity of Einstein's equations of GR which set the relations between the Newtonian potential, matter inhomogeneities and curvature perturbations.
The tools that we developed in this study allowed us to forecast the power of such surveys to detect and constrain departures from GR on cosmological scales. Our analysis is model independent and determines how many parameters describing deviations from GR can be constrained, as well as
the redshifts and scales on which data is most sensitive to the departures. Reversely, this analysis can be used in survey design to focus the experiment on the region in time and space for which some theory makes a specific prediction.

***

BKR1138

Discovery of a new quasicrystal, Sc12Zn88

Icosahedral quasicrystals, discovered 25 years ago by Dan Schectman,
literally transformed the science of crystallography. These materials
manifest long-range positional and orientational order, but lack the
periodic translational order of crystalline solids. Although they form,
almost readily, in a wide variety of ternary and quaternary metallic
alloys, examples of stable binary icosahedral quasicrystals are quite
rare. Indeed, it has been nearly a decade since the discovery of the
only known stable binary icosahedral phases in Cd-Yb and Cd-Ca by A.P.
Tsai’s group in Japan.

In the ___ issue of Physical Review B this month, a group of researchers
at DOE’s Ames Laboratory reported on the discovery of millimeter-sized,
facetted grains of the icosahedral phase in the Sc-Zn binary system
using a novel approach…. solution-growth. Although the Sc-Zn system has
been associated with quasicrystal formation in ternary and quaternary
alloys for some time, the binary “parent” icosahedral phase in the Sc-Zn
system had eluded detection. Using high-energy x-rays from the Advanced
Photon Source at Argonne National Laboratory, the Ames group confirmed
that the facetted grains were, in fact, icosahedral quasicrystals with
crystallographic symmetries that reflect their external growth habits.
The discovery of this new binary quasicrystal by solution-growth renews
the hope that other binary quasicrystals are lurking out there, waiting
to be uncovered.

***

BB11094

Direct visualization of exotic electron crystals in graphene

In conventional solids, atoms or molecules "freeze" at well defined
positions, thus forming a lattice. However, electrons, one of the
smallest constituents of atoms, may also solidify and form a so-called
Wigner crystal. The magnetic field can enhance this tendency of
crystallization, and even more exotic solids may be found, with bubbles
containing two or more electrons per lattice site.


The recent discovery of graphene, a two-dimensional material where the
electrons live at the surface, has raised the hope to "see" electrons at
work. In contrast to conventional two-dimensional electron gases, that
are buried in a semiconductor, the electrons in graphene are directly
accessible, e.g., by a scanning tunneling microscope. Thus graphene
yields the promising prospect to directly observe exotic electronic-solid phases.

The article studies in detail the local density of
states -- a density map at a fixed energy -- of high-field electron
crystals in graphene. They calculated the density patterns for the
Wigner and bubble crystals and found that the local density of states
exhibits a scaling relation: it is possible to infer the behavior of a
complex bubble-crystal, by knowing the behavior of a Wigner-crystal.
These density patterns may find and experimental verification in future
spectroscopic measurements.

November 17, 2009

LC12643B

When load acts to decrease friction

High school physics teaches that the friction force is the product of a friction coefficient and the normal force. Newer ideas that evolved roughly half a century ago showed that the friction force is in fact proportional to the contact area. The older approach still used in high schools today was then realized to be the special case of rough surfaces in which the contact area happens to increase linearly with the load. A group from Lamar university lead by Dr. Tadmor found a system in which the lateral force decreases with the normal force in spite of the fact that the contact area increases. This happens for drops on surfaces. To enable the discovery, the group built a special device that allows complete decoupling of normal and lateral retention forces.


***

ZH10036

GENERATING THE WORLD'S BRIGHTEST LIGHT

Funding has recently been granted to build what will become the
world's highest-brightness storage-ring light source in Lund, Sweden.
Upon completion in 2015, the new MAX IV facility will offer ultra-
intense synchrotron radiation to a broad and international research
community. This unique radiation, which covers the entire frequency
spectrum from infrared to visible, ultraviolet, and all the way to
soft and hard x-rays, is used throughout science and engineering as
the prime tool to investigate materials, surfaces, and interfaces.
Like a giant x-ray microscope, synchrotrons allow researchers to peer
inside matter such as crystal structures, molecular compounds,
polymeres, proteins, etc.

Like its predecessors MAX I-III operated at Lund University's MAX-lab
today, the MAX IV facility is built around an electron storage ring
which generates ultra-intense and highly-focused synchrotron
radiation. MAX IV will not only be the brightest such source in the
world, it also takes a novel approach in generating the radiation.
Rather than build a single accelerator that can cover only certain
wavelengths and pulse durations, the MAX IV facility consists of three
different accelerators for the generation of various types of
synchrotron radiation. One for ultra-short highly-intense laser-like
pulses and two for highly-focused and widely-tunable radiation.

This paper reports on the design of the MAX IV 3 GeV storage ring at
the heart of the MAX IV facility. It details the novel approach to
generating intense and ultra-small electron beams as a source of
synchrotron radiation.

***

LG12841

Did life begin in a whirlpool?

Many entities in Nature have a well-defined handedness since they cannot be superposed to their mirror image. They are said to be chiral. Notably, molecular components of life typically appear in only one of two possible enantiomers . It remains a puzzle at what point in evolution and through which natural force did this symmetry break, although it is known that whirling flows can promote the self-assembly in solution of molecular aggregates of a defined handedness.

In this work we report on the selection of chirality on initially non chiral soft surfaces as a consequence of vortical stirring. A class of surfactant molecules self-assemble at the air/water interface covering the surface with sub-millimeter domains with liquid-crystal order, arranging with a supramolecular configuration of random handedness. In the presence of a vortical flow under the surface, domains of a particular chirality prevail. The latter correlates with the sign of the stirring through a kinetic mechanism described in this work. The resulting decorated interfaces have a potential to be used as patterns to build more developed chiral entities through biomineralization (crystallization) or through catalytic biochemical reactions.

***

LC12643B

Keeping hot wires locally cool - a new route to higher Tc
superconductors?

Keeping superconducting wires superconducting at high-temperatures have
been a major goal of condensed matter physics, bearing, however, little
success in increasing the operating temperature close to room temperature.
We suggest that in clean superconducting wires, local cooling of the wire
should be enough to induce low temperatures throughout the wires,
and keep the wire in a superconducting state even when the external
temperature rise. A novel theoretical method allows us to calculate the
superconducting properties of a wire in contact with several heat baths,
and we show that the wire “favors” the cold temperature, which means that
it remains superconducting even if the average temperature at which it
is held is much larger than its critical temperature. We suggest that
for practical application thermo-electricity based local refrigerators
should be used. We also suggest a simple experiment to test our
predictions, which may shed light not only on the superconducting
properties of a wire, but also on other fundamental issues of heat
transport in microscopic scales.

November 12, 2009

LX11371

Counterfactual quantum cryptography

According to quantum mechanics, events that might have occurred can have actual physical effects, even though they do not in fact occur. What has been termed as an interaction-free measurement is a typical example of such striking counterfactual phenomena: the presence of an object can be determined without a photon being scattered by the object. It has also been shown that the outcome of a quantum computation can sometimes be inferred without the running of a computer. This counterfactual computation exhibits a surprising counterintuitive quantum computational effect, but it seems that it does not have a practical advantage for a specific computational purpose in its present form. Here we apply the fundamental concept of quantum counterfactuality to a real-world communication task in what may be called a ‘counterfactual communication.’ We present a novel class of counterfactual protocols of quantum cryptography that relies on the ‘non-transmission’ of a signal particle (the carrier of secret information): the
mere possibility for signal particles to be transmitted is sufficient to create a secret key.

***

LJ11767

Realization of Universal Ion Trap Quantum Computation

Any residual coupling of a quantum computer to the environment results in computational
errors. Encoding quantum information in a so-called decoherence-free subspace provides means to avoid these errors. Despite tremendous progress in employing this technique to extend memory storage times by orders of magnitude, computation within such subspaces has been scarce. Here, we demonstrate the realization of a universal set of quantum gates acting on decoherence-free ion qubits. We combine these gates to realize the first controlled-NOT gate towards a decoherence-free, scalable quantum computer.

November 11, 2009

LD12736


Big surprises come in small nuclei

Electron scattering measurements from nuclei have shown a clear difference in
how quarks are distributed in large nuclei compared to small nuclei, a
phenomenon called the EMC effect. Our understanding of the origins of the
effect is limited by the difficulty of modeling large, complex nuclei.
Therefore, even basic information about the effect has been elusive, such as
the underlying mechanism that causes the difference and whether it depends on
the mass or density of the nucleus.

Jefferson Lab experiment E03-103 made precise new measurements of the EMC
effect for four light nuclei. The large difference between 3He and 4He, the
lightest nuclei measured, rules out mass-dependent explanations. The large
difference between 3He and 9Be, which have similar densities but very
different masses, rules out density-dependent models. The low density of 9Be
is related to its unusual structure; most of the time it is in a configuration
with two 4He-like clusters and an additional neutron orbiting around each
other. The orbiting clusters yield a large radius and an anomalously low
average density, even though most nucleons are contained within the high local
densities of the clusters. This clearly demonstrates that the microscopic
structure of nuclei, usually neglected in high energy measurements, is of
critical importance. The results also suggest that the EMC effect may be
entirely generated within these small, high-density clusters, where densities
can briefly approach those in a neutron star.

***

lg12369

Blast Shocks and marbles falling upon sand

In the 1940s, G. I. Taylor, a famous physicist and hydrodynamicist,
managed to deduce the energy released by a nuclear bomb blast,
classified information then, from photographs of such explosions.
Since then, blasts have been studied in a variety of settings and are
believed to arise in the explosion of stars raising important
questions about the relevance of the analysis developed by Taylor. In
a recent paper to be published in Physical Review Letters, we show
that such a complex phenomenon can be produced in the lab by simply
letting a marble impact a sand layer. If the sand layer is made to
flow, the impact of the marble makes a hole in the layer. This hole
expands rapidly as if it were a small explosion. And the expansion of
the hole indeed follows the rules for blast shocks. With one major
difference, energy losses are present and can be accounted for in a
simple way. It may well be that such experiments help in the
understanding of phenomena as far away as a dying star and provide a
simple system to further study the blast problem.

***

LG12995

Taming the charmonium wilderness

Since its discovery in the early 70s until the beginning of the new century,
charmonium spectroscopy was considered a safe haven in the tumultuous
landscape of hadronic physics. It seemed to be fairly well understood and no
unusual or unexplained features seemed to exist. However, during the last
years experiments have proved how utterly wrong we were. No less than
eighteen new mesons have been recently reported in this once upon a time
quite and peaceful energy region. Their understanding as simple
quark-antiquark pairs has proved to be nearly impossible and therefore
several different attempts to dissect them have been performed. Their success
in the ambitious task of obtaining a full picture of the whole charmonium
spectra has been, at best, limited.

In our work we have attempted a different approach. Making use of standard
few-body techniques developed, for example, to study the deuteron, we have
examined the possible existence of meson-meson molecular states hidden within
the charmonium spectra. Hence, our idea was not to describe the properties of
a carefully chosen set of states, but to determine the possible existence of
molecular structures. We have carefully examined all possible quantum
numbers establishing boundaries for the existence of molecular states
that might be identified with some of the new reported mesons. This is
the first time such a global and ambitious study has been performed
and we hope it will encourage an experimental effort to confirm or rule out
the molecular character of some particular states.


***

LK11836

Refrigerated ions last longer!

We have measured the lifetime of negative helium ions stored in an ion trap
at very low temperature to avoid any influence on the result by the thermal
radiation from the surroundings. Even though no visible light is seen in the
thermal radiation of an object at room temperature, the infrared radiation
is sufficiently intense and energetic to destroy a weekly bound system like
negative helium rapidly enough to severely disturb a measurement of the
intrinsic lifetime (found to be 359 microsecs in the present experiment). By
building a very small electrostatic ion-beam trap and mounting it in a
vacuum chamber which can be cooled down to 10 Kelvin, we have for the first
time reached a situation where the thermal radiation can be neglected and
the lifetime determined directly without the need for any assumptions or
corrections.

November 9, 2009

LG13016

Movies of Quantum Decoherence

Complex superpositions of photonic quantum states that display strong
quantum interference have been “filmed” as they evolve and decohere in
time, allowing detailed observations of their decay as they interact with
their environment.

Using a superconducting phase qubit, experimenters at UC Santa Barbara
have created a range of quantum superpositions of photon states in a
microwave resonator. They can analyze the photon states by employing a
technique known as Wigner tomography, from which two-dimensional Wigner
quasiprobability functions can be calculated; these display the
artistically appealing characteristics of quantum interference. By
performing tomography on the states as they evolve in time, the
experimenters can construct “movies” of the state evolution, providing
striking illustrations of the decoherence and decay of the delicate
quantum states in the resonator. These detailed measurements also allow
the experimenters to extract the time-dependent density matrix for the
resonator, with non-zero on- and off-diagonal elements, and they find good
agreement with theory for the detailed time evolution of every element in
the density matrix. This paper and its accompanying movies present an
appealing and intuitive demonstration of quantum state evolution and
decay.

***

BG11217

Atomic configuration, conductance and tensile force of platinum wires with single-atom width

An atomic-scale direct visual experiment was realized for possible smallest artificial structures, i.e., single-atom-width wires. Only six platinum atoms were pulled out from a substrate in line using a nanotip. For individual atomic wires, the dynamic process was directly observed by transmission electron microscopy while simultaneous measurement of electric conductance and mechanical strength. Thus, now, our method allowed us to produce smallest artificial structures, to observe them, and to analyze electrical and mechanical properties one by one. Metallic atom wires have been classified into two large groups: those which exhibit the quantization of conductance or not. Platinum atomic wires typify non-quantized conductance group. Due to deficiency of method, the picture of wires exhibiting non-quantized conductance have not been imaged, although their electricity have been intensively studied. The present study demonstrated an experimental evidence for the picture of atomic wires exhibiting non-quantized conductance.


***

AH10402

Magic travel on "Poincare sphere"

Researchers recently investigated evolution of the optical polarization in periodically poled optical superlattice with electric field by observing polarization trajectories on Poincare sphere and novel behaviors of optical polarization were surprisingly discovered.
Optical polarization which reflects the vector nature of the electromagnetic field has been attracting researchers' curiosity for centuries and many remarkable effects regarding optical polarization has been discovered. Recently, researchers from Shanghai Jiao Tong University introduced such striking topic into an optical superlattice. Here, the superlattice functioned as a "magic box" which was able to flexibly manipulate the polarization of a light through electric field. To get an insight into such "magic", researchers observed the evolution trajectories of polarization using "Poincare sphere".
Experiments have revealed that after passing through the "magic box", optical polarization of some wavelengths motivated by electric fields will travel along a closed path parallel to the equator of Poincare sphere like planets around stars; perturbation of the initial optical polarization may force the evolution to take a different orbit. More interestingly, optical polarization of other wavelengths will however travel along a series of discrete paths with a common point of tangency. Here, the superlattice provides a spaceship for polarization and the electric field functions as an energy booster which supports the travel.
This study will promote a novel method for precise and flexible polarization control and may find applications in many scientific realms.

***

[BBR1149] R19942PRB

Doppler-Effect in Superconductivity

If waves like light or sound are reflected from a moving surface a shift of wavelength and frequency occurs, which is known as the Doppler-effect. In a recent experiment, we demonstrated that the Doppler shift also occurs in the wave mechanics of electrons, i.e. in the Andreev reflection at a normal-metal/superconductor interface. Andreev reflection means that the incoming electron is reflected as a hole, while a Cooper pair joins the superconducting condensate. Andreev reflections allow supercurrent flow through a Josephson junction formed by normal metal sandwiched between two superconducting contacts. If the condensate in the contacts is set in motion, e.g., by an external magnetic field, the wavelength of the reflected hole is drastically affected. As a consequence the allowed energy levels in the normal metal shift. In our experiment the Doppler-shift results in an unanticipated sensitivity of the supercurrent capacity of the junctions and other properties, which may be exploited in the construction of ultra-sensitive magnetic field detectors.

***

LJ11730

Dark matter nuggets shine brightly in gamma-rays

Ultracompact nuggets of dark matter formed when the Universe was in its infancy could be detected by existing high-energy telescopes. If the nuggets (“ultracompact minihalos”) are made of weakly-interacting massive particles (WIMPs), they should produce gamma-rays when WIMPs annihilate with one another. Our paper shows that depending on exactly when the nuggets formed, they could produce enough gamma-rays today to be detected by the Large Area Telescope aboard the Fermi satellite, or existing ground-based gamma-ray observatories such as the HESS telescope, in Namibia.

Dark matter makes up almost a quarter of the Universe, but so far we have no idea what it really is; WIMPs are the leading theoretical candidate. A detection of gamma-rays from WIMP annihilation would not only solve the dark matter problem, but revolutionise our current understanding of particle physics. Ultracompact minihalos are a recently-proposed dark matter structure; our paper shows that their compactness results in very strong gamma-ray signals.

October 27, 2009

LA12100

New phase of hydrogen-storage material could be missing link in
understanding dehydrogenation process

Sodium alanate (NaAlH4) has attracted a great deal of attention as a
template material for engineering practical hydrogen-storage devices
based on a class of materials known as complex light-metal hydrides.
Using molecular dynamics simulations derived from first principles
(i.e., a fully quantum-mechanical description), we have discovered and
characterized a hitherto unknown phase of this important material, which
we present as a vital step towards understanding the complex process of
hydrogen release in NaAlH4. Our calculations indicate that this new
phase, which is formed at surfaces and grain boundaries, should become
the favored structure once temperatures relevant for dehydrogenation are
approached. Structurally similar to the product of the complete hydrogen
release process in sodium alanate, the new phase could be a key missing
link in a comprehensive picture of the phase dynamics of hydrogen uptake
and release. Our findings offer a new interpretation of recent
experiments on NaAlH4 and provide an additional avenue for future
research and optimization efforts.

***

LF12333


Energy Landscape of Social Balance

The shifting of alliances and rivalries in a social group
can be viewed as arising from an energy minimization
process. For example, suppose you have two friends who
happen to detest each other. The resulting awkwardness
often resolves itself in one of two ways: either you drop one
of your friends, or they find a way to reconcile. In such
scenarios, the overall social stress corresponds to a kind of
energy that relaxes over time as relationships switch from
hostility to friendship or vice versa.

We model a close-knit community of friends and enemies as a fully connected network with positive
and negative signs on its edges. Theories from social psychology suggest that certain sign patterns are
more stable than others. This notion of social ‘‘balance’’ allows us to define an energy landscape for such
networks. Its structure is complex: numerical experiments reveal a landscape dimpled with local minima
of widely varying energy levels. We derive rigorous bounds on the energies of these local minima and
prove that they have a modular structure that can be used to classify them.

October 21, 2009

LF12510ER

Boolean Chaos

Deterministic chaos - responsible for the so-called butterfly effect –
appears in a wide variety of simple dynamical systems. Most chaotic
systems have variables taking on continuous values. However, the
switching-like behavior in many systems, such as logic circuits and
genetic regulatory networks, makes it very useful to assume the
variables taking on only Boolean values (e.g., “on” and “off”), with
information exchanged between elements connected in a network. It is
generally believed that Boolean systems cannot display chaos because
the range of possible system states is usually finite. Here, the
authors show an experimental electronic network using a very simple
digital device to generate deterministic chaotic signals. The device
is composed of a small number of commercially available, high-speed
logic gates. The circuit displays Boolean-like behavior, where the
complexity and divergence occur in the timing of switches (events)
rather than in the amplitude of the variables. The dynamics of the
device can be faithfully captured by autonomously updated Boolean
models that include history-dependent time delays. The models strongly
suggest that a large class of devices will exhibit chaos without a
need for careful selection of parameters. These models may be relevant
for representing the dynamics of complex networks in other contexts,
including genetic regulatory systems as well as electronics. The newly
discovered chaos-generating device could be the basis for a new class
of sensor or secure communications systems.

LF12532 and LE12368


Physics of a Ruck (bump) in a Rug

LF12532: The motion of a ruck in a rug is used as an analogy to explain the role of dislocations in crystalline
solids. We take literally one side of this analogy and study the shape and motion of a bump, wrinkle or
ruck in a thin sheet in partial contact with a rough substrate in a gravitational field. Using a combination of
experiments, scaling analysis and numerical solutions of the governing equations, we quantify the static
shape of a ruck on a horizontal plane. When the plane is inclined, the ruck becomes asymmetric and
moves by rolling only when the inclination of the plane reaches a critical angle, at a speed determined by a
simple power balance. We find that the angle at which rolling starts is larger than the angle at which the
ruck stops; i.e., static rolling friction is larger than dynamic rolling friction. We conclude with a
generalization of our results to wrinkles in soft adherent extensible films.

LE12368:We consider the familiar problem of a bump, or ruck, in a rug. Under lateral compression, a rug bends to
form a ruck—a localized region in which it is no longer in contact with the floor. We show that when the
external force that created the ruck is removed, the ruck flattens out unless the initial compression is
greater than a critical value, which we determine. We also study the inertial motion of a ruck that is
generated when one end of the rug is moved rapidly. We show that the equations of motion admit a
traveling ruck solution for which a linear combination of the tension and kinetic energy is determined by
the ruck size. We confirm these findings experimentally. We end by discussing the potential implications
of our work for the analogous propagation of localized slip pulses in the sliding of two bodies in contact.

***

LW11697


Quick Clay and Landslides of Clayey Soils

We study the rheology of quick clay, an unstable soil responsible for many landslides. We show that
above a critical stress the material starts flowing abruptly with a very large viscosity decrease caused by
the flow. This leads to avalanche behavior that accounts for the instability of quick clay soils. Reproducing
landslides on a small scale in the laboratory shows that an additional factor that determines the violence of
the slides is the inhomogeneity of the flow.We propose a simple yield stress model capable of reproducing
the laboratory landslide data, allowing us to relate landslides to the measured rheology.

***

LH12203

Molecules in a glass are arranged differently from those in a liquid

Conventional wisdom states that a glass is a frozen liquid, in the
sense that the arrangement of molecules relative to each other is the
same in both cases. A team of scientists from Northwestern
University, Argonne National Laboratory and Brookhaven National
Laboratory has found that in fact there are distinct differences in
the surface structure that can be observed using X-ray scattering.

Both liquids and glasses are thought to be isotropic and disordered,
but when isotropic liquids are cooled to sufficiently low
temperatures, they develop molecular-scale layers (anisotropic,
liquid-crystalline order) near the surface. When such a liquid is
cooled to below the glass transition, this layering becomes sharper
and penetrates much further into the bulk of the glass. This change
happens suddenly at the transition temperature, i.e. it is an
apparently discontinuous signal of the glass transition.

Glassy materials play an important role in everyday life, from
containers to lenses to insulating fibers, and the glass transition
is important in materials processing and also a common phenomenon in
everyday life (for example, wrinkles in clothes are removed by
heating them with an iron to above the glass transition). The new
observations may provide a test of the various competing theoretical
models of glasses, and thus lead to a clearer view of this
poorly-understood transition.

October 14, 2009

LF12228

Patterns in Flowing Sand

Patterns are ubiquitous in nature from convection rolls in thin layers
heated from below to the ripples of sand dunes shaped by the wind. Often
the formation of these patterns reveals the inner workings of the material
in which the patterns form. For granular systems, this is of particular
interest because complete descriptions of the equations of motion elude
us, making the behavior of granular flow decidedly unpredictable. Thus,
our experimental and numerical observations of robust stripe patterns of
sand flowing easily down a rough inclined plane may provide an important
ingredient for improved theory and practical modeling of this challenging
material. The stripes we observe are parallel to the downstream flow
direction, have alternating faster and slower regions, and appear to slide
along on top of highly-agitated "boiling" granular material - reminiscent
of the Leidenfrost effect where liquid is lifted by its vapor above a hot
surface.

***

BG11570

Magnet moves superconducting vortex like a hockey stick moves a puck

A movie created by magnetic force microscopy (MFM) shows the effect of its magnet (hockey stick) passing a vortex (puck) back and forth and pushing it between nanoholes in a superconductor. Analyzing such motion allows us to measure the tiny force (1 pN) that traps a vortex in a hole. The escape of vortices from traps limits the utility of superconductors because it can give rise to energy dissipation. Therefore, understanding vortex motion and trapping is important for technologies such as magnetic resonance imaging (MRI), power transmission and superconducting quantum interference devices (SQUIDs). In this work we use MFM at temperatures down to 5 K to image vortices in a superconducting thin film patterned with an array of 50 nm holes that are spaced 100 nm apart. Although the array is well ordered, we are sensitive to slight imperfections which enhance the trapping of vortices a specific sites and affect the way a vortex moves.

October 13, 2009

EG10540

Diffusion of scientific credits and the ranking of scientists


Recently, the abundance of digital data enabled the
implementation of graph based ranking algorithms that provide system
level analysis for ranking publications and authors. Here we take
advantage of the entire Physical Review publication archive (1893-2006)
to construct authors' networks where weighted edges, as measured
from opportunely normalized citation counts, definee a proxy for the
mechanism of scientific credit transfer. On this network we define a
ranking method based on a diffuusion algorithm that mimics the spreading
of scientific credits on the network. We compare the results obtained
with our algorithm with those obtained by local measures such as the
citation count and provide a statistical analysis of the assignment
of major career awards in the area of Physics. A web site where the
algorithm is made available to perform customized rank analysis can be
found at the address http://www.physauthorsrank.org.


***

LJ11750

Forget about the computational benefits of time travel

Closed timelike curves (CTCs), if they exist, would be of little or no help in computation or quantum state discrimination. Unlike, say, perpetual motion, or faster-than-light communication, CTCs don't appear to violate any fundamental laws, so it is reasonable to explore their computational consequences, but they are hard to think about because they make quantum evolution nonlinear. Some have argued that a quantum computer equipped with a "time traveling" CTC register, or undergoing nonlinear evolution for some other reason, would have vastly enhanced computational and state-discriminatory powers. We show this is not so, at least if computation and state discrimination are defined in a natural way, as generating the correct output corresponding to a given input, when the input is chosen from a set of possible inputs by an independent referee at the start of the computation, rather than having been built deterministically into the structure of the computer. The impression that nonlinear or CTC assisted computers had great computational and discriminatory power arose from overlooking this subtlety, which makes a nonlinearly evolving computer’s output depend not only on the input, but on the process by which the input is chosen.


***

LC12596BR

Electronic interferometer mystery solved

In our paper we presented an exact solution for a realistic model of electronic
Mach-Zehnder interferometer. This solution explains in detail peculiar behavior
observed in experiments which evaded theoretical understanding for several years.

These interferometers utilize quantum nature of electrons which allows them
to behave as coherent light-waves. By applying voltage to a specially
fabricated microchip, held in a high magnetic field and at very low temperature,
researchers could control average number of electrons inside the system. With only
few particles in the interferometer, they observed a perfect interference pattern
of oscillating current. However, with increasing voltage these oscillations disappeared
completely and then, reappeared again. This came as a great surprise.

Several theoretical proposals had been put forward aiming to explain the
unexpected behavior. All of them used simplifying assumptions or specific
details of the interferometer geometry and the physical picture remained incomplete.
The difficulty lies in the fact that here one has to solve a problem of
collective motion of macroscopic number of quantum particles out of equilibrium,
which is known to be a very nontrivial task. With the exact solution we give
an unbiased proof that collapse and revival of the interference observed in
experiments is a collective effect which is a result of intrinsic Coulomb
interactions between electrons.

One of the main and most interesting features of electronic interferometers is their
ability to generate Schroedinger cat like, highly entangled quantum states. They will also
allow to braid nonabelian anyons which can be useful for applications in quantum computers.

***

EGR1065

Microbes: linking performance and architecture

Do the characteristics of a living organism influence or at
least bear signatures of their network architecture and vice
versa? In a recent rapid communication to appear in Physical
Review E, physicists explore this question using systems
biology approaches. They use publicly available data for ten
phenotypes like genome size, GC content, motility, competence,
aerobicity, modularity etc. They study the metabolic networks
of 32 microbial species (both archaea and bacteria), using 11
independent topological metrics from complex networks
(including higher moments of some metrics). Using
well-established procedures from statistics and
machine-learning, the authors identify relevant subgroups of
these metrics and establish that they associate with various
microbial phenotypes surprisingly well. These studies also
show why a systematic and simultaneous study of topological
network metrics and their higher moments are important. This
wor is a good starting point towards better modeling of
phenotypes. The results also point the way towards a thorough
cataloging of biologically relevant topological features,
that can eventually yield vocabularies which cross-reference
network architecture against biological function. Finally, the
methods used are very general and can be applied to networks
from other disciplines.

October 9, 2009

LA11689


How do lotus leaves achieve anti-dew water repellency?

Many plants exhibit remarkable water repellency owing to their rough surface. The textured surface traps air underneath water drops and the air cushioning gives rise to the water repellency. A long-standing puzzle is that, unlike their biological counterparts, engineered rough surfaces do not retain water repellency when subjected to naturally occurring condensations. Researchers at Duke University uncovered an ingenious mechanism used by lotus leaves to stay dry after repeated condensations. Physicists have now shown that even the lotus leaf does not retain water repellency under condensation when fixed; however, the same leaf becomes water-repellent again when vibrated. There is a reason for the big leaf sitting on a slim stem – natural vibrations supply energy for dewetting the condensate-penetrated textures and restoring the air cushioning. The research on anti-dew water repellency is pointing toward a new direction in engineering robust self-cleaning systems.

***

LE12614


Mushrooms' spore ejection process reproduced on water-repellent surfaces

Mushrooms are known to use surface energy to discharge a spore from the tip of its sterigma. The discharge process is triggered by the coalescence of the wetted spore with a condensate drop at its base. Researchers at Duke University discovered a similar process on man-made water-repellent surfaces. Researchers have now observed that when condensate drops coalesced on water-repellent surfaces, the merged drop spontaneously jumped out of the surface. Like the spore ejection, the jumping drops are powered by surface energy released upon drop coalescence. The research is the first known engineering reproduction of the ballistospore ejection process. Their work also has immediate applications in energy harvesting and thermal management. For example, the spontaneous jumping motion offers an internal mechanism, independent of gravity, to remove liquid condensate from the condensers in power plants and spacecrafts.


***

LF12743AR

Radio-controlled atoms

Scientists have succeeded in using
radio-frequency radiation to control the way atoms collide with each
other. Cold gases of atoms have been a hot topic for many years,
particularly since 1995, when a new state of matter known as a
Bose-Einstein condensate was formed for the first time. Many experiments
since then have depended on being able to precisely control the
interactions between the atoms – tuning them to be strong or weak,
attractive or repulsive. One common way of doing this is by tuning a
magnetic field to certain values where the interaction properties change
quickly. In this new work, the authors demonstrate that radiofrequency
radiation provides a second degree of control. This can be used together
with a magnetic field to provide greater levels of control. One can, for
instance independently control the interactions between different
components of a gas with three or more types of atom. This is of
interest to other programs using cold gases, such as many-body physics
and quantum information

***

LF11943

Exploring the limits of antiferromagnetism in nanostructured materials




Researchers at Argonne National Laboratory and Politecnico di Milano in Italy have recently explored the limits of antiferromagnetism in a nanostructured material for the first time, measuring the temperature required to support antiferromagnetic order in atomic monolayers of manganese on tungsten as the dimensions of the structures are reduced. While these boundaries are well understood in ferromagnetic materials, antiferromagnetic materials Ð where neighboring magnetic moments cancel rather than add together Ð have proven much more challenging to unravel. In this work, the authors exploited the unique properties of manganese spin spirals on tungsten to correlate spin-sensitive scanning tunneling microscopy techniques on the atomic scale with electronic signatures, showing that the ordering temperature for the antiferromagnetic structure depends both on its size and its orientation with respect to the crystal lattice. Atomic-scale spin-sensitive investigations such as this will help guide the way to next generation platforms for ultra-high-density data storage and novel sensing capabilities.


Figure: (Top Panel) Schematic of the spin structure of the Mn monolayer on W(110). Nearest neighbor spins are slightly canted resulting in a cycloidal spin spiral with alternating out-of-plane and in-plane regions that repeat with a period of about 6 nm. (a) Topography and (b) differential conductance of the Mn on W(110) at 40 K. In the inset, high resolution topographic data taken with a spin-sensitive tip shows the atomically resolved spin structure, the electronic signature of which can be seen as light and dark stripes in the differential conductance map. The contrast of these stripes is used to determine the degree of antiferromagnetic order in the nanostructure.



***

LE12264BR

"Mirror, Mirror on the Wall": The Many Faces of Silver Nanoparticles

Imagine peering into a mirror and seeing a different image look back at
you! And if that is not enough, the image changes with time, twinkling
like stars in the night-time sky.

150 years ago, Bernhard Tollens invented a simple chemical route to
growing large silver mirrors. We demonstrate that such a mirror -
composed of myriads of tiny "nanoparticles" - may itself shine: rather
than merely reflecting light, the mirror actually generates light.

This effect arises because light waves behave a bit like lightning: give
them a conducting surface of a particular shape, and the metal will
absorb and transmit radiation. The consequence is that light energy
becomes highly focused, leading to optical amplification phenomena we do
not usually experience. One such phenomenon is hyperscattering of light:
much like blowing too hard on a whistle generates an overtone,
scattering intense light fields generates radiation of twice the
frequency – the mirror changes the color of light at discrete “hot spots”.

Remarkably, the radiation from these “hot spots” is not static: the
mirror constantly but reversibly changes the way in which it responds to
the incident light field on nanometer length scales.


***

EE10730

Understanding of complex behaviour of vehicular trajectories measured on multi-lane American freeways

The physics of spatiotemporal phase transitions in traffic flow on multi-lane freeways is revealed based on data analyses of vehicular traffic in the framework of three-phase traffic theory. The complex dynamics of moving jams observed in single vehicle data measured by video-cameras on American highways (http://ngsim.camsys.com) is explained by the nucleation-interruption effect in synchronized flow, i.e., the spontaneous nucleation of a narrow moving jam with the subsequent jam dissolution.
A dual role of lane changing in vehicular traffic is revealed: (1) lane changing can lead to the emergence of a nucleus for a phase transition, in particular for moving jam emergence; and in contrast, (2) lane changing can lead to moving jam dissolution.
This dual role of lane changing is responsible for a very complex spatiotemporal behaviour of vehicular traffic on multi-lane roads.

September 29, 2009

LF12390

EXACT FUNDAMENTAL LIMIT TO THE SPEED OF PHYSICAL COMPUTATION IS DETERMINED

We have established the exact fundamental limit on the rate (number per
operations per second) of any computing system. No system can exceeed this
limit. However, a system with suitably chosen parameters and states can approach
the limit as closely as desired.

The limit is set by the minimum time required for a physical computing system
to perform an elementary operation, namely, the transition from a given state
to another state distinguishable from it with certainty. This minimum time
follows from the laws of quantum mechanics and depends on the average energy
and the energy spread (uncertainty) of the system's state. Since the maximum
rate is determined by the smallest of those two parameters, the optimum is
achieved when they are equal. Then, adding one joule of energy to the system
allows its rate to increase by 6*10^32 elementary operations per second.

Today's computers are slower by many orders of magnitude. Our research reveals
the prospects for future development---what is possible to achieve and what is
required for that.

Note that our results amount to a general "law of nature": the limit does not
depend on a specific model of computation or a specific technology.

***

CH10233

Radioactive half-lives do not depend on temperature after all

If the half-lives of radioactive isotopes could be altered by changes in the temperature of their surroundings, then it might be possible to speed up the disposal of radioactive waste. Hopes for this simple solution to one of society’s problems were raised several years ago by published reports of appreciable half-life changes having been observed at low temperatures. These claims have now been scotched.
The original reports encompassed the three most-common forms of radioactivity, referred to as alpha, beta and electron-capture decays. Since then, however, well controlled and more precise measurements have discredited the claims one by one. First, it was demonstrated that beta-decay half-lives are, in fact, stable to within less than a tenth of a percent at low temperature; then the claimed changes in alpha-decay half-lives were shown to be wrong. Now, the third class of radioactivity, electron-capture decay, has also been proven stable to within a tenth of a percent (CH10233).
For nearly a century, physicists have considered the half-lives of radioactive isotopes to be independent of temperature, but without modern proofs. Now, it turns out that they were right all along.

***

LG12785

Magnetic Monopoles in the Mirror

A novel class of materials with topological order, called topological
insulators, has recently been proposed theoretically and has been
observed experimentally. One striking aspect of topological insulators
is that when an electric charge is brought close to their surface it
behaves as if it had a magnetic monopole partner on the other side of
the interface: when seen in the mirror of a topological insulator, an
electron looks like a monopole. In this work I re-obtain expressions for
this mirror charge in a formalism that treats electric and magnetic
charges on an equal footing. This “duality covariant formalism” helps to
present compact expressions for the final result and hopefully paves the
way for a deeper understanding of these fascinating materials.

***

LE12655

Creating superpositions of quantum operations to directly prove the
commutation relation


The commutation relation is one of the pillars of quantum mechanics and
brings about a fundamental quantum paradox, the uncertainty principle,
according to which precise simultaneous measurements of two
non-commutative observables is impossible.

Although fundamental and ubiquitous, the commutation relation is often
learnt as a mere mathematical notion and little physical insight is ever
gained. For a light field, it really means that annihilating a photon
after its creation is different from the opposite sequence of
operations, and the difference is equal to the identity operator.

Here we provide the first direct experimental verification of the
commutation relation by means of a general scheme for superposing
distinct quantum operations. In the language of the Schrödinger's cat
paradox, it corresponds to having implemented a new versatile and
powerful tool, allowing one to superpose the two operations "to kill"
and "not to kill", so that they may be eventually applied to any
"animal" (i.e., to any quantum state, including macroscopic classical
ones), not just unfortunate "cats".

Besides clearly illustrating one of the main textbook concepts of
quantum mechanics, the possibility of implementing general operator
superpositions is an alternative promising tool to control and engineer
quantum information for future technologies.

***

CJ10179

Addressing Nuclear Data Needs for Thorium-Based Nuclear Reactors

The energy crisis is in the forefront of the international arena and thorium-based nuclear energy systems provide a channel for massive energy generation without the overproduction of ozone-depleting greenhouse gases. The thorium-uranium fuel cycle has several advantages, with respect to nuclear nonproliferation and radioactive waste management, when compared with the conventional uranium-plutonium fuel cycle. However, much of the experimental nuclear data required for design calculations for thorium-based reactor systems are not precise or even absent. To achieve improved design calculations for thorium-based reactors, the determination or reevaluation of neutron-induced fission cross sections for short-lived radioactive isotopes of thorium is required. In this paper, we present the first measurement of the 231Th(n,f) cross section, formerly a major source of uncertainty in thorium-based reactor design calculations, to an unprecedented level of accuracy. This work fosters responsible environmental practices through sustainable nuclear energy.

***

LC12048

Experimental indication of the Majorana fermions on a surface of superfluid 3He.

Although particles are usually different from their antiparticles, Majorana fermions have a unique property: they are their own antiparticles.
They were originally posed in the field of the elementally particle physics but have not been identified yet. Theoretical developments, however,
have shown that Majorana fermions should also appear on a surface of condensed materials if their bulk properties are topologically nontrivial.
One of such material is superfluid 3He at ultra-low temperatures which has been a model system of quantum condensates with internal degrees of
freedom. In this paper, we made a systematic study of the surface states of superfluid 3He by the transverse acoustic impedance measurement with
controlling a roughness of a wall. Indication of gapless excitations with a linear dispersion, which are referred to as the Majorana cone, was
experimentally obtained if the roughness is small enough.

***

EGJ1041

Cell Adhesion

Adhesion is fundamental in cell functioning, and in various inter-cell activities of biological tissues. When an external pulling force is applied onto a cell stuck to its substrate, a reacting "suction-cup” force, due to the slow penetration of the surrounding fluid between the cell and the substrate, opposes to the separation. It can overcome other known adhesive forces when the process is sufficiently violent (typically 105N/m2). The physical origin of this effect may be compared with that leaning a suction-cup against a bathroom wall. Indeed, when the cell begins to move, the pressure under the cell diminishes, which leads to penetration of the surrounding liquid between the cell and its substrate. Then, the induced pressure difference between the top and the underside of the cell generates the suction-cup force. In contrast to similar hydrodynamic forces caused, for instance, by shear flow, the suction-cup force is purely attractive. The suction-cup force can answer to problems and questions raised
by experimentalists.

***

LZ11637BR

Graphene quantum dot

Quantum dots defined by electrostatic potential were realized in a
graphene nano-ribbon device sandwiched between a narrow top gate and a
back gate. The dot consists of a small local island filled with holes,
separated by pn junctions from the neighboring regions filled with
electrons. This is one of the first experiments where pn junctions are
intentionally used for confining electrons and holes. The quantum dot
devices demonstrated here would enable one to measure single particle
energies of Dirac particles in graphene, study their spin and valley
degrees of freedom, and explore their potential for quantum coherent
control. Our work is an important starting point for further quantum
dots applications where great tunability is desired.

On the other hand, we also found that without the intentional barriers,
in the high density limit, electrons become localized in one large
island, in contrast to the multiple localized islands found in previous
work. The reason for this localization is not yet understood.

September 23, 2009

LG12145BR

How “lasagne” become “Bolognese”:
x-ray spectra reveal how magnetic phases mix together in magnetoresistive
artificial layered structures

Manganites, a class of materials based on manganese and oxygen, are famous
for magnetoresistance, i.e. for changing resistivity under external
magnetic fields. However manganites of simplest composition, such as
LaMnO3 or SrMnO3, are insulating and antiferromagnetic; and only at
intermediate chemical composition, e.g. (LaSr)MnO3, they become
conductive, ferromagnetic and magnetoresistive. At atomic scale
magnetoresistive manganites are chemically disordered and appear as
“Bolognese” spaghetti, a random mixture of pasta and meat sauce, i.e. of
LaMnO3 and SrMnO3. Recently it was shown that magnetoresistance can be
obtained also in artificial layered structures, where LaMnO3 and SrMnO3
are alternated similarly to pasta and meat in “lasagne”. Indeed the trick
works only if the layers are thin enough. Using synchrotron radiation for
measuring x-ray absorption spectra, we have directly seen, at atomic
scale, how LaMnO3/SrMnO3 superlattices become similar to (LaSr)MnO3, i.e.
how it happens that “lasagne” can taste as “Bolognese”. Our measurements
provide a detailed description of the distribution of antiferromagnetic
and ferromagnetic regions across the material, providing useful
information on how to possibly improve the manganite recipe. A high
density of interfaces is the “secret ingredient”: the more of them you
have the better your “lasagne” will taste.

***

LD12763

Shortest light flashes ever from ultra-hot state of matter

Recent calculations show that the quark gluon plasma, a state of matter
created in heavy ion collisions at the particle accelerator RHIC and soon at
the LHC, emits light with pulse duration a million times shorter than present-
day ultra-fast lasers. Researchers strive for ever shorter and more energetic
light pulses, as these allow for measurements with better resolution in space
and time. Quark gluon plasmas are intensely studied as they are thought to
resemble the state of our universe at its very beginning. In this work, we
show that their remarkable properties advance the presently available light
sources to durations in the yoctosecond range (a number with 24 zeros in front
of the first non-zero digit), and to photon energies in the high-energy range.
This could open up the time-resolved study, e.g., of nuclear processes.
Remarkably, under certain conditions, controlled double light flashes could be
emitted. Such double flashes might eventually lead to the creation of the first
slow-motion movie ever of nuclear dynamics.


***

LE12337

Bacteria Thin Liquids

Common swimming bacteria such as Bacillus Subtilis
dramatically reduce the viscosity of liquid they swim in. A seven-fold
viscosity reduction was observed in two independent experiments with the
suspensions of bacteria confined in thin soap-like fluid films. The
scientists also discovered that the viscosity depends on the
concentration and the swimming speed of bacteria. The primary mechanism
of the viscosity reduction is related to transformation by bacteria of
chemical energy of the nutrient into kinetic energy of fluid motion.

The study sheds a new light on physical properties of a variety
biological fluids. 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:
schematics of "bacterial viscosimeter". Tiny movable probe creates a
vortex in thin soap-like film, the viscosity is extracted from the
vortex decay time.

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.