Berry curvature inside a PT -symmetry protected exceptional surface

Abstract

A three-dimensional non-Hermitian Hamiltonian with parity-time symmetry can exhibit a closed exceptional surface (EP surface) in momentum space, which is a non-Hermitian deformation of the degeneracy line. Since the degeneracy line lacks an internal space, the distributions of Berry curvature inside the EP surface become particularly intriguing. This paper studies the distributions taking a toruslike EP surface as an example. In a meridian cross section, the Berry connection exhibits a vortexlike field with only angular components, while the Berry curvature is perpendicular to this cross section; in a equatorial cross section, the Berry curvature forms a closed curve surrounding the central genus. Both Berry connection and curvature converge along the coplanar axis and diverge at the surface. We find the Berry flux depends on the radius of the integration region and is not quantized inside the EP torus. Approaching the surface, the Berry flux tends to infinity and the dynamical phase oscillates violently. We point out that the streamlines of Berry curvature can be used to estimate the zero or nonzero Berry flux. We generalize the above patterns to the case of EP surfaces with complex shapes and present a proposal of realizing the EP surface in an electrical circuit. Our research outcomes enhance the comprehension of EP surfaces and the topological characteristics of non-Hermitian systems with parity-time (PT) symmetry. Published by the American Physical Society 2024