Structural disorder and topology in Bi₂Se₃ and BiTeI.

The use of crystal symmetries and band representations have enabled a classification scheme for crystalline topological materials, leading to large scale topological materials discovery [1]. We address the role of structural disorder in topological insulators and show that amorphous topological materials, which lack of long-range order, exist in the solid state. Amorphous Bi₂Se₃ shows a high but temperature independent resistance, weak antilocalization consistent with decoupled conductance channels, and ARPES data showing a dispersive spin-polarized electronic structure [2]. These experimental results are consistent with an amorphous tight binding model. Next, we show how breaking of crystalline symmetries in the trivial insulator BiTeI leads to a topological phase transition via first principles calculations. Structural disorder removes the degeneracy of the orbitals near the Fermi level and increases the crystal field pushing the bands closer together and incorporating spin-orbit coupling leads to a band inversion [3]. Our work provides an understanding on how local environments produce topological phases and is a key step for predicting disordered and amorphous topological materials to be used in scalable topological devices.