Isotope effect in LixZrNCl superconductors

The N-isotope effect has been investigated for a series of layered superconductors LixZrNCl 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 N-15 substitution. On the other hand, the decrease in the superconducting critical temperature (T-c) upon N-15 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 alpha=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.