Capacity and measure approximations for Schrödinger operators

Abstract

We prove that logarithmic capacity convergence for phase-union spectra of quasi-periodic Schrödinger operators in the zero Lyapunov exponent regime is robust, requiring only continuity of the potential. Let $S^+(p/q)$ denote the union, over the phase, of the spectra at rational frequency $p/q$. We show that if the Lyapunov exponent vanishes on the spectrum $Σ(α)$ at an irrational frequency $α$, then for every sequence $p_n/q_n\toα$, the logarithmic capacities $Cap(S^+(p_n/q_n))\longrightarrow Cap(Σ(α)).$ We also prove convergence of the corresponding harmonic measures. As a consequence, the equilibrium measures of $S^+(p_n/q_n)$ converge in the weak$^*$ topology to the density of states measure of the quasi-periodic Schrödinger operator. We extend these results to multi-frequency Schrödinger operators and prove analogous convergence theorems, for logarithmic capacity, harmonic measure, and equilibrium measure, for ergodic Schrödinger operators in a general setting where the almost sure spectrum is approximated in the Hausdorff metric by union spectra of periodic operators. This abstract formulation applies, in particular, to uniformly almost periodic potentials along sequences of almost periods. We also provide counterexamples when the limiting frequency is rational.