Nonsymmorphic Topological Phases of Non-Hermitian Systems

Non-Hermiticity appears ubiquitously in various open classical and quantum systems and enriches classification of topological phases. However, the role of nonsymmorphic symmetry, crystalline symmetry accompanying fractional lattice translations, has remained largely unexplored. Here, we systematically classify non-Hermitian topological crystalline phases protected by nonsymmorphic symmetry and reveal unique phases that have no counterparts in either Hermitian topological crystalline phases or non-Hermitian topological phases protected solely by internal symmetry. Specifically, we elucidate the ${\mathbb{Z}}_2$ and ${\mathbb{Z}}_4$ non-Hermitian topological phases and their associated anomalous boundary states characterized by distinctive complex-valued energy dispersions.