Random band matrices in the delocalized phase, II: Generalized resolvent estimates

Abstract

This is the second part of a three part series abut delocalization for band matrices. In this paper, we consider a general class of $N\times N$ random band matrices $H=(H_{ij})$ whose entries are centered random variables, independent up to a symmetry constraint. We assume that the variances $\mathbb{E}|H_{ij}{|}^2$ form a band matrix with typical band width $1\ll W\ll N$. We consider the generalized resolvent of $H$ defined as $G(Z):=(H-Z{)}^{-1}$, where $Z$ is a deterministic diagonal matrix such that $Z_{ij}=\left(z{1}_{1\le i\le W}+\tilde{z}{1}_{i>W}\right){\delta }_{ij}$, with two distinct spectral parameters $z\in {\mathbb{C}}_{+}:=\{z\in \mathbb{C}:\mathrm{Im}z>0\}$ and $\tilde{z}\in {\mathbb{C}}_{+}\cup \mathbb{R}$. In this paper, we prove a sharp bound for the local law of the generalized resolvent $G$ for $W\gg N^{3/4}$. This bound is a key input for the proof of delocalization and bulk universality of random band matrices in \cite{PartI}. Our proof depends on a fluctuations averaging bound on certain averages of polynomials in the resolvent entries, which will be proved in \cite{PartIII}.