Superconducting properties of transition-metal nitrides from first principles: prospects for strain and isotope modification of superconducting critical temperatures

Epitaxial integration of metallic and superconducting transition metal nitrides into the Group IIIA-nitride semiconductor family opens new avenues for low-temperature electronics and quantum information systems. A challenge for superconducting devices in the nitride platform arises due to the incompatibility between the hexagonal GaN crystal structure and the cubic NbN phase, the main focus of existing experimental work. This incompatibility leads to twin domains that may hamper device design and functionality. Furthermore, due to epitaxial lattice mismatch, the superconducting material may be strained by several percent. Here, we use first-principles theory to explore the structural polytypes of Nb_xN to gather insights into the microscopic physics at play in their metallic and superconducting properties. We focus this work on the prospect of using strain and isotopes to modify electron-phonon interactions and superconducting properties. We discuss the limitations of the theoretical approximations and present our results within the framework of current experimental capabilities. We weigh the desired properties against the constraints imposed by structural symmetry-dictated domains, aiming for the development of domain- and defect-free epitaxial devices.