Effect of stoichiometry on the superconducting transition temperature in single crystalline 2H-NbS2

Single crystals of 2H-NbS2 are prepared by chemical vapor transport method under varying conditions. The residual resistivity ratio (RRR) of the as-grown single crystals is observed to change from 10 to 60, while the superconducting transition temperatures (Tc) remain around 6.2 K. Changes in stoichiometry were obtained by annealing in different conditions, resulting in sulfur deficient 2H-NbS2−y single crystals. Both Tc and RRR of the annealed samples are found to drop drastically when y increases from 0.05 to 0.10, which is argued to be the effect of enhanced electron scattering caused by the decreased interlayer coupling evidenced by the increase of the c-axis lattice constant after annealing, possibly due to Nb intercalation between layers.