Comparison of critical field experiments on the many types of anisotropic superconductors shows that these disparate materials look surprisingly similar. In particular positive curvature of Hc2(T) and temperature-dependent anisotropy appear to be common to most of the members of the class. Somewhat different features occur in those materials in which the superconducting layers decouple at low temperatures. These materials undergo a transition which is marked by a discontinuity in Hc2(T) and a change from an angular dependence of Hc2 from 3D anisotropic (Eqn. 4.3) to thin-filmlike (Eqn. 4.9). Except for those of the TMDC's in which a CDW competes with superconductivity, diluting the superconducting layers with non-superconducting layers depresses Tc, in accordance with proximity-effect theories. As discussed further later on, the KHg-GIC's do not follow the proximity effect Tc-dependence, with the Tc even of stage 3 higher than that of the pristine KHg alloy.[248] The findings from the other anisotropic superconductors suggest that the lack of Tc depression with increasing stage may be a crucial clue.
Most of the findings from previous studies on anisotropic superconductors can be summarized by examining the progression from Nb metal to NbSe2 to alkali-intercalated TMDC's to organic-molecule-intercalated NbSe2. Niobium metal in bulk form is an anisotropic 3D superconductor described by Fermi surface anisotropy theories.[33,129] NbSe2 shows the same features, only with a greater degree of positive curvature (or extended linearity) of Hc2(T) and larger anisotropy.[51,181,114] The alkali-metal and alkaline-earth TMDCIC's are even more anisotropic and have even larger anomalies in Hc2(T).[221,266] The organic TMDCIC's are 2D-coupled superconductors, like Nb thin films, and so are qualitatively different from the others, which are 3D-coupled. Yet the organic TMDCIC's are also clearly the endpoint of a continuum which has bulk Nb as its other limit.
The materials in this chapter also have many features in common with the superconducting GIC's which are the subject of the rest of this work. Many of the theories developed for application to these other systems will be useful in the study of the superconducting GIC's, so a familiarity with the previous work is mandatory. The similarity between the alkali-metal GIC's and the alkali-metal TMDCIC's is already apparent, but an attempt will also be made to show the likeness of the organic-intercalated TMDCIC's and the non-superconducting acceptor GIC's.
The consideration of the properties of the superconducting GIC's will begin with the preliminaries, sample preparation and characterization.