A Unification of the Holstein Polaron and Dynamic Disorder Pictures of Charge Transport in Organic Crystals

We present a unified and nonperturbative method for calculating spectral and transport properties of Hamiltonians with simultaneous Holstein (diagonal) and Peierls (off-diagonal) electron-phonon coupling. Our approach is motivated by the separation of energy scales in organic molecular crystals, in which electrons couple to high-frequency intramolecular Holstein modes and to low-frequency intermolecular Peierls modes. We treat Peierls modes as quasiclassical dynamic disorder, while Holstein modes are included with a Lang-Firsov polaron transformation and no narrow-band approximation. Our method reduces to the popular polaron picture due to Holstein coupling and the dynamic disorder picture due to Peierls coupling. We derive an expression for efficient numerical evaluation of the frequency-resolved optical conductivity based on the Kubo formula and obtain the dc mobility from its zero-frequency component. We also use our method to calculate the electron-addition Green's function corresponding to the inverse photoemission spectrum. For realistic parameters, temperature-dependent dc mobility is largely determined by the Peierls-induced dynamic disorder with minor quantitative corrections due to polaronic band narrowing, and an activated regime is not observed at relevant temperatures. In contrast, for frequency-resolved observables, a quantum-mechanical treatment of the Holstein coupling is qualitatively important for capturing the phonon replica satellite structure.