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The promise of molecular electronics has generated intense interest in the interplay of molecular vibrations and resonant electronic transport. Two customary limits of phonon-assisted tunneling are the adiabatic and anti-adiabatic regimes, where the phonon frequency is either sufficiently small or sufficiently large as compared to the bare electronic hopping rate. In the former case the phonon is too slow to track the electronic motion, having little effect on its rate. In the latter case the phonon can efficiently respond to hopping
events by forming a polaron, thus suppressing the bare tunneling rate.
We conducted a systematic study of the crossover between the two regimes in the framework of the resonant-level model in order (1) to reveal the physical parameters governing the transition, (2) to expose its manifestations in linear transport, and (3) to track the evolution of the low-energy scale corresponding to the renormalized resonance width. In particular, focusing on strong electron-phonon coupling we find an extended regime where the bare electronic tunneling rate is large as compared to the phonon frequency and yet the physics is essentially that of the anti-adiabatic limit. |
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