Metallic and superconducting properties of transition-metal-nitride structural phases: an ab initio study

The recent epitaxial integration of metallic and superconducting Nb_xN crystalline phases into the IIIA-nitride semiconductor family opens new directions for integrated low-temperature electronics. Furthermore, the possibility of engineered hybrid quantum systems based on these materials is spurred by the recent demonstration of concurrent superconductivity and the quantum Hall effect in a single epitaxial device. A challenge for superconducting devices in this platform comes from the incompatibility of the hexagonal GaN crystal structure with the cubic NbN phase that has been the focus of existing experimental work, which leads to twin domains that may hamper device design. Here we use first-principles theory to explore the structural polytypes of transition-metal nitrides, focused on their metallic and superconducting properties, to gather insights into the microscopic physics at play. We discuss this in the context of current experimental capabilities, weighing desired properties with the structural symmetry-dictated domains towards the goal of domain- and defect-free epitaxial devices.