Coexistence of zero Lyapunov exponent and positive Lyapunov exponent for new quasi-periodic Schr$\ddot{o}$dinger operator

Abstract

In this paper we solve a problem about the Schr$\ddot{o}$dinger operator with potential $v(\theta)=2\lambda cos2\pi\theta/(1-\alpha cos2\pi\theta),\ (|\alpha|<1)$ in physics. With the help of the formula of Lyapunov exponent in the spectrum, the coexistence of zero Lyapunov exponent and positive Lyapunov exponent for some parameters is first proved, and there exists a curve that separates them. The spectrum in the region of positive Lyapunov exponent is purely pure point spectrum with exponentially decaying eigenfunctions for almost every frequency and almost every phase. From the research, we realize that the infinite potential $v(\theta)=2\lambda tan^2(\pi\theta)$ has zero Lyapunov exponent for some energies if $0<|\lambda|<1$.