Enhanced coupling and anisotropy in the low carrier density 2D superconductors

Electron system with low carrier density and low dimension is a candidate for the exotic superconductor. Li intercalated ZrNCl and HfNCl can offer the model systems for such research of unconventional superconducting phenomena, because they exhibit the relatively high superconducting transition temperatures above 15 K even at the low carrier density in order of 10²⁰ cm^-3 and the strong anisotropy. However, the studies on these systems so far have been carried out in polycrystalline samples in many cases. Therefore, it is highly desired to study the intrinsic properties of superconductivity containing the anisotropic parameter, the coupling strength of the Cooper pairs and the relation between them using single crystals, which has been left to be uncovered. Here we report the transport and tunneling spectroscopy studies on Li intercalated ZrNCl and HfNCl single crystals by utilizing the in-situ characterization techniques of intercalation processes established recently[1]. We succeeded in tuning T_c, anisotropy and the superconducting gap, as a function of carrier density, both in Zr and Hf systems.
[1] W. Shi et al., Sci. Rep. 5, 12534 (2015).