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Non-Hermitian second-order skin and topological modes

The skin effect and topological edge states in non-Hermitian system have been well-studied, and the second-order skin effect and corner modes have also been proposed in non-Hermitian system recently. In this paper, we construct the nested tight-binding formalism to research the second-order corner modes analytically, which is a direct description of the generic non-Hermitian tight-binding model without other assumptions. Within this formalism, we obtain the exact solutions of second-order topological zero-energy corner modes for the non-Hermitian four-band model. We validate the nested tight-binding formalism in the hybrid skin-topological corner modes for the four-band model and a non-Hermitian two-dimensional (2D) extrinsic model. In addition, we exactly illustrate the corner modes induced by second-order skin effect for a simplest 2D non-Hermitian model by the nested tight-binding formalism.