INTERIMPURITY TRANSFER IN CONDENSED MEDIA - BREAKDOWN OF COHERENT TUNNELING AND CONDITIONS FOR THE CREATION OF LOCALIZED STATES

This paper deals with the problem of interimpurity transport of quasip articles (electron, exciton, etc.) in condensed media. A realistic sit uation is simulated by the simple two-level model interacting with an environment whose influence is described by a set of harmonic oscillat ors. The model is general enough to include most of the situations of interest by a convenient choice of parameters. A variational approach is developed which enables one to formulate the set of evolution equat ions for the variables describing the essential system properties in p ractically the whole parameter space, which consists of the coupling c onstant and the adiabaticity parameter (representing the ratio of the interimpurity-transfer integral to the phonon bandwidth). It was found that the parameter space (plane of the coupling constant vs adiabatic ity parameter) is divided into two regions with quite distinct physica l properties. In the first one the dynamics is dominated by the quantu m nature of the phonon subsystem and the localization transition is ac hieved through the reduction of the effective transfer integral. In th e second, the so-called symmetry-breaking region, the classical nature of the phonon field prevails. Therefore, the, dynamics and localizati on are described by the discrete nonlinear Schrodinger equation. The s ymmetry breaking itself is induced by nature of the phonon field, whic h, in the strong coupling limit, behaves classically. The results are presented in such a form that it allows for the precise determination of the system behavior for any set of realistic system parameters, ena bling comparison with previous results.