Thickness-dependent oscillation of the superconducting Tc in ultrathin NbC films: A first-principles study

The dimensionality effect on superconductivity has been a long-term interesting topic. Here we have systematically investigated the electronic and superconducting properties of ultrathin NbC films by using first-principles electronic structure calculations. We find that the superconducting transition temperatures of ultrathin NbC films show an interesting thickness-dependent oscillation, whose amplitude can reach as large as 10 K. Notably, the superconducting Tc of trilayer NbC film is predicted to be 16.8 K, even higher than that (11.5 K) of bulk NbC. Further analyses indicate that the ultrathin NbC films with relatively high superconducting Tc's all have strong phonon softening and enhanced electron-phonon coupling, for which the special phonon modes and electronic states as well as their real-space patterns are clarified. Our results provide an insight for modulating the superconductivity of transition metal carbides with the reduced dimensionality.