Experimental Evidence of Topological Surface States in Mg_3Bi_2 Films Grown by Molecular Beam Epitaxy

Nodal line semimetal (NLS) is a new quantum state hosting one-dimensional closed loops formed by the crossing of two bands. The so-called type-II NLS means that these two crossing bands have the same sign in their slopes along the radial direction of the loop, which requires that the crossing bands are either right-tilted or left-tilted at the same time. According to the theoretical prediction, Mg_3Bi_2 is an ideal candidate for studying the type-II NLS by tuning its spin-orbit coupling (SOC). High-quality Mg_3Bi_2 films are grown by molecular beam epitaxy (MBE). By in-situ angle resolved photoemission spectroscopy (ARPES), a pair of surface resonance bands around the \bar\it \Gamma point are clearly seen. This shows that Mg_3Bi_2 films grown by MBE are Mg(1)-terminated by comparing the ARPES spectra with the first principles calculations results. Moreover, the temperature dependent weak anti-localization effect in Mg_3Bi_2 films is observed under magneto-transport measurements, which shows clear two-dimensional (2D) e–e scattering characteristics by fitting with the Hikami–Larkin–Nagaoka model. Therefore, by combining with ARPES, magneto-transport measurements and the first principles calculations, this work proves that Mg_3Bi_2 is a semimetal with topological surface states. This paves the way for Mg_3Bi_2 to be used as an ideal material platform to study the exotic features of type-II nodal line semimetals and the topological phase transition by tuning its SOC.