Superconductivity and low temperature properties of niobium diselenide single crystals grown by direct vapor transport

A procedure for the growth of high purity single crystals of 2H-NbSe2 by a direct vapor transport (DVT) method was developed. Single crystal growth rate and the degree of crystal perfection were studied as deviations from stoichiometry were made over the compositional range of Nb0.95Se2 to Nb1.05Se2. Single crystal perfection was also studied as changes were made in the reaction and annealing times. Residual resistivity ratio (RRR) values were observed over the range of 21 to 140 and superconductivity was observed over the range of 6.60 deg K to 7.12 deg K as growth conditions were varied. It was concluded that selenium acts as a transporting agent for the growth of the single crystals. X-ray photoelectron spectroscopy (XPS) measurements were made of the oxidation state of niobium on the surface and in the bulk of 2H-NbSe2 samples prepared by the DVT and iodine vapor transport (IVT) procedures. Measurements were also made of iodine incorporated in the lattice of the IVT samples.