Quantum Transport in Air-Stable NaBi Thin Films.

NaBi has attracted significant interest in both bulk form as a three-dimensional topological Dirac semimetal and ultrathin form as a wide-band gap two-dimensional topological insulator. Its extreme air sensitivity has limited experimental efforts on thin and ultrathin films grown via molecular beam epitaxy to ultrahigh vacuum environments. Here, we demonstrate air-stable NaBi thin films passivated with magnesium difluoride (MgF) or silicon (Si) capping layers. Electrical measurements show that deposition of MgF or Si has minimal impact on the transport properties of NaBi while in ultrahigh vacuum. Importantly, the MgF-passivated NaBi films are air-stable and remain metallic for over 100 h after exposure to air, as compared to near instantaneous degradation when they are unpassivated. Air stability enables transfer of films to a conventional high-magnetic field cryostat, enabling quantum transport measurements, which verify that the Dirac semimetal character of NaBi films is retained after air exposure.