441 research outputs found
Electronic and magnetic properties of GaMnAs: Annealing effects
The effect of short-time and long-time annealing at 250C on the conductivity,
hole density, and Curie temperature of GaMnAs single layers and GaMnAs/InGaMnAs
heterostructures is studied by in-situ conductivity measurements as well as
Raman and SQUID measurements before and after annealing. Whereas the
conductivity monotonously increases with increasing annealing time, the hole
density and the Curie temperature show a saturation after annealing for 30
minutes. The incorporation of thin InGaMnAs layers drastically enhances the
Curie temperature of the GaMnAs layers.Comment: 4 pages, 6 figures, submitted to Physica
Evaluation of transition rates from nonequilibrium instantons
Equilibrium rate theories play a crucial role in understanding rare, reactive
events. However, they are inapplicable to a range of irreversible processes in
systems driven far from thermodynamic equilibrium like active and biological
matter. Here, we develop a general, computationally efficient nonequilibrium
rate theory in the weak-noise limit based on an instanton approximation to the
stochastic path integral and illustrate its wide range of application in the
study of rare nonequilibrium events. We demonstrate excellent agreement of the
instanton rates with numerically exact results for a particle under a
non-conservative force. We also study phase transitions in an active field
theory. We elucidate how activity alters the stability of the two phases and
their rates of interconversion in a manner that can be well-described by
modifying classical nucleation theoryComment: 6 pages, 3 figures, Supplementary material availabl
Effect of annealing on the depth profile of hole concentration in (Ga,Mn)As
The effect of annealing at 250 C on the carrier depth profile, Mn
distribution, electrical conductivity, and Curie temperature of (Ga,Mn)As
layers with thicknesses > 200 nm, grown by molecular-beam epitaxy at low
temperatures, is studied by a variety of analytical methods. The vertical
gradient in hole concentration, revealed by electrochemical capacitance-voltage
profiling, is shown to play a key role in the understanding of conductivity and
magnetization data. The gradient, basically already present in as-grown
samples, is strongly influenced by post-growth annealing. From secondary ion
mass spectroscopy it can be concluded that, at least in thick layers, the
change in carrier depth profile and thus in conductivity is not primarily due
to out-diffusion of Mn interstitials during annealing. Two alternative possible
models are discussed.Comment: 8 pages, 8 figures, to appear in Phys. Rev.
Chemical weathering and provenance evolution of Holocene–Recent sediments from the Western Indus Shelf, Northern Arabian Sea inferred from physical and mineralogical properties
We present a multi-proxy mineral record based on X-ray diffraction and diffuse reflectance spectrophotometry analysis for two cores from the western Indus Shelf in order to reconstruct changing weathering intensities, sediment transport, and provenance variations since 13 ka. Core Indus-10 is located northwest of the Indus Canyon and exhibits fluctuations in smectite/(illite + chlorite) ratios that correlate with monsoon intensity. Higher smectite/(illite + chlorite) and lower illite crystallinity, normally associated with stronger weathering, peaked during the Early–Mid Holocene, the period of maximum summer monsoon. Hematite/goethite and magnetic susceptibility do not show clear co-variation, although they both increase at Indus-10 after 10 ka, as the monsoon weakened. At Indus-23, located on a clinoform just west of the canyon, hematite/goethite increased during a period of monsoon strengthening from 10 to 8 ka, consistent with increased seasonality and/or reworking of sediment deposited prior to or during the glacial maximum. After 2 ka terrigenous sediment accumulation rates in both cores increased together with redness and hematite/goethite, which we attribute to widespread cultivation of the floodplain triggering reworking, especially after 200 years ago. Over Holocene timescales sediment composition and mineralogy in two localities on the high-energy shelf were controlled by varying degrees of reworking, as well as climatically modulated chemical weathering
On the Statistical Mechanics of Mass Accommodation at Liquid-Vapor Interfaces
We propose a framework for describing the dynamics associated with the
adsorption of small molecules to liquid-vapor interfaces, using an intermediate
resolution between traditional continuum theories that are bereft of molecular
detail and molecular dynamics simulations that are replete with them. In
particular, we develop an effective single particle equation of motion capable
of describing the physical processes that determine thermal and mass
accommodation probabilities. The effective equation is parameterized with
quantities that vary through space away from the liquid-vapor interface. Of
particular importance in describing the early time dynamics is the spatially
dependent friction, for which we propose a numerical scheme to evaluate from
molecular simulation. Taken together with potentials of mean force computable
with importance sampling methods, we illustrate how to compute the mass
accommodation coefficient and residence time distribution. Throughout, we
highlight the case of ozone adsorption in aqueous solutions and its dependence
on electrolyte composition.Comment: 9 pages, 7 figure
Fermion loop simulation of the lattice Gross-Neveu model
We present a numerical simulation of the Gross-Neveu model on the lattice
using a new representation in terms of fermion loops. In the loop
representation all signs due to Pauli statistics are eliminated completely and
the partition function is a sum over closed loops with only positive weights.
We demonstrate that the new formulation allows to simulate volumes which are
two orders of magnitude larger than those accessible with standard methods
A large deviation theory perspective on nanoscale transport phenomena
Understanding transport processes in complex nanoscale systems, like ionic
conductivities in nanofluidic devices or heat conduction in low dimensional
solids, poses the problem of examining fluctuations of currents within
nonequilibrium steady states and relating those fluctuations to nonlinear or
anomalous responses. We have developed a systematic framework for computing
distributions of time integrated currents in molecular models and relating
cumulants of those distributions to nonlinear transport coefficients. The
approach elaborated upon in this perspective follows from the theory of
dynamical large deviations, benefits from substantial previous formal
development, and has been illustrated in several applications. The framework
provides a microscopic basis for going beyond traditional hydrodynamics in
instances where local equilibrium assumptions break down, which are ubiquitous
at the nanoscale.Comment: Small revisions for clarit
Enhancement of the Curie temperature in GaMnAs/InGaMnAs superlattices
We report on an enhancement of the Curie temperature in GaMnAs/InGaMnAs
superlattices grown by low-temperature molecular beam epitaxy, which is due to
thin InGaMnAs or InGaAs films embedded into the GaMnAs layers. The pronounced
increase of the Curie temperature is strongly correlated to the In
concentration in the embedded layers. Curie temperatures up to 110 K are
observed in such structures compared to 60 K in GaMnAs single layers grown
under the same conditions. A further increase in T up to 130 K can be
achieved using post-growth annealing at temperatures near the growth
temperature. Pronounced thickness fringes in the high resolution X-ray
diffraction spectra indicate good crystalline quality and sharp interfaces in
the structures.Comment: 4 pages, 4 figures, submitted to Appl. Phys. Let
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