Superconducting and Fermi Surface Properties of Pyrite-type Compounds CuS2 and CuSe2

We grew high-quality single crystals of the pyrite-type compounds CuS₂ and CuSe₂ under a pressure of 5 GPa and studied their superconducting and Fermi surface properties by measuring their electrical resistivity, specific heat, and de Haas-van Alphen (dHvA) effect. The upper critical field H_c2 at 0 K [H_c2(0)] is found to be small, H_c2(0) = 160 Oe in CuS₂ with the superconducting transition temperature T_sc of 1.5 K and H_c2(0) = 610 Oe in CuSe₂ with T_sc of 2.4 K. The detected dHvA branches are small in number in CuS₂, but the main dHvA branches, which are expected to exist in the energy band calculations, are hardly observed in CuSe₂. The cyclotron effective masses are not large, ranging from 0.8 to 2.4 m₀, revealing that the correlation between Cu-3d conduction electrons is weaker than those of the other pyrite-type compounds of the ferromagnet CoS₂ and the Mott-Hubbard insulator NiS₂.