Thermally Activated Flux Motion in High-T_c-Superconductors

Abstract

The discovery of high-T_c superconductivity by Willer et al. [1] has led to a renewed interest in relaxation phenomena in superconductors. Large non-exponential relaxation effects in the magnetization M(t) of these new superconductors were reported by many investigators [1,2,3,4]. These observations, together with the presence of an irreversibility line which showed a close resemblance with the de Almeida-Thouless line in spin glasses, led to the assumption of a glassy state in high-T_c superconductors and to the suggestion [1,5] that M(t) ~ exp[-(t/τ)^β], i.e. a Kohlrausch behaviour. Although this idea appeared quite reasonable for ceramic samples consisting of weakly linked superconducting grains, large relaxation effects were reported also for a single crystal [6] of YBa₂Cu₃O_7−δ . Many authors, among which Rossel and Chaudhari [7], Hagen et al. [8], Yeshurun and Malozemoff [6] and Tinkham [9], pointed out that these relaxation effects might also arise from thermally activated flux motion (TAFM), which was known to occur in conventional superconductors, though on a much smaller scale. Anderson [10] and Beasley et al. [11] showed that in the limit where the activation energy E for TAFM is much larger than kT, the magnetic relaxation follows a logarithmic law. As a Kohlrausch behaviour with β ≪ 1 is difficult to distinguish from a logarithmic time dependence, it has not been possible yet to identify unambiguously the origin of the large magnetic relaxation effects observed in all high-T_c superconductors. Measurements by Rossel, Maeno and Morgenstern [12] of memory effects in the magnetic relaxation seem to indicate that glassy behaviour occurs only in relatively weak magnetic fields (B ≃ 0.2 T) at temperatures close to T_c.