2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/21856Using a supersymmetry formalism, we reduce exactly the problem of electron motion in an external potential to a new supermatrix model valid at all distances. All approximate nonlinear sigma models obtained previously for disordered systems can be derived from our exact model using a coarse-graining procedure. As an example, we consider a model for a smooth disorder and demonstrate that using our approach does not lead to a 'mode-locking' problem. As a new application, we consider scattering on strong impurities for which the Born approximation cannot be used. Our method provides a new calculational scheme for disordered and chaotic systems.4 pages, no figure, REVTeX4; title changed, revision for publicationMesoscale and Nanoscale PhysicsDisordered Systems and Neural Networkstext