Quasiparticle spectroscopy, transport, and magnetic properties of Nb films used in superconducting transmon qubits*

Niobium films, 160 nm thick, used as leads to aluminum Josephson junctions in superconducting qubits (transmons) have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, quasiparticle spectroscopy, and magneto-optical imaging. In the Meissner state, the films show a very sharp superconducting transition at T_c = 9.35 K, a fairly clean superconducting gap, and signs of stronger coupling consistent with the recent theory of anisotropic strong-coupling superconductivity in Nb. The behavior in the applied magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat channeling to a silicone substrate leading to thermo-magnetic dendritic avalanches. The latter may signify an issue for transmons driven at several GHz. Possible mitigation strategies are discussed.