The N-isotope effect has been investigated for a series of layered superconductors Lix 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 N15 substitution. On the other hand, the decrease in the superconducting critical temperature (Tc) upon N15 substitution has been found to be only 0.06±0.03 K (=0.5±0.3%). This small change in Tc corresponds to an isotope shift coefficient α=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.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2007 Aug 8|