Experimental electronic structure of the mineral superconductor covellite CuS

Covellite (CuS) is the first known natural mineral superconductor. Despite its simple chemical formula, covellite exhibits a rich crystal structure at the origin of several remarkable properties. The ionic arrangement in CuS crystals leads to a mixed valence of Cu and a second-order structural transition at 55 K. Despite the abundance of structural studies and theoretical reports on its electronic structure, there are scarce references on its experimental band structure. By means of Angle Resolved PhotoEmission Spectroscopy (ARPES), we have probed the experimental electronic structure of covellite. We compare our results with the predictions of density-functional theory (DFT) calculations. Our experimental data are in remarkable agreement with the calculations, revealing subtle fingerprints of the structural phase transition, and confirming the quasi-2D nature of the electronic structure of CuS.