Increase in Tc upon reduction of doping in LixZrNCl superconductors

We revealed a detailed phase diagram of the very lightly doped regime in Li-intercalated superconductors, LixZrNCl, to which previous studies have never gained access owing to the difficulty in synthesizing single-phase samples. A continuous and uniform Li intercalation without any indication of phase separation was carefully confirmed by means of synchrotron x-ray diffraction and Raman scattering experiments. Upon reducing the carrier density below x=0.12, we found a rapid increase in the superconducting transition temperature (T-c) immediately followed by the superconductor-to-insulator transition (SIT). Such an increase in T-c on the verge of SIT seems to be difficult to explain by the conventional theory, but may be indicative of the charge fluctuation contribution to superconductivity in low-carrier-density systems.