1,224 research outputs found
Electrodynamics of Fulde-Ferrell-Larkin-Ovchinnikov superconducting state
We develop the Ginzburg-Landau theory of the vortex lattice in clean
isotropic three-dimensional superconductors at large Maki parameter, when
inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov state is favored. We show that
diamagnetic superfluid currents mainly come from paramagnetic interaction of
electron spins with local magnetic field, and not from kinetic energy response
to the external field as usual. We find that the stable vortex lattice keeps
its triangular structure as in usual Abrikosov mixed state, while the internal
magnetic field acquires components perpendicular to applied magnetic field.
Experimental possibilities related to this prediction are discussed.Comment: 5 pages, 1 figur
Vortex core deformation and stepper motor behavior in a superconducting ratchet
We investigated experimentally the frequency dependence of a superconducting
vortex ratchet effect by means of electrical transport measurements and modeled
it theoretically using the time dependent Ginzburg-Landau formalism. We
demonstrate that the high frequency vortex behavior can be described as a
discrete motion of a particle in a periodic potential, i.e. the so called
stepper motor behavior. Strikingly, in the more conventional low frequency
response a transition takes place from an Abrikosov vortex rectifier to a phase
slip line rectifier. This transition is characterized by a strong increase in
the rectified voltage and the appearance of a pronounced hysteretic behavior.Comment: 4 pages, 4 figure
Discrete transverse superconducting modes in nano-cylinders
Spatial variation in the superconducting order parameter becomes significant
when the system is confined at dimensions well below the typical
superconducting coherence length. Motivated by recent experimental success in
growing single-crystal metallic nanorods, we study quantum confinement effects
on superconductivity in a cylindrical nanowire in the clean limit. For large
diameters, where the transverse level spacing is smaller than superconducting
order parameter, the usual approximations of Ginzburg-Landau theory are
recovered. However, under external magnetic field the order parameter develops
a spatial variation much stronger than that predicted by Ginzburg-Landau
theory, and gapless superconductivity is obtained above a certain field
strength. At small diameters, the discrete nature of the transverse modes
produces significant spatial variations in the order parameter with increased
average magnitude and multiple shoulders in the magnetic response.Comment: 10 pages, 8 figure
FFLO state in thin superconducting films
We present the analysis of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) superconducting state in thin superconducting films in the parallel
magnetic field. For the tetragonal crystal symmetry (relevant to CeCoIn -
the most probable candidate for the FFLO state formation) we predict a very
peculiar in-plane angular dependence of the FFLO critical field due to the
orbital effect. In the uniform superconducting state the critical field should
be isotropic. The magnetic field pins also the direction of the FFLO modulation
permitting thus to study the critical current anisotropy. Our calculations
reveal a strong critical current anisotropy in the FFLO state in sharp contrast
with the usual superconducting state. The predicted characteristic anisotropies
of the critical field and critical current may provide an unambiguous probe of
the FFLO phase formation.Comment: 7 pages, 2 figures, to be published in Europhys. Let
Magnetic impurities in a superconductor: Effect of domain walls and interference
We consider the effect of magnetic impurities, modeled by classical spins, in
a conventional superconductor. We study their effect on the quasiparticles,
specifically on the spin density and local density of states (LDOS). As
previously emphasized, the impurities induce multiple scatterings of the
quasiparticle wave functions leading to complex interference phenomena. Also,
the impurities induce quantum phase transitions in the many-body system.
Previous authors studied the effect of either a small number of impurities
(from one to three) or a finite concentration of impurities, typically in a
disordered distribution. In this work we assume a regular set of spins
distributed inside the superconductor in such a way that the spins are
oriented, forming different types of domain walls, assumed stable. This
situation may be particularly interesting in the context of spin transfer due
to polarized currents traversing the material.Comment: 26 pages, 26 figures (72 in total
Upper critical field divergence induced by mesoscopic phase separation in the organic superconductor (TMTSF)2ReO4
Due to the competition of two anion orders, (TMTSF)2ReO4, presents a phase
coexistence between semiconducting and metallic (superconducting) regions
(filaments or droplets) in a wide range of pressure. In this regime, the
superconducting upper critical field for H parallel to both c* and b' axes
present a linear part at low fields followed by a divergence above a cross-over
field. This cross-over corresponds to the 3D-2D decoupling transition expected
in filamentary or granular superconductors. The sharpness of the transition
also demonstrates that all filaments are of similar sizes and self organize in
a very ordered way. The distance between the filaments and their cross-section
are estimated.Comment: 4 pages, 4 figure
Fulde-Ferrell-Larkin-Ovchinnikov states in one-dimensional spin-polarized ultracold atomic Fermi gases
We present a systematic study of quantum phases in a one-dimensional
spin-polarized Fermi gas. Three comparative theoretical methods are used to
explore the phase diagram at zero temperature: the mean-field theory with
either an order parameter in a single-plane-wave form or a self-consistently
determined order parameter using the Bogoliubov-de Gennes equations, as well as
the exact soluble Bethe ansatz method. We find that a spatially inhomogeneous
Fulde-Ferrell-Larkin-Ovchinnikov phase, which lies between the fully paired BCS
state and the fully polarized normal state, dominates most of the phase diagram
of a uniform gas. The phase transition from the BCS state to the
Fulde-Ferrell-Larkin-Ovchinnikov phase is of second order, and therefore there
are no phase separation states in one-dimensional homogeneous polarized gases.
This is in sharp contrast to the three-dimensional situation, where a phase
separation regime is predicted to occupy a very large space in the phase
diagram. We conjecture that the prediction of the dominance of the phase
separation phases in three dimension could be an artifact of the
non-self-consistent mean-field approximation, which is heavily used in the
study of three-dimensional polarized Fermi gases. We consider also the effect
of a harmonic trapping potential on the phase diagram, and find that in this
case the trap generally leads to phase separation, in accord with the
experimental observations for a trapped gas in three dimension. We finally
investigate the local fermionic density of states of the
Fulde-Ferrell-Larkin-Ovchinnikov ansatz. A two-energy-gap structure is shown
up, which could be used as an experimental probe of the
Fulde-Ferrell-Larkin-Ovchinnikov states.Comment: 22 papes, 19 figure
Nematic-Isotropic Transition with Quenched Disorder
Nematic elastomers do not show the discontinuous, first-order, phase
transition that the Landau-De Gennes mean field theory predicts for a
quadrupolar ordering in 3D. We attribute this behavior to the presence of
network crosslinks, which act as sources of quenched orientational disorder. We
show that the addition of weak random anisotropy results in a singular
renormalization of the Landau-De Gennes expression, adding an energy term
proportional to the inverse quartic power of order parameter Q. This reduces
the first-order discontinuity in Q. For sufficiently high disorder strength the
jump disappears altogether and the phase transition becomes continuous, in some
ways resembling the supercritical transitions in external field.Comment: 12 pages, 4 figures, to be published on PR
Supercurrent induced domain wall motion
We study the dynamics of a magnetic domain wall, inserted in, or juxtaposed
to, a conventional superconductor, via the passage of a spin polarized current
through a FSF junction. Solving the Landau-Lifshitz-Gilbert equation of motion
for the magnetic moments we calculate the velocity of the domain wall and
compare it with the case of a FNF junction. We find that in several regimes the
domain wall velocity is larger when it is driven by a supercurrent.Comment: 10 pages, 8 figure
Exploring the Structure of Distant Galaxies with Adaptive Optics on the Keck-II Telescope
We report on the first observation of cosmologically distant field galaxies
with an high order Adaptive Optics (AO) system on an 8-10 meter class
telescope. Two galaxies were observed at 1.6 microns at an angular resolution
as high as 50 milliarcsec using the AO system on the Keck-II telescope. Radial
profiles of both objects are consistent with those of local spiral galaxies and
are decomposed into a classic exponential disk and a central bulge. A
star-forming cluster or companion galaxy as well as a compact core are detected
in one of the galaxies at a redshift of 0.37+/-0.05. We discuss possible
explanations for the core including a small bulge, a nuclear starburst, or an
active nucleus. The same galaxy shows a peak disk surface brightness that is
brighter than local disks of comparable size. These observations demonstrate
the power of AO to reveal details of the morphology of distant faint galaxies
and to explore galaxy evolution.Comment: 5 pages, Latex, 3 figures. Accepted for publication in P.A.S.
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