Asymptotics of spectral gaps of 1D Dirac operator with two exponential terms potential (1312.2219v1)

Abstract: The one-dimensional Dirac operator \begin{equation*} L = i \begin{pmatrix} 1 & 0 \ 0 & -1 \end{pmatrix} \frac{d}{dx} +\begin{pmatrix} 0 & P(x) \ Q(x) & 0 \end{pmatrix}, \quad P,Q \in L^2 ([0,\pi]), \end{equation*} considered on $[0,\pi]$ with periodic and antiperiodic boundary conditions, has discrete spectra. For large enough $|n|,\, n \in \mathbb{Z}, $ there are two (counted with multiplicity) eigenvalues $\lambda_n^-,\lambda_n^+ $ (periodic if $n$ is even, or antiperiodic if $n$ is odd) such that $|\lambda_n^\pm - n |<1/2.$ We study the asymptotics of spectral gaps $\gamma_n =\lambda_n^+ - \lambda_n^-$ in the case $$P(x)=a e^{-2ix} + A e^{2ix}, \quad Q(x)=b e^{-2ix} + B e^{2ix},$$ where $a, A, b, B$ are nonzero complex numbers. We show, for large enough $m,$ that $\gamma_{\pm 2m}=0 $ and \begin{align*} \gamma_{2m+1} = \pm 2 \frac{\sqrt{(Ab)^m (aB)^{m+1}}}{4^{2m} (m!)^2 } \left[ 1 + O \left( \frac{\log^2 m}{m^2}\right) \right], \end{align*} \begin{align*} \gamma_{-(2m+1)} = \pm 2\frac{\sqrt{(Ab)^{m+1} (aB)^m}}{4^{2m} (m!)^2} \left[ 1 + O \left( \frac{\log^2 m}{m^2}\right) \right]. \end{align*}