104 research outputs found
Vortices in magnetically coupled superconducting layered systems
Pancake vortices in stacks of thin superconducting films or layers are
considered. It is stressed that in the absence of Josephson coupling
topological restrictions upon possible configurations of vortices are removed
and various examples of structures forbidden in bulk superconductors are given.
In particular, it is shown that vortices may skip surface layers in samples of
less than a certain size R_c which might be macroscopic. The Josephson coupling
suppresses R_c estimates
Superconducting thin rings with finite penetration depth
Recently Babaei Brojeny and Clem [Phys. Rev. B 68, 174514 (2003)] considered
superconducting thin-film rings in perpendicular magnetic fields in the ideal
Meissner state with negligibly small magnetic penetration depth and presented
useful analytical limiting expressions and numerical results for the
magnetic-field and sheet-current profiles, trapped magnetic flux,
self-inductance, magnetic moment, and focusing of magnetic flux into the hole
when no net current flows in the ring. The present paper generalizes all these
results to rings with arbitrary values of the two-dimensional effective
penetration depth \Lambda = \lambda^2 /d (\lambda is the London depth and d <
\lambda/2 the film thickness) using a straightforward matrix inversion method.
We also present results for the energy of a superconducting ring as a function
of the applied magnetic induction B_a and the quantum number N defining the
size of the fluxoid N \phi_0 trapped in the hole.Comment: with 19 figures, gives 11.5 page
Meissner response of anisotropic superconductors
The response field of a half-space anisotropic superconductor is evaluated
for an arbitrary weak external field source. Example sources of a point
magnetic moment and a circular current are considered in detail. For the
penetration depth with being any other relevant distance
(the source size, or the distance between the source and the superconductor),
the major contribution to the response is the independent field of
the source image. It is shown that the absolute value of cannot be
extracted from the response field with a better accuracy than that for the
source position. Similar problems are considered for thin films.Comment: 8 pages, 0 figures. 7 pages: section removed, refs. adde
Vortex Trapping an Expulsion in Thin-Film Type-II Supercondu cting Strips
Vortex trapping is investigated in thin-film strips of superconducting material. We present a model for the critical field above which vortex trapping occurs in these strips. This model includes the pairing energy of vortex-antivortex pairs in addition to the energy of single vortices. Experimental verification of the model with a scanning SQUID microscope shows very good agreement between the model and experiments on YBa2Cu3O7-delta and Nb strips. Statistical analysis of the vortex distribution in the strips above the critical field has been performed and a comparison has been made between Nb and YBa2Cu3O7-delta for the distributions in the lateral and longitudinal directions
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Voltage-probe-position dependence and magnetic-flux contribution to the measured voltage in ac transport measurements: which measuring circuit determines the real losses?
The voltage V{sub ab} measured between two voltage taps a and b during magnetic flux transport in a type-II superconductor carrying current I is the sum of two contributions, the line integral from a to b of the electric field along an arbitrary path C{sub s} through the superconductor and a term proportional to the time rate of change of magnetic flux through the area bounded by the path C{sub s} and the measuring circuit leads. When the current I(t) is oscillating with time t, the apparent ac loss (the time average of the product IV{sub ab}) depends upon the measuring circuit used. Only when the measuring-circuit leads are brought out far from the surface does the apparent power dissipation approach the real (or true) ac loss associated with the length of sample probed. Calculations showing comparisons between the apparent and real ac losses in a flat strip of rectangular cross section will be presented, showing the behavior as a function of the measuring-circuit dimensions. Corresponding calculations also are presented for a sample of elliptical cross section
Josephson vortices and solitons inside pancake vortex lattice in layered superconductors
In very anisotropic layered superconductors a tilted magnetic field generates
crossing vortex lattices of pancake and Josephson vortices (JVs). We study the
properties of an isolated JV in the lattice of pancake vortices. JV induces
deformations in the pancake vortex crystal, which, in turn, substantially
modify the JV structure. The phase field of the JV is composed of two types of
phase deformations: the regular phase and vortex phase. The phase deformations
with smaller stiffness dominate. The contribution from the vortex phase
smoothly takes over with increasing magnetic field. We find that the structure
of the cores experiences a smooth yet qualitative evolution with decrease of
the anisotropy. At large anisotropies pancakes have only small deformations
with respect to position of the ideal crystal while at smaller anisotropies the
pancake stacks in the central row smoothly transfer between the neighboring
lattice positions forming a solitonlike structure. We also find that even at
high anisotropies pancake vortices strongly pin JVs and strongly increase their
viscous friction.Comment: 22 pages, 11 figures, to appear in Phys. Rev.
Phase Transition Study of Superconducting Microstructures
The presented results are part of a feasibility study of superheated
superconducting microstructure detectors. The microstructures (dots) were
fabricated using thin film patterning techniques with diameters ranging from
m up to m and thickness of m. We used arrays and single
dots to study the dynamics of the superheating and supercooling phase
transitions in a magnetic field parallel to the dot surface. The phase transi-
tions were produced by either varying the applied magnetic field strength at a
constant temperature or changing the bath temperature at a constant field.
Preliminary results on the dynamics of the phase transitions of arrays and
single indium dots will be reported.Comment: 7pages in LaTex format, five figures available upon request by
[email protected], preprint Bu-He 93/
Inter- and Intragranular Effects in Superconducting Compacted Platinum Powders
Compacted platinum powders exhibit a sharp onset of diamagnetic screening at
mK in zero magnetic field in all samples investigated. This
sharp onset is interpreted in terms of the intragranular transition into the
superconducting state. At lower temperatures, the magnetic ac susceptibility
strongly depends on the ac field amplitude and reflects the small intergranular
critical current density . This critical current density shows a strong
dependence on the packing fraction f of the granular samples. Surprisingly,
increases significantly with decreasing f ( A/cm for f = 0.67 and A/cm for f
= 0.50). The temperature dependence of shows strong positive curvature
over a wide temperature range for both samples. The phase diagrams of inter-
and intragranular superconductivity for different samples indicate that the
granular structure might play the key role for an understanding of the origin
of superconductivity in the platinum compacts.Comment: 11 pages including 9 figures. To appear in Phys. Rev. B in Nov. 0
Magnetic-field and current-density distributions in thin-film superconducting rings and disks
We show how to calculate the magnetic-field and sheet-current distributions
for a thin-film superconducting annular ring (inner radius a, outer radius b,
and thickness d<<a) when either the penetration depth obeys lambda < d/2 or, if
lambda > d/2, the two-dimensional screening length obeys Lambda = 2 lambda^2/d
<< a for the following cases: (a) magnetic flux trapped in the hole in the
absence of an applied magnetic field, (b) zero magnetic flux in the hole when
the ring is subjected to an applied magnetic field, and (c) focusing of
magnetic flux into the hole when a magnetic field is applied but no net current
flows around the ring. We use a similar method to calculate the magnetic-field
and sheet-current distributions and magnetization loops for a thin,
bulk-pinning-free superconducting disk (radius b) containing a dome of magnetic
flux of radius a when flux entry is impeded by a geometrical barrier.Comment: 10 pages, 13 figure
Phase Transitions in Isolated Vortex Chains
In very anisotropic layered superconductors (e.g. BiSrCaCuO)
a tilted magnetic field can penetrate as two co-existing lattices of vortices
parallel and perpendicular to the layers. At low out-of-plane fields the
perpendicular vortices form a set of isolated vortex chains, which have
recently been observed in detail with scanning Hall-probe measurements. We
present calculations that show a very delicate stability of this isolated-chain
state. As the vortex density increases along the chain there is a first-order
transition to a buckled chain, and then the chain will expel vortices in a
continuous transition to a composite-chain state. At low densities there is an
instability towards clustering, due to a long-range attraction between the
vortices on the chain, and at very low densities it becomes energetically
favorable to form a tilted chain, which may explain the sudden disappearance of
vortices along the chains seen in recent experiments.Comment: 9 pages, 10 figure
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