7-14-08

LR11808
Connecting Qubits to Physics

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

***

LS11513
MANIPULATING SPINS WITH A LASER, IN SUPERFLUID HELIUM NANODROPLETS

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

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

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


***

BR10825

The structure of a large family of TS lattices is equivalent

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

***

LF11807
Electrical control of nano-pendulum motions

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

***

LP11401 (Embargoed until August 5, 2008)

Chaotic Dance of Nuclear Spins

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


***


LG11893 (Embargoed until August 6, 2008)

Shaping up graphite using light

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




***

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

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


***

ZJ10029

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


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

***

BM10877

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

***

Lq11479
Polariton Light-Matter Particles Moving in Step

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

7-11-08

LP11401
Chaotic Dance of Nuclear Spins

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


***

LP11113 (Embargoed until July 23, 2008)
PREDICTION AND UNDERSTANDING OF NOVEL MATERIALS TO BE USED FOR NEW, HIGH
EFFICIENCY PHOTOVOLTAIC CELLS

A new type of material which can provide highly efficiency solar cells has
been developed. This material will have an intermediate energy band which is
one possible way of enhancing the efficiency of photovoltaic cells. The basic
operation of a conventional photovoltaic cell relies on the electron
promotion from a lower energy state (valence band) to a higher energy one
(conduction band) through absorption of photons with sufficient energy. The
intermediate band is located between these bands and helps to absorb, not
only the sun’s photons with energy higher than that of the gap width, but
also the lower energy ones. This will enhance the photovoltaic conversion
efficiency in these new cells in relation to the conventional ones. In this
paper, using quantum calculations, we have obtained structural, electronic
and optical properties of a new system based on an indium sulphide
semiconductor substituted with transition metal atoms. The computed optical
absorption of these compounds compared to the corresponding undoped material,
predicts a significant absorption below the band-gap of the parent
semiconductor and an enhancement of the optical absorption across the whole
solar-spectrum range. These systems seem promising for developing more
efficient novel optoelectronic devices. Their experimental synthesis has
already been reported.



***

BR10591
The solid solution hardening SSH is a long standing problem of statistical physics.
The early time of the SSH theory sends us back to the analytical works of the late
Sir Nevill Francis Mott and Frank Reginald Nunes Nabarro. The strength of a single
crystal was then determined through some continuous models and it was predicted
to vary as a fractional power law of the solute concentration. An important number
of experimental studies were performed and the compiling of their results reveals
the conundrums left aside by the early theory, as for instance the temperature effect.
The current revival of the SSH theory is mainly supported by of the development of
the 3 dimensional atomistic simulations. The atomistic simulations allows to shed a new
light on the SSH and to advance on questions that remain.
In our paper, we studied throughout numerical simulations an alloy extensively used
in aeronautics as matrix for wings and engines, i.e. the Nickel-Aluminum system.
This allowed us to emphasize that the common belief for the weakness
of the screw dislocation pinning strength does not hold for the solid solution we studied.
We determined which version of the SSH theory is the more adapted to predict the strength
of our system. Such a result should serve as a guide to further the theoretical
developments on the dislocation mobility in random media.

***


LR11643
Colorful approach to the fractional quantum Hall effect

The different types of fractional quantum Hall effect may be described in
terms of multi-color quantum liquids. This is the result of a trial wave
function approach proposed in a recent paper by Regnault, Goerbig, and
Jolicoeur (CNRS France), which yields a complementary vision of the
fractional quantum Hall effect. Indeed, each color group of two-dimensional
particles in a strong magnetic field forms a Laughlin liquid. However, in a
typical system, there is only one type of particles, say gray, and the colors
need to be viewed as an artificial marking. In order to get rid of these
artificial colors, the authors have proposed a procedure which renders all
particles gray again. This may be viewed as taking a black-and-white photo of
a colorful painting. Amazingly, the black-and-white photo reveals an internal
structure of the quantum liquid - it consists of distinct droplets the number
of particles of which is that of the original colors. The performed numerical
calculations indicate that this structure may be a common feature of the
different types of fractional quantum Hall states, such as the
composite-fermion states or else the 5/2 state with its exotic excitations.



***

LS11531
ORIGIN OF GIANT OCEAN WAVES

Scientists at Lancaster University have made a discovery that
illuminates the origin of rogue waves -- the giant waves that
occasionally appear on the ocean and are suspected of being
responsible for many unexplained losses of large ships. These
waves are quite different from the tsunami created by undersea
earthquakes, which in the open sea are usually so low that they
are almost invisible. In contrast, survivors describe a giant wave
as being like ``a wall of water'', perhaps 100 feet or more or
more in height. There is intense interest in the origin of giant
waves on account of the commercial importance of this
extraordinary phenomenon. The Lancaster team is studying nonlinear
wave interactions through experiments on superfluid helium. These
enable fundamental wave processes to be studied under controlled
conditions, in contrast to giant ocean waves which require
hundreds of miles of open sea to appear and disappear. The
scientists were astonished to discover that wave energy could
sometimes concentrate to create giant waves in the laboratory. If
the new understanding can be exploited to explain how rogue waves
arise on the ocean, it may be possible to predict them. If so,
there will be many grateful mariners and insurance companies.


***

LP10938A
Confined light could rotate microscopic rods on a chip

Laser light can apply mechanical pressure on microscopic objects, causing them to be trapped and even rotated. In this paper, we theoretically demonstrate that light that is being guided by a tiny rod and confined around it, could rotate the rod itself, forming a new type of microscopic machines that could be on a chip. The rod, possibly made of a glass-like material, could be a fraction of a micrometer in diameter and only a few micrometers long. Laser light with a "rotating" wave front needs to be injected into the rod, and then if the rod absorbs some the light, it will tend to rotate. This is similar to a plastic collision in mechanics, such that the rotating light particles "stick" to the rod and consequently apply a rotational force. Another effect that results from the interaction of the light with the absorptive rod is that the light tends to push the rod forward. Several light-driven motors may be conceived based on this concept, such as a microscopic drill that is being pushed forward and rotated by the guided and confined light.


***
LE10914AR
FROG-CRAB catches the light



Researchers at JILA, University of Colorado at Boulder, have made a giant leap
in simplifying the generation of some of the shortest light flashes achievable to date:
x-ray pulses that are just about one femtosecond (a millionth of a billionth of a second)
long can now be generated by simply shining high-power laser pulses into a gas-filled hollow
capillary. Now, if you generate an event this short, then not only making it, but also
measuring it is a challenge. The researchers accomplished this using a method called
FROG-CRAB. The ultrashort x-ray flash is scanned across the electric field oscillations
of a second, much longer infrared light pulse, and the combined electric field of the long
and short pulse knocks out photoelectrons from a noble-gas. The recorded “spectrogram”,
shown in Figure 1, contains all the information required to know the shape and duration of
the ultrashort x-ray flash (shown in Figure 2). This kind of ultrafast radiation can now
serve as a flashlight to track some of the fastest events occurring in our everyday world,
as they happen e.g. in molecular and materials dynamics, or in the motion of electrons in a
chemical reaction.

***

LP11298
Crackling dynamics of cracks

From broken dishes to collapsing buildings, materials failure bothers,
damages or devastates our life. While the failure of homogeneous solids
is well described by Linear Elastic Fracture Mechanics, the case of
heterogeneous materials remains far more complex. In particular, the
cracks propagation displays there an intermittent dynamics -- so-called
crackling dynamics -- with seemingly random discrete jumps of a variety
of sizes. Indeed, the distribution of energy released through these
jumps forms a power law with no characteristic size scale, as observed
for instance in the acoustic emission accompagnying the failure of
various materials or - at a much larger scale - in the seismic activity
associated with earthquakes. Here we derive - and confront to
experiments - a simple stochastic description for crack growth in
heterogeneous media which suggests that this crackling dynamics observed
in fracture exhibits statistical features insensitive to the mechanistic
details. This "universality" proves that model experiments in
laboratories and more complex failure phenomena as earthquakes share
common, and to some extent predictable, features.

7-9-08

LE10865A

An interferometer-free experiment is proposed aiming at violating a
Franson-type Bell inequality using photons generated from four-wave mixing
in an ensemble of two-level atoms. Previous research showed that when a
retro-reflected pump is passed through the ensemble, pairs of
counter-propagating photons emerge off of the pump axis. These photons can
either be produced at the same wavelength as the pump beam or at
wavelengths differing from the pump by the Rabi frequency that the pump
induces. In this paper, we show that interference (Figure 1) between these
two processes leads to oscillations in the photon correlation time since
photons emitted at the same wavelength will experience the same group
delay in the ensemble whereas those emitted at different wavelengths will
experience different group delays. The violation of Bell¿s inequality
further implies the result for non-locality. Since the energy separation
between photons with different wavelengths is tunable, this experiment
might be an interesting way of probing the quantum nature of the detection
process. The interference (Figure 1) will disappear when the separation
approaches the fundamental timescales for photon absorption in the
detector.

7-7-08

LF11471
Superconducting silane is layered

An international research team has predicted from first principles the
superconducting properties of silicon-based hydrogen rich alloy ┐ silane
(SiH4) with a layered structure, fueling up the possible realization of
metallization and superconductivity in dense hydrogen which has long been
a major driving force in high-pressure physics and remains an important
challenge in modern physics and astrophysics.

Silane offers a source for high purity silicon which is at the base of
electronics and microdevices. It is gas at ambient condition. The new
structure having layered network is formed when silane is subjected to
pressures above 600,000 times atmospheric pressure at sea level. The
researchers obtained the superconducting transition temperature in the
range of 20 and 75 K in the layered metallic phase. They demonstrated that
silane is a good example to metallize and superconduct hydrogen at modest
pressure much lower than necessary for solid hydrogen because it has been
already chemically precompressed.

The research, publishing in Physical Review Letters, suggested that the
layered feature could be essential for superconductivity in other
hydrogen-dominant compounds.


***


LQ11339
Light rulers on a microchip

A frequency comb is a laser source that emits a spectrum of many different discrete frequencies (corresponding to different colors), which are perfectly uniformly spaced and can serve as a ruler to measure optical frequencies. Frequency combs have become a universal tool for optical frequency metrology, spectrometer calibration, gas sensing and arbitrary optical waveform generation within the last decade and part of the Nobel price in physics in 2005 has been dedicated to this invention.
Recently we presented frequency comb generation in circular microresonators made of fused silica. These sub-millimeter resonators are fabricated on microchips using processes that are well known from computer chip production and are promising elements for integrated photonic computers. However, to generate a uniform frequency comb, the time a photon needs for one round-trip in these resonators has to be stabilized. In our work we present in a remarkably simple approach how this stabilization can be done by controlling the optical power sent into the resonator and using a thermal effect that controls its effective size. Using this approach we were able control the average photon round-trip time during one second down to a level of 10-23 seconds (0.00000000000000000000001 s). This represents an important step towards phase stabilized on-chip frequency comb generators.

***

EQ10358
More Bandgaps in Periodic Structures

Periodic structures featuring forbidden bandgaps have found a great many
applications in diverse areas of physics and engineering. Almost all
bandgaps found hitherto are induced by the well-known Bragg resonance which
occurs between waves of identical field profiles transverse to propagation
direction. Physically, resonance can also occur between waves of distinct
transverse wave profiles. However, this non-Bragg nature resonance was
usually overlooked until its existence was proved in electromagnetic waves
[1]. Extension was recently made to sound waves in an axially hard-walled
duct [2]. In the present work we make a further extension, both
theoretically and experimentally, to surface waves in a water trough with
corrugated sidewalls. Our results demonstrate the coexistence of both types
of resonances in water-waves, thus concluding the ubiquity of the phenomenon
for classic waves. We also find that the bandgaps are highly tunable by a
simple geometric arrangement, and the non-Bragg bandgap can even be made as
wide as the Bragg one. What surprises us most, just as in the case for sound
waves, is the impressively greater transmission loss within the non-Bragg
gap, which shows the higher efficiency in localizing wave energy. The
richness of transverse modes in waveguides implies the feasibility of
implementing more bandgaps with improved techniques, thus opening a new
avenue of control over band structures.


***

LQ11818
A Hot Union

The phenomenon of superconductivity requires two essential ingredients: the
binding of electrons into pairs and the establishment of coherence between
the phases of the pairs' wave-functions. Unfortunately, systems in which
pairing is strong typically exhibit a low transition temperature due to
significant phase fluctuations. On the other hand, large phase stiffness is
commonly accompanied by weak pairing. An intriguing question then arises:
Is it possible for a composite system, made of a strong-pairing component
and a phase-stiff element, to inherit the best of the two worlds and posses
a transition temperature higher than those of both its constituents? We have
demonstrated that such an enhancement indeed takes place at the interface
between underdoped and heavily overdoped cuprate high-temperature
superconductors. Our results corroborate the notion that the underdopd
regime of the cuprates is governed by a high pairing scale and strong phase
fluctuations, while the overdoped region is more conventional in the sense
that pairing and phase order occur simultaneously. Interestingly, by varying
the doping level of the underdoped layer we found that the maximal
transition temperature of the bilayer is obtained at the same doping level
where the transition temperature of the bare underdoped films exhibits an
anomalous suppression. This suppression is usually associated with the
spontaneous segregation of the electrons into quasi-one-dimensional
"stripes", and our findings may shed new light on their role in the
mechanism of high-temperature superconductivity. From a practical point of
view, the approach pursued in this study may offer guidance for the design
of higher temperature superconductors.


***

LE11370BR
Watching superconductivity to appear in copper oxides

Summary:
High temperature superconductivity occurs when an
antiferromagnetic insulator is doped with charge carriers beyond a certain
level by
slightly modifying its chemical composition. In this paper we show
that, while the magnetic order disappears continuously, there
are discontinuous changes in the
electronic, lattice and magnetic properties at the onset
of superconductivity. This is not at all
obvious since the new state emerges at zero absolute temperature
where transitions are often governed by quantum fluctuations
favoring continuous variations. In our light scattering
experiments we follow the evolution of spin, charge, and lattice
excitations with doping. We find that all vary continuously
up to the onset point of superconductivity. Here, all of a
sudden spin excitations and one of the lattice vibrations become
strongly damped and the charges start behaving as those of a normal metal.
In many respects this is reminiscent of a first order phase transition
such as the one between ice and water.


***

LQ11301
Biodiversity in ecological systems is often maintained by the
self-arrangement of the interacting individuals into spatial patterns.
In our article, theoretically and generally, we investigate the effects
of such self-organizing spatial patterns, and find that, in most cases,
they support species diversity. However, we also identify a situation
where an instability of patterns results in rapid species extinction.

Our work builds on recent microbial experiments. There, three strains of
colicinogenic Escherichia coli display a competition similar to the
children's game "rock-paper-scissors". Growing on a (essentially
two-dimensional) Petri dish, spatial patterns form. In each spatial
region, one of the three strains dominates. These patterns help to
protect one strain from the others, and therefore enable stable
coexistence, in other words, the diversity. In our theoretical work, we
investigate a more general situation where three species exhibit cyclic
dominance. We demonstrate that, in most cases, the formation of spatial
patterns helps the maintenance of biodiversity (see Figure). In
contrast, and for the first time, we also show that in a certain regime,
the opposite is true: the self-formation of patterns leads to rapid
extinction of all but one species. We provide a fundamental
understanding of both effects in terms of an analytical description via
a complex Ginzburg-Landau equation.


***

LQ11182
RELATIVISTIC CONDITIONS FROM LONG-WAVELENGTH LASERS

It is shown in a recent PRL paper (Manuscript LQ11182) that
long-wavelength, strong-field lasers can provide access to
a relativistic domain of phenomena that has not previously
been examined. In particular, the transverse fields created
by a long-wavelength laser can produce an environment in
which the magnetic field of the laser becomes important.
With increased wavelength, true relativistic conditions can
be established. This is contrary to expectations based on a
theoretical analysis using a so-called "tunneling method",
thought to be valid for long wavelengths. The new PRL
article: 'Limits on tunneling theories of strong-field
ionization', shows that a tunneling method is not
applicable when the laser has a sufficiently long
wavelength. Instead, powerful long-wavelength lasers - such
as existing infrared FELs - can approach and enter a
low-frequency relativistic domain about which almost
nothing is now known.

***

BN10869
Large-scale Atomistic Simulation for Ferromagnetic Materials at Elevated Temperatures

Magnetic materials, on both nano- and macro- scales, have been used for a growing number of applications of increasing sophistication, ranging from the storage, recovery, communication, manipulation and processing of information, to quantum computing, and to irradiation damage-resistant ferritic-martensitic steels for nuclear reactors. However, modeling dynamical processes in these materials at high temperature and/or under irradiation proves difficult because the correlated dynamics of motion of atoms and spins is characterized by broadly similar timescales, making the evolution of the corresponding variables inseparable. The spin-lattice dynamics approach developed in this paper links the real-space motion of the atoms and the precession of their spins in one time-dependent simulation, yielding an interactive description of the lattice and spin subsystems.

The example simulations described in the paper include spin-lattice relaxation of domain walls, equilibrium and time-dependent spin correlation functions, temperature dependent magnetization curves for both the infinite periodic and finite-size atomic systems, the analysis of short range spin order below and above the Curie temperature, and the effect of magnetism on elastic modulus and thermal expansion of the material. The results show that the method will likely replace conventional molecular dynamics simulations in iron-based alloys, steels and other materials where magnetism strongly affects structural, mechanical, and various other properties relevant to applications.




***

LR11184
Do we know what it really means by alloy?

Alloying effect is generally considered well understood, and the subject
is obviously of less interest than nanomaterials these days.
Conventional wisdom tells us that an alloy state of a disordered
structure can be connected to a well defined Bloch state belonging to an
ordered structure in a so-called virtual-crystal approximation. In this
paper, we have revealed that such a understanding is invalid for alloy
states in general, based on a first ever performed systematic
examination of a prototype semiconductor alloy system Ga(x)In(1-x)P
throughout the whole composition range (0 < x < 1) and in a broad
spectral range. This material happens to be of one of the few most
important semiconductor alloy systems, if not the most important one, in
major technology applications: telecommunications, photovoltaics, and
solid-state lighting. It is the key component that leads to the recent
breakthrough in > 40% multijunction solar cell efficiency and offers a
realistic potential to exceed 45%.


***

LQ11940BR
THEORETICAL STUDIES OF ORGANIC INTERFACES AID THE DEVELOPMENT OF
ORGANIC ELECTRONICS

The strong push to use organic materials in the electronics and
optoelectronics industries means that an in-depth understanding of the
physical processes underlying the performance of novel devices such as
flexible displays and organic solar cells is required. In the present
work, the interface formed between one of the most widely used organic
semiconductors, pentacene, and the surface of a carbon electrode
(graphite) was studied using quantum-mechanical calculations.
Understanding such interfaces is critical since electronic devices
present multiple layers of materials and the interfaces between these
layers strongly control the motion of electrical charges across the
device and directly impact device performance. A novel methodology
capable of describing interfacial electronic processes was developed
based on a molecular-level picture; its validity was assessed through a
comparison with available experimental data. Subtle effects due to the
weak electronic interactions between pentacene and graphite were
uncovered and have significant implications for charge transport. The
current work is expected to help in the development of novel
optoelectronic devices by providing a toolbox of computational methods
allowing the description of key electronic processes.

7-3-08

LQ11254
"Stroboscopic" wavepackets make sense of the quantum transport of electrons

The flow of electricity through nanoscale wires and structures is dominated by
the quantum nature of matter, in which the charge-carrying electrons must be
described by probability waves. The need for a quantum approach makes the
accurate simulation or even qualitative theory of such problems very
challenging. In our Letter we present a formally exact approach based on
special quantum-mechanical "wavepackets" which are snapshots of the wave of a
single electron taken at regular time intervals -- as if by a stroboscope --
within a controlled energy range. When many electrons are present, each
electron passes from one wavepacket to the next as the stroboscope flashes.
Crucially, this guarantees the exact fulfilment of Pauli's famous exclusion
principle which prevents two electrons from being in the same place at the
same instant. Viewed now in terms of these wavepackets, many complex
problems in quantum transport gain a simple "common-sense" interpretation.
We illustrate this by deriving new results for how the current builds up, as
a function of time, when a voltage is applied to a nanostructure, and also
for the accumulation of electrons of a preferred spin at the edge of a
current-carrying layer.


***

LM11702
Liquid crystals on droplets: designing superatoms

Chemists and materials scientists have only 80 stable atoms available
with which to make molecules and materials. The molecules are formed
by directional quantum mechanical bonds which share electrons between
neighboring atoms. Each atom has a very limited number of possible
bonds it can form, determined by its atomic number.
In recent years attention has turned to creating a new kind of
building block (a superatom) which may be 100-100,000 times larger
than conventional atoms. The bonding between superatoms is driven by
the rich statistical mechanical physics of nanometer to micron scale
structures that undergo complex thermal fluctuations. Since they are
not restricted by quantum mechanics the number of potential superatoms
is enormous and the corresponding supramolecular chemistry and
materials science is only just beginning to be explored. One way to
create superatoms is to self-assemble liquid crystals, like those
found in your digital watch, on spherical water droplets in oil in the
same way mustard seeds can stabilize oil droplets in egg yolk (mostly
water) in the classic sauce mayonnaise. Liquid crystal molecules on
spherical droplets have unavoidable irregularities (defects) in their
patterns at which additional molecules can be attached to link droplets
into molecules and bulk materials. In our paper we show one can control
the location of the defects by varying the material properties of the
liquid crystal, thus allowing for the controlled design of a variety
of superatoms.


***

BR10744
Evidence of local superconductivity in granular Bi nanowires fabricated
by electrodeposition


Bulk Bi is a semimetal down to at least 50 mK without showing any
evidence of superconductivity. However the electrical property of Bi
films and nanowires is very sensitive to the exact conditions on how the
samples were fabricated. Recently, we systematically investigated the
electrical properties of granular Bi nanowires consisting of
rhombohedral Bi grains of a few nanometer in size, fabricated by
electrochemically depositing Bi into porous polycarbonate membranes. It
was found that the granular Bi nanowires exhibit superconducting,
insulating or even super-resistive behavior sensitively depending on the
details of its morphology or specific configurations of the nanowires.
The superconductivity comes from the interfacial structures between the
grains of the nanowires due to structure distortion or lattice strain,
but the grain itself with a rhombohedral structure may still remain its
semimetal or semiconducting character. The superresistivity is the
consequence of the nucleation of local superconductivity at the grain
boundary area without long-range phase coherence, a reminiscent of the
¿Cooper-pair insulator¿ observed previously in ultra thin
two-dimensional (2D) superconducting films.


***

LP11053
Critical dynamics of vesicle stretching transition in elongation flow

Biological membranes often develop nanotubes and form dynamical tubular networks, which connect over
long distances various cell types, including neuronal and immune cells, to provide signal communication
as well as virological transport. In this paper we report an effective way to produce nanosize lipid tethers from
tubular vesicles by elongation flow at its stagnation point. A surprising observation is that during the
stretching a sequence of transitions to various conformational states occurs (Fig.1). First transition from
tubular-to-dumbbell shape takes place at the critical strain rate. The striking features of the stretching
transition are critical slowing down in vesicle relaxation towards a steady state and increased fluctuations
of vesicle stretching, the features similar to well known in the continuous thermodynamic phase transition.
Thus, we suggest that the critical effects are a universal dynamical phenomenon, which occurs in any
microscopic and mesoscopic objects where thermal noise leads to large variety of available configurations
close to the conformation transition, where the entropic force (elasticity) is balanced by the hydrodynamic
drag (stretching force). At higher strain rates, ext order modes become unstable that manifests in a much
faster rate of a tube extension and leads to the well-known pearling state. The larger the strain rate,
the higher order mode becomes unstable, the more pearls initially appear, and the faster the rate of a tube
extension towards the even longer and thinner tube.

***

LP10964AR
Building quantum processors in a noisy environment

Quantum information processing involves manipulation of entities (atoms,
ions, photons etc.) which obey laws of quantum mechanics. Building
stable, scalable "quantum components" like registers and processors is
one of the biggest challenges in physics. The major obstacle to quantum
processing is due to the deleterious effect of the surrounding
environment that destroys correlations arising out of their
¿quantumness¿. This is called decoherence and has been known for a long
time. The schemes proposed to counter decoherence fall into two broad
categories: error correction and error avoidance. In this paper, we
investigate the latter under some realistic assumption. The
error-avoidance schemes rely on the fact that in some cases it is
possible to encode information in quantum states that do not degrade
(decohere). We show that for most realistic models this can only be
approximated. The validity of the approximations depends on the relative
strengths of the interactions. We give various conditions and
estimations on these approximation schemes. The implication is that for
succesful quantum processing components we need more detailed modelling
of the quantum system and its interactions with the environment.

It might be much harder to build a working quantum processor than we expect.

7-1-08

LQ11508B
A pulsed semiconductor disk laser shows hysteresis in the output characteristics

Hysteresis in a semiconductor disk laser pulsed with a semiconductor saturable absorber
mirror is reported for the first time. This kind of bistable laser can potentially be utilized
in optical memories and clocks for computers.
Semiconductor disk lasers combine the wavelength versatility of semiconductor lasers with
the high optical power and excellent beam quality of traditional thin disk lasers, which makes
them ideal for projector and display applications. Pulsed semiconductor disk lasers can reach
multigigahertz pulse repetition rates and are well suited for high-speed communications,
switching, sampling, and clocking. The laser studied in this paper could operate at three different
pulsing frequencies in the gigahertz range, exhibiting bistability and hysteresis between
these states in the output power as well as in the laser wavelength and pulse width.
Furthermore, the size of the hysteresis loops could be tuned by varying the design of the light
intensifying semiconductor structure, holding promise for controllable devices in the future.

***

LQ11508B
Improvements in magnetic nano-oscillators as a function of the
orientation of an applied magnetic field

In devices consisting of an alternating stack of magnetic and
nonmagnetic metal layers, a perpendicularly-flowing direct electrical
current can become partially spin polarized upon passing through one
magnetic layer and can then transfer some of that spin into the next
magnetic layer downstream. As a result, under certain experimental
conditions the magnetization of the second layer can be excited into
steady-state oscillation and it emits microwaves. This phenomenon is
under development for making frequency-tunable nanoscale microwave
sources. For these applications, it is desirable that the
oscillations be highly reproducible and periodic, i.e. that they have
narrow linewidths in their frequency spectrum. In this paper, we
study how these linewidths depend on the angle of an applied magnetic
field. We found that the most-commonly studied field orientation, in
the plane of the magnetic layers and parallel to the magnetic easy
axis of the oscillating layer, actually produces the worst microwave
properties. As the field angle is rotated by 90 degrees in the sample
plane, the linewidths can decrease dramatically, by a factor of 20-50
in some devices. This enables a simple way to improve the microwave
source. Computer simulations attribute the improvement to a
transition from spatially incoherent dynamics within the moving
magnetic layer to nearly spatially-uniform oscillations.


***

LQ11476

SLOW ANTIPROTONS UNRAVEL THE SECRETS OF ATOMIC COLLISIONS.

We have used a beam of slow antiprotons to learn more about the dynamics of
atomic collisions in general.

One of the first successes of quantum theory was the accurate description of
the structure of the isolated hydrogen atom. Due to difficulties with the
inclusion of the interaction between its two electrons, it took years to
reach a similar accuracy for the helium atom.

Today, 90 years later, similar progress has not yet been achieved in the
case of atomic collisions, where a description of dynamic systems involving
several electrons are needed. Even for the "theorists dream system", namely
the collision of a slow antiproton on a helium atom leading to the
liberation of one electron, for which a large number of advanced theoretical
calculations exist, these differ by up to 30%.

Using a new scheme for the creation of a DC beam of slow (a few keV)
antiprotons at CERN, this group have obtained benchmark data for the
development of more accurate theoretical calculations.

The use of antiprotons instead of other ions is important, since
antiprotons, due to their negative charge, do not support the process where
electrons are transferred to the slow projectile. This simplifies the
already difficult task of the theorists.


***

BQR1070
Temperature-induced spin coherence dissipation in quantum dots

Electron spins in quantum dots are an attractive candidate for
implementing quantum information technologies in a solid state
environment. This attractivity is mainly based on the long living spin
coherence in these systems, as most of the spin relaxation mechanisms
which work very efficiently in systems of higher dimensionality are
suppressed in quantum dots. The interaction which cannot be avoided at
cryogenic temperatures is the hyperfine interaction with the
background of lattice nuclei limiting the coherence time $T_2$ to the
microseconds range in GaAs based quantum dots. Despite of these
results, the understanding of the spin dynamics is still fragmentary.

For practical applications also the temperature dependence of the
coherence time is of key importance. In this work the authors report
the first measurement of the variation of $T_2$ for quantum dot
electrons with increasing temperature. Such measurements became
possible by addressing quantum dot ensembles and implementing the
recently developed technique of mode-locking by which the spin
ensemble is synchronized with a periodic excitation laser. This
technique allows a refocussing of the spins in the inhomogeneous
ensemble by which the single spin dynamics can be extracted. It was
found that $T_2$ is constant in the microseconds range up to 20 K, but
for higher temperatures shows a sharp drop. Comparison with detailed
model caluculations shows that this drop arises from fluctuations of
the hyperfine interaction due to the temperature induced lattice
vibrations. These results are a key step towards understanding the
limitations of spin relaxation which are under lively debate at the
moment. From this understanding one might develop tools to extend the
coherence towards longer times, as required only not for quantum
information but also for spin-based electronics



***

LS11288
Demystifying the Plutonium Enigma

Plutonium is an element which, while controversial, is central to our national
security, both in terms of energy production and defense. Despite almost half
a century of research, plutonium is remains a fundamental problem
of condensed matter physics. In this paper, we have performed one of the most
accurate simulations to date of the electrons in solid Pu using the Dynamical
Mean-Field Theory (DMFT) and Density Functional Theory (DFT). This achievement
was made possible by both recent theoretical developments and the massive
supercomputing resources at Lawrence Livermore National Laboratory. We
show that the electrons in Pu are delocalized, but are somewhat heavy due to
their interactions. The behavior of the electrons are then studied as the
volume of the Pu lattice is expanded, which is a feat that is not easily
achieved under controlled experimental conditions. The electrons become
progressively heavier as the volume increases, and for a 30% expansion they
behave in a localized fashion at ambient temperatures. Predicted trends of
various physical properties are in favorable agreement with experimental
observation. The success of this study is simply the beginning of a much
longer journey of discovery. Delta Pu displays many anomalies which are not
sufficiently understood, such as a density which is less than liquid Pu (like
water) and the fact that it shrinks when it is heated! Finally, it appears that
we may have a tool, DFT+DMFT, which is capable of capturing the salient physics
of Pu and revealing its secrets.


***

LP11489
The universe is necessarily unpredictable

From the flight of a falling apple to the rising of the sun, the world
around us appears to be highly predictable. However, in contrast to
these examples, we show that there are particular natural processes
which are entirely unpredictable.

A remarkable feature of our world is that numerous physical phenomena
can be calculated with virtual certainty. In fact, such certainty can
only apply to large objects; for smaller ones, our ability to make
precise predictions becomes inherently worse. This is because small
objects are described by an altogether different theory, quantum
mechanics. Consider, for example, the radioactive decay of some atom.
Quantum mechanics tells us the probability of decay within, say, the
next hour, but not the exact time of decay. This is an undesirable
feature and led several scholars, including Albert Einstein, to
speculate that there may exist a (yet unknown) more complete
description of quantum processes. However, as our article shows, this
is impossible. The work of Kochen and Specker and of Bell showed that
outcomes of physical experiments cannot generally be predicted *with
certainty*. We have extended their result to make a stronger claim,
that the outcomes of particular physical experiments are *entirely
unpredictable*.


***

BP10887
A rule of selecting preferred copolymers for organic optoelectronic devices

Organic light-emitting diodes (OLEDs) and photovoltaic cells are optoelectronic devices with contrary operation processes. Surprisingly, it has been revealed that donor-acceptor copolymers are species of promising materials that favor both OLEDs and photovoltaic cells. How to choose efficiently a copolymer for a specific purpose is a realistic issue that must be addressed by experimentalist. In this paper, by employing a dynamic simulation method, we propose a general role associated with the electronic structure of donor-acceptor copolymers, such as the ratio of level offset to polaron or exciton binding energy, and the interfacial electronic coupling, to help experimentalist on this issue. It is found that the most favored level offset for photovoltaic cells is the one with level offset greater than the exciton binding energy while for OLEDs is that between polaron and exciton binding energy. The electronic coupling at the interface is also an important factor that can affect the operation of optoelectronic devices. Our study presents a helpful guidence to experiments in the preliminary selection of preferred copolymers for organic optoelectronic devices.

6-30-08

LQ11225B
Unusual dependence of the superconductivity on the thickness of MoGe films.

Thin films of MoGe show progressively reduced superconducting transition temperatures as their thickness is decreased. The optical conductivity has been obtained from far-infrared transmission and reflection measurements for a set of such films. The absorptive part of this conductivity shows a clear energy gap whereas the dispersive part measures the superfluid density. The superfluid density decreases as thickness (and transition temperature) is reduced, whereas the ratio of the gap to the transition temperature, a measure of the strength of the pairing interaction, is unchanged. The normal state properties (e.g., the resistivity) also do not change. Although the result that the transition temperature is suppressed in thin films, even though the basic electronic properties and the strength of the superconducting pairing interactions in the metal do not change, initially seems to contain contradicitons, detailed theoretical calculations, using a sophisticated theory of superconductivity, do provide an explanation of the results. However, the measurements also find that the expected coherence peak in the optical scattering rate is lost in the thinner samples, a result which cannot be explained within conventional theories.

6-27-08

LN11370ER
Toroidal crystals: physics on the surface of a donut

Are there defects in the most effective packing of sugar grains on
the surface of a donut? Remarkably the ordered structure of many
natural systems can be related to this simple and appetizing
question and the answer is frequently yes! In the language of
geometry the surface of a donut is called a torus. Crystalline
assemblages of identical sub-units packed together and bent in the
form of a torus have been discovered in the past ten years in the
protein coats of viruses, self-assembled fatty acids and carbon
nanorings. In our article we provide for the first time a unified
description of the structural properties of toroidal crystals based
on the elasticity of disclination defects. In two-dimensional
crystals disclinations are lattice sites with more or less than the
average number (6) of neighbors. Disclinations typically have 5 or 7
neighbors in a triangular lattice. On a flat surface disclinations
are energetically prohibitive and never appear in the ground state.
As soon as the crystal is curved, however, disclinations may appear
and fundamentally alter the basic order. Disclinations may serve as
active biological sites or places for chemical linkages so that
mesoscopic toroidal surfaces can spontaneously link to form novel
molecules and bulk materials. Donuts continue to surprise.

***

BQR1070
Temperature-induced spin coherence dissipation in quantum dots

Phys. Rev. B Rapid Communication and Editor's Suggestion

Spins in ensembles of quantum dots offer one possible pathway to
implementing quantum information technologies in a solid-state
environment. Unfortunately, the spin interaction with the host lattice
leads to coherence lost of the quantum bit. In this work the authors
reported the first measurement of the temperature dependence of the
electron spin decoherence time T2 in semiconductor quantum dots and
they compared their experimental results to recent theoretical
calculations. It has been possible thanks to: (i) a refocusing
technique using laser pulses called "mode-locking technique" developed
by this research group which allows to avoid the inhomogeneities
within an ensemble, (ii) the investigation of InAs quantum dots where
the electronic confinement is much larger than GaAs dots (gate-defined
GaAs dots which allow only very low temperature measurements). It was
found that T2 remains constant up to 20 K and then it presents a sharp
drop due fluctuations of the electron spin interaction with the
lattice nuclei spins. The topic of the paper is of timely nature. The
results are quite important for the corresponding science community
considering the actual debate in the community about the mechanisms
responsible for spin decoherence.


***

LN11086B
Noise self-pumping in long Josephson junctions

The noise self-pumping effect in a spatially extended system is investigated.
This effect leads to significant degradation of noise properties of spatially
extended systems. In particular, in long Josephson junctions it is realized
in a similar fashion as usual ac self-pumping effect: fluctuating solitons,
radiating from the junction, induce fluctuating magnetic field, which in turn
modulates the dynamics of the soliton chain and increases the spectral
linewidth. Contrary to the theory for short Josephson junctions,
predicting linear decrease of the spectral linewidth with increase
of junction length, the minimum of the linewidth versus the length
is observed both for uniform and non-uniform bias feed
distributions.



***

LH11065E
Re-entrance in vitro

In a recent communication, researchers studied the effect of temperature
on single stranded DNA which can form a hairpin structure as seen in the case of
Molecular Beacon. They showed that in the constant force ensemble (appropriate for the set up like magnetic tweezers) the reaction co-ordinate i.e. extension may increase or decrease with temperature as seen in the recent experiment. Though an increase in extension with temperature is well understood theoretically, but there is no clear understanding about the decrease in the extension with temperature. Their exact solution based on the model Molecular Beacon for short chains showed that the decrease in the extension is an entropic effect. This advancement in knowledge now may resolve a long standing issue related to the prediction of re-entrance in force induced transitions where a double stranded DNA (dsDNA) goes to the zipped state from the unzipped state and again to the unzipped state with temperature. Notably now the prediction of re-entrance is not
only confined to dsDNA but also for other bio-polymers e.g. proteins and homopolymers but it experimentally remained elusive so far. Using the large conformational change
in the reaction co-ordinate and the formation of hairpin because of solvent, they showed
that Molecular Beacon is an ideal candidate to observe re-entrance in vitro.


***

LS11341
A UNIVERSAL ELASTIC ANISOTROPY INDEX FOUND
Practically all elastic materials are anisotropic, which means that their properties are directionally dependent. This calls for an appropriate universal measure to uniquely quantify the degree of anisotropy for any particular material, e.g. crystal. Three well known anisotropy measures used for years lack universality as they are non-unique and ignore contributions from the bulk part of the elastic stiffness (or compliance) tensor. In this letter we introduce a universal anisotropy index that overcomes these limitations and allows one to fully quantify the single crystal anisotropy. Furthermore, we establish special relationships between the proposed anisotropy index and the existing anisotropy measures for special cases. An elastic anisotropy diagram is constructed for over 100 different crystals (from cubic through triclinic), demonstrating that the proposed anisotropy measure is applicable to all types of elastic single crystals, and thus fills an important void in the existing literature.


***

LL11424E
Moving at a constant pace may be key to the formation of coherent
patterns in swarming organisms

Self-propelled particle (SPP) models describe leaderless pattern
formation in biology, such as fish schools and insect swarms. SPP
models are classified into two distinct categories: kinematic and
dynamic. Kinematic SPP models assume that each particle travels at a
near constant speed and senses the position and orientation of its
local neighbors to adjust their orientation. Dynamic SPP models
describe the motion of particles based on Newtonian mechanics that
involve attractive and repulsive forces between particles rather than
alignment principles. In their paper, Newman and Sayama developed a
new dynamic SPP model in which a sensory blind zone is introduced into
each particle's zone of interaction, and studied the effects of the
blind zone on the formation of coherent vortex patterns. The result
indicates that even a slight sensory deficiency makes swarms unable to
form a pattern, which is quite different from kinematic models that
can have large sensory deficits and still form vortex patterns. This
comparison presents a conjecture that certain biological swarming
behaviors may sensitively depend on the ability of individuals to
maintain a constant velocity. Specifically, for organisms that keep
moving autonomously at a near constant pace, coherent pattern
formation emerges relatively easily even with significant sensory
blind zones, but for organisms whose motion strongly depends on
environmental stimuli, coherent pattern formation requires a nearly
complete panoramic range of interaction in order for particles to gain
enough propulsion from behind.

6-25-08

BSR1113
Oxygen-free iron arsenide superconductors discovered

A family of oxygen-free iron arsenide superconductors with a not yet
optimized critical temperature (Tc) of 38 K was discovered. The material
is closely related to the recently found iron-arsenide-oxides, but
adopts an even simpler structure. It is meanwhile accepted, that the
crucial building block to superconductivity in LaOFeAs is the
iron-arsenide layer. The latter is sandwiched between lanthanum-oxide
layers, but only by barium atoms in the new material BaFe2As2. In our
recent paper (PRB, BSR1113, in press, arxiv:0805.4021), we show that the
physical properties of the new parent compound BaFe2As2 are amazingly
similar to LaOFeAs. Thus we predicted that BaFe2As2 would be a
superconductor by doping as known from the LaOFeAs materials. Very
recently, we succeed in inducing superconductivity at 38 K in
Ba0.6K0.4Fe2As2 by hole doping. (PRL, submitted, arxiv:0805.4630). This
discovery opens new avenues to find further superconductors in the large
family of BaFe2As2-related compounds. But more important, the new
materials have a simpler structure and are easier to synthesize also as
large single crystals. This gives a world of opportunity for rapid
experimental and theoretical progress and thus a fresh impetus for
finally solving the mystery of high-Tc superconductivity.

***


LF11238


Violation of macroscopic realism in every-day life?


The laws of physics are of quantum nature and microscopic systems cannot be
described classically. But the macroscopic objects around us seem to have
objective properties prior to and independent of measurement which can be
inferred without altering them. Is it possible to experience a violation of
this macroscopic realism in every-day life?
Under realistic conditions we are only able to perform coarse-grained
measurements that do not resolve individual quantum levels of a macroscopic
object. We show that this usually allows a realistic description of the
object's dynamics: At every instant of time the quantum state - even if it
is a macroscopic Schrödinger cat-like superposition - appears as a classical
mixture and the time evolution of this mixture can be explained classically.
However, we demonstrate that there exist non-classical time evolutions which
allow to see a violation of macroscopic realism even under classical
coarse-grained measurements. The question why we then do not see such
violations arises again. We finally suggest that the reason for this is that
non-classical time evolutions are of high computational complexity.
Figuratively, this means that if nature spontaneously "chooses" a time
evolution, it is much more likely that a low complex, i.e. a classical, time
evolution is realized and thus our every-day world appears classical under
coarse-grained measurements.

Figure Caption:
The two graphs on the top show the quantum states of a Schrödinger cat-like
superposition of a spin pointing to the north and to the south (left) and a
classical fifty-fifty mixture in which half of the spins is along north and
the other half is along south (right). Under every-day coarse-grained
measurements both states have the same classical description in terms of a
fifty-fifty probability distribution (bottom left and right). However, there
exist non-classical time evolutions producing time-dependent superposition
states that allow to violate macroscopic realism even under coarse-grained
measurements.



***

BQ10572

Riding the wave of silica glass

Surface dynamics measurements can provide insight into the surface structure where direct imaging techniques fail. Silica glass, the chief component of the vast variety of silicate glasses, is an excellent insulator and its surface features very low surface tension values. For these as well as other reasons, commonly applied surface science techniques did not yet succeed in obtaining high-resolution information of the melt-formed silica surface.
In this paper, we have used neutral helium atoms as probes for investigating the silica glass surface. In the course of the experiment, a coherent matter wave of helium particles is created, gets scattered at the surface and the energy distribution of the scattered particles is obtained by measuring their time of flight. General surface dynamics parameters have been determined from an analysis of elastically scattered particles. Furthermore, a new surface dynamics effect peculiar to disordered matter – the surface boson peak – has been discovered; properties of which have also been reported in two recent issues of Physical Review Letters [PRL 99, 035503 (2007) and PRL 100, 135504 (2008)].

***

LQ11728

Observing the quantization states of 3d electrons in monoatomic Cu chains.

It is well known that the behavior of d electons in solid is very important and associated with magnetism, superconductivity and transport phenomena. For the first time, this paper reported the quantization of one-dimensional 3d electrons states in monatomic metallic Cu wires. Low dimensional electronic systems with novel and tuneable properties attract high fundamental interest with challenging questions. For example, sp electrons in one dimensional Au atom chains on Si(111) step surface exhibit a number of exotic features, such as Peieris transitions, periodic lattice distortions, and charge density waves. By applying angle-resolved photoelectron spectroscopy, the authors in this paper for the first time observed an anisotropic one-dimensional 3d electron band quantization states in a monatomic array of Cu chains on the Pt(997) step surface. The 3d electrons of Cu are delocalized along the chains and localized in perpendicular direction of chains. Moreover, the authors demonstrated that the one dimensional confinement of Cu 3d electrons quantization states is introduced by the direct wave-function overlap between next-neighbor Cu atoms in the chains. These results are susceptible to have more general impact on the microscopic understanding of electronic low-dimensional systems.

***

BS10835
Sub-picosecond ac spin current pulses produced.

Conventional electronics utilize electron's charge. Recently, there is a
general interest in using spin of electrons for the next generation
electronic technology - spintronics. Spin currents, flow of electron's
angular momentum, or spin, is the counterpart of charge currents in
conventional electronic devices. Although generation of charge currents
only requires application of a voltage, spin currents are not easy to
produce. In the past, several methods have been developed to generate
spin currents that are dc in nature and steady state. Now, ac spin
current pulses shorter than 1 picosecond have been demonstrated by using
coherent control techniques. The current pulses are generated optically
in intrinsic GaAs bulk and quantum wells, without applying any external
voltage or using magnetic materials. The new scheme may find
applications in high-speed spintronic designs where ac currents are
desired.


***

BSR1064

Strong correlations and order in the iron pnictide superconductors.

One of the key questions raised by the discovery of the new iron
based high
temperature superconductors is whether their physics is akin to the
copper
based high temperature superconductors discovered over 2 decades ago.

In our paper we argue that structural and spin ordering transitions
observed in
recent neutron scattering experiments suggest that these two classes
do indeed
share important features of ``strong correlation physics''. We show
that a
model of localized, and strongly correlated, electrons naturally
exhibits a
transition at relatively high temperatures where the square lattice
symmetry of
the Fe ions is reduced to a rectangular symmetry, just as is
observed. We extend
our model to describe a number of properties of the superconducting
and metallic
states, including the pairing symmetry of the Cooper pairs.


***

BR11049

In this paper, we present for the first time a complete description of
non-specular diffracted beams, both theoretically and experimentally.


An interesting and almost unexplored phenomenon occurring in photonic
crystals at wavelengths on the order or smaller than the lattice
parameter is the opening of diffraction channels, that is, a finite
number of diffracted beams emerge from the crystal slab when the
photon energy is greater than a threshold energy or diffraction
cut-off. These diffracted beams are propagating waves that can be
projected on a screen in order to measure their intensities. However,
up to date, most of the experimental and theoretical analyses in this
high energy range have been focused on the intensities of the
specularly reflected and forwardly (or ballistically) transmitted
beams.

We found in the spectral analysis of the diffracted beams strong
intensity modulations that arise from resonances of the three
dimensionally ordered sphere ensemble. Such novel diffraction
phenomena can be put into practice in a number of optoelectronic
devices.

***

CP10108

Pygmy dipole resonance is a very important phenomenon for astrophysical
models of rapid neutron capture occuring in core-collapse supernovae
which is responsible for the production of roughly one half of all heavy
elements beyond iron in the Universe. Since strongly interacting protons
and neutrons, the constituents of atomic nuclei, respond differently to
an external electromagnetic field, their relative motion exhibits different
modes of excitation at various energies. The best-known example is the giant dipole resonance caused by an oscillation of the proton and neutron liquids
against each other. A neutron or proton excess localized on the
nuclear surface forms a kind of a soft nuclear skin and in an external
electromagnetic field this skin can oscillate against the core
formed by an equal numbers of protons and neutrons. This oscillation
is called pygmy resonance. It occurs at low energies and
resembles the motion of sea water during the tides caused by the
gravitational attraction of the moon.

In this paper, we develop an approach which allows us to describe
many-body dynamics of giant and pygmy dipole resonances as well as
other nuclear vibrational excitations. Mode coupling between the
giant and pygmy dipole resonances and other surface vibrations is
taken into account in a fully consistent way. The respective model
predictions are in a very good agreement with the available
experimental data. Due to the consistency and the relatively small
number of the model parameters, which are universal within the
entire nuclear chart, the method allows also a reliable description
of nuclei not yet accessible to the experiment.


***

BP10529
Classification and Analytical Description of States in a Photonic Crystal

Photonic crystal (PC) has revolutionized the entire field of optics. Numerous methods are proposed for the theoretical modeling of PCs. Unfortunately none of these methods is simple, as they all involves numerical method without any analytical details or a formula-like description.

This work solves this problem by reporting a systematic and generalized way of getting analytical descriptions for two-dimensional PCs. The photonic states in the limit of zero modulation were analyzed and a systematic classification system based on integers was introduced for the states with distinct frequencies. The integers can be factored and states with the same factor can be categorized as they have the same form of solution when the spatial modulation is switched on. As it is shown in the paper, such states can be easily solved and typically lead to analytical solutions.
With the presented work, it is now possible to have an analytical description or formulas for many novel optical properties of PC explored previously using numerical methods such as band gaps, refractions, group velocities and density of states. This in turn, may enable researchers to engineer PCs and PC-based devices analytically.

6-23-08

AR10216
Quantum Billiard Pocket Computer

We propose and describe for the first time how quantum computers could
be physically realized by playing billiard with single atoms. The required
miniature billiard tables must have the proportions of an A4 paper sheet.
Experimental means to create such tables with cushions made of laser light
do already exist. So far, they have been used to study the chaotic motion
of atoms confined to the interior of appropriately shaped laser corrals.
Our "A4-billiards", on the contrary, lead to perfectly predictable and
time-periodic motional patterns. Their dynamic behavior becomes particularly
transparent if observed stroboscopically, with a natural time-unit ("clock
cycle") proportional to the billiard area. The character of the resulting
motion is governed by the laws of quantum mechanics. A striking consequence
is that an atomic billiard-ball (i.e., a "blob" of quantum matter) can
"split" and --in a way-- end up at two or more locations at the same time.
Adding a controlled, minimal intervention from outside (e.g., shining short
laser pulses on individual "blobs" at appropriate instants of time) it
becomes possible to manipulate the system's quantum state at will. The
A4-proportion of these billiards allows one also to increase the system's
complexity by introducing onion-skin-like "billiard in a billiard"
architectures.


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BS10711

Relaxor Ferroelctric-like Behaviour in Ca Doped TbMnO3

TbMnO3 is known as the first magnetic induced ferroelectric material. Many ferroelectric materials have large switching (coercive) fields. In this manuscript we show that small amounts of doping with divalent Ca2+ results in relaxor type behaviour. This behaviour is associated with low coercive fields. The manuscript present neutron diffraction, magnetic and electric properties for 2, 5, and 10% doping.


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LG11819B

Mysterious threshold switching finally unveiled

Apply a large voltage to an amorphous semiconductor, and
the electrons hopping through traps in the material will
heat up and gain mobility. Above a certain threshold
voltage, this process culminates in a tremendous
enhancement of conductivity, which goes under the name of
⿿threshold switching⿿. Threshold switching was first
discovered in 1968 by S. Ovshinsky in chalcogenide
glasses, namely selenium- and tellurium-based alloys
lacking an atomic order. Researchers have long debated the
root cause of this mysterious effect, invoking thermal,
electric breakdown or phase transitions. A report appeared
recently in Phys. Rev. B may have unveiled the intimate
physics of threshold switching. Amorphous semiconductors
conduct electricity by electrons/holes hopping through
localized states. The higher the carrier energy, the
higher its ability to escape from traps, hence its
velocity. This is similar to what happens in a river,
where deep waters tend to remain trapped between rocks and
flow slowly, while shallow waters run more freely. High
electric fields can drive electrons/holes to high energy,
thus promoting their mobility and eventually leading to a
flash-like increase of conductivity. This discovery may
help the development of faster and less consuming non
volatile memories and cognitive devices based on
chalcogenide switching.


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BR10880
Photodiode based on carbon monolayer.

Fabricated a few years ago, graphene, a one-atom-thick carbon
layer of surprisingly high electronic and crystal quality, is a
possible candidate for the base material in future
nanoelectronics. However, the absence of the energy gap between
the valence and conduction bands in this two-dimensional
semiconductor hinders the possibility to control currents in
graphene-based devices, e.g. diodes and field-effect transistors.
To resolve this problem
we propose to use external electromagnetic
radiation, which allows to create a dynamical gap in the graphene
energy spectrum. The value of the gap depends on the power and the
frequency of radiation. We show that the electromagnetic field of
a wide frequency range generates photocurrent in this gapless
semiconductor. Such a photocurrent arises as a result of inelastic
electron tunneling accompanied by one- or two-photon absorption.
Applying a sufficiently large radiation power, one can also fully
suppress electron transport in a graphene-based junction.


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LG11487
Visualizing atomic-scale acoustic waves in nanostructures

The highest frequency acoustic waves in materials, with nearly
atomic-scale wavelengths, are beginning to be utilized as a new kind
of tool to measure the properties of nanostructures but practical
detection of these waves with ultrahigh THz frequencies (10^12 Hz) is
extremely challenging. We have discovered a new physical phenomenon
that enables observation of such high frequency waves.

The highest frequency acoustic waves can form spontaneously at the
front of shock waves or be generated by sub-picosecond pulse length
lasers. Under some circumstances, when such a wave crosses an
interface between two materials, tiny electric currents are generated
at the interface. These currents produce electromagnetic radiation
of THz frequencies that can be detected a few millimeters away from
the interface. Using molecular dynamics simulations, we show that
the time-history of the wave can be determined with potentially
sub-picosecond, nearly atomic time and space resolution by measuring
the electromagnetic field coherently generated. We have studied the
effect for an interface between AlN and GaN which are used in LED
(light-emitting diode) nanostructures and are piezoelectric, i.e.
electric currents are generated when they are squeezed.
Piezoelectric materials have been employed for decades as arrival
time gauges for shock wave experiments but have been limited by
electrical equipment to detection of acoustic frequencies less than
10 GHz, precluding observation of the highest frequency acoustic
waves.


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BSR1110B

Precision STM measurements resolve atomic structure debate

Recent scanning tunneling microscopy measurements by Choi et al. have
resolved a long-running debate concerning the atomic structure of
one-monolayer films of copper nitride (Cu2N). Copper nitride has attracted
considerable interest for nanoscale templating applications, because
one-monolayer films can be grown that self-assemble into a regular array of
square islands. Despite the variety of techniques applied to study this
system over the last 20+ years, the mechanisms for self-assembly are not
well understood. Choi et al., use an STM operating in a low-temperature,
ultrahigh vacuum environment to directly measure the lattice constant of
copper nitride films. The measurements suggest that strain due to the 3%
lattice mismatch with the underlying Cu substrate contributes to
self-assembly in this system.

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BM10809
Electron-hole drops in multivalley semiconductors


The article develops a new formalism that gives the total energy of an
electron gas in common multivalley semiconductors such as Si, Ge, and GaAs.
To demonstrate its usage, the formalism is applied to electron-hole drops.

Every electron in a solid feels forces from all other electrons, creating
correlations, which makes the total energy of the electrons analytically
tractable in just a small handful of systems. In multivalley semiconductors
the electrons can be distinguished by a number categorizing them according
to their band structure valley; the limit of many valleys permits an
approximation that allows the exact total energy to be found. The energy
penalty of a changing electron density is also found, which allows systems
with spatially varying electron density to be analyzed.

The electron density profile for experimentally realisable electron-hole
drops is derived. Drops can be experimentally probed by straining their
semiconductor, which is naturally investigated within the formalism since
strain reduces the main parameter of the theory, the number of valleys.


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AR10399

Focusing evanescent waves to a tiny spot

Evanescent waves can be shaped and focused to tiny spots using planar
gratings. This effect was first demonstrated by Merlin (Science, Vol.
3127, 927, 2007), who coined it ?radiationless electromagnetic
interference?. Conventionally, focusing is only achieved using
propagating waves, and to date evanescent waves have been completely
forgotten in this respect. In the current work we demonstrate that
evanescent waves exhibiting certain symmetric polarization patterns
can generate focal spots or holes which are much smaller than the
wavelength of light. The work has applications in optical microscopy
and data storage, where intensity distributions smaller than the
wavelength of light are required to resolve nanoscale objects.