Effect of Early Transition-Metal (M=Ti, V and Cr) Doping on the Electronic Structure of Charge-Transfer Type Compound NiS Studied by Thermoelectric Power and X-Ray Photoemission Measurements

概要

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The M 2p core-level XPS peaks of doped elements in Ni_<1-x> M_x S are observed at the binding energies of M in "M_2S_3" rather than those of "MS" below and above the nonmetal-metal transition temperature (T), indicating that the M atom are ionized to M^<3+> and acts as shallow donor impuritites. Below T_t the T^<1/2> contribution with negative sign to the thermoelectric power(S) is superimposed on the usual degenerate semiconductive contribution with positive sign. It overcomes the latter contribution above a concentration (x_c). The value of x_c increases in the order of M=Ti, V and Cr. These results indicate that the T^<1/2> contribution to S comes from variable range hopping through the localized states formed near E_F within the narrow charge-transfer gap of the parent NiS. Above T_t the sign of S is negative in the electron-doped Ni_<1-x>M_x S as well as in the hole-doped Ni_<1-x>S, suggesting that they have the Fermi surface topology.
社団法人日本物理学会の論文
1994-04-15