Phase Transitions and Critical Behavior in Quasi-One-Dimensional Two-Channel Systems with Quasiperiodic Disorder

Abstract: We investigate the localization properties of a quasi-one-dimensional two-channel system with symmetric and asymmetric onsite energies using the Aubry-Andr\'{e} model. By analyzing the Lyapunov exponent and localization length, we characterize the phase transitions and critical behavior of the system. For the symmetric model, we obtain the phase diagram for the entire spectrum, revealing mobility edges between delocalized and localized states. In contrast, for the asymmetric model, we identify a critical line $ \lambda_1^c + \lambda_2^c \approx 0.5 $ marking the phase transition between delocalized and localized states. We also study the effects of the inter-channel coupling $ \tilde{t} $, and observe that increasing $ \tilde{t} $ reduces the delocalized phase space, shifting the transition from $ \lambda_1 = \lambda_2 = 1 $ at $ \tilde{t} = 0 $ to $ \lambda_1^c + \lambda_2^c \approx 0.5 $ at larger $ \tilde{t} $. Furthermore, the phase transition point is found to be sensitive to both $ \tilde{t} $ and the incommensurate modulation parameter $ b $. While the general phase transition behavior is preserved, subtle differences arise for different values of $ b $, indicating a dependence of the phase boundary on both parameters. Using the cost function approach, we calculate the critical potential strength $ \lambda_c $ and the critical exponent $ \nu $, with $ \nu \approx 0.5 $ for the middle of the spectrum in both symmetric and asymmetric models.