BaHgSn: A Dirac semimetal with surface hourglass fermions

TL;DR

BaHgSn is a Dirac semimetal with protected hourglass surface states, distinguished by a unique band inversion and topological features that can transition to a strong topological insulator under strain.

Contribution

This work identifies BaHgSn as a Dirac semimetal with novel surface hourglass fermions and details its topological properties and strain-induced phase transition.

Findings

BaHgSn hosts hourglass surface states protected by nonsymmorphic symmetry.

It exhibits bulk Dirac nodes due to band inversion at the Γ point.

Strain can induce a transition to a strong topological insulator with coexisting surface states.

Abstract

We proposed that BaHgSn is a Dirac semimetal (DSM) which can host hourglass-like surface states (HSSs) as protected by nonsymmorphic glide symmetry. Compared to KHgSb, an isostructural topological crystalline insulator with the same HSSs, BaHgSn has an additional band inversion at $\Gamma$ point. This band inversion is induced by the stronger interlayer coupling among Hg-Sn honeycomb layers than that among Hg-Sb-layers in KHgSb, which leads to bulk Dirac nodes in BaHgSn along the layer stacking direction $\Gamma$-$A$. In addition, the mirror Chern number $C_{i}$ protected by the mirror plane $\overline{M}_{z}$ ($k_z$=0) changes from 2 in KHgSb to 3 in BaHgSn. Therefore, when a compressive uniaxial strain is applied along the $y$ axis to break the rotation symmetry protecting the DSM state, BaHgSn becomes a strong topological insulator with $Z_{2}$ indices of $(1;000)$ and the topological surface Dirac cone co-exists with HSSs on the (010) surface. The Wilson-loop spectra have been calculated to verify these topological features. The calculated surface states, the Fermi surfaces and their quasiparticle interference patterns are ready to be compared with experimental measurements.