Fluctuation-induced diamagnetism and dimensionality in superconducting layered compounds: TaS2(pyridine)12 and NbSe2

Abstract
The results of an investigation of fluctuation-induced diamagnetism in superconducting layered compounds are presented. These results are used to establish the dimensionality of the superconducting fluctuation effects near Tc. Compounds studied include the intercalated compound TaS2(pyridine)12, and unintercalated NbSe2 and TaS1.6 Se0.4. The susceptibility above Tc, and magnetization in the superconducting state, were measured in fields up to 327 Oe with a superconducting quantum-interference magnetometer suitable for a broad range of sensitive magnetic measurements. The operation and performance of this magnetometer are described. Results for TaS2(pyridine)12 indicate that in low fields and near Tc, the fluctuation effects are three-dimensional in nature, where this is established via a comparison with previous results for an isotropic alloy, Pb-5-at.% Tl. This conclusion as to the dimensionality of the superconducting effects is in accord with the qualitative predictions of the Lawrence-Doniach theory for weakly-coupled superconducting layers. Data for NbSe2 have a number of complicating features, and firm conclusions regarding this material could not be drawn.