We develop a variational approach to an extended Holstein model, comprising both local and nonlocal electron-phonon coupling. The approach is based on the minimization of a Bogoliubov bound to the Helmholtz free energy. The ambivalent character of nonlocal coupling, which both promotes and hinders transport, is clearly observed. Furthermore, a salient feature of our results is that the local and nonlocal couplings can compensate each other, leading to a reduction of polaronic effects and a quasi-free character of the excitation. Our findings have implications for organic crystals of p-conjugated molecules, where this electron-phonon coupling mechanism plays an important role.
|Tijdschrift||Physical Review B|
|Nummer van het tijdschrift||14|
|Status||Gepubliceerd - 2004|