Structural and electronic properties of the two-dimensional superconductor CuS with 1 1/3-valent copper

We present first-principles calculations of the structural and electronic properties of the CuS covellite material. The symmetry-lowering structural transition is well reproduced. However, the microscopic origin of the transition is unclear. The calculations firmly establish that the previously controversial Cu valency in this compound is 1.33. We also argue that recently reported high-temperature superconductivity (HTSC) in CuS is unlikely to occur in the stoichiometric defect-free material, since the determined Cu valency is too close to 1 to ensure proximity to a Mott-Hubbard state and superexchange spin fluctuations of considerable strength. On the other hand, one can imagine a related system with more holes per Cu in the same structural motif (e.g., due to defects or O impurities), in which case the combination of superexchange and a Fermi surface enlarged compared to that of CuS may lead to unconventional superconductivity, as in HTSC cuprates, but, unlike them, of an f-wave symmetry.