NMR Investigation of a New Superconductor MoNiP Synthesized under High Pressure : Condensed Matter: Electronic Properties, etc.

概要

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^<95>Mo, ^<61>Ni and ^<31>P nuclear magnetic resonance observations in a ternary molybdenum phosphide MoNiP with Fe_2P-type structure have been carried out to investigate the mechanism of superconductivity (T_C≃8.5 K, H_<C2>≃ 7.4 k Oe) that appears only when it is synthesized at high pressures. A large negative Knight shift of ^<95>Mo in the superconducting MoNiP, K = -0.155%, is indicative of a dominant d spin core-polarization interaction. The ^<95>Mo spin-Iattice relaxation rate (T_1T)^<-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 ^<61>Ni, 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 (T_1T)^<-1> of ^<95>Mo 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_CV_<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.
社団法人日本物理学会の論文
2000-10-15