Almost mobility edges and existence of critical regions in one-dimensional quasiperiodic lattices
We study a one-dimensional quasiperiodic system described by the Aubry-Andr\'e model in the small wave vector limit and demonstrate the existence of almost mobility edges and critical regions in the system. It is well known that the eigenstates of the Aubry-Andr\'e model are either extended or localized depending on the strength of incommensurate potential $V$ being less or bigger than a critical value $V_c$, and thus no mobility edge exists. However, it was shown in a recent work that this conclusion does not hold true when the wave vector $\alpha$ of the incommensurate potential is small, and for the system with $V<V_c$, there exist almost mobility edges at the energy $E_{c_{\pm}}$, which separate the robustly delocalized states from "almost localized" states. We find that, besides $E_{c_{\pm}}$, there exist additionally another energy edges $E_{c'_{\pm}}$, at which abrupt change of inverse participation ratio occurs. By using the inverse participation ratio and carrying out multifractal analyses, we identify the existence of critical regions among $|E_{c_{\pm}}| \leq |E| \leq |E_{c'_{\pm}}|$ with the almost mobility edges $E_{c_{\pm}}$ and $E_{c'_{\pm}}$ separating the critical region from the extended and localized regions, respectively. We also study the system with $V>V_c$, for which all eigenstates are localized states, but can be divided into extended, critical and localized states in their dual space by utilizing the self-duality property of the Aubry-Andr\'e model.