NMR Investigation of a New Superconductor MoNiP
Synthesized under High Pressure

Abstract

⁹⁵Mo, ⁶¹Ni and ³¹P nuclear magnetic resonance observations in a ternary molybdenum phosphide MoNiP with Fe₂P-type structure have been carried out to investigate the mechanism of superconductivity (T_C ≅ 8.5 K, H_C2 ≅ 7.4 kOe) that appears only when it is synthesized at high pressures. A large negative Knight shift of ⁹⁵Mo in the superconducting MoNiP, K = -0.155%, is indicative of a dominant d spin core-polarization interaction. The ⁹⁵Mo spin-lattice relaxation rate, (T₁T)^-1 = 0.15 (sK)^-1, gives a large density of states at the Fermi level, N(E_F) ∼ 1.17 states/eV. A positive Knight shift of ⁶¹Ni, K = 0.29%, on the other hand, is dominated by the Van Vleck orbital interaction. Thus it is concluded that N(E_F) consists mainly of a narrow Mo 4d band. In a non-superconducting MoNiP synthesized at an ambient pressure, K and (T₁T)^-1 of ⁹⁵Mo increase to -0.202% and to 0.19 (sK)^-1, respectively, indicating that the decrease in N(E_F) by ∼ 10% induces the superconductivity, contrary to the prediction of the Bardeen-Cooper-Schrieffer theory. In MoNiP, strong electron correlations in the narrow d band of Mo are thought to be antagonistic to the electron-phonon coupling, V_C ∼ V_ph, and the synthesis at high pressures provides an additional screening of the Coulomb potential, which leads to the condition of V_ph > V_C.