Pressure-enhanced superconductivity and superlattice structures in transition metal dichalcogenide layer crystals

The pressure dependence of the superconducting transition temperature T c is investigated for the metallic layer crystals 2H NbS2 and 2H NbSe2. In 2H NbS2 T c is pressure-independent, as in the 3R polytype. It is suggested that the pressure-enhanced T c in 2H NbSe2 may be related to recent observations in electron diffraction and N.M.R. of a phase change to a superlattice structure in this material at low temperatures. Our results would tend to rule out the model of pressure-dependent Josephson tunnelling between layers suggested by previous authors. The aniso-tropy and pressure dependence of the critical field H c2 are described.