Fluctuation-Induced Diamagnetism aboveTcin Superconductors

Abstract

The results of an extensive investigation of the enhanced diamagnetism of superconductors above $T_c$ due to thermal fluctuations are presented. Data are reported on a variety of super-conductors, including In, Pb, Nb, and several In-Tl and Pb-Tl alloys. The versatile super-conducting quantum magnetometer developed for these experiments, which is suitable for a variety of low-temperature magnetic measurements, is also described. The various theoretical treatments of the fluctuation-induced diamagnetism in superconductors are summarized and their predictions compared with experiment. It is found that at high fields [$H\gtrsim H_{c2\mathrm{}}\mathrm{}\left(0\right)\mathrm{}$] and high temperatures ($T\gtrsim 2T_c$), the observed diamagnetism deviates markedly from that expected on the basis of the Ginzburg-Landau (GL) theory of superconductivity due to the important contribution of high-energy fluctuations in these regimes. These high-energy fluctuations are not properly described by the simple GL theory. Recent theoretical calculations based on the microscopic Gor'kov theory that attempt to treat these fluctuations properly are found to give a good account of our results for clean materials, once the effects of nonlocal electrodynamics are included, as in the work of Lee and Payne and of Kurkijärvi, Ambegaokar, and Eilenberger. For alloys, however, the theoretical picture is less satisfactory, and it appears possible that even Gor'kov theory may fail to provide an adequate description of high-energy fluctuations in dirty superconductors.