256 research outputs found
Determination of critical current density in melt-processed HTS bulks from levitation force measurements
A simple approach to describe the levitation force measurements on
melt-processed HTS bulks was developed. A couple of methods to determine the
critical current density were introduced. The averaged -plane
values for the field parallel to this plane were determined. The first and
second levitation force hysteresis loops calculated with these values
coincide remarkably well with the experimental data.Comment: 10 pages (tex), 2 figures (in jpeg
Singular Fermi Surfaces II. The Two--Dimensional Case
We consider many--fermion systems with singular Fermi surfaces, which contain
Van Hove points where the gradient of the band function
vanishes. In a previous paper, we have treated the case of spatial dimension . In this paper, we focus on the more singular case and establish
properties of the fermionic self--energy to all orders in perturbation theory.
We show that there is an asymmetry between the spatial and frequency
derivatives of the self--energy. The derivative with respect to the Matsubara
frequency diverges at the Van Hove points, but, surprisingly, the self--energy
is in the spatial momentum to all orders in perturbation theory, provided
the Fermi surface is curved away from the Van Hove points. In a prototypical
example, the second spatial derivative behaves similarly to the first frequency
derivative. We discuss the physical significance of these findings.Comment: 68 pages LaTeX with figure
Electronic band structure of optimal superconductors: from cuprates to ferropnictides and back again
While the beginning decade of the high-Tc cuprates era passed under
domination of local theories, Abrikosov was one of the few who took seriously
the electronic band structure of cuprates, stressing the importance of an
extended Van Hove singularity near the Fermi level. These ideas have not been
widely accepted that time mainly because of a lack of experimental evidence for
correlation between saddle point position and superconductivity. In this short
contribution, based on the detailed comparison of the electronic band
structures of different families of cuprates and iron based superconductors I
argue that a general mechanism of the Tc enhancement in all known high-Tc
superconductors is likely related with the proximity of certain Van Hove
singularities to the Fermi level. While this mechanism remains to be fully
understood, one may conclude that it is not related with the electron density
of states but likely with some kind of resonances caused by a proximity of the
Fermi surface to topological Lifshitz transition. One may also notice that the
electronic correlations often shifts the electronic bands to optimal for
superconductivity positions
Simple technique for superconducting joints quality estimation in bulk melt-processed high temperature superconductors
We propose an empirical approach to estimate the quality of superconducting
joints (welds) between blocks of bulk high temperature superconductors (HTS).
As a measuring value, we introduce a joint's quality factor and show its
natural correlation with joint's critical current density. Being simple and
non-destructive, this approach is considered to be quite important to solve the
problem of utilization of HTS in large scale applications. The approach has
been applied to characterize the joint's quality of melt-processed Y-123 joined
by Tm-123 solder.Comment: 3 pages with 2 figures (revtex
Superconducting joining of melt-textured Y-Ba-Cu-O bulk material
The Tm-Ba-Cu-O solder can be successfully used to produce a superconductive
joint between MT-YBCO parts. The peculiarities of solidification, phase
formation, structure transformations and electromagnetic properties of MT-YBCO
soldered with TmBa2Cu3O7-d are discussed.Comment: PS of 6 pages text and 5 figures, presented at ICMC'2000, Brasi
Surface Influence on Flux Penetration into HTS Bulks
The influence of surface treatment on AC loss in melt-processed quasi-single
crystal HTS was investigated with resonance oscillations technique. We have
found that amplitude dependencies of AC loss on magnetic field amplitude become
rather complicated after surface polishing. The experimental data show well
distinguished dynamic crossover from absence of barrier at low rates of field
variation to its appearence at higher rates. An explaination of such a dynamic
surface barrier appearance based on consideration of along surface vortex
propagation was suggested.Comment: 2 pages with 1 EPS figure, to be presented at LT2
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