Superconducting characteristics in electron-doped layered hafnium nitride:: 15N isotope effect studies -: art. no. 100509

We report N-15 nuclear magnetic resonance (NMR) and the nitrogen-isotope effect studies on c-axis oriented sample of Li-doped HfNCl with superconducting transition temperature T(c)similar to25.5 K. N-15 NMR Knight shift decreases toward zero below T-c, giving evidence that the pairing symmetry is an even-parity spin singlet, and that the nitrogen site plays an important role in the occurrence of superconductivity. A nitrogen isotope shift is found to be quite small, DeltaT(c)similar to0.1 K (alpha(N) = 0.07 +/- 0.02), which cannot reproduce such a high T-c within the traditional BCS framework.