HALL-EFFECT AND CONDUCTIVITY IN PYRITE NIS2

We report Hall-effect measurements on single crystals of antiferromagnetic pyrite-type NiS2 (T-N = 40 K), in which the resistivity appears to show a metal-insulator transition at T approximate to 100 K. Our transport measurements demonstrate that this transition is not intrinsic to the bulk: At low temperatures the transport is dominated by a metalliclike conduction at the surface, with carrier density n(S) approximate to 5 x 10(14) cm(-2) and mobility mu(S) x 1.3 cm(2)/V s at T = 0. For T > 100 K the sample conductance is dominated by the bulk, which behaves Like a doped semiconductor; the acceptorlike impurity states have binding energy E(B) = 80 meV. The valence-band mobility is mu x 0.002 cm(2)/Vs.