NMR investigation of a new superconductor MoNiP synthesized under high pressure

Mo-95, Ni-61 and P-31 nuclear magnetic resonance observations in a ternary molybdenum phosphide MoNiP with Fe2P-type structure have been carried out t o investigate the mechanism of superconductivity (T(c)similar or equal to8. 5K, H(c2)similar or equal to7.4kOe) that appears only when it is synthesize d at high pressures. A large negative Knight shift of Mo-95 in the supercon ducting MoNiP, K = -0.155%, is indicative of a dominant d spin core-polariz ation interaction. The Mo-95 spin-lattice relaxation rate, (T1T)(-1) 0.15 ( sK)(-1), gives a large density of states at the Fermi level, N(E-F)similar to1.17 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 conclud ed that N(EF) consists mainly of a narrow Mo 4d band. In a non-superconduct ing MoNiP synthesized at an ambient pressure, It and (T1T)(-1) of Mo-95 inc rease to -0.202% and to 0.19 (sK)(-1), respectively, indicating that the de crease in N(EF) by similar to 10% induces the superconductivity, contrary t o the prediction of the Bardeen-Cooper-Schrieffer theory. In MoNiP, strong electron correlations in the narrow d band of Mo are thought to be antagoni stic to the electron-phonon coupling, V(C)similar toV(ph), and the synthesi s at high pressures provides an additional screening of the Coulomb potenti al, which leads to the condition of V-ph > V-C.