Inelastic effects in electron transport studied with wave packet propagation

TL;DR

This paper employs a time-dependent wave packet propagation method to analyze inelastic effects in electron transport through simple models, revealing detailed insights into vibrational interactions, phase shifts, and spectral features across various coupling regimes.

Contribution

It introduces a comprehensive time-dependent approach to study inelastic electron transport, clarifying the physical origins of spectral features and the validity of simplified models in different regimes.

Findings

Vibrational excitation sequences can be traced over time.

Spectral features in current derivatives relate to vibrational effects.

Finite-band effects cause electron back-scattering at high conductance.

Abstract

A time-dependent approach is used to explore inelastic effects during electron transport through few-level systems. We study a tight-binding chain with one and two sites connected to vibrations. This simple but transparent model gives insight about inelastic effects, their meaning and the approximations currently used to treat them. Our time-dependent approach allows us to trace back the time sequence of vibrational excitation and electronic interference, the ibrationally introduced time delay and the electronic phase shift. We explore a full range of parameters going from weak to strong electron-vibration coupling, from tunneling to contact, from one-vibration description to the need of including all vibrations for a correct description of inelastic effects in transport. We explore the validity of single-site resonant models as well as its extension to more sites via molecular orbitals and the conditions under which multi-orbital, multi-vibrational descriptions cannot be simplified. We explain the physical meaning of the spectral features in the second derivative of the electron current with respect to the bias voltage. This permits us to nuance the meaning of the energy value of dips and peaks. Finally, we show that finite-band effects lead to electron back-scattering off the molecular vibrations in the regime of high-conductance, although the drop in conductance at the vibrational threshold is rather due to the rapid variation of the vibronic density of states.