Interlayer-spacing dependence of Tc in LixMyHfNCl (M: molecule) superconductors

We have systematically investigated the global phase diagram for Li(x)M(y)HfNCl (M: molecule), demonstrating the independent controllability of carrier density x and interlayer spacing d. In Li(x)HfNCl, the superconducting phase with almost constant T(c) of 20 K prevails for 0.15 <= x <= 0.50. Accompanied by an increase in d upon the cointercalation, an enhancement of T(c) up to 30% was observed with its x independence preserved. This result indicates that pairing interaction strongly depends on the Fermi surface warping along the k(z) direction, implying the relevance of charge and/or spin fluctuation.