I will discuss our work on films of Na3Bi (a topological Dirac semimetal) which, when thinned to a few atomic layers, becomes a large bandgap (>300 meV) quantum spin Hall insulator, with an electric-field tuned topological-to-conventional transition, making it a promising platform for topological electronics[1-5]. We study thin films of Na3Bi grown in ultra-high vacuum by molecular beam epitaxy[1], characterized with electronic transport, scanning tunneling microscopy (STM), and angle-resolved photoemission spectroscopy. When thinned to a few atomic layers Na3Bi is a large gap (>300 meV) 2D topological insulator with topologically protected edge modes observable in STM[4]. Electric field applied perpendicular to the Na3Bi film, by potassium doping or by proximity of an STM tip, closes the bandgap completely and reopens it as a conventional insulator[4]. To enable electrical transport experiments, we prepare ultra-thin Na3Bi on insulating sapphire. A non-local transport measurement design allows us to observe edge conduction in millimeter-scale samples when the Fermi level is brought into the bulk gap by doping with acceptor molecule F4-TCNQ. In the edge conduction regime, helical conduction gives rise to a giant negative magnetoresistance due to suppression of spin-flip scattering which becomes inelastic in a magnetic field[5]. Comparison to a simple theoretical model indicates >98% of the resistance is due to spin-flip scattering, well beyond the limit of 2/3 for a generic non-helical metal with exchange-mediated scattering, providing an unambiguous signature of helical transport.
References:
- Nano Letters 16 (5), 3210-3214 (2016).
- Physical Review Materials 1, 054203 (2017).
- Science Advances 3, eaao6661 (2017).
- Nature 564, 390-394 (2018).
- arXiv:1906.01214