11 research outputs found
Charge degrees in the quarter-filled checkerboard lattice
For a systematic study of charge degrees of freedom in lattices with
geometric frustration, we consider spinless fermions on the checkerboard
lattice with nearest-neighbor hopping and nearest-neighbor repulsion at
quarter-filling. An effective Hamiltonian for the limit is given to
lowest non-vanishing order by the ring exchange (). We show
that the system can equivalently be described by hard-core bosons and map the
model to a confining U(1) lattice gauge theory.Comment: Proceedings of ICM200
Magnetic-interference patterns in Josephson junctions with d+is symmetry
The magnetic interference pattern and the spontaneous flux in unconventional
Josephson junctions of superconductors with d+is symmetry are calculated for
different reduced junction lengths and the relative factor of the d and s wave
components. This is a time reversal broken symmetry state. We study the
stability of the fractional vortex and antivortex which are spontaneously
formed and examine their evolution as we change the length and the relative
factor of d and s wave components. The asymmetry in the field modulated
diffraction pattern exists for lengths as long as L=10\lambda_J.Comment: 8 pages, 6 eps files, submitted to PR
Distinct Magnetic Phase Transition at the Surface of an Antiferromagnet
In the majority of magnetic systems the surface is required to order at the same temperature as the bulk. In the present Letter, we report a distinct and unexpected surface magnetic phase transition at a lower temperature than the Néel temperature. Employing grazing incidence x-ray resonant magnetic scattering, we have observed the near-surface behavior of uranium dioxide. UO2 is a noncollinear, triple-q, antiferromagnet with the U ions on a face-centered cubic lattice. Theoretical investigations establish that at the surface the energy increase—due to the lost bonds—is reduced when the spins near the surface rotate, gradually losing their component normal to the surface. At the surface the lowest-energy spin configuration has a double-q (planar) structure. With increasing temperature, thermal fluctuations saturate the in-plane crystal field anisotropy at the surface, leading to soft excitations that have ferromagnetic XY character and are decoupled from the bulk. The structure factor of a finite two-dimensional XY model fits the experimental data well for several orders of magnitude of the scattered intensity. Our results support a distinct magnetic transition at the surface in the Kosterlitz-Thouless universality class
Microscopic mechanisms of spin-dependent electric polarization in 3d oxides
We present a short critical overview of different microscopic models for
nonrelativistic and relativistic magnetoelectric coupling including the
so-called "spin current scenario", ab-initio calculations, and several recent
microscopic approaches to a spin-dependent electric polarization in 3d oxides.Comment: 8 pages, 3 figure
Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors
Quasiclassic Uzadel equations for two-band superconductors in the dirty limit
with the account of both intraband and interband scattering by nonmagnetic
impurities are derived for any anisotropic Fermi surface. From these equations
the Ginzburg-Landau equations, and the critical temperature are obtained.
An equation for the upper critical field, which determines both the temperature
dependence of and the orientational dependence of
as a function of the angle between and the c-axis is
obtained. It is shown that the shape of the curve essentially
depends on the ratio of the intraband electron diffusivities and ,
and can be very different from the standard one-gap dirty limit theory. In
particular, the value can considerably exceed ,
which can have important consequences for applications of . A scaling
relation is proposed which enables one to obtain the angular dependence of
from the equation for at . It is shown
that, depending on the relation between and , the ratio of the upper
critical field for and can both increase and decrease as the temperature decreases. Implications
of the obtained results for are discussed
Ginzburg-Landau theory of vortices in a multi-gap superconductor
The Ginzburg-Landau functional for a two-gap superconductor is derived within
the weak-coupling BCS model. The two-gap Ginzburg-Landau theory is, then,
applied to investigate various magnetic properties of MgB2 including an upturn
temperature dependence of the transverse upper critical field and a core
structure of an isolated vortex. Orientation of vortex lattice relative to
crystallographic axes is studied for magnetic fields parallel to the c-axis. A
peculiar 30-degree rotation of the vortex lattice with increasing strength of
an applied field observed by neutron scattering is attributed to the multi-gap
nature of superconductivity in MgB2.Comment: 11 page
Ginzburg-Landau theory and effects of pressure on a two-band superconductor : application to <tex>MgB_{2}$</tex>
Structural properties and melting of a quasi-one dimensional classical Wigner crystal
The properties of a quasi-one-dimensional (Q1D) classical Wigner crystal are studied. The charged particles move in a plane and interact with a screened Coulomb potential (Yukawa type) and with an additional external confining parabolic potential in one direction. At T = 0 we obtain the phase diagram in the space of inverse screening length kappa and density (n) over tilde (e). The phonon spectrum was calculated for the different phases of the electron solid and its magnetic field dependence was obtained. We found that the melting of Q1D Wigner lattice shows reentrant behavior as function of the density. (C) 2003 Published by Elsevier B.V.</p
Emergence of a hidden magnetic phase in LaFe11.8Si1.2 investigated by inelastic neutron scattering as a function of magnetic field and temperature
The NaZn13 type itinerant magnet LaFe13−xSix has seen considerable interest due to its unique combination of large magnetocaloric effect and low hysteresis. Here, this alloy with a combination of magnetometry, bespoke microcalorimetry, and inelastic neutron scattering is investigated. Inelastic neutron scattering reveals the presence of broad quasielastic scattering that persists across the magnetic transition, which is attributed to spin fluctuations. In addition, a quasielastic peak is observed at Q = 0.52 Å−1 for x = 1.2 that exists only in the paramagnetic state in proximity to the itinerant metamagnetic transition and argue that this indicates emergence of a hidden mag the netic phase that drives the first-order phase transition in this system