Fluctuation-induced diamagnetism and dimensionality in superconducting layered compounds:TaS2(pyridine)12and 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 ${T}_{c}$. Compounds studied include the intercalated compound $\mathrm{Ta}{\mathrm{S}}_{2}{(\mathrm{pyridine})}_{\frac{1}{2}}$, and unintercalated Nb${\mathrm{Se}}_{2}$ and Ta${\mathrm{S}}_{1.6}$${\mathrm{Se}}_{0.4}$. The susceptibility above ${T}_{c}$, 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 $\mathrm{Ta}{\mathrm{S}}_{2}{(\mathrm{pyridine})}_{\frac{1}{2}}$ indicate that in low fields and near ${T}_{c}$, 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 Nb${\mathrm{Se}}_{2}$ have a number of complicating features, and firm conclusions regarding this material could not be drawn.