HOPPING TRANSPORT IN THE 3-SITE MODEL IN THE PRESENCE OF ELECTRIC AND MAGNETIC-FIELDS - RATE-EQUATION AND TRANSPORT PHENOMENA FOR SMALL POLARONS

A rate equation for the electronic density at the sites in the presence of electric and magnetic fields has been formulated for any strength of coupling with phonons, where three-site transitions are included. A classification of the irreducible diagrams is given, so that the transition probabilities contain also phonon-free processes. It has been shown that the sign of the magnetoresistance changes for jumps far from the Fermi level, if one goes over from electrons to holes. Generally speaking, it is not justified to treat the Hall effect and the magnetoresistance in disordered systems independently within the presented formalism. A concrete calculation of the current in ordered systems has been carried out in the small-polaron model for any strength of the fields E and H.