The structure of topologically insulating amorphous Bi₂Se₃

The explosion of research into topologically nontrivial electronic states over the past decades has focused overwhelmingly on crystalline materials. However, the translational symmetry that defines crystals is not required for topological phases of matter; amorphous materials can also support topological phases if certain other symmetries are present, such as time-reversal symmetry. Recently, spin-momentum locked surface states, a hallmark of topological insulators, were observed in amorphous Bi₂Se₃ [1], expanding the realm of experimentally observed topological materials into amorphous solids. Here, we elucidate in detail the local structure of amorphous Bi₂Se₃ via x-ray pair distribution function analysis of thin films. Topologically insulating amorphous Bi₂Se₃ retains the same sixfold octahedral coordination of neighboring Bi and Se atoms observed in crystalline Bi₂Se₃, indicating that the electrons experience a similar local structural environment in both amorphous and crystalline Bi₂Se₃. In addition, the arrangement of these octahedral coordination units approximates a single layer of crystalline Bi₂Se₃ with a structural coherence length of 2-3 nm. No remnants of the interlayer structural correlations present in crystalline Bi₂Se₃ are seen in the data for amorphous Bi₂Se₃. Previously reported Raman measurements [1] are consistent with this picture of the local structure. Taken together, these results shed light on the origin of topological states in amorphous Bi₂Se₃ and provide guidance for the search for nontrivial topology in other amorphous systems.



[1] Paul Corbae et al., “Observation of spin-momentum locked surface states in amorphous Bi₂Se₃”, Nature Materials 22, 200 (2023).