FeSe as a Polymorphous Network

The observed electronic structure of FeSe has lower apparent symmetry than the one that would be suggested by its macroscopic crystallographic structure. It has been argued that such nematicity must be electronic symmetry lowering, driven by strong correlations, rather than a local structural symmetry lowering, the latter being judged on the basis of global structural probes to be too small. Standard structure predictions use small unit cells that cannot accommodate structural symmetry lowering. Using a predictive first principles minimization of the internal total energy without restricting it to a small unit cell reveals that the lowest energy configuration whose average macroscopic symmetry is tetragonal consists, in fact, of a distribution of different local low-symmetries. This polymorphous network explains the PDF pattern in both the local and long-range regions without a fit. When used as input to electronic structure calculations, the predicted polymorphous structure reveals electronic symmetry breaking that is unique to this unusual compound.