Universal edge scaling limit of discrete 1d random Schr\"odinger operator with vanishing potentials

Abstract

Consider random Schr\"odinger operators $H_n$ defined on $[0,n]\cap\mathbb{Z}$ with zero boundary conditions: $$ (H_n\psi)_\ell=\psi_{\ell-1}+\psi_{\ell+1}+\sigma\frac{\mathfrak{a}(\ell)}{n^{\alpha}}\psi_{\ell},\quad \ell=1,\cdots,n,\quad \quad \psi_{0}=\psi_{n+1}=0, $$ where $\sigma>0$ is a fixed constant, $\mathfrak{a}(\ell)$, $\ell=1,\cdots,n$, are i.i.d. random variables with mean $0$, variance $1$ and fast decay. The bulk scaling limit has been investigated in \cite{kritchevski2011scaling}: at the critical exponent $\alpha= \frac{1}{2}$, the spectrum of $H_n$, centered at $E\in(-2,2)\setminus\{0\}$ and rescaled by $n$, converges to the $\operatorname{Sch}_\tau$ process and does not depend on the distribution of $\mathfrak{a}(\ell).$ We study the scaling limit at the edge. We show that at the critical value $\alpha=\frac{3}{2}$, if we center the spectrum at 2 and rescale by $n^2$, then the spectrum converges to a new random process depending on $\sigma$ but not the distribution of $\mathfrak{a}(\ell)$. We use two methods to describe this edge scaling limit. The first uses the method of moments, where we compute the Laplace transform of the point process, and represent it in terms of integrated local times of Brownian bridges. Then we show that the rescaled largest eigenvalues correspond to the lowest eigenvalues of the random Schr\"odinger operator $-\frac{d^2}{dx^2}+\sigma b_x'$ defined on $[0,1]$ with zero boundary condition, where $b_x$ is a standard Brownian motion. This allows us to compute precise left and right tails of the rescaled largest eigenvalue and compare them to Tracy-Widom beta laws. We also show if we shift the potential $\mathfrak{a}(\ell)$ by a state-dependent constant and take $\alpha=\frac{1}{2}$, then for a particularly chosen state-dependent shift, the rescaled largest eigenvalues converge to the Tracy-Widom beta distribution.