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Phase transitions and generalized biorthogonal polarization in non-Hermitian systems

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Edvardsson, E., Kunst, F., Yoshida, T., & Bergholtz, E. J. (2020). Phase transitions and generalized biorthogonal polarization in non-Hermitian systems. Physical Review Research, 2(4): 043046. doi:10.1103/PhysRevResearch.2.043046.

Cite as: https://hdl.handle.net/21.11116/0000-0009-8BE2-B
Non-Hermitian (NH) Hamiltonians can be used to describe dissipative systems, notably including systems with gain and loss, and are currently intensively studied in the context of topology. A salient difference between Hermitian and NH models is the breakdown of the conventional bulk-boundary correspondence, invalidating the use of topological invariants computed from the Bloch bands to characterize boundary modes in generic NH systems. One way to overcome this difficulty is to use the framework of biorthogonal quantum mechanics to define a biorthogonal polarization, which functions as a real-space invariant signaling the presence of boundary states. Here, we generalize the concept of the biorthogonal polarization beyond the previous results to systems with any number of boundary modes and show that it is invariant under basis transformations as well as local unitary transformations. Additionally, we focus on the anisotropic Su-Schrieffer-Heeger chain and study gap closings analytically. We also propose a generalization of a previously developed method with which to find all the bulk states of the system with open boundaries to NH models. Using the exact solutions for the bulk and boundary states, we elucidate genuinely NH aspects of the interplay between the bulk and boundary at the phase transitions.