Two-scale estimates for limit-periodic Schrödinger operators

Abstract

We prove strong ballistic transport for one-dimensional continuum Schrödinger operators by comparing the spectral geometry at two consecutive periods. Enlarging a period decomposes each fiber into the eigenspaces of translation by the smaller cell. Away from the larger-period symmetry momenta, a simple ordered band remains in one translation sector and agrees with one translated smaller-period band. This permits us to combine large-period ordering and non-overlap with a smaller-period derivative bound. We obtain lower bounds for folded band widths and confine all near-collisions to a common neighborhood of the symmetry momenta, with the exponential cost at the smaller period. For nested periods $p_n$ with $p_{n+1}/p_n\in\mathbb{N}$ and $p_{n+1}\ge2p_n$, approximation errors $o(p_{n+1}^{-2}e^{-ηp_n})$, where $η>7\sqrt{2R}$ and $\|V\|_\infty\le R$, give a nonzero strong asymptotic velocity on a weighted $H^2$ class containing the Schwartz space. Every position moment of order $0<r\le4$ has two-sided ballistic growth. A period-uniform form estimate, ordered-gap summation, and a linear dynamical comparison complete the argument. No upper bound on successive period ratios is assumed, and the sufficient approximation exponents are not claimed to be optimal.