6,410 research outputs found
Thickness-dependent magnetic structure of ultrathin Fe/Ir(001) films: from spin-spiral states towards ferromagnetic order
We present a detailed study of the ground-state magnetic structure of
ultrathin Fe films on the surface of fcc Ir(001). We use the spin-cluster
expansion technique in combination with the relativistic disordered local
moment scheme to obtain parameters of spin models and then determine the
favored magnetic structure of the system by means of a mean field approach and
atomistic spin dynamics simulations. For the case of a single monolayer of Fe
we find that layer relaxations very strongly influence the ground-state spin
configurations, whereas Dzyaloshinskii-Moriya (DM) interactions and biquadratic
couplings also have remarkable effects. To characterize the latter effect we
introduce and analyze spin collinearity maps of the system. While for two
monolayers of Fe we find a single-q spin spiral as ground state due to DM
interactions, for the case of four monolayers the system shows a noncollinear
spin structure with nonzero net magnetization. These findings are consistent
with experimental measurements indicating ferromagnetic order in films of four
monolayers and thicker.Comment: 9 pages, 7 figure
Structure of a liquid crystalline fluid around a macroparticle: Density functional theory study
The structure of a molecular liquid, in both the nematic liquid crystalline
and isotropic phases, around a cylindrical macroparticle, is studied using
density functional theory. In the nematic phase the structure of the fluid is
highly anisotropic with respect to the director, in agreement with results from
simulation and phenomenological theories. On going into the isotropic phase the
structure becomes rotationally invariant around the macroparticle with an
oriented layer at the surface.Comment: 10 pages, 6 figues. Submitted to Phys. Rev.
Scattering matrix approach to interacting electron transport
We investigate the modification in mesoscopic electronic transport due to
electron-electron interactions making use of scattering states. We demonstrate
that for a specific (finite range) interaction kernel, the knowledge of the
scattering matrix is sufficient to take interaction effects into account. We
calculate perturbatively the corrections to the current and current-current
correlator; in agreement with previous work, we find that, in linear response,
interaction effects can be accounted for by an effective (renormalized)
transmission probability. Beyond linear response, simple renormalization of
scattering coefficients is not sufficient to describe the current-current
correlator, as additional corrections arise due to irreducible two-particle
processes. Furthermore, we find that the correlations between opposite-spin
currents induced by interaction are enhanced for an asymmetric scatterer,
generating a nonzero result already to lowest order in the interaction
The probability distribution of the number of electron-positron pairs produced in a uniform electric field
The probability-generating function of the number of electron-positron pairs
produced in a uniform electric field is constructed. The mean and variance of
the numbers of pairs are calculated, and analytical expressions for the
probability of low numbers of electron-positron pairs are given. A recursive
formula is derived for evaluating the probability of any number of pairs. In
electric fields of supercritical strength |eE| > \pi m^2/ \ln 2, where e is the
electron charge, E is the electric field, and m is the electron mass, a
branch-point singularity of the probability-generating function penetrates the
unit circle |z| = 1, which leads to the asymptotic divergence of the cumulative
probability. This divergence indicates a failure of the continuum limit
approximation. In the continuum limit and for any field strength, the positive
definiteness of the probability is violated in the tail of the distribution.
Analyticity, convergence, and positive definiteness are restored upon the
summation over discrete levels of electrons in the normalization volume.
Numerical examples illustrating the field strength dependence of the asymptotic
behavior of the probability distribution are presented.Comment: 7 pages, REVTeX, 4 figures; new references added; a short version of
this e-print has appeared in PR
Dynamics of Two-Level System Interacting with Random Classical Field
The dynamics of a particle interacting with random classical field in a
two-well potential is studied by the functional integration method. The
probability of particle localization in either of the wells is studied in
detail. Certain field-averaged correlation functions for quantum-mechanical
probabilities and the distribution function for the probabilities of final
states (which can be considered as random variables in the presence of a random
field) are calculated. The calculated correlators are used to discuss the
dependence of the final state on the initial state. One of the main results of
this work is that, although the off-diagonal elements of density matrix
disappear with time, a particle in the system is localized incompletely
(wave-packet reduction does not occur), and the distribution function for the
probability of finding particle in one of the wells is a constant at infinite
time.Comment: 5 page
Wave Function Collapse in a Mesoscopic Device
We determine the non-local in time and space current-current cross correlator
in a mesoscopic conductor with a
scattering center at the origin. Its excess part appearing at finite voltage
exhibits a unique dependence on the retarded variable , with the Fermi
velocity. The non-monotonic dependence of the retardation on and its
absence at the symmetric position is a signature of the wave
function collapse, which thus becomes amenable to observation in a mesoscopic
solid state device.Comment: 7 pages, 2 fugure
N-particle scattering matrix for electrons interacting on a quantum dot
We present a non-perturbative expression for the scattering matrix of
particles interacting inside a quantum dot. Characterizing the dot by its
resonances, we find a compact form for the scattering matrix in a real-time
representation. We study the transmission probabilities and interaction-induced
orbital entanglement of two electrons incident on the dot in a spin-singlet
state.Comment: 4 page
Bulk viscosity of superfluid neutron stars
The hydrodynamics, describing dynamical effects in superfluid neutron stars,
essentially differs from the standard one-fluid hydrodynamics. In particular,
we have four bulk viscosity coefficients in the theory instead of one. In this
paper we calculate these coefficients, for the first time, assuming they are
due to non-equilibrium beta-processes (such as modified or direct Urca
process). The results of our analysis are used to estimate characteristic
damping times of sound waves in superfluid neutron stars. It is demonstrated
that all four bulk viscosity coefficients lead to comparable dissipation of
sound waves and should be considered on the same footing.Comment: 11 pages, 1 figure, this version with some minor stylistic changes is
published in Phys. Rev.
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