DISORDER-INDUCED SMALL-POLARON FORMATION

Small-polaron formation is studied for a disordered multidimensional v ersion of Holstein's molecular-crystal model (MCM). The MCM is appropr iate for the study of polaronic effects in covalent semiconductors, wh ere the electron-lattice interactions are short ranged. Our considerat ions focus on the ''adiabatic'' regime since it prevails in most semic onductors. In this domain the near-neighbor electronic-transfer energi es are much greater than the characteristic phonon energy. We consider a situation in which the characteristic near-neighbor electronic-tran sfer energy t0 and the small-polaron binding energy E(b) are such that the stable carriers in a crystal would be quasifree rather than small polarons: zt0 > E(b), where z is the number of nearest neighbors. We then address how disordering of the transfer energies affects small-po laron formation. In particular, disorder is taken to replace t0 with a distribution of electronic-transfer energies. Small electronic-transf er energies about some sites by themselves stabilize small-polaron for mations at these sites. Moreover, stabilization of a small polaron at a site is found to foster the stabilization of a small polaron at site s adjacent to it. This effect enhances the effectiveness with which di sorder can trigger the collapse of a steady-state carrier from being q uasifree to being a small polaron. That is, disorder reduces the stren gth of the electron-lattice coupling needed to stabilize global small- polaron formation. With the stabilization of small-polaronic carriers, the electronic transport changes from being that of quasifree carrier s (that may occasionally be trapped at small-polaronic sites) to small -polaron hopping.