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

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Virginia Polytechnic Institute and State University

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°K to 7.12°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. These measurements indicated the niobium on the surface of the IVT samples was more completely oxidized than the niobium on the surface of the DVT samples, whereas, XPS spectra of cleaved DVT and IVT surfaces were similar. In both cases more niobium, which was identified as Nb°, appeared to be present on the cleaved surfaces than is consistent with the chemical formula NbSe₂ and the superconducting transition tempera- tures observed for these samples. X-ray photoelectron spectroscopy measurements made of the 3d3/2 level of iodine incorporated in the lattice of the IVT samples indicated the iodine is present as iodide ion.

A critical current density dependence upon crystal thickness was observed for the DVT and IVT samples. The higher critical current densities that were observed for the DVT crystals were attributed primarily to thickness.

The superconducting transition temperatures of the DVT crystals with the highest degree of crystal perfection were observed to be about 0.1°K lower than those observed for the IVT crystals. It was demonstrated by means of iodine doping of the DVT crystals that the superconducting transition temperature of 2H-NbSe₂ was elevated by iodine.