Diffusion Profile for Random Band Matrices: a Short Proof

Let $H$ be a Hermitian random matrix whose entries $H_{xy}$ are independent, centred random variables with variances $S_{xy} = \mathbb E|H_{xy}|^2$, where $x, y \in (\mathbb Z/L\mathbb Z)^d$ and $d \geq 1$. The variance $S_{xy}$ is negligible if $|x - y|$ is bigger than the band width $W$. For $d = 1$ we prove that if $L \ll W^{1 + \frac{2}{7}}$ then the eigenvectors of $H$ are delocalized and that an averaged version of $|G_{xy}(z)|^2$ exhibits a diffusive behaviour, where $G(z) = (H-z)^{-1}$ is the resolvent of $H$. This improves the previous assumption $L \ll W^{1 + \frac{1}{4}}$ by Erd\H{o}s et al. (2013). In higher dimensions $d \geq 2$, we obtain similar results that improve the corresponding by Erd\H{o}s et al. Our results hold for general variance profiles $S_{xy}$ and distributions of the entries $H_{xy}$. The proof is considerably simpler and shorter than that by Erd\H{o}s et al. It relies on a detailed Fourier space analysis combined with isotropic estimates for the fluctuating error terms. It avoids the intricate fluctuation averaging machinery used by Erd\H{o}s and collaborators.