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 (Tc) immediately followed by the superconductor-to-insulator transition (SIT). Such an increase in Tc 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.

The N-isotope effect has been investigated for a series of layered superconductors Li x ZrNCl with small carrier density. The N-vibration phonon mode around 615 cm −1 interacting most strongly with electronic system has shown softening by about 20 cm −1 ͑=3.3% ͒ upon 15 N substitution. On the other hand, the decrease in the superconducting critical temperature ͑T c ͒ upon 15 N substitution has been found to be only 0.06± 0.03 K ͑=0.5± 0.3% ͒. This small change in T c corresponds to an isotope shift coefficient ␣ = 0.07± 0.04, which is much smaller than the conventional value of 0.5, implying the relevance of other fluctuations than the phonon to the pairing interaction.

Transport properties in magnetic fields have been investigated for possibly exotic Li x ZrNCl superconductors with a wide range of doping concentration of 0.06ഛ x ഛ 0.37. Application of magnetic field along c axis rapidly suppressed the superconducting transition temperature ͑T c ͒ while strongly broadening the width. Zerotemperature limit of upper critical field ͓H c2 ͑0͔͒ showed systematic and monotonic decrease upon increasing x. Detailed analysis with a phenomenological argument gave a simple relation of k F L ϰ x and almost x-independent ͑k F , L, and denote Fermi wave number, mean free path, and carrier mobility, respectively͒. These results combined with the temperature-independent Hall coefficient clearly indicate that the present system can be viewed as modulation-doped semiconductors in which conduction electrons suffer from minimal disorder scattering. This could be a key to the relatively high-T c values realized in this class of layered superconductors.

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