Quantum Transport in Air-Stable Na₃Bi Thin Films

Na₃Bi 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 Na₃Bi thin films passivated with magnesium difluoride (MgF₂) or silicon (Si) capping layers. Electrical measurements show that deposition of MgF₂ or Si has minimal impact on the transport properties of Na₃Bi while in ultrahigh vacuum. Importantly, the MgF₂-passivated Na₃Bi 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 Na₃Bi films is retained after air exposure.