3,190 research outputs found
Localization Properties of Quantized Magnetostatic Modes in Nanocubes
We investigate the dynamical properties of a system of interacting magnetic
dipoles disposed in sites of an sc lattice and forming a cubic-shaped sample of
size determined by the cube edge length (N-1)a (a being the lattice constant, N
representing the number of dipolar planes). The dipolar field resulting from
the dipole-dipole interactions is calculated numerically in points of the axis
connecting opposite cube face centers (central axis) by collecting individual
contributions to this field coming from each of the N atomic planes
perpendicular to the central axis. The applied magnetic field is assumed to be
oriented along the central axis, magnetizing uniformly the whole sample, all
the dipoles being aligned parallelly in the direction of the applied field. The
frequency spectrum of magnetostatic waves propagating in the direction of the
applied field is found numerically by solving the Landau-Lifshitz equation of
motion including the local (nonhomogeneous) dipolar field component; the mode
amplitude spatial distributions (mode profiles) are depicted as well. It is
found that only the two energetically highest modes have bulk-extended
character. All the remaining modes are of localized nature; more precisely, the
modes forming the lower part of the spectrum are localized in the subsurface
region, while the upper-spectrum modes are localized around the sample center.
We show that the mode localization regions narrow down as the cube size, N,
increases (we investigated the range of N=21 to N=101), and in sufficiently
large cubes one obtains practically only center-localized and surface-localized
magnetostatic modes.Comment: 20 pages, 9 figures in postscript, useing Revtex4.cl
(In)finite extensions of algebras from their Inonu-Wigner contractions
The way to obtain massive non-relativistic states from the Poincare algebra
is twofold. First, following Inonu and Wigner the Poincare algebra has to be
contracted to the Galilean one. Second, the Galilean algebra is to be extended
to include the central mass operator. We show that the central extension might
be properly encoded in the non-relativistic contraction. In fact, any
Inonu-Wigner contraction of one algebra to another, corresponds to an infinite
tower of abelian extensions of the latter. The proposed method is
straightforward and holds for both central and non-central extensions. Apart
from the Bargmann (non-zero mass) extension of the Galilean algebra, our list
of examples includes the Weyl algebra obtained from an extension of the
contracted SO(3) algebra, the Carrollian (ultra-relativistic) contraction of
the Poincare algebra, the exotic Newton-Hooke algebra and some others. The
paper is dedicated to the memory of Laurent Houart (1967-2011).Comment: 7 pages, revtex style; v2: Minor corrections, references added; v3:
Typos correcte
Area Distribution of Elastic Brownian Motion
We calculate the excursion and meander area distributions of the elastic
Brownian motion by using the self adjoint extension of the Hamiltonian of the
free quantum particle on the half line. We also give some comments on the area
of the Brownian motion bridge on the real line with the origin removed. We will
stress on the power of self adjoint extension to investigate different possible
boundary conditions for the stochastic processes.Comment: 18 pages, published versio
A theory of non-local linear drift wave transport
Transport events in turbulent tokamak plasmas often exhibit non-local or
non-diffusive action at a distance features that so far have eluded a
conclusive theoretical description. In this paper a theory of non-local
transport is investigated through a Fokker-Planck equation with fractional
velocity derivatives. A dispersion relation for density gradient driven linear
drift modes is derived including the effects of the fractional velocity
derivative in the Fokker-Planck equation. It is found that a small deviation (a
few percent) from the Maxwellian distribution function alters the dispersion
relation such that the growth rates are substantially increased and thereby may
cause enhanced levels of transport.Comment: 22 pages, 2 figures. Manuscript submitted to Physics of Plasma
Pressure Evolution of the Ferromagnetic and Field Re-entrant Superconductivity in URhGe
Fine pressure () and magnetic field () tuning on the ferromagnetic
superconductor URhGe are reported in order to clarify the interplay between the
mass enhancement, low field superconductivity (SC) and field reentrant
superconductivity (RSC) by electrical resistivity measurements. With increasing
, the transition temperature and the upper critical field of the low field
SC decrease slightly, while the RSC dome drastically shifts to higher fields
and shrinks. The spin reorientation field also increases. At a
pressure GPa, the RSC has collapsed while the low field SC persists
and may disappear only above 4 GPa. Via careful studies of the
inelastic resistivity term, it is demonstrated that this drastic change
is directly related with the dependence of the effective mass which
determines the critical field of the low field SC and RSC on the basis of
triplet SC without Pauli limiting field.Comment: 5 pages, 6 figures, to appear in Journal of the Physical Society of
Japa
Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe
Magnetoresistivity measurements with fine tuning of the field direction on
high quality single crystals of the ferromagnetic superconductor UCoGe show
anomalous anisotropy of the upper critical field H_c2. H_c2 for H // b-axis
(H_c2^b) in the orthorhombic crystal structure is strongly enhanced with
decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The
temperature dependence of H_c2^a shows upward curvature with a low temperature
value exceeding 30 T, while H_c2^c at 0 K is very small (~ 0.6 T). Contrary to
conventional ferromagnets, the decrease of the Curie temperature with
increasing field for H // b-axis marked by an enhancement of the effective mass
of the conduction electrons appears to be the origin of the S-shaped H_c2^b
curve. These results indicate that the field-induced ferromagnetic instability
or magnetic quantum criticality reinforces superconductivity.Comment: 5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Modulating the phase transition temperature of giant magnetocaloric thin films by ion irradiation
Magnetic refrigeration based on the magnetocaloric effect at room temperature
is one of the most attractive alternative to the current gas
compression/expansion method routinely employed. Nevertheless, in giant
magnetocaloric materials, optimal refrigeration is restricted to the narrow
temperature window of the phase transition (Tc). In this work, we present the
possibility of varying this transition temperature into a same giant
magnetocaloric material by ion irradiation. We demonstrate that the transition
temperature of iron rhodium thin films can be tuned by the bombardment of ions
of Ne 5+ with varying fluences up to 10 14 ions cm --2 , leading to optimal
refrigeration over a large 270--380 K temperature window. The Tc modification
is found to be due to the ion-induced disorder and to the density of new
point-like defects. The variation of the phase transition temperature with the
number of incident ions opens new perspectives in the conception of devices
using giant magnetocaloric materials
The fractional Schr\"{o}dinger operator and Toeplitz matrices
Confining a quantum particle in a compact subinterval of the real line with
Dirichlet boundary conditions, we identify the connection of the
one-dimensional fractional Schr\"odinger operator with the truncated Toeplitz
matrices. We determine the asymptotic behaviour of the product of eigenvalues
for the -stable symmetric laws by employing the Szeg\"o's strong limit
theorem. The results of the present work can be applied to a recently proposed
model for a particle hopping on a bounded interval in one dimension whose
hopping probability is given a discrete representation of the fractional
Laplacian.Comment: 10 pages, 2 figure
Anyon wave equations and the noncommutative plane
The ``Jackiw-Nair'' non-relativistic limit of the relativistic anyon
equations provides us with infinite-component wave equations of the
Dirac-Majorana-Levy-Leblond type for the ``exotic'' particle, associated with
the two-fold central extension of the planar Galilei group. An infinite
dimensional representation of the Galilei group is found. The velocity operator
is studied, and the observable coordinates describing a noncommutative plane
are identified.Comment: 11 pages, typos correcte
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