1,272 research outputs found
Spin Transport in Disordered Two-Dimensional Hopping Systems with Rashba Spin-Orbit Interaction
The influence of Rashba spin-orbit interaction on the spin dynamics of a
topologically disordered hopping system is studied in this paper. This is a
significant generalization of a previous investigation, where an ordered
(polaronic) hopping system has been considered instead. It is found, that in
the limit, where the Rashba length is large compared to the typical hopping
length, the spin dynamics of a disordered system can still be described by the
expressions derived for an ordered system, under the provision that one takes
into account the frequency dependence of the diffusion constant and the
mobility (which are determined by charge transport and are independent of
spin). With these results we are able to make explicit the influence of
disorder on spin related quantities as, e.g., the spin life-time in hopping
systems.Comment: 12 pages, 6 figures, some clarifications adde
Relationship between solidification microstructure and hot cracking susceptibility for continuous casting of low-carbon and high-strength low-alloyed steels: A phase-field study
© The Minerals, Metals & Materials Society and ASM International 2013Hot cracking is one of the major defects in continuous casting of steels, frequently limiting the productivity. To understand the factors leading to this defect, microstructure formation is simulated for a low-carbon and two high-strength low-alloyed steels. 2D simulation of the initial stage of solidification is performed in a moving slice of the slab using proprietary multiphase-field software and taking into account all elements which are expected to have a relevant effect on the mechanical properties and structure formation during solidification. To account for the correct thermodynamic and kinetic properties of the multicomponent alloy grades, the simulation software is online coupled to commercial thermodynamic and mobility databases. A moving-frame boundary condition allows traveling through the entire solidification history starting from the slab surface, and tracking the morphology changes during growth of the shell. From the simulation results, significant microstructure differences between the steel grades are quantitatively evaluated and correlated with their hot cracking behavior according to the Rappaz-Drezet-Gremaud (RDG) hot cracking criterion. The possible role of the microalloying elements in hot cracking, in particular of traces of Ti, is analyzed. With the assumption that TiN precipitates trigger coalescence of the primary dendrites, quantitative evaluation of the critical strain rates leads to a full agreement with the observed hot cracking behavior. © 2013 The Minerals, Metals & Materials Society and ASM International
On the structure of the energy distribution function in the hopping regime
The impact of the dispersion of the transport coefficients on the structure
of the energy distribution function for charge carriers far from equilibrium
has been investigated in effective-medium approximation for model densities of
states. The investigations show that two regimes can be observed in energy
relaxation processes. Below a characteristic temperature the structure of the
energy distribution function is determined by the dispersion of the transport
coefficients. Thermal energy diffusion is irrelevant in this regime. Above the
characteristic temperature the structure of the energy distribution function is
determined by energy diffusion. The characteristic temperature depends on the
degree of disorder and increases with increasing disorder. Explicit expressions
for the energy distribution function in both regimes are derived for a constant
and an exponential density of states.Comment: 16 page
Revival of Silenced Echo and Quantum Memory for Light
We propose an original quantum memory protocol. It belongs to the class of
rephasing processes and is closely related to two-pulse photon echo. It is
known that the strong population inversion produced by the rephasing pulse
prevents the plain two-pulse photon echo from serving as a quantum memory
scheme. Indeed gain and spontaneous emission generate prohibitive noise. A
second -pulse can be used to simultaneously reverse the atomic phase and
bring the atoms back into the ground state. Then a secondary echo is radiated
from a non-inverted medium, avoiding contamination by gain and spontaneous
emission noise. However, one must kill the primary echo, in order to preserve
all the information for the secondary signal. In the present work, spatial
phase mismatching is used to silence the standard two-pulse echo. An
experimental demonstration is presented.Comment: 13 pages, 6 figure
Theory of electric-field-induced spin accumulation and spin current in the two-dimensional Rashba model
Based on the spin-density-matrix approach, both the electric-field-induced
spin accumulation and the spin current are systematically studied for the
two-dimensional Rashba model. Eigenmodes of spin excitations give rise to
resonances in the frequency domain. Utilizing a general and physically
well-founded definition of the spin current, we obtain results that differ
remarkably from previous findings. It is shown that there is a close
relationship between the spin accumulation and the spin current, which is due
to the prescription of a quasi-chemical potential and which does not result
from a conservation law. Physical ambiguities are removed that plagued former
approaches with respect to a spin-Hall current that is independent of the
electric field. For the clean Rashba model, the intrinsic spin-Hall
conductivity exhibits a logarithmic divergency in the low-frequency regime.Comment: 19 pages including figure
Tuberculosis vaccine strain Mycobacterium bovis BCG Russia is a natural recA mutant
BACKGROUND: The current tuberculosis vaccine is a live vaccine derived from Mycobacterium bovis and attenuated by serial in vitro passaging. All vaccine substrains in use stem from one source, strain Bacille Calmette-Guérin. However, they differ in regions of genomic deletions, antigen expression levels, immunogenicity, and protective efficacy. RESULTS: As a RecA phenotype increases genetic stability and may contribute restricting the ongoing evolution of the various BCG substrains while maintaining their protective efficacy, we aimed to inactivate recA by allelic replacement in BCG vaccine strains representing different phylogenetic lineages (Pasteur, Frappier, Denmark, Russia). Homologous gene replacement was achieved successfully in three out of four strains. However, only illegitimate recombination was observed in BCG substrain Russia. Sequence analyses of recA revealed that a single nucleotide insertion in the 5' part of recA led to a translational frameshift with an early stop codon making BCG Russia a natural recA mutant. At the protein level BCG Russia failed to express RecA. CONCLUSION: According to phylogenetic analyses BCG Russia is an ancient vaccine strain most closely related to the parental M. bovis. We hypothesize that recA inactivation in BCG Russia occurred early and is in part responsible for its high degree of genomic stability, resulting in a substrain that has less genetic alterations than other vaccine substrains with respect to M. bovis AF2122/97 wild-type
Measuring the Accuracy of Object Detectors and Trackers
The accuracy of object detectors and trackers is most commonly evaluated by
the Intersection over Union (IoU) criterion. To date, most approaches are
restricted to axis-aligned or oriented boxes and, as a consequence, many
datasets are only labeled with boxes. Nevertheless, axis-aligned or oriented
boxes cannot accurately capture an object's shape. To address this, a number of
densely segmented datasets has started to emerge in both the object detection
and the object tracking communities. However, evaluating the accuracy of object
detectors and trackers that are restricted to boxes on densely segmented data
is not straightforward. To close this gap, we introduce the relative
Intersection over Union (rIoU) accuracy measure. The measure normalizes the IoU
with the optimal box for the segmentation to generate an accuracy measure that
ranges between 0 and 1 and allows a more precise measurement of accuracies.
Furthermore, it enables an efficient and easy way to understand scenes and the
strengths and weaknesses of an object detection or tracking approach. We
display how the new measure can be efficiently calculated and present an
easy-to-use evaluation framework. The framework is tested on the DAVIS and the
VOT2016 segmentations and has been made available to the community.Comment: 10 pages, 7 Figure
Thermoelectric three-terminal hopping transport through one-dimensional nanosystems
A two-site nanostructure (e.g, a "molecule") bridging two conducting leads
and connected to a phonon bath is considered. The two relevant levels closest
to the Fermi energy are connected each to its lead. The leads have slightly
different temperatures and chemical potentials and the nanos- tructure is also
coupled to a thermal (third) phonon bath. The 3 x 3 linear transport
("Onsager") matrix is evaluated, along with the ensuing new figure of merit,
and found to be very favorable for thermoelectric energy conversion.Comment: Accepted by Phys. Rev.
{\it Ab initio} NMR chemical shifts and quadrupolar parameters for phases and their precursors
The Gauge-Including Projector Augmented Wave (GIPAW) method, within the
Density Functional Theory (DFT) Generalized Gradient Approximation (GGA)
framework, is applied to compute solid state NMR parameters for in
the , , and aluminium oxide phases and their gibbsite
and boehmite precursors. The results for well-established crystalline phases
compare very well with available experimental data and provide confidence in
the accuracy of the method. For -alumina, four structural models
proposed in the literature are discussed in terms of their ability to reproduce
the experimental spectra also reported in the literature. Among the considered
models, the structure proposed by Paglia {\it et al.} [Phys. Rev.
B {\bf 71}, 224115 (2005)] shows the best agreement. We attempt to link the
theoretical NMR parameters to the local geometry. Chemical shifts depend on
coordination number but no further correlation is found with geometrical
parameters. Instead our calculations reveal that, within a given coordination
number, a linear correlation exists between chemical shifts and Born effective
charges
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