Energy spectra of an open chain with length L = 80 (unit cell) as a function of t ′ by numerical diagonalization of the Hamiltonian: (a)  in Eq. (1), (b) of the non-Hermitian SSH model for ladder I, (c) of the non-Hermitian SSH model for ladder II. Here t 1 = 1.3 , t 2 = 0.6 , t z = 1.2 , γ 1 = 4 / 3 , and γ 2 = 2 / 3 . The other parameters are set to be t 1 ′ = t 2 ′ = t ′ .