Controlling physical parameters of layer-structured nitride-halide superconductors

Metal-intercalation into band insulators sometimes affords superconductors, well-known examples of which are carbon-based materials, such as fullerides and graphite. Layer-structured nitride-halide Li$_x$ZrNCl and Li$_xM_y$HfNCl ($M$ denotes molecule) belong to another class of intercalation-induced superconductors with relatively high $T_c$, in which doping level and interlayer distance (and hence interlayer hopping interaction) can independently be controlled by changing Li concentration and the size of the co- intercalated molecule. The controllability provides a unique and interesting opportunity to investigate the effect of the two important physical parameters on $T_c$ in a single system. Recent progress in the synthesis technique enabled us to obtain for the first time a series of single-phase samples of Li$_x$ZrNCl with finely controlled doping-levels which were notoriously difficult to prepare. Using these samples, we have established[1] an electronic phase diagram to find anomalous doping evolution of $T_c$, which takes a maximum value on the verge of superconductor- insulator transition. Based on this phase diagram and the results of systematic Raman scattering and transport measurements, we will discuss possible roles in producing relatively high $T_c$ played by charge fluctuation and reduced disorder scattering in the layered structure reminiscent of modulation-doped semiconductors. We will also briefly refer to our very recent results on the Hf-based materials in which both of the doping level and interlayer distance were varied. \newline [1] Y. Taguchi {\it et al.}, Phys. Rev. Lett. {\bf 97}, 107001 (2006)