Fig. 1. Topological phase diagram and band structures of ZrTe₅.

(A) Universal phase diagram of topological insulators proposed by Murakami for a 3D system (2, 3, 10). The control parameter 𝛿 describes the breaking of inversion symmetry. The control parameter ϵ does not break inversion symmetry. (B) Crystal structure of ZrTe₅. Chains of ZrTe₃ prisms (consisting of Te_a and Te_d atoms) extend along the a axis. These chains are connected by Te_z atoms along the c axis to form layers. These layers are vdW bonded in the b axis direction. (C) Size of bandgap E_g at the Γ point as functions of strains in the a and c lattice directions. The dashed gray arrow indicates the anisotropic strain induced by a uniaxial stress along the a axis direction, as governed by the calculated Poisson’s ratio ϵ_aa = −4.0ϵ_cc. (D) Band structures for different strain states taken at points along the Poisson’s ratio path. These points (from left to right) correspond to STI, Dirac semimetal, and WTI, respectively. Fermi level is defined as the zero energy, and the k-point labeling is based on the primitive unit cell. A band inversion involving Te_d and Te_z p orbitals (shown as red and green colors, respectively) is seen in the STI phase.