6,745 research outputs found
Lifetimes of Magnons in Two-Dimensional Diluted Ferromagnetic Systems
Spin dynamics in low dimensional magnetic systems has been of fundamental
importance for a long time and has currently received an impetus owing to the
emerging field of nanoelectronics. Knowledge of the spin wave lifetimes, in
particular, can be favorable for future potential applications. We investigate
the low-temperature spin wave excitations in two-dimensional disordered
ferromagnetic systems, with a particular focus on the long wavelength magnon
lifetimes. A semi-analytical Green's functions based approach is used to
determine the dynamical spectral functions, for different magnetic impurity
concentrations, from which the intrinsic linewidth is extracted. We obtain an
unambiguous scaling of the magnon linewidth which is ascribed to the
disorder induced damping of the spin waves, thereby settling a longstanding
unresolved issue on the wave-vector dependence. Our findings are also in good
agreement with previous theoretical studies on Heisenberg ferromagnets.
Additionally, we demonstrate the futility of using the low moments associated
with the spectral densities to evaluate the magnon dispersions and lifetimes.Comment: 9 pages, 9 figures, improved discussion, references added, and
revised to match the published versio
In-plane magnetoelectric response in bilayer graphene
A graphene bilayer shows an unusual magnetoelectric response whose magnitude
is controlled by the valley-isospin density, making it possible to link
magnetoelectric behavior to valleytronics. Complementary to previous studies,
we consider the effect of static homogeneous electric and magnetic fields that
are oriented parallel to the bilayer's plane. Starting from a tight-binding
description and using quasi-degenerate perturbation theory, the low-energy
Hamiltonian is derived including all relevant magnetoelectric terms whose
prefactors are expressed in terms of tight-binding parameters. We confirm the
existence of an expected axion-type pseudoscalar term, which turns out to have
the same sign and about twice the magnitude of the previously obtained
out-of-plane counterpart. Additionally, small anisotropic corrections to the
magnetoelectric tensor are found that are fundamentally related to the skew
interlayer hopping parameter . We discuss possible ways to identify
magnetoelectric effects by distinctive features in the optical conductivity.Comment: 14 pages, 7 figure
MIMO Transmission with Residual Transmit-RF Impairments
Physical transceiver implementations for multiple-input multiple-output
(MIMO) wireless communication systems suffer from transmit-RF (Tx-RF)
impairments. In this paper, we study the effect on channel capacity and
error-rate performance of residual Tx-RF impairments that defy proper
compensation. In particular, we demonstrate that such residual distortions
severely degrade the performance of (near-)optimum MIMO detection algorithms.
To mitigate this performance loss, we propose an efficient algorithm, which is
based on an i.i.d. Gaussian model for the distortion caused by these
impairments. In order to validate this model, we provide measurement results
based on a 4-stream Tx-RF chain implementation for MIMO orthogonal
frequency-division multiplexing (OFDM).Comment: to be presented at the International ITG Workshop on Smart Antennas -
WSA 201
Fast Frechet Distance Between Curves With Long Edges
Computing the Fr\'echet distance between two polygonal curves takes roughly
quadratic time. In this paper, we show that for a special class of curves the
Fr\'echet distance computations become easier. Let and be two polygonal
curves in with and vertices, respectively. We prove four
results for the case when all edges of both curves are long compared to the
Fr\'echet distance between them: (1) a linear-time algorithm for deciding the
Fr\'echet distance between two curves, (2) an algorithm that computes the
Fr\'echet distance in time, (3) a linear-time
-approximation algorithm, and (4) a data structure that supports
-time decision queries, where is the number of vertices of
the query curve and the number of vertices of the preprocessed curve
- …