Upper critical fields and reduced dimensionality of the superconducting layered compounds

A detailed study of the critical magnetic fields of the superconducting layered compounds is reported and used to investigate the quasi-two-dimensional nature of these superconductors. The compounds studied include Ta${\mathrm{S}}_{2}$ and Ta${\mathrm{S}}_{1.6}$${\mathrm{Se}}_{0.4}$ intercalated with collidine, pyridine, or aniline and the unintercalated compounds Ta${\mathrm{S}}_{1.6}$${\mathrm{Se}}_{0.4}$ and Nb${\mathrm{Se}}_{2}$. The parallel critical fields of the intercalated compounds are found to be extremely high, rising faster with decreasing temperature (up to 200 kOe/K in the case of the Ta${\mathrm{S}}_{1.6}$${\mathrm{Se}}_{0.4}$ intercalation complexes) than for any other known superconductor. Ginzburg-Landau coherence lengths derived from the data indicate that the intercalated compounds should behave two dimensionally at low temperatures. It is found that the critical-field behavior is in qualitative agreement with the theoretical predictions of Klemm, Beasley, and Luther, based on the model of layered compounds as two-dimensional superconducting layers weakly coupled via Josephson tunneling. However, not all features of the data can be explained by this model. In addition, it is found that the observed critical fields are too high to be consistent with the conventional theory of Pauli paramagnetic limiting of the critical fields in superconductors.