Universal velocity correlations in disordered and chaotic systems

The response of weakly disordered metallic grains to Aharonov-Bohm flux suggests a rescaling in which statistical correlators become universal. We derive an exact expression for a correlator of level ``velocities,'' and provide numerical evidence which suggests that the universality extends to a wider class of systems and generalizes to arbitrary perturbations providing a new characterization of quantum chaos. These results are applied to Fermi velocities of complex lattices.