Uncovering surface states of the Dirac semimetal BaMg2Bi2

Abstract

BaMg2Bi2 is a Dirac semimetal characterized by a simple Dirac cone crossing the Fermi level at the center of the Brillouin zone, protected by C3 rotational symmetry. Together with its Sr-based analogue SrMg2Bi2, it has been proposed as a promising candidate for a chemically driven topological switch: while SrMg2Bi2 is an insulator, BaMg2Bi2 exhibits non-trivial topological features. A detailed understanding of its electronic structure is essential to elucidate its electronic and transport properties. Previous photoemission studies confirmed the Dirac nature of BaMg2Bi2, but were limited to high photon energies, which hindered direct comparison with density functional theory calculations (DFT), due to reduced resolution and higher-frequency matrix-element modulation in that regime. In this work, we combine high-resolution angle-resolved photoemission spectroscopy (ARPES) and DFT calculations to get full insight on the valence band states, providing a comprehensive picture of the low-energy electronic structure. Our measurements reveal the presence of previously unobserved surface states. We found that they are topologically trivial, but they unlock a more comprehensive understanding of the material's behavior, reconciling previous discrepancies between experiment and theory.

Figures (4)

• Figure 1: (a) Side and top view of the structure and unit cell of BaMg$_2$Bi$_2$ crystal, showing the typical CaAl$_2$Si$_2$ structure. (b) Resulting Brillouin zone with high symmetry directions indicated. The surface Brillouin zone is also indicated. (c) Bulk electronic structure along the in-plane high symmetry directions. (d) Same as (c) but along the out of plane direction.
• Figure 2: (a) $E$--$k$ dispersions of BaMg$_2$Bi$_2$ along the $\Gamma$--$K$ (top) and $\Gamma$--$M$ (bottom) directions for both LH and LV polarizations. Spectra were acquired with 75 eV photons, corresponding to a bulk $\Gamma$ point. (b) Same dispersions acquired with 85 eV photons, corresponding to the $A$ point. (c) Spectra obtained with 95 eV photons, corresponding to the next bulk $\Gamma$ point. Red and blue arrows highlight additional features not previously reported.
• Figure 3: (a) Electronic structure of BaMg$_2$Bi$_2$ along the $\Gamma$-K and (b) $\Gamma$-M directions for both light polarizations indicated in the figure. DFT calculations have been overlaid and the gray color indicates the projected bulk continuum.
• Figure 4: (a) Electronic structure of BaMg$_2$Bi$_2$ along the $\Gamma$-K and (b) $\Gamma$-M (bottom) directions for both light polarizations indicated in the figure. DFT calculations have been overlaid and the gray colour indicates the projected bulk continuum. The green colour indicate the surface states, calculated as described in the text.