Phonon Anomaly on Na x HfNCl Superconductors with T c = 22 K

Inelastic neutron scattering measurements have been performed on superconducting Na_xHfNCl powder samples with x = 0.27 and 0.38 (T_c = 22 K): We have observed several temperature dependent phonon anomalies, e.g. at E ∼ 15, 29, 76, and 81 meV for the former sample. A phonon peak with E = 14.5 meV at Q ∼ 6Å⁻¹ increases for both samples with decreasing temperature below T = 40 K; which is much higher than T_c. The phonon peak is assigned to an ab-plane vibrational mode, E_u, of chlorine and hafnium atoms, which strongly modifies the electronic band structure. The phonon anomaly is discussed in terms of carrier inhomogeneity.