Superconducting characteristics in electron-doped layered hafnium nitride:15Nisotope effect studies

We report 1 5 N 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 ∼25.5 K. 1 5 N NMR Knight shift decreasestoward 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, ΔT c ∼0.1 K (α N =0.07′0.02), which cannot reproduce such a high T c within the traditional BCS framework.