Anisotropic Superconductivity in Layer Compounds

Measurements of the angular dependence of the upper critical field ${H}_{c2}$ in the transition metal dichalcogenides Ta${\mathrm{S}}_{2}$, Ta${\mathrm{S}}_{2}$-(pyridine), Ta${\mathrm{S}}_{1}$${\mathrm{Se}}_{1}$, Ta${\mathrm{S}}_{1}$${\mathrm{Se}}_{1}$-(pyridine), Ta${\mathrm{S}}_{1.2}$${\mathrm{Se}}_{0.8}$, and Ta${\mathrm{S}}_{0.8}$${\mathrm{Se}}_{1.2}$ have been made in magnetic fields up to 150 kOe. The observed angular dependence has been compared to simple theoretical models in order to estimate the anisotropy of the coherence length $\ensuremath{\xi}$ in these materials. The values of the $\frac{{\ensuremath{\xi}}_{\ensuremath{\parallel}}}{{\ensuremath{\xi}}_{\ensuremath{\perp}}}$ calculated from the data were 20 for Ta${\mathrm{S}}_{2}$-(pyridine) and approximately 7 for the Ta${\mathrm{S}}_{2}$, Ta${\mathrm{S}}_{1}$${\mathrm{Se}}_{1}$, and Ta${\mathrm{S}}_{1}$${\mathrm{Se}}_{1}$-(pyridine). The alloys with different ratios of sulfur and selenium showed no major change in behavior as compared to Ta${\mathrm{S}}_{1}$${\mathrm{Se}}_{1}$. The results are also discussed in relation to a generalized effective-mass model of anisotropic electronic conduction. The resistive transitions in the intercalated materials for field orientations less than 10\ifmmode^\circ\else\textdegree\fi{} from the parallel orientation are extremely broad and also exhibit unusual structure connected with enhanced flux-flow resistance. The magnetoresistance behavior in the normal state of the pure and intercalated layer compounds has also been measured and the changes in anisotropy due to intercalation are discussed.

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