758 research outputs found
Spin Transfer Torque for Continuously Variable Magnetization
We report quantum and semi-classical calculations of spin current and
spin-transfer torque in a free-electron Stoner model for systems where the
magnetization varies continuously in one dimension.Analytic results are
obtained for an infinite spin spiral and numerical results are obtained for
realistic domain wall profiles. The adiabatic limit describes conduction
electron spins that follow the sum of the exchange field and an effective,
velocity-dependent field produced by the gradient of the magnetization in the
wall. Non-adiabatic effects arise for short domain walls but their magnitude
decreases exponentially as the wall width increases. Our results cast doubt on
the existence of a recently proposed non-adiabatic contribution to the
spin-transfer torque due to spin flip scattering.Comment: 11 pages, 9 figure
Macrospin Models of Spin Transfer Dynamics
The current-induced magnetization dynamics of a spin valve are studied using
a macrospin (single domain) approximation and numerical solutions of a
generalized Landau-Lifshitz-Gilbert equation. For the purpose of quantitative
comparison with experiment [Kiselev {\it et al.} Nature {\bf 425}, 380 (2003)],
we calculate the resistance and microwave power as a function of current and
external field including the effects of anisotropies, damping, spin-transfer
torque, thermal fluctuations, spin-pumping, and incomplete absorption of
transverse spin current. While many features of experiment appear in the
simulations, there are two significant discrepancies: the current dependence of
the precession frequency and the presence/absence of a microwave quiet magnetic
phase with a distinct magnetoresistance signature. Comparison is made with
micromagnetic simulations designed to model the same experiment.Comment: 14 pages, 14 figures. Email [email protected] for a
pdf with higher quality figure
Scaling of the Hysteresis Loop in Two-dimensional Solidification
The first order phase transitions between a two-dimensional (2d) gas and the
2d solid of the first monolayer have been studied for the noble gases Ar, Kr
and Xe on a NaCl(100) surface in quasi-equilibrium with the three-dimensional
gas phase. Using linear temperature ramps, we show that the widths of the
hysteresis loops of these transitions as a function of the heating rate, r,
scales with a power law r^alpha with alpha between 0.4 and 0.5 depending on the
system. The hysteresis loops for different heating rates are similar. The
island area of the condensed layer was found to grow initially with a t^4 time
dependence. These results are in agreement with theory, which predicts alpha =
0.5 and hysteresis loop similarity.Comment: 4 pages, 5 figures, Revte
Maximum Likelihood Estimation in Gaussian Chain Graph Models under the Alternative Markov Property
The AMP Markov property is a recently proposed alternative Markov property
for chain graphs. In the case of continuous variables with a joint multivariate
Gaussian distribution, it is the AMP rather than the earlier introduced LWF
Markov property that is coherent with data-generation by natural
block-recursive regressions. In this paper, we show that maximum likelihood
estimates in Gaussian AMP chain graph models can be obtained by combining
generalized least squares and iterative proportional fitting to an iterative
algorithm. In an appendix, we give useful convergence results for iterative
partial maximization algorithms that apply in particular to the described
algorithm.Comment: 15 pages, article will appear in Scandinavian Journal of Statistic
Mn induced modifications of Ga 3d photoemission from (Ga, Mn)As: evidence for long range effects
Using synchrotron based photoemission, we have investigated the Mn-induced
changes in Ga 3d core level spectra from as-grown . Although Mn is located in Ga substitutional sites, and does
therefore not have any Ga nearest neighbours, the impact of Mn on the Ga core
level spectra is pronounced even at Mn concentrations in the range of 0.5%. The
analysis shows that each Mn atom affects a volume corresponding to a sphere
with around 1.4 nm diameter.Comment: Submitted to Physical Review B, Brief Repor
Dynamical Casimir-Polder interaction between an atom and surface plasmons
We investigate the time-dependent Casimir-Polder potential of a polarizable
two-level atom placed near a surface of arbitrary material, after a sudden
change in the parameters of the system. Different initial conditions are taken
into account. For an initially bare ground-state atom, the time-dependent
Casimir-Polder energy reveals how the atom is "being dressed" by virtual,
matter-assisted photons. We also study the transient behavior of the
Casimir-Polder interaction between the atom and the surface starting from a
partially dressed state, after an externally induced change in the atomic level
structure or transition dipoles. The Heisenberg equations are solved through an
iterative technique for both atomic and field operators in the medium-assisted
electromagnetic field quantization scheme. We analyze in particular how the
time evolution of the interaction energy depends on the optical properties of
the surface, in particular on the dispersion relationof surface plasmon
polaritons. The physical significance and the limits of validity of the
obtained results are discussed in detail.Comment: 12 pages, 8 figure
The Dynamics of Charges Induced by a Charged Particle Traversing a Dielectric Slab
We studied the dynamics of surfacea and wake charges induced by a charged
particle traversing a dielectric slab. It is shown that after the crossing of
the slab first boundary, the induced on the slab surface charge (image charge)
is transformed into the wake charge, which overflows to the second boundary
when the particle crosses it. It is also shown, that the polarization of the
slab is of an oscillatory nature, and the net induced charge in a slab remains
zero at all stages of the motion.Comment: 12 pages, 1 figur
Center of mass and relative motion in time dependent density functional theory
It is shown that the exchange-correlation part of the action functional
in time-dependent density functional theory , where
is the time-dependent density, is invariant under the
transformation to an accelerated frame of reference , where is an arbitrary
function of time. This invariance implies that the exchange-correlation
potential in the Kohn-Sham equation transforms in the following manner:
. Some of the
approximate formulas that have been proposed for satisfy this exact
transformation property, others do not. Those which transform in the correct
manner automatically satisfy the ``harmonic potential theorem", i.e. the
separation of the center of mass motion for a system of interacting particles
in the presence of a harmonic external potential. A general method to generate
functionals which possess the correct symmetry is proposed
Temperature dependence of the magnetic Casimir-Polder interaction
We analyze the magnetic dipole contribution to atom-surface dispersion
forces. Unlike its electrical counterpart, it involves small transition
frequencies that are comparable to thermal energy scales. A significant
temperature dependence is found near surfaces with a nonzero DC conductivity,
leading to a strong suppression of the dispersion force at T > 0. We use
thermal response theory for the surface material and discuss both normal metals
and superconductors. The asymptotes of the free energy of interaction and of
the entropy are calculated analytically over a large range of distances. Near a
superconductor, the onset of dissipation at the phase transition strongly
changes the interaction, including a discontinuous entropy. We discuss the
similarities with the Casimir interaction beween two surfaces and suggest that
precision measurements of the atom-surface interaction may shed new light upon
open questions around the temperature dependence of dispersion forces between
lossy media.Comment: 11 figure
The Effective Particle-Hole Interaction and the Optical Response of Simple Metal Clusters
Following Sham and Rice [L. J. Sham, T. M. Rice, Phys. Rev. 144 (1966) 708]
the correlated motion of particle-hole pairs is studied, starting from the
general two-particle Greens function. In this way we derive a matrix equation
for eigenvalues and wave functions, respectively, of the general type of
collective excitation of a N-particle system. The interplay between excitons
and plasmons is fully described by this new set of equations. As a by-product
we obtain - at least a-posteriori - a justification for the use of the TDLDA
for simple-metal clusters.Comment: RevTeX, 15 pages, 5 figures in uufiles format, 1 figure avaible from
[email protected]
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