# Hourglass semimetals with nonsymmorphic symmetries in three dimensions

## TL;DR

This paper introduces hourglass semimetals, 3D systems with bulk hourglass-like dispersions protected by nonsymmorphic and time-reversal symmetries, revealing new topological features like hourglass Weyl points and nodal lines.

## Contribution

It constructs 3D lattice models demonstrating bulk hourglass dispersions protected by nonsymmorphic symmetries, expanding the understanding of topological semimetals.

## Key findings

- Hourglass nodal lines in glide-invariant planes
- Hourglass Weyl points on screw-invariant axes
- Surface Fermi arcs stable against weak symmetry-breaking perturbations

## Abstract

It was recently shown that nonsymmorphic space group symmetries can protect novel surface states with hourglass-like dispersions. In this paper, we show that such dispersions can also appear in the bulk of three-dimensional (3D) systems which respect nonsymmorphic symmetries. Specifically, we construct 3D lattice models featuring hourglass-like dispersions in the bulk, which are protected by nonsymmorphic and time-reversal symmetries. We call such systems hourglass semimetals, as they have point or line nodes associated with hourglass-like dispersions. Hourglass nodal lines appear in glide-invariant planes, while hourglass Weyl points can occur on screw-invariant axes. The Weyl points and surface Fermi arcs in hourglass Weyl semimetals are stable against weak perturbations breaking those nonsymmorphic symmetries. Our results may shed light on searching for exotic Weyl semimetals in nonsymmorphic materials.