23,879 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
Analytic Solution for the Critical State in Superconducting Elliptic Films
A thin superconductor platelet with elliptic shape in a perpendicular
magnetic field is considered. Using a method originally applied to circular
disks, we obtain an approximate analytic solution for the two-dimensional
critical state of this ellipse. In the limits of the circular disk and the long
strip this solution is exact, i.e. the current density is constant in the
region penetrated by flux. For ellipses with arbitrary axis ratio the obtained
current density is constant to typically 0.001, and the magnetic moment
deviates by less than 0.001 from the exact value. This analytic solution is
thus very accurate. In increasing applied magnetic field, the penetrating flux
fronts are approximately concentric ellipses whose axis ratio b/a < 1 decreases
and shrinks to zero when the flux front reaches the center, the long axis
staying finite in the fully penetrated state. Analytic expressions for these
axes, the sheet current, the magnetic moment, and the perpendicular magnetic
field are presented and discussed. This solution applies also to
superconductors with anisotropic critical current if the anisotropy has a
particular, rather realistic form.Comment: Revtex file and 13 postscript figures, gives 10 pages of text with
figures built i
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
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
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
Hard Thermal Loops in the n-Dimensional phi3 Theory
We derive a closed-form result for the leading thermal contributions which
appear in the n-dimensional phi3 theory at high temperature. These
contributions become local only in the long wavelength and in the static
limits, being given by different expressions in these two limits.Comment: 3 pages, one figure. To be published in the Brazilian Journal of
Physic
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]
Current density inhomogeneity throughout the thickness of superconducting films and its effect on their irreversible magnetic properties
We calculate the distribution of the current density in superconducting
films along the direction of an external field applied perpendicular to the
film plane. Our analysis reveals that in the presence of bulk pinning is
inhomogeneous on a length scale of order the inter vortex distance. This
inhomogeneity is significantly enhanced in the presence of surface pinning. We
introduce new critical state model, which takes into account the current
density variations throughout the film thickness, and show how these variations
give rise to the experimentally observed thickness dependence of and
magnetic relaxation rate.Comment: RevTex, 9 PS figures. To appear in Phys. Rev.
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