Enhanced electron-phonon coupling near the lattice instability of superconducting NbC(1-x)N_x from density-functional calculations

摘要

Using density-functional theory within the local-density approximation, we study the electron-phonon coupling in NbC(1-x)N_x and NbN crystals in the rocksalt structure. The Fermi surface of these systems exhibits important nesting. The associated Kohn anomaly greatly increases the electron-phonon coupling and induces a structural instability when the electronic density of states reaches a critical value. Our results reproduce the observed rise in T_c from 11.2 to 17.3 K as the nitrogen doping is increased in NbC(1-x)N_x. To further understand the contribution of the structural instability to the rise of the superconducting temperature, we develop a model for the Eliashberg spectral function in which the effect of the unstable phonons is set apart. We show that this model together with the McMillan formula can reproduce the increase of T_c near the structural phase transition.