Movable but not removable band degeneracies in a symmorphic crystal

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.Comment: 11 pages (Paper plus Supplemental Material); 6 figures (4 in the Paper, 2 in the Supplemental Material); new Introduction, added references, revised tex