116 research outputs found
Current-voltage characteristics of quasi-one-dimensional superconductors: An S-curve in the constant voltage regime
Applying a constant voltage to superconducting nanowires we find that its
IV-characteristic exhibits an unusual S-behavior. This behavior is the direct
consequence of the dynamics of the superconducting condensate and of the
existence of two different critical currents: j_{c2} at which the pure
superconducting state becomes unstable and j_{c1}<j_{c2} at which the phase
slip state is realized in the system.Comment: 4 pages, 5 figures, replaced with minor change
Localization of quasiparticles in a disordered vortex
We study the diffusive motion of low-energy normal quasiparticles along the
core of a single vortex in a dirty, type-II, s-wave superconductor. The physics
of this system is argued to be described by a one-dimensional supersymmetric
nonlinear sigma model, which differs from the sigma models known for disordered
metallic wires. For an isolated vortex and quasiparticle energies less than the
Thouless energy, we recover the spectral correlations that are predicted by
random matrix theory for the universality class C. We then consider the
transport problem of transmission of quasiparticles through a vortex connected
to particle reservoirs at both ends. The transmittance at zero energy exhibits
a weak localization correction reminiscent of quasi-one-dimensional metallic
systems with symmetry index beta = 1. Weak localization disappears with
increasing energy over a scale set by the Thouless energy. This crossover
should be observable in measurements of the longitudinal heat conductivity of
an ensemble of vortices under mesoscopic conditions. In the regime of strong
localization, the localization length is shown to decrease by a factor of 8 as
the quasiparticle energy goes to zero.Comment: 38 pages, LaTeX2e + epsf, 4 eps figures, one reference adde
Onset of Vortices in Thin Superconducting Strips and Wires
Spontaneous nucleation and the consequent penetration of vortices into thin
superconducting films and wires, subjected to a magnetic field, can be
considered as a nonlinear stage of primary instability of the current-carrying
superconducting state. The development of the instability leads to the
formation of a chain of vortices in strips and helicoidal vortex lines in
wires. The boundary of instability was obtained analytically. The nonlinear
stage was investigated by simulations of the time-dependent generalized
Ginzburg-Landau equation.Comment: REVTeX 3.0, 12 pages, 5Postscript figures (uuencoded). Accepted for
Phys. Rev.
Antiferromagnetic order and dielectric gap within the vortex core of antiferromagnetic superconductor
The structure of a superconducting vortex has been studied theoretically for
a dirty antiferromagnetic superconductor (AFSC), modelling an AFSC as a doped
semi-metal with s-wave superconducting pairing and antiferromagnetic
(dielectric) interaction between electrons (holes). It is also supposed that
the quasiparticles dispersion law possesses the property of nesting. The
distribution of the superconducting and magnetic order parameters near the
vortex core is calculated. It is shown that the antiferromagnetic order, been
suppressed at large distances, is restored around the superconducting flux and
the vortex core is in fact insulating and antiferromagnetic, in stark contrast
to the normal metal cores of traditional superconductors. Moreover, our model
calculations predict that as the temperature decreases the flux region of the
superconductivity and antiferromagnetism coexistence increases.Comment: 9 pages, 3 Postscript figures,NATO Advanced Research Workshop on
"Vortex dynamics in superconductors and other complex systems" Yalta, Crimea,
Ukraine, 13-17 September 200
Dynamics of 2D pancake vortices in layered superconductors
The dynamics of 2D pancake vortices in Josephson-coupled
superconducting/normal - metal multilayers is considered within the
time-dependent Ginzburg-Landau theory. For temperatures close to a
viscous drag force acting on a moving 2D vortex is shown to depend strongly on
the conductivity of normal metal layers. For a tilted vortex line consisting of
2D vortices the equation of viscous motion in the presence of a transport
current parallel to the layers is obtained. The specific structure of the
vortex line core leads to a new dynamic behavior and to substantial deviations
from the Bardeen-Stephen theory. The viscosity coefficient is found to depend
essentially on the angle between the magnetic field and the
axis normal to the layers. For field orientations close to the layers
the nonlinear effects in the vortex motion appear even for slowly moving vortex
lines (when the in-plane transport current is much smaller than the
Ginzburg-Landau critical current). In this nonlinear regime the viscosity
coefficient depends logarithmically on the vortex velocity .Comment: 15 pages, revtex, no figure
Properties of the Ideal Ginzburg-Landau Vortex Lattice
The magnetization curves M(H) for ideal type-II superconductors and the
maximum, minimum, and saddle point magnetic fields of the vortex lattice are
calculated from Ginzburg-Landau theory for the entire ranges of applied
magnetic fields Hc1 <= H < Hc2 or inductions 0 <= B < Hc2 and Ginzburg-Landau
parameters sqrt(1/2) <= kappa <= 1000. Results for the triangular and square
flux-line lattices are compared with the results of the circular cell
approximation. The exact magnetic field B(x,y) and magnetization M(H, kappa)
are compared with often used approximate expressions, some of which deviate
considerably or have limited validity. Useful limiting expressions and
analytical interpolation formulas are presented.Comment: 11 pages, 8 figure
Nucleation and Growth of the Superconducting Phase in the Presence of a Current
We study the localized stationary solutions of the one-dimensional
time-dependent Ginzburg-Landau equations in the presence of a current. These
threshold perturbations separate undercritical perturbations which return to
the normal phase from overcritical perturbations which lead to the
superconducting phase. Careful numerical work in the small-current limit shows
that the amplitude of these solutions is exponentially small in the current; we
provide an approximate analysis which captures this behavior. As the current is
increased toward the stall current J*, the width of these solutions diverges
resulting in widely separated normal-superconducting interfaces. We map out
numerically the dependence of J* on u (a parameter characterizing the material)
and use asymptotic analysis to derive the behaviors for large u (J* ~ u^-1/4)
and small u (J -> J_c, the critical deparing current), which agree with the
numerical work in these regimes. For currents other than J* the interface
moves, and in this case we study the interface velocity as a function of u and
J. We find that the velocities are bounded both as J -> 0 and as J -> J_c,
contrary to previous claims.Comment: 13 pages, 10 figures, Revte
The Current Carried by Bound States of a Superconducting Vortex
We investigate the spectrum of quasiparticle excitations in the core of
isolated pancake vortices in clean layered superconductors. Analysis of the
spectral current density shows that both the circular current around the vortex
center as well as any transport current through the vortex core is carried by
localized states bound to the core by Andreev scattering. Hence the physical
properties of the core are governed in clean high- superconductors
(e.g. the cuprate superconductors) by the Andreev bound states, and not by
normal electrons as it is the case for traditional (dirty) high-
superconductors.Comment: 17 pages in a RevTex (3.0) file plus 5 Figures in PostScript.
Submitted to Physical Review
Approximate Ginzburg-Landau solution for the regular flux-line lattice. Circular cell method
A variational model is proposed to describe the magnetic properties of
type-II superconductors in the entire field range between and
for any values of the Ginzburg-Landau parameter . The
hexagonal unit cell of the triangular flux-line lattice is replaced by a circle
of the same area, and the periodic solutions to the Ginzburg-Landau equations
within this cell are approximated by rotationally symmetric solutions. The
Ginzburg-Landau equations are solved by a trial function for the order
parameter. The calculated spatial distributions of the order parameter and the
magnetic field are compared with the corresponding distributions obtained by
numerical solution of the Ginzburg-Landau equations. The comparison reveals
good agreement with an accuracy of a few percent for all values
exceeding . The model can be extended to anisotropic
superconductors when the vortices are directed along one of the principal axes.
The reversible magnetization curve is calculated and an analytical formula for
the magnetization is proposed. At low fields, the theory reduces to the London
approach at , provided that the exact value of is used.
At high fields, our model reproduces the main features of the well-known
Abrikosov theory. The magnetic field dependences of the reversible
magnetization found numerically and by our variational method practically
coincide. The model also refines the limits of some approximations which have
been widely used. The calculated magnetization curves are in a good agreement
with experimental data on high-T superconductors.Comment: 8 pages, RevTex, 6 figures, submitted to Phys. Rev.
Effects of gap anisotropy upon the electronic structure around a superconducting vortex
An isolated single vortex is considered within the framework of the
quasiclassical theory. The local density of states around a vortex is
calculated in a clean type II superconductor with an anisotropy. The anisotropy
of a superconducting energy gap is crucial for bound states around a vortex. A
characteristic structure of the local density of states, observed in the
layered hexagonal superconductor 2H-NbSe2 by scanning tunneling microscopy
(STM), is well reproduced if one assumes an anisotropic s-wave gap in the
hexagonal plane. The local density of states (or the bound states) around the
vortex is interpreted in terms of quasiparticle trajectories to facilitate an
understanding of the rich electronic structure observed in STM experiments. It
is pointed out that further fine structures and extra peaks in the local
density of states should be observed by STM.Comment: 11 pages, REVTeX; 20 PostScript figures; An Animated GIFS file for
the star-shaped vortex bound states is available at
http://mp.okayama-u.ac.jp/~hayashi/vortex.htm
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