COOPER-PAIR DELOCALIZATION IN DISORDERED MEDIA

We discuss the effect of disorder on the coherent propagation of the b ound state of two attracting particles. It is shown that a result anal ogous to the Anderson theorem for dirty superconductors is also valid for the Cooper problem, namely, that the pair wave function is extende d beyond the single-particle localization length if the latter is larg e. A physical justification is given in terms of Thouless block-scalin g picture of localization. These arguments are supplemented by numeric al simulations. With increasing disorder we find a transition from a r egime in which the interaction delocalizes the pair to a regime in whi ch the interaction enhances localization. (C) 1998 Elsevier Science Lt d. All rights reserved.