916 research outputs found
Theory of magnetic phases of hexagonal rare earth manganites
The magnetic phases of hexagonal perovskites RMnO_3 (R=Ho, Er, Tm, Yb, Sc, Y)
are analysed using group theory and the Landau theory of phase transitions. The
competition between various magnetic order parameters is discussed in the
context of antiferromagnetic interactions. A phenomenological model based on
four one-dimensional magnetic order parameters is developed and studied
numerically. It is shown that coupling of the various order parameters leads to
a complex magnetic field-temperature phase diagram and the results are compared
to experiment.Comment: 11 pages, 14 figures. Manuscript with higher quality figures can be
obtained here: http://www.physics.mun.ca/~curnoe/papers/RMnO3.submit.pd
Vortex Fractionalization in a Josephson Ladder
We show numerically that, in a Josephson ladder with periodic boundary
conditions and subject to a suitable transverse magnetic field, a vortex
excitation can spontaneously break up into two or more fractional excitations.
If the ladder has N plaquettes, and N is divisible by an integer q, then in an
applied transverse field of 1/q flux quanta per plaquette the ground state is a
regular pattern of one fluxon every q plaquettes. When one additional fluxon is
added to the ladder, it breaks up into q fractional fluxons, each carrying 1/q
units of vorticity. The fractional fluxons are basically walls between
different domains of the ground state of the underlying 1/q lattice. The
fractional fluxons are all depinned at the same applied current and move as a
unit. For certain applied fields and ladder lengths, we show that there are
isolated fractional fluxons. It is shown that the fractional fluxons would
produce a time-averaged voltage related in a characteristic way to the ac
voltage frequency.Comment: 13 Figures. 10 page
Length-scale-dependent phase transition in BSCCO single crystals
Electrical transport measurements using a multiterminal configuration are
presented, which prove that in BSCCO single crystals near the transition
temperature in zero external magnetic field the secondary voltage is induced by
thermally activated vortex loop unbinding. The phase transition between the
bound and unbound states of the vortex loops was found to be below the
temperature where the phase coherence of the superconducting order parameter
extends over the whole volume of the sample. We show experimentally that 3D/2D
phase transition in vortex dimensionality is a length-scale-dependent layer
decoupling process and takes place simultaneously with the 3D/2D phase
transition in superconductivity at the same temperature.Comment: 14 pages, 4 figures, to be published in Philos. Ma
Rocking ratchets in 2D Josephson networks: collective effects and current reversal
A detailed numerical study on the directed motion of ac-driven vortices and
antivortices in 2D Josephson junction arrays (JJA) with an asymmetric periodic
pinning potential is reported. Dc-voltage rectification shows a strong
dependence on vortex density as well as an inversion of the vortex flow
direction with ac amplitude for a wide range of vortex density around =1/2
(=), in good agreement with recent experiments by Shal\'om
and Pastoriza [Phys. Rev. Lett. {\bf 94}, 177001 (2005)]. The study of vortex
structures, spatial and temporal correlations, and vortex-antivortex pairs
formation gives insight into a purely collective mechanism behind the current
reversal effect.Comment: 4 pages, 5 figures. Accepted for publication in Phys. Rev. Let
Analytical Results for Cold Asymmetrical Fermion Superfluids at the Mean-Field Level
We present the analytical results at the mean-field level for the
asymmetrical fermion system with attractive contact interaction at the zero
temperature. The results can be expressed in terms of linear combinations of
the elliptic integrals of the first and second kinds. In the limit of small gap
parameter, we discuss how the asymmetry in fermion species affects the phases
of the ground state. In the limit of large gap parameter, we show that two
candidate phases are competing for the system's ground state. The Sarma phase
containing a pure Fermi fluid and a mixed condensate is favored at large degree
of asymmetry. The separated phase consisting of a pure Fermi fluid and a boson
condensate supports the system at smaller degree of asymmetry. The two phases
are degenerate in the limit of infinite pairing gap.Comment: 23 pages, no figur
Josephson effect in ballistic graphene
We solve the Dirac-Bogoliubov-De-Gennes equation in an impurity-free
superconductor-normal-superconductor (SNS) junction, to determine the maximal
supercurrent that can flow through an undoped strip of graphene with heavily
doped superconducting electrodes. The result is determined by the
superconducting gap and by the aspect ratio of the junction (length L, small
relative to the width W and to the superconducting coherence length). Moving
away from the Dirac point of zero doping, we recover the usual ballistic result
in which the Fermi wave length takes over from L. The product of critical
current and normal-state resistance retains its universal value (up to a
numerical prefactor) on approaching the Dirac point.Comment: 4 pages, 2 figure
Two-bands effect on the superconducting fluctuating diamagnetism in MgB₂
The field dependence of the magnetization above the transition temperature Tc
in MgB₂ is shown to evidence a diamagnetic contribution consistent with
superconducting fluctuations reflecting both the σ and π bands. In
particular, the upturn field Hup in the magnetization curve, related to the
incipient effect of the magnetic field in quenching the fluctuating pairs,
displays a double structure, in correspondence to two correlation lengths. The
experimental findings are satisfactorily described by the extension to the
diamagnetism of a recent theory for paraconductivity, in the framework of a
zero-dimensional model for the fluctuating superconducting droplets above Tc
Collective pinning of the vortex lattice by columnar defects in layered superconductors
The mixed phase of layered superconductors with no magnetic screening is
studied through a partial duality analysis of the corresponding frustrated XY
model in the presence of random columnar pins. A small fraction of pinned
vortex lines is assumed. Thermally induced plastic creep of the vortex lattice
within isolated layers results in an intermediate Bose glass phase that
exhibits weak superconductivity across layers in the limit of weak Josephson
coupling. The correlation volume of the vortex lattice is estimated in the
strongly-coupled Bose-glass regime at lower temperature. In the absence of
additional point pins, no peak effect in the critical current density is
predicted to occur on this basis as a function of the Josephson coupling. Also,
the phase transition observed recently inside of the vortex-liquid phase of
high-temperature superconductors pierced by sparse columnar defects is argued
to be a sign of dimensional cross-over.Comment: 16 pages, 1 figure, account of transition to ``nanoliquid'' in BSCCO,
to appear in PR
Incoherent multiple Andreev reflection in an array of SNS junctions
Last years many interesting effects related to incoherent MAR have been
experimentally found, but only few of them were theoretically explained. It was
shown, for example, that if the voltage at the edges of a linear array is
then subgarmonic structures in the current -voltage characteristics appear not
only at usual for nonstationary Josephson effect positions, ,
where is integer, but also at voltages other than . A step towards
description of electron transport in a dirty array of SNS junctions is done in
this letter. It is shown that subgarmonic structures may indeed appear at
``unusual'' voltages
Manipulating superconductivity through the domain structure of a ferromagnet: experimental aspects and theoretical implications
In the present work we study experimentally the influence that the domain
structure of a fer- romagnet (FM) has on the properties of a superconductor
(SC) in bilayers and multilayers of La0.60Ca0.40MnO3/Nb and FePt/Nb proximity
hybrids. Specific experimental protocols that were employed in the performed
magnetization measurements enabled us to directly uncover a generic property of
FM/SC hybrids: in the absence of an external magnetic field, the multidomain
struc- ture of the FM promotes the nucleation of superconductivity, while its
monodomain state strongly suppresses it. Our experimental findings support
recent theoretical studies proposing that when an inhomogeneous exchange field
is offered by the FM to the SC the superconducting pairs are not susceptible to
pair-breaking.Comment: 4 pages, 4 figure
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