64 research outputs found
Irreversible magnetostriction and magnetization of the superconduting 2H-NbSe₂ single crystals in a peak-effect regime
Magnetostriction mea sure ments in the mixed state of superconducting 2H-NbSe₂ single crystals under in-plane magnetic fields 0-12 T have revealed a peak on the magnetostriction versus magnetic field dependences in the vicinity of the upper critical field Hc₂. The peak value of the longitudinal magnetostriction is higher by more than an order of magnitude in comparison with that of the trans verse magnetostriction when measured along the hexagonal axis. Analysis of the measured field dependences of the magnetostriction and magnetization of 2H-NbSe₂ allows one to relate the observed peculiarities of magnetostriction with the loss of order in the lat tice of Abrikosov vortices, which occurs by a first-order phase transition.
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Magnetostriction in the mixed state of superconducting 2H-NbSe₂ single crystals
Magnetostriction measurements on 2H-NbSe₂ single crystals in the temperature range 1.5-8 K in a magnetic field up to 14 T are reported. Peak and oscillations in the measured field dependences of magnetostriction were observed near Hc₂. The reversible and irreversible components are separated and analyzed in the region of peak. The scaling parameters are defined, the contribution of the elastic constants dependence on magnetic field is demonstrated. The oscillatory component is discussed regarding Landau quantization of electronic spectrum
The puzzle of 90 degree reorientation in the vortex lattice of borocarbide superconductors
We explain 90 degree reorientation in the vortex lattice of borocarbide
superconductors on the basis of a phenomenological extension of the nonlocal
London model that takes full account of the symmetry of the system. We propose
microscopic mechanisms that could generate the correction terms and point out
the important role of the superconducting gap anisotropy.Comment: 4 pages, 2 eps figure
Theory of Double-Sided Flux Decorations
A novel two-sided Bitter decoration technique was recently employed by Yao et
al. to study the structure of the magnetic vortex array in high-temperature
superconductors. Here we discuss the analysis of such experiments. We show that
two-sided decorations can be used to infer {\it quantitative} information about
the bulk properties of flux arrays, and discuss how a least squares analysis of
the local density differences can be used to bring the two sides into registry.
Information about the tilt, compressional and shear moduli of bulk vortex
configurations can be extracted from these measurements.Comment: 17 pages, 3 figures not included (to request send email to
[email protected]
Influence of nonlocal electrodynamics on the anisotropic vortex pinning in
We have studied the pinning force density Fp of YNi_2B_2C superconductors for
various field orientations. We observe anisotropies both between the c-axis and
the basal plane and within the plane, that cannot be explained by usual mass
anisotropy. For magnetic field , the reorientation structural
transition in the vortex lattice due to nonlocality, which occurs at a field
, manifests itself as a kink in Fp(H). When , Fp is
much larger and has a quite different H dependence, indicating that other
pinning mechanisms are present. In this case the signature of nonlocal effects
is the presence of a fourfold periodicity of Fp within the basal plane.Comment: 4 pages, 3 figure
Fluctuations and Intrinsic Pinning in Layered Superconductors
A flux liquid can condense into a smectic crystal in a pure layered
superconductors with the magnetic field oriented nearly parallel to the layers.
If the smectic order is commensurate with the layering, this crystal is {\sl
stable} to point disorder. By tilting and adjusting the magnitude of the
applied field, both incommensurate and tilted smectic and crystalline phases
are found. We discuss transport near the second order smectic freezing
transition, and show that permeation modes lead to a small non--zero
resistivity and large but finite tilt modulus in the smectic crystal.Comment: 4 pages + 1 style file + 1 figure (as uufile) appended, REVTEX 3.
Low field vortex matter in YBCO: an atomic beam magnetic resonance study
We report measurements of the low field structure of the magnetic vortex
lattice in an untwinned YBCO single-crystal platelet. Measurements were carried
out using a novel atomic beam magnetic resonance (ABMR) technique. For a 10.7 G
field applied parallel to the c-axis of the sample, we find a triangular
lattice with orientational order extending across the entire sample. We find
the triangular lattice to be weakly distorted by the a-b anisotropy of the
material and measure a distortion factor, f = 1.16. Model-experiment
comparisons determine a penetration depth, lambda_ab = 140 (+-20) nm. The paper
includes the first detailed description of the ABMR technique. We discuss both
technical details of the experiment and the modeling used to interpret the
measurements.Comment: 44 pages, 13 figures, submitted to Phys. Rev. B Revision includes
Postscript wrapped figures + minor typo
Instabilities in the Flux Line Lattice of Anisotropic Superconductors
The stability of the flux line lattice has been investigated within
anisotropic London theory. This is the first full-scale investigation of
instabilities in the `chain' state. It has been found that the lattice is
stable at large fields, but that instabilities occur as the field is reduced.
The field at which these instabilities first arise, ,
depends on the anisotropy and the angle at which the
lattice is tilted away from the -axis. These instabilities initially occur
at wavevector , and the component of along the
average direction of the flux lines, , is always finite. As the
instability occurs at finite the dependence of the cutoff on is
important, and we have used a cutoff suggested by Sudb\ospace and Brandt. The
instabilities only occur for values of the anisotropy appropriate to
a material like BSCCO, and not for anisotropies more appropriate to YBCO. The
lower critical field is calculated as a function of the angle
at which the applied field is tilted away from the crystal axis. The
presence of kinks in is seen to be related to instabilities in
the equilibrium flux line structure.Comment: Extensively revised paper, with modified analysis of elastic
instabilities. Calculation of the lower critical field is included, and the
presence of kinks in is seen to be related to the elastic
instabilities. 29 pages including 16 figures, LaTeX with epsf styl
Critical depinning force and vortex lattice order in disordered superconductors
We simulate the ordering of vortices and its effects on the critical current
in superconductors with varied vortex-vortex interaction strength and varied
pinning strengths for a two-dimensional system. For strong pinning the vortex
lattice is always disordered and the critical depinning force only weakly
increases with decreasing vortex-vortex interactions. For weak pinning the
vortex lattice is defect free until the vortex-vortex interactions have been
reduced to a low value, when defects begin to appear with a simultaneous rapid
increase in the critical depinning force. In each case the depinning force
shows a maximum for non-interacting vortices. The relative height of the peak
increases and the peak width decreases for decreasing pinning strength in
excellent agreement with experimental trends associated with the peak effect.
We show that scaling relations exist between the distance between defects in
the vortex lattice and the critical depinning force.Comment: 5 pages, 6 figure
Elasticity-driven interaction between vortices in type-II superconductors
The contribution to the vortex lattice energy which is due to the
vortex-induced strains is calculated covering all the magnetic field range
which defines the vortex state. This contribution is compared with previously
reported ones what shows that, in the most part of the vortex state, it has
been notably underestimated until now. The reason of such underestimation is
the assumption that only the vortex cores induce strains. In contrast to what
is generally assumed, both core and non-core regions are important sources of
strains in high- superconductors.Comment: 10 pages, 1 figure, revtex
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