n-hourglass Weyl fermions in nonsymmorphic materials

摘要

Hourglasslike band structures protected by nonsymmorphic space group symmetries can appear along the high-symmetry lines or on the high-symmetry surfaces of the Brillouin zone. In this work, from symmetry analysis, we demonstrate that n-hourglasslike band structures, a generalization of hourglasslike band structures, are enforced along screw-invariant lines in nonmagnetic materials with an N-fold screw axis when spin-orbit coupling is finite, where n, a nonunity factor of N, denotes the degree of the screw-invariant line. The n-hourglass has a minimum of n - 1 crossings, which are Weyl points with monopole charge ±1. Using compatibility relations, we identify all the space groups that host n-hourglasslike band structures induced by screw symmetries, with n = 2, 3, 4. For N = 3 we find Weyl points that are indirectly enforced by symmetries. We construct an effective model, from which we see how the n-hourglasses appear when spin-orbit coupling is turned on. As examples, BiPd, the cinnabar phase of HgTe, and the high-temperature phase of Tl_3PbBr_5 are shown from first-principles calculations to exhibit n-hourglasslike band structures, with N = 2, 3, 4, respectively, which confirms our symmetry analysis. For N = 3 and 4, there are minimally two particle and two hole Fermi pockets enclosing Weyl points at proper fillings, where submanifold nesting may lead to topological density waves.