35,943 research outputs found
Detection of a Spin Accumulation in Nondegenerate Semiconductors
Electrical detection of a spin accumulation in a nondegenerate semiconductor using a tunnel barrier and ferromagnetic contact is shown to be fundamentally affected by the energy barrier associated with the depletion region. This prevents the ferromagnet from probing the spin accumulation directly, strongly suppresses the magnetoresistance in current or potentiometric detection, and introduces nonmonotonic variation of spin signals with voltage and temperature. Having no analogue in metallic systems, we identify energy mismatch as an obstacle for spin detection, necessitating control of the energy landscape of spin-tunnel contacts to semiconductors
Accurate and efficient algorithm for Bader charge integration
We propose an efficient, accurate method to integrate the basins of
attraction of a smooth function defined on a general discrete grid, and apply
it to the Bader charge partitioning for the electron charge density. Starting
with the evolution of trajectories in space following the gradient of charge
density, we derive an expression for the fraction of space neighboring each
grid point that flows to its neighbors. This serves as the basis to compute the
fraction of each grid volume that belongs to a basin (Bader volume), and as a
weight for the discrete integration of functions over the Bader volume.
Compared with other grid-based algorithms, our approach is robust, more
computationally efficient with linear computational effort, accurate, and has
quadratic convergence. Moreover, it is straightforward to extend to non-uniform
grids, such as from a mesh-refinement approach, and can be used to both
identify basins of attraction of fixed points and integrate functions over the
basins.Comment: 19 pages, 8 figure
Sign of tunnel spin polarization of low-work-function Gd/Co nanolayers in a magnetic tunnel junction
Magnetic tunnel junctions having a low-work-function Gd/Co nanolayer at the interface with an Al2O3 tunnel barrier are shown to exhibit both positive and negative values of the tunnel magnetoresistance. The sign of the tunnel spin polarization of the Gd/Co nanolayer electrode depends on the thickness of the Gd and Co layers, temperature, and applied voltage. This reflects the nature of the interaction between the conduction electrons of the rare-earth and transition metals. \u
Test vectors for Rankin-Selberg -functions
We study the local zeta integrals attached to a pair of generic
representations of , , over a -adic
field. Through a process of unipotent averaging we produce a pair of
corresponding Whittaker functions whose zeta integral is non-zero, and we
express this integral in terms of the Langlands parameters of and .
In many cases, these Whittaker functions also serve as a test vector for the
associated Rankin-Selberg (local) -function.Comment: arXiv admin note: text overlap with arXiv:1804.0772
Effects of edge magnetism and external electric field on energy gaps in multilayer graphene nanoribbons
Using first-principles density-functional theory, we study the electronic
structure of multilayer graphene nanoribbons as a function of the ribbon width
and the external electric field, applied perpendicular to the ribbon layers. We
consider two types of edges (armchair and zigzag), each with two edge
alignments (referred to as alpha- and beta-alignments). We show that, as in
monolayer and bilayer armchair nanoribbons, multilayer armchair nanoribbons
exhibit three classes of energy gaps which decrease with increasing width.
Nonmagnetic multilayer zigzag nanoribbons have band structures that are
sensitive to the edge alignments and the number of layers, indicating different
magnetic properties and resulting energy gaps. We find that energy gaps can be
induced in ABC-stacked ribbons with a perpendicular external electric field
while in other stacking sequences, the gaps decrease or remain closed as the
external electric field increases.Comment: 7 pages, 9 figures, text revised, last version before publicatio
Publicness, Privacy and Confidentiality in the Single-Serving Quantum Broadcast Channel
The 2-receiver broadcast channel is studied: a network with three parties
where the transmitter and one of the receivers are the primarily involved
parties and the other receiver considered as third party. The messages that are
determined to be communicated are classified into public, private and
confidential based on the information they convey. The public message contains
information intended for both parties and is required to be decoded correctly
by both of them, the private message is intended for the primary party only,
however, there is no secrecy requirement imposed upon it meaning that it can
possibly be exposed to the third party and finally the confidential message
containing information intended exclusively for the primary party such that
this information must be kept completely secret from the other receiver. A
trade-off arises between the rates of the three messages, when one of the rates
is high, the other rates may need to be reduced to guarantee the reliable
transmission of all three messages. The encoder performs the necessary
equivocation by virtue of dummy random numbers whose rate is assumed to be
limited and should be considered in the trade-off as well. We study this
trade-off in the one-shot regime of a quantum broadcast channel by providing
achievability and (weak) converse regions. In the achievability, we prove and
use a conditional version of the convex-split lemma as well as position-based
decoding. By studying the asymptotic behaviour of our bounds, we will recover
several well-known asymptotic results in the literature.Comment: 23 pages, 1 figure, journa
Atmospheric Chemistry for Astrophysicists: A Self-consistent Formalism and Analytical Solutions for Arbitrary C/O
We present a self-consistent formalism for computing and understanding the
atmospheric chemistry of exoplanets from the viewpoint of an astrophysicist.
Starting from the first law of thermodynamics, we demonstrate that the van't
Hoff equation (which describes the equilibrium constant), Arrhenius equation
(which describes the rate coefficients) and procedures associated with the
Gibbs free energy (minimisation, rescaling) have a common physical and
mathematical origin. We address an ambiguity associated with the equilibrium
constant, which is used to relate the forward and reverse rate coefficients,
and restate its two definitions. By necessity, one of the equilibrium constants
must be dimensionless and equate to an exponential function involving the Gibbs
free energy, while the other is a ratio of rate coefficients and must therefore
possess physical units. We demonstrate that the Arrhenius equation takes on a
functional form that is more general than previously stated without recourse to
tagging on ad hoc functional forms. Finally, we derive analytical models of
chemical systems, in equilibrium, with carbon, hydrogen and oxygen. We include
acetylene and are able to reproduce several key trends, versus temperature and
carbon-to-oxygen ratio, published in the literature. The rich variety of
behavior that mixing ratios exhibit as a function of the carbon-to-oxygen ratio
is merely the outcome of stoichiometric book-keeping and not the direct
consequence of temperature or pressure variations.Comment: Accepted by ApJ. 9 pages, 4 figure
Kinetics and thermodynamics of ceramic/metal interface reactions related to high T(sub c) superconducting applications
Superconducting ceramic materials, no matter what their form, size or shape, must eventually make contact with non-superconducting materials in order to accomplish current transfer to other parts of a real operating system, or for testing and measurement of properties. Thus, whether the configuration is a clad wire, a bulk superconducting disc, tape, or a thick or thin superconducting film on a substrate, the physical and mechanical behavior of interface (interconnections, joints, etc.) between superconductors and normal conductor materials of all kinds is of extreme importance to the technological development of these systems. Fabrication heat treatments associated with the particular joining process allow possible reactions between the superconducting ceramic and the contact to occur, and consequently influence properties at the interface region. The nature of these reactions is therefore of great broad interest, as these may be a primary determinant for the real capability of these materials. Research related both to fabrication of composite sheathed wire products, and the joining contacts for physical property measurements, as well as, a review of other related literature in the field are described. Comparison are made between 1-2-3, Bi-, and Tl-based ceramic superconductors joined to a variety of metals including Cu, Ni, Fe, Cr, Ag, Ag-Pd, Au, In, and Ga. The morphology of reaction products and the nature of interface degradation as a function of time will be highlighted
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