Susceptibility of Superconductors via Scattering Approach

Abstract

The measurement of magnetic properties of superconductors is usually understood in context with flux penetration length and in case of type-II superconductors also with vortex matter. We investigate the behavior of superconductors in very weak magnetic fields, typically µ0H < 1µT, near the critical temperature. The aim is to explore the details of SN transition and phase diagram (B, T ) of vortex matter in extreme type-II superconductors like cuprates. The data recorded on HTS, even if extensively challenged with theory, are hard to understand as a vortex matter behavior. On the other hand, interpretation on the basis of electron conductivity arising from electron scattering is consistent even with data obtained by x-ray diffraction, photoemission, tunnelling spectroscopy and specific heat. PACS : 74.25.Fy, 74.25.Ha,In type-I superconductors, like Pb or Ga which are "good" BCS superconductors, the superconducting-normal (SN) transition measured by the ac susceptibility in weak magnetic fields is very sharp. In pure (6N) long thin Ga single crystal it is 90% complete in a temperature interval of 2×10 −6 K [1]. On the other hand, the magnetic properties of type-II superconductors, particularly of cuprates which are extreme type II, are expected to be strongly affected by thermodynamic fluctuations in the vicinity of the critical temperature T ≈ T c . Fluctuations should considerably broaden the transition and influence the phase diagram (B, T ) of vortex matter [2]. However, recent low field high-resolution magnetic measurements show a distinct character of SN transition in conventional BCS superconductors and cuprates [3]. Since the transition shape depends on the sample geometry (demagnetization * ) Presented at 12