Electronic stiffness of a superconducting niobium nitride single crystal under pressure

We report a quantitative study of pressure effects on the superconducting transition temperature Tc and the electronic stiffness of niobium nitride. It is found that Tc increases initially with pressure and then saturates up to 42GPa. Combining phonon and structural information on the samples obtained from the same single crystal, we derive a nonmonotonic pressure dependence of the electronic stiffness, rising moderately at low pressure while dropping slightly at high pressure. The theory of Gaspari and Gyorffy is found to reproduce the observed low-pressure results qualitatively but fails to predict the high-pressure data. The observed pressure effect on Tc is attributed to the pressure-induced interplay of the electronic stiffness and phonon frequencies.