30 research outputs found
Superconductivity and Stoichiometry in the BSCCO-family Materials
We report on magnetization, c-axis and ab-plane resistivity, critical
current, electronic band structure and superconducting gap properties. Bulk
measurements and photoemission data were taken on similar samples.Comment: 4 pages, latex, to be published in Journal of Superconductivity. two
figures available from Jian Ma at [email protected]
Ergodic versus nonergodic behavior in oxygen deficient high-T_c superconductors
The oxygen defects induced phase transition from nonergodic to ergodic state
in superconductors with intragrain granularity is considered within the
superconductive glass model. The model predictions are found to be in a
qualitative agreement with some experimental observations in deoxygenated
high-T_c single crystals
Theory of plastic vortex creep
We develop a theory for plastic flux creep in a topologically disordered
vortex solid phase in type-II superconductors. We propose a detailed
description of the plastic vortex creep of the dislocated, amorphous vortex
glass in terms of motion of dislocations driven by a transport current . The
{\em plastic barriers} show power-law divergence at
small drives with exponents for single dislocation creep and for creep of dislocation bundles. The suppression of the creep rate is a
hallmark of the transition from the topologically ordered vortex lattice to an
amorphous vortex glass, reflecting a jump in from ,
characterizing creep in the topologically ordered vortex lattice near the
transition, to its plastic values. The lower creep rates explain the observed
increase in apparent critical currents in the dislocated vortex glass.Comment: 4 pages, 1 figur
Magnetic field induced polarization effects in intrinsically granular superconductors
Based on the previously suggested model of nanoscale dislocations induced
Josephson junctions and their arrays, we study the magnetic field induced
electric polarization effects in intrinsically granular superconductors. In
addition to a new phenomenon of chemomagnetoelectricity, the model predicts
also a few other interesting effects, including charge analogues of Meissner
paramagnetism (at low fields) and "fishtail" anomaly (at high fields). The
conditions under which these effects can be experimentally measured in
non-stoichiometric high-T_c superconductors are discussed.Comment: 10 pages (REVTEX), 5 EPS figures; revised version accepted for
publication in JET
Pinning of spiral fluxons by giant screw dislocations in YBa_2Cu_3O_7 single crystals: Josephson analog of the fishtail effect
By using a highly sensitive homemade AC magnetic susceptibility technique,
the magnetic flux penetration has been measured in YBa_2Cu_3O_7 single crystals
with giant screw dislocations (having the structure of the Archimedean spirals)
exhibiting a=3 spiral turnings, the pitch b=18.7 microns and the step height
c=1.2nm (the last parameter is responsible for creation of extended weak-link
structure around the giant defects). The magnetic field applied parallel to the
surface enters winding around the weak-link regions of the screw in the form of
the so-called spiral Josephson fluxons characterized by the temperature
dependent pitch b_f(T). For a given temperature, a stabilization of the fluxon
structure occurs when b_f(T) matches b (meaning an optimal pinning by the screw
dislocations) and manifests itself as a pronounced low-field peak in the
dependence of the susceptibility on magnetic field (applied normally to the
surface) in the form resembling the high-field (Abrikosov) fishtail effect.Comment: see also http://www.jetpletters.ac.ru/ps/1886/article_28701.shtm
Static and dynamic coupling transitions of vortex lattices in disordered anisotropic superconductors
We use three-dimensional molecular dynamics simulations of magnetically
interacting pancake vortices to study vortex matter in disordered, highly
anisotropic materials such as BSCCO. We observe a sharp 2D-3D transition from
vortex lines to decoupled pancakes as a function of relative interlayer
coupling strength, with an accompanying large increase in the critical current
remniscent of a second peak effect. We find that decoupled pancakes, when
driven, simultaneously recouple and order into a crystalline-like state at high
drives. We construct a dynamic phase diagram and show that the dynamic
recoupling transition is associated with a double peak in dV/dI.Comment: 4 pages, 4 postscript figure
Pinning-induced transition to disordered vortex phase in layered superconductors
Destruction of the vortex lattice by random point pinning is considered as a
mechanism of the ``second peak'' transition observed experimentally in weakly
coupled layered high temperature superconductors. The transition field
separating the topologically ordered quasilattice from the amorphous vortex
configuration is strongly influenced by the layered structure and by the
nonlocal nature of the vortex tilt energy due to the magnetic interlayer
coupling. We found three different regimes of transition depending on the
relative strength of the Josephson and magnetic couplings. The regimes can be
distinguished by the dependence of the transition fieldComment: 8 pages, 3 Postscript figures. Accepted to Phys. Rev.B. (regular
article
Anomalous behavior of the irreversible magnetization and time relaxation in YBaCuO single crystals with splayed tracks
We have studied the angular dependence of the irreversible magnetization and
its time relaxation in YBaCuO single crystals with one or two
families of columnar defects inclined with respect to the c-axis. At high
magnetic fields, the magnetization shows the usual maximum centered at the mean
tracks' orientation and an associated minimum in the normalized relaxation
rate. In contrast, at low fields we observe an anomalous local minimum in the
magnetization and a maximum in the relaxation rate. We present a model to
explain this anomaly based on the slowing down of the creep processes arising
from the increase of the vortex-vortex interactions as the applied field is
tilted away from the mean tracks' direction.Comment: 15 pages, 6 figures. Submitted to Phys. Rev .
Phase Behavior of Type-II Superconductors with Quenched Point Pinning Disorder: A Phenomenological Proposal
A general phenomenology for phase behaviour in the mixed phase of type-II
superconductors with weak point pinning disorder is outlined. We propose that
the ``Bragg glass'' phase generically transforms via two separate thermodynamic
phase transitions into a disordered liquid on increasing the temperature. The
first transition is into a glassy phase, topologically disordered at the
largest length scales; current evidence suggests that it lacks the long-ranged
phase correlations expected of a ``vortex glass''. This phase has a significant
degree of short-ranged translational order, unlike the disordered liquid, but
no quasi-long range order, in contrast to the Bragg glass. This glassy phase,
which we call a ``multi-domain glass'', is confined to a narrow sliver at
intermediate fields, but broadens out both for much larger and much smaller
field values. The multi-domain glass may be a ``hexatic glass''; alternatively,
its glassy properties may originate in the replica symmetry breaking envisaged
in recent theories of the structural glass transition. Estimates for
translational correlation lengths in the multi-domain glass indicate that they
can be far larger than the interline spacing for weak disorder, suggesting a
plausible mechanism by which signals of a two-step transition can be obscured.
Calculations of the Bragg glass-multi-domain glass and the multi-domain
glass-disordered liquid phase boundaries are presented and compared to
experimental data. We argue that these proposals provide a unified picture of
the available experimental data on both high-T and low-T materials,
simulations and current theoretical understanding.Comment: 70 pages, 9 postscript figures, modified title and minor changes in
published versio
Peak Effect, Fishtail Effect and Plateau Effect : The Reentrant Amorphization of Vortex Matter in 2H-NbSe_2
The magnetic field dependence of the critical current is studied in single
crystal samples of the weak pinning type-II superconductor 2H-NbSe in the
high temperature and the low field region of the (H,T) phase space, in the
vicinity of the reentrant peak effect. The experimental results demonstrate
various pinning regimes : a collective pinned quasi-ordered solid in the
intermediate field that is destabilized in favor of disordered vortex phases in
both high fields near H and at low fields near H. The temperature
evolution of the pinning behavior demonstrates how the amorphous limit (where
the correlation volume is nearly field independent) is approached around the
so-called nose region of the reentrant peak effect boundary. Furthermore, the
data show that the rapid approach to the amorphous limit naturally yields a
peak effect, i.e., a peak in the critical current, in the high field regime,
but yields a ``plateau effect'' instead in the low field regime in an analogous
way. With increasing effective disorder the peak effect shifts away from
H and resembles a ``fishtail'' anomaly.Comment: 6 pages of text and 4 figures. Paper submitted to Phys. Rev.