3,379 research outputs found
Tunnel Magnetoresistance of a Single-Molecule Junction
Based on the non-equilibrium Green's function (NEGF) technique and the
Landauer-B\"{u}ttiker theory, the possibility of a molecular spin-electronic
device, which consists of a single C molecule attached to two
ferromagnetic electrodes with finite cross sections, is investigated. By
studying the coherent spin-dependent transport through the energy levels of the
molecule, it is shown that the tunnel magnetoresistance (TMR) of the molecular
junction depends on the applied voltages and the number of contact points
between the device electrodes and the molecule. The TMR values more than 60%
are obtained by adjusting the related parameters.Comment: 5 pages, 3 figure
Analyzing the Reduced Required BS Density due to CoMP in Cellular Networks
In this paper we investigate the benefit of base station (BS) cooperation in
the uplink of coordinated multi-point (CoMP) networks. Our figure of merit is
the required BS density required to meet a chosen rate coverage. Our model
assumes a 2-D network of BSs on a regular hexagonal lattice in which path loss,
lognormal shadowing and Rayleigh fading affect the signal received from users.
Accurate closed-form expressions are first presented for the sum-rate coverage
probability and ergodic sum-rate at each point of the cooperation region. Then,
for a chosen quality of user rate, the required density of BS is derived based
on the minimum value of rate coverage probability in the cooperation region.
The approach guarantees that the achievable rate in the entire coverage region
is above a target rate with chosen probability. The formulation allows
comparison between different orders of BS cooperation, quantifying the reduced
required BS density from higher orders of cooperation.Comment: Accepted for presentation in IEEE Globecom Conf., to be held in
Atlanta, USA, Dec. 2013. arXiv admin note: text overlap with arXiv:1302.159
-Groups for which each outer -automorphism centralizes only elements
An automorphism of a group is called outer if it is not an inner
automorphism.
Let be a finite -group. Then for every outer -automorphism
of the subgroup has order if and
only if is of order at most
Electronic transport calculations for lightly-doped thermoelectrics using density functional theory: Application to high-performing Cu-doped zinc antimonides
We propose a new method for accurately calculating electrical transport
properties of a lightly-doped thermoelectric material from density functional
theory (DFT) calculations, based on experimental data and density functional
theory results for the corresponding undoped material. We employ this approach
because hybrid DFT calculations are prohibitive for the large supercells
required to model low dopant concentrations comparable to those achieved
experimentally for high-performing thermoelectrics. Using zinc antimonide as
our base material, we find that the electrical transport properties calculated
with DFT and Boltzmann transport theory exhibit the same trends with changes in
chemical potential as those computed with hybrid DFT, and propose a fitting
algorithm that involves adjusting the computed Fermi energy so that the
resulting Seebeck coefficient trends with temperature match experimental
trends. We confirm the validity of this approach in reproducing the
experimental trends in electrical conductivity and Seebeck coefficient versus
temperature for Bi-doped ZnSb. We then screen various
transition metal cation dopants, including copper and nickel, and find that a
Cu dopant concentration of 2.56% in ZnSb exhibited a 14% increase
in the thermoelectric power factor for temperatures between 300-400 K. We thus
propose that transition metal dopants may significantly improve the
thermoelectric performance of the host material, compared to heavy and/or
rare-earth dopants
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