Superconducting critical fields of alkali and alkaline-earth intercalates of MoS2

The critical-field anisotropy and temperature dependence of the alkali and alkaline-earth intercalates of MoS2 have been measured. There are three principal results: First, an unusual positive curvature in the critical-field-temperature boundary has been observed. This curvature appears to be a universal feature of layered superconductors and possible theoretical explanations are discussed. Secondly, the critical-field anisotropy is compared with two models, a coupled-layer model and an uncoupled-layer or thin-film model. Which model fits the data depends on the material, and it is apparent that very accurate experimental data must be taken to distinguish between the models. Our third principal result is the grouping of properties of the alkali and alkaline-earth series of intercalates. The materials with the largest ionic intercalate atom diameters and hexagonal structures (K, Rb, and Cs compounds) have the highest critical temperatures, critical fields, and critical boundary slopes. In all cases the hexagonal materials have critical fields exceeding the paramagnetic limiting fields. The second group, consisting of the nonhexagonal Na, Sr, and Ca intercalates, have lower critical temperatures and fields, and the paramagnetic limit is not exceeded.