8 research outputs found
Nonequilibrium steady states of driven magnetic flux lines in disordered type-II superconductors
We investigate driven magnetic flux lines in layered type-II superconductors
subject to various configurations of strong point or columnar pinning centers
by means of a three-dimensional elastic line model and Metropolis Monte Carlo
simulations. We characterize the resulting nonequilibrium steady states by
means of the force-velocity / current-voltage curve, static structure factor,
mean vortex radius of gyration, number of double-kink and half-loop
excitations, and velocity / voltage noise spectrum. We compare the results for
the above observables for randomly distributed point and columnar defects, and
demonstrate that the three-dimensional flux line structures and their
fluctuations lead to a remarkable variety of complex phenomena in the
steady-state transport properties of bulk superconductors.Comment: 23 pages, IOP style, 18 figures include
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
Systematic characterization of upper critical fields for MgB thin films using the two-band superconducting theory
We present experimental results of the upper critical fields of
various MgB thin films prepared by the molecular beam epitaxy,
multiple-targets sputtering, and co-evaporation deposition apparatus.
Experimental data of the are successfully analyzed by applying
the Gurevich theory of dirty two-band superconductivity in the case of
, where and are the intraband
electron diffusivities for and bands, respectively. We find that
the parameters obtained from the analysis are strongly correlated to the
superconducting transition temperature of the films. We also
discuss the anormalous narrowing of the transition width at intermediate
temperatures confirmed by the magnetoresistance measurements.Comment: 7 pages, 7 figures, submitted to Phys. Rev.
Applicability of layered sine-Gordon models to layered superconductors: II. The case of magnetic coupling
In this paper, we propose a quantum field theoretical renormalization group
approach to the vortex dynamics of magnetically coupled layered
superconductors, to supplement our earlier investigations on the
Josephson-coupled case. We construct a two-dimensional multi-layer sine-Gordon
type model which we map onto a gas of topological excitations. With a special
choice of the mass matrix for our field theoretical model, vortex dominated
properties of magnetically coupled layered superconductors can be described.
The well known interaction potentials of fractional flux vortices are
consistently obtained from our field-theoretical analysis, and the physical
parameters (vortex fugacity and temperature parameter) are also identified. We
analyse the phase structure of the multi-layer sine--Gordon model by a
differential renormalization group method for the magnetically coupled case
from first principles. The dependence of the transition temperature on the
number of layers is found to be in agreement with known results based on other
methods.Comment: 7 pages, 1 figure, published in J. Phys.: Condens. Matte
Phase dynamics of inductively coupled intrinsic Josephson junctions and terahertz electromagnetic radiation
The Josephson effects associated with quantum tunneling of Cooper pairs
manifest as nonlinear relations between the superconductivity phase difference
and the bias current and voltage. Many novel phenomena appear, such as Shapiro
steps in dc cuurent-voltage (IV) characteristics of a Josephson junction under
microwave shining, which can be used as a voltage standard. Inversely, the
Josephson effects provide a unique way to generate high-frequency
electromagnetic (EM) radiation by dc bias voltage. The discovery of cuprate
high-Tc superconductors accelerated the effort to develop novel source of EM
waves based on a stack of atomically dense-packed intrinsic Josephson junctions
(IJJs), since the large superconductivity gap covers the whole terahertz
frequency band. Very recently, strong and coherent terahertz radiations have
been successfully generated from a mesa structure of
single crystal which works both as the source
of energy gain and as the cavity for resonance. It is then found theoretically
that, due to huge inductive coupling of IJJs produced by the nanometer junction
separation and the large London penetration depth of order of of
the material, a novel dynamic state is stabilized in the coupled sine-Gordon
system, in which kinks in phase differences are developed responding
to the standing wave of Josephson plasma and are stacked alternatively in the
c-axis. This novel solution of the inductively coupled sine-Gordon equations
captures the important features of experimental observations. The theory
predicts an optimal radiation power larger than the one available to date by
orders of magnitude, and thus suggests the technological relevance of the
phenomena.Comment: review article (69 pages, 30 figures
Vortex Chains in Anisotropic Superconductors
High-T_c superconductors in small magnetic fields directed away from the
crystal symmetry axes have been found to exhibit inhomogeneous chains of flux
lines (vortices), in contrast to the usual regular triangular flux-line
lattice. We review the experimental observations of these chains, and summarize
the theoretical background that explains their appearance. We treat separately
two classes of chains: those that appear in superconductors with moderate
anisotropy due to an attractive part of the interaction between tilted flux
lines, and those with high anisotropy where the tilted magnetic flux is created
by two independent and perpendicular crossing lattices. In the second case it
is the indirect attraction between a flux line along the layers (Josephson
vortex) and a flux line perpendicular to the layers (pancake vortex stack) that
leads to the formation of chains of the pancake vortex stacks. This complex
system contains a rich variety of phenomena, with several different equilibrium
phases, and an extraordinary dynamic interplay between the two sets of crossing
vortices. We compare the theoretical predictions of these phenomena with the
experimental observations made to date. We also contrast the different
techniques used to make these observations. While it is clear that this system
forms a wonderful playground for probing the formation of structures with
competing interactions, we conclude that there are important practical
implications of the vortex chains that appear in highly anisotropic
superconductors.Comment: Topical review for Journal of Physics: Condensed Matter; large pdf
file 1.9M