Observation of Tunneling Gap in Epitaxial Ultrathin Films of Pyrite-Type Copper Disulfide

We report scanning tunneling microscopy investigation on epitaxial ultrathin films of pyrite-type copper disulfide. Layer-by-layer growth of CuS_2 films with a preferential orientation of (111) on SrTiO_3(001) and Bi_2Sr_2CaCu_2O_8+\delta substrates is achieved by molecular beam epitaxy growth. For ultrathin films on both kinds of substrates, we observe symmetric tunneling gap around the Fermi level that persists up to \sim15 K. The tunneling gap degrades with either increasing temperature or increasing thickness, suggesting new matter states at the extreme two-dimensional limit.