Charge transfer and coherence for open quantum systems

Abstract

Quantum coherences have long been studied and coherences are essential descriptions for relationships among quantum states. Although there are plenty of studies on coherence among different quantum states, it remains a question whether charged-state coherences, defined as the coherences between states with different number of charges, actually exist in quantum systems. In this thesis, it is shown by basic quantum theory and properties of partial trace that for part of a closed system, no charged-state coherence could possibly exist. Evaluation on a simple three-state model and first-principles dynamic simulation on a non-equilibrium open system both indicate the validity of this result. The simulation of charge transfer and two-dimensional spectroscopy is based on the theory of first-principles time-dependent quantum transport for open systems and phase-matching approach. Charge transfer may occur when electrons stay in population states and when couplings exist among states, which may be reflected by coherences on two-dimensional electronic spectra. Several aspects may affect charge transfer and coherence, including energy gap among coupled states, coupling strength among states, number of coupled states and initial temperature of the system. Together with Born Approximation and Lowest-Order Approximation, first-principles weak electron-phonon interaction can be included in the formalism, which depicts dissipative charge transfer in quantum systems. Charge transfer becomes extremely inefficient among coupled states with energy gap, but phonon may assist electrons to cross coupled states with energy gaps. Phonon may either be absorbed by electrons for excitation or be emitted by electrons for relaxation. Electron-phonon coupling strength, photon frequency and initial temperature of the system may affect the efficiency of phonon-assisted charge transfer, which can be revealed by coherences on two-dimensional electronic spectra. Electron migration between quantum device and electrode may result in photocurrent signal. Similar to optical spectra, two-dimensional photocurrent spectra with a specific phase-matching condition may be efficiently evaluated by summation on signals from several simulations by complex electric field pulses. Simulations on an open three-level model with asymmetric system-electrode coupling indicates that photocurrent spectroscopy reveals not only the energy structures and optical transitions, but also the system-electrode coupling of quantum systems. Moreover, qualitative reproduction of experimental two-dimensional photocurrent spectra of PbS quantum dots in previously-published literature has been performed to reveal their possible interaction pathways with the incident field of different frequencies.