641 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
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
Spin transport in magnetic multilayers
We study by extensive Monte Carlo simulations the transport of itinerant
spins travelling inside a multilayer composed of three ferromagnetic films
antiferromagnetically coupled to each other in a sandwich structure. The two
exterior films interact with the middle one through non magnetic spacers. The
spin model is the Ising one and the in-plane transport is considered. Various
interactions are taken into account. We show that the current of the itinerant
spins going through this system depends strongly on the magnetic ordering of
the multilayer: at temperatures below (above) the transition temperature
, a strong (weak) current is observed. This results in a strong jump of
the resistance across . Moreover, we observe an anomalous variation,
namely a peak, of the spin current in the critical region just above . We
show that this peak is due to the formation of domains in the temperature
region between the low- ordered phase and the true paramagnetic disordered
phase. The existence of such domains is known in the theory of critical
phenomena. The behavior of the resistance obtained here is compared to a recent
experiment. An excellent agreement with our physical interpretation is
observed. We also show and discuss effects of various physical parameters
entering our model such as interaction range, strength of electric and magnetic
fields and magnetic film and non magnetic spacer thicknesses.Comment: 8 pages, 17 figures, submitted to J. Phys.: Cond Matte
Effects of Frustrated Surface in Heisenberg Thin Films
We study by extensive Monte Carlo (MC) simulations and analytical Green
function (GF) method effects of frustrated surfaces on the properties of thin
films made of stacked triangular layers of atoms bearing Heisenberg spins with
an Ising-like interaction anisotropy. We suppose that the in-plane surface
interaction can be antiferromagnetic or ferromagnetic while all other
interactions are ferromagnetic. We show that the ground-state spin
configuration is non linear when is lower than a critical value .
The film surfaces are then frustrated. In the frustrated case, there are two
phase transitions related to disorderings of surface and interior layers. There
is a good agreement between MC and GF results. In addition, we show from MC
histogram calculation that the value of the ratio of critical exponents
of the observed transitions is deviated from the values of two and
three Ising universality classes. The origin of this deviation is discussed
with general physical arguments.Comment: 9 pages, 16 figure
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
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
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]
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
Enhancement of the Deuteron-Fusion Reactions in Metals and its Experimental Implications
Recent measurements of the reaction d(d,p)t in metallic environments at very
low energies performed by different experimental groups point to an enhanced
electron screening effect. However, the resulting screening energies differ
strongly for divers host metals and different experiments. Here, we present new
experimental results and investigations of interfering processes in the
irradiated targets. These measurements inside metals set special challenges and
pitfalls which make them and the data analysis particularly error-prone. There
are multi-parameter collateral effects which are crucial for the correct
interpretation of the observed experimental yields. They mainly originate from
target surface contaminations due to residual gases in the vacuum as well as
from inhomogeneities and instabilities in the deuteron density distribution in
the targets. In order to address these problems an improved differential
analysis method beyond the standard procedures has been implemented. Profound
scrutiny of the other experiments demonstrates that the observed unusual
changes in the reaction yields are mainly due to deuteron density dynamics
simulating the alleged screening energy values. The experimental results are
compared with different theoretical models of the electron screening in metals.
The Debye-H\"{u}ckel model that has been previously proposed to explain the
influence of the electron screening on both nuclear reactions and radioactive
decays could be clearly excluded.Comment: 22 pages, 12 figures, REVTeX4, 2-column format. Submitted to Phys.
Rev. C; accepte
Adiabatic Domain Wall Motion and Landau-Lifshitz Damping
Recent theory and measurements of the velocity of current-driven domain walls
in magnetic nanowires have re-opened the unresolved question of whether
Landau-Lifshitz damping or Gilbert damping provides the more natural
description of dissipative magnetization dynamics. In this paper, we argue that
(as in the past) experiment cannot distinguish the two, but that
Landau-Lifshitz damping nevertheless provides the most physically sensible
interpretation of the equation of motion. From this perspective, (i) adiabatic
spin-transfer torque dominates the dynamics with small corrections from
non-adiabatic effects; (ii) the damping always decreases the magnetic free
energy, and (iii) microscopic calculations of damping become consistent with
general statistical and thermodynamic considerations
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