419 research outputs found
Comment on "Ferromagnetic film on a superconducting substrate"
A superconducting substrate is not able to shrink drastically domains in a
ferromagnetic film, contrary to the prediction of Bulaevskii and Chudnovsky
[Phys. Rev. B, 63, issue1 (2001)]. This is shown on the basis of the exact
solution for the stripe domain structure.Comment: 8 pages, 1 figure, the version published in Phys. Rev.
Ferromagnetic Film on a Superconducting Substrate
We study the equilibrium domain structure and magnetic flux around a
ferromagnetic (FM) film with perpendicular magnetization M_0 on a
superconducting (SC) substrate. At 4{\pi}M_0<H_{c1} the SC is in the Meissner
state and the equilibrium domain width in the film, l, scales as
(l/4{\pi}{\lambda}_{L}) = (l_{N}/4{\pi}{\lambda}_{L})^{2/3} with the domain
width on a normal (non-superconducting) substrate, l_{N}/4\pi\lambda_L >> 1.
Here \lambda_L is the London penetration length. For 4{\pi}M_0 > H_{c1} and
l_{N} in excess of about 35 {\lambda}_{L}, the domains are connected by SC
vortices. We argue that pinning of vortices by magnetic domains in FM/SC
multilayers can provide high critical currents.Comment: 4 pages, 2 figures, submitted to PR
Inhomogeneous LOFF phase revisited for surface superconductivity
We consider 2D surface superconductivity in high magnetic fields parallel to
the surface. We demonstrate that the spin-orbit interaction at the surface
changes the properties of the inhomogeneous superconducting
Larkin-Ovchinnikov-Fulde-Ferrell state that develops above fields given by the
paramagnetic criterion. Strong spin-orbit interaction significantly broadens
the range of existence of the LOFF phase, which takes the form of periodic
superconducting stripes running along the field direction on the surface,
leading to the anisotropy of its properties. In connection with experiments by
J.H. Schon et al. [Nature 914, 434 (2001)] on superconductivity of electrically
doped films of the cuprate material CaCuO2, we also discuss this problem for
the d-wave pairing to indicate the possibility of a re-orientation transition
as the magnetic field direction is rotated in the plane parallel to the
surface. Our results provide a tool for studying surface superconductivity as a
function of doping.Comment: 4 pages, 1 fig, revtex
Hysteretic nonequilibrium Ising-Bloch transition
We show that a parametrically driven cubic-quintic complex Ginzburg-Landau
equation exhibits a hysteretic nonequilibrium Ising-Bloch transition for large
enough quintic nonlinearity. These results help to understand the recent
experimental observation of this pheomenon [A. Esteban-Martin et al., Phys.
Rev. Lett. 94, 223903 (2005)].Comment: 3 pages + six figure
INTERLAYER VORTICES AND EDGE DISLOCATIONS IN HIGH TEMPERATURE SUPERCONDUCTORS
The interaction of an edge dislocation made of half the superconducting plane
with a magnetic interlayer vortex is considered within the framework of the
Lawrence-Doniach model with negative as well as positive Josephson interlayer
coupling. In the first case the binding energy of the vortex and the
dislocation has been calculated by employing a variational procedure. The
current distribution around the bound vortex turns out to be asymmetric. In the
second case the dislocation carries a spontaneous magnetic half-vortex, whose
binding energy with the dislocation turns out to be infinite. The half-vortex
energy has been calculated by the same variational procedure. Implications of
the possible presence of such half-vortices for the properties of high
temperature superconductors are discussed.Comment: 14 Latex pages, 1 figure available upon request
A strong-coupling expansion for the Hubbard model
We reconsider the strong-coupling expansion for the Hubbard model recently
introduced by Sarker and Pairault {\it et al.} By introducing slave particles
that act as projection operators onto the empty, singly occupied and doubly
occupied atomic states, the perturbation theory around the atomic limit
distinguishes between processes that do conserve or do not conserve the total
number of doubly occupied sites. This allows for a systematic expansion
that does not break down at low temperature ( being the intersite hopping
amplitude and the local Coulomb repulsion). The fermionic field becomes a
two-component field, which reflects the presence of the two Hubbard bands. The
single-particle propagator is naturally expressed as a function of a matrix self-energy. Furthermore, by introducing a time- and
space-fluctuating spin-quantization axis in the functional integral, we can
expand around a ``non-degenerate'' ground-state where each singly occupied site
has a well defined spin direction (which may fluctuate in time). This formalism
is used to derive the effective action of charge carriers in the lower Hubbard
band to first order in . We recover the action of the t-J model in the
spin-hole coherent-state path integral. We also compare our results with those
previously obtained by studying fluctuations around the large- Hartree-Fock
saddle point.Comment: 20 pages RevTex, 3 figure
Optical Resonances in Reflectivity near Crystal Modes with Spatial Dispersion
We study the effect of spatial dispersion of crystal modes on optical
properties such as the reflectivity . As an example for isotropic media, we
investigate the simplest model for phonons in ionic crystals and compare with
previous results for highly anisotropic plasmons, which are now understood from
a more general point of view. As a consequence of the wave vector dependence of
the dielectric function small changes in the lineshape are predicted. Beyond
that, if the frequency of minimal is near a pole of the dispersionless
dielectric function, the relative amplitude of dips in with normal and
anomalous dispersion differ significantly, if dissipation and disorder are low.Comment: 4 pages, 7 eps figures, minor change
Novel Phases of Planar Fermionic Systems
We discuss a {\em family} of planar (two-dimensional) systems with the
following phase strucure: a Fermi liquid, which goes by a second order
transition (with non classical exponent even in mean-field) to an intermediate,
inhomogeneous state (with nonstandard ordering momentum) , which in turn goes
by a first order transition to a state with canonical order parameter. We
analyze two examples: (i) a superconductor in a parallel magnetic field (which
was discussed independently by Bulaevskii)for which the inhomogeneous state is
obtained for where is the critical temperature (in Kelvin) of the superconductor
without a field and is measured in Tesla, and (ii) spinless (or, as is
explained, spin polarized) fermions near half-filling where a similar, sizeable
window (which grows in size with anisotropy) exists for the intermediate CDW
phase at an ordering momentum different from . We discuss the
experimental conditions for realizing and observing these phases and the
Renormalization Group approach to the transitions.Comment: ([email protected],[email protected]) 29 p Latex 4 figs
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Thermally Assisted Penetration and Exclusion of Single Vortex in Mesoscopic Superconductors
A single vortex overcoming the surface barrier in a mesoscopic superconductor
with lateral dimensions of several coherence lengths and thickness of several
nanometers provides an ideal platform to study thermal activation of a single
vortex. In the presence of thermal fluctuations, there is non-zero probability
for vortex penetration into or exclusion from the superconductor even when the
surface barrier does not vanish. We consider the thermal activation of a single
vortex in a mesoscopic superconducting disk of circular shape. To obtain
statistics for the penetration and exclusion magnetic fields, slow and periodic
magnetic fields are applied to the superconductor. We calculate the
distribution of the penetration and exclusion fields from the thermal
activation rate. This distribution can also be measured experimentally, which
allows for a quantitative comparison.Comment: 7 pages, 4 figure
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