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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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%.
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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.
