  The propagation of nonrelativistic excitations in material media with
topological defects can be modeled in terms of an external torsion field
modifying the Schroedinger equation. Through a perturbative approach, we find a
solution for the wave function which gives corrections in the interference
patterns of the order of 0.1 Angstrom, for a possible experimental setup at
atomic scales. Finally, we demonstrate how this geometric, but effective,
approach can indeed accommodate a probabilistic interpretation of the wave
function although the perturbative theory is nonunitary.
