Abstract
Magnetizations and current-voltage characteristics of superconducting tapes from Nb-50% wt.Ti alloy are studied. In the region of external magnetic fields close to the upper critical field, there is a so called irreversibility field, which is often associated with the phase transition of vortex matter. However, in this study it is shown that the whole volume of experimental data can be described if we abandon the hypothesis of a phase transition and go over to the hypothesis of macroscopic inhomogeneity. This hypothesis implies the existence of two superconducting components with different upper critical fields, which naturally arise as a result of the production process.
Highlights
One of the most puzzling questions in the modern thermodynamics of vortex matter in type-II superconductors is the nature of the so-called irreversibility line in the field-temperature (H-T) phase diagram
Regardless of the mechanisms, it seemed that consensus was reached on what is the main cause of this phenomenon: a large anisotropy in electronic properties and a high impact of thermal fluctuations
It was later discovered that the irreversibility field is not unique to high-Tc oxides, and exists in conventional superconductors such as Nb [3], Nb3Sn [4], Nb-Ta [5] and Nb-Ti [4,5,6], which are supposed to be isotropic and weakly subject to thermal fluctuations due to low operating temperatures
Summary
One of the most puzzling questions in the modern thermodynamics of vortex matter in type-II superconductors is the nature of the so-called irreversibility line in the field-temperature (H-T) phase diagram. It was later discovered that the irreversibility field is not unique to high-Tc oxides, and exists in conventional superconductors such as Nb [3], Nb3Sn [4], Nb-Ta [5] and Nb-Ti [4,5,6], which are supposed to be isotropic and weakly subject to thermal fluctuations due to low operating temperatures. Attempts to apply existing models of the irreversibility field to low-Tc superconductors have led to the fact that the most adequate phenomenological description is given by the model of the flux lattice melting [3, 4], the validity of this model gives rise to doubt, as due to high pinning in commercial superconductors, the vortex matter should be in a glass state below H*(T) [7]
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