Movable but not removable band degeneracies in a symmorphic crystal

TL;DR

This paper demonstrates that movable but non-removable band degeneracies can occur in symmorphic crystals, challenging the previous belief that such features require nonsymmorphic symmetries, through tight-binding models with specific symmetries.

Contribution

The study shows that certain symmetries in symmorphic crystals can produce movable, non-removable band degeneracies, expanding understanding of band crossing phenomena.

Findings

Band crossings can occur in symmorphic crystals without nonsymmorphic symmetry.

Chiral, time-reversal, and site-mirror symmetries are sufficient for such degeneracies.

Theoretical models reveal conditions for movable but non-removable degeneracies.

Abstract

Crossings of energy bands in solids that are not pinned at symmetry points in the Brillouin zone and yet cannot be removed by perturbations are thought to be conditioned on the presence of a nonsymmorphic symmetry. In this Letter we show that such band crossings can actually appear also in a symmorphic crystal. A study of a class of tight-binding multiband one-dimensional lattice models of spinful electrons reveals that chiral, time-reversal and site-mirror symmetries are suffcient to produce such movable but not removable band degeneracies.