20,525 research outputs found
Theory of Type-II Superconductors with Finite London Penetration Depth
Previous continuum theory of type-II superconductors of various shapes with
and without vortex pinning in an applied magnetic field and with transport
current, is generalized to account for a finite London penetration depth
lambda. This extension is particularly important at low inductions B, where the
transition to the Meissner state is now described correctly, and for films with
thickness comparable to or smaller than lambda. The finite width of the surface
layer with screening currents and the correct dc and ac responses in various
geometries follow naturally from an equation of motion for the current density
in which the integral kernel now accounts for finite lambda. New geometries
considered here are thick and thin strips with applied current, and `washers',
i.e. thin film squares with a slot and central hole as used for SQUIDs.Comment: 14 pages, including 15 high-resolution figure
Critical State in Thin Anisotropic Superconductors of Arbitrary Shape
A thin flat superconductor of arbitrary shape and with arbitrary in-plane and
out-of-plane anisotropy of flux-line pinning is considered, in an external
magnetic field normal to its plane.
It is shown that the general three-dimensional critical state problem for
this superconductor reduces to the two-dimensional problem of an infinitely
thin sample of the same shape but with a modified induction dependence of the
critical sheet current. The methods of solving the latter problem are well
known. This finding thus enables one to study the critical states in realistic
samples of high-Tc superconductors with various types of anisotropic flux-line
pinning. As examples, we investigate the critical states of long strips and
rectangular platelets of high-Tc superconductors with pinning either by the
ab-planes or by extended defects aligned with the c-axis.Comment: 13 pages including 13 figure files in the tex
Nanomechanics of an individual vortex in an anisotropic type-II superconductor
As shown in recent experiments [Auslaender et al., Nature Physics 5, 35
(2009)] magnetic force microscopy permits one not only to image but also to
manipulate an individual vortex in type-II superconductors, and this
manipulation provides a new powerful tool to study vortex dynamics and pinning.
We derive equations that describe the deformation of an individual vortex in an
anisotropic biaxial type-II superconductor under the action of the microscope's
magnetic tip. These equations take into account the driving force generated by
the tip, the elastic force caused by the vortex deformation, and the pinning
force exerted by point defects. Using these equations, we reproduce the main
features of the experimental data obtained by Auslaender et al.Comment: 11 pages, 13 figure
Meissner-London currents in superconductors with rectangular cross section
Exact analytic solutions are presented for the magnetic moment and screening
currents in the Meissner state of superconductor strips with rectangular cross
section in a perpendicular magnetic field and/or with transport current. The
extension to finite London penetration is achieved by an elegant numerical
method which works also for disks. The surface current in the specimen corners
diverges as l^(-1/3) where l is the distance from the corner. This enhancement
reduces the barrier for vortex penetration and should increase the nonlinear
Meissner effect in d-wave superconductors
Anisotropic superconducting strip in an oblique magnetic field
The critical state of a thin superconducting strip in an oblique applied
magnetic field H_a is analyzed without any restrictions on the dependence of
the critical current density j_c on the local magnetic induction {\bf B}. In
such a strip, j_c is not constant across the thickness of the sample and
differs from J_c/d, where J_c is the critical sheet current. It is shown that
in contrast to the case of {\bf B}-independent j_c, the profiles H_z(x) of the
magnetic-field component perpendicular to the strip plane generally depend on
the in-plane component H_{ax} of the applied magnetic field H_a, and on how H_a
is switched on. On the basis of this analysis, we explain how and under what
conditions one can extract j_c({\bf B}) from the magnetic-field profiles H_z(x)
measured by magneto-optical imaging or by Hall-sensor arrays at the upper
surface of the strip.Comment: 7 pages with 4 figure
Critical state in type-II superconductors of arbitrary shape
The well-known Bean critical state equations in general are not sufficient to
describe the critical state of type-II superconductors when the sample shape is
not symmetric. We show how one can find the critical state in superconductors
of arbitrary shape. Analyzing a simple example of nonsymmetry, we demonstrate
that in the general case, a perturbation of the current distribution in the
critical state propagates into the sample smoothly in a diffusive way. This is
in contrast to the usual Bean critical state where the current distribution
changes abruptly at a narrow front.Comment: 4 pages, 1 figure, appears in Phys. Rev. B 71, issue 1 (2005
The theory of the reentrant effect in susceptibility of cylindrical mesoscopic samples
A theory has been developed to explain the anomalous behavior of the magnetic
susceptibility of a normal metal-superconductor () structure in weak
magnetic fields at millikelvin temperatures. The effect was discovered
experimentally by A.C. Mota et al \cite{10}. In cylindrical superconducting
samples covered with a thin normal pure metal layer, the susceptibility
exhibited a reentrant effect: it started to increase unexpectedly when the
temperature lowered below 100 mK. The effect was observed in mesoscopic
structures when the and metals were in good electric contact. The
theory proposed is essentially based on the properties of the Andreev levels in
the normal metal. When the magnetic field (or temperature) changes, each of the
Andreev levels coincides from time to time with the chemical potential of the
metal. As a result, the state of the structure experiences strong
degeneracy, and the quasiparticle density of states exhibits resonance spikes.
This generates a large paramagnetic contribution to the susceptibility, which
adds up to the diamagnetic contribution thus leading to the reentrant effect.
The explanation proposed was obtained within the model of free electrons. The
theory provides a good description for experimental results [10]
Ginzburg-Landau Vortex Lattice in Superconductor Films of Finite Thickness
The Ginzburg-Landau equations are solved for ideally periodic vortex lattices
in superconducting films of arbitrary thickness in a perpendicular magnetic
field. The order parameter, current density, magnetic moment, and the
3-dimensional magnetic field inside and outside the film are obtained in the
entire ranges of the applied magnetic field, Ginzburg Landau parameter kappa,
and film thickness. The superconducting order parameter varies very little near
the surface (by about 0.01) and the energy of the film surface is small. The
shear modulus c66 of the triangular vortex lattice in thin films coincides with
the bulk c66 taken at large kappa. In thin type-I superconductor films with
kappa < 0.707, c66 can be positive at low fields and negative at high fields.Comment: 12 pages including 14 Figures, corrected, Fig.14 added, appears in
Phys. Rev. B 71, issue 1 (2005
Buckling instability in type-II superconductors with strong pinning
We predict a novel buckling instability in the critical state of thin type-II
superconductors with strong pinning. This elastic instability appears in high
perpendicular magnetic fields and may cause an almost periodic series of flux
jumps visible in the magnetization curve. As an illustration we apply the
obtained criteria to a long rectangular strip.Comment: Submitted to Phys. Rev. Let
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