Stability, Metallicity, and Magnetism in Niobium Silicide Nanofilms

Modern superconducting charge qubits based on transmons involve the growth of niobium thin films on resistive silicon substrates under variable processing conditions. The atomic precision of the Nb-Si heterointerface can limit qubit-coherence times. Bulk binary intermetallic niobium silicide phases exhibit a range of stable compositions that are often processed at high temperature; however, the thermodynamic phase stability and properties of possible ultrathin silicides, such as those that form at the Nb-Si heterointerface during deposition, have not yet been reported. Here we report the first ab initio-based finite-size effect studies and predict a novel nonequilibrium silicide stabilized at the nanoscale. This result is consistent with our direct experimental growth of the silicide from a bulk Nb source by pulsed laser deposition. We also find the surfaces of the silicides are magnetic, which may lead to an additional dissipation channel. Our work suggests that Nb-Si heterointerfaces in transmons may not be atomically sharp and interfacial composition and morphology have important implications on achieving long qubit-coherence times.