Mixed quantum-classical dynamics of energy transport in open quantum systems

• Author / Creator

  Carpio Martinez, Pablo

• The study of energy transport through molecules has gathered much attention in recent years due to its crucial role in the operation of a host of nano-devices. Understanding the details of such processes can aid in the development of novel molecular electronics and nanophononic devices. Over the years, various approaches have been used to simulate energy transport dynamics in both simple and complex models in order to gain insight into how system and environmental properties affect the transport rates/mechanisms. In the present thesis, we studied (i) nonequilibrium heat transport and (ii) vibrational energy transfer in a variety of model molecular systems using mixed quantum-classical dynamics. For direction (i), we assessed the ability of a recently developed mixed quantum-classical dynamics method, known as Deterministic Evolution of Coordinates with Initial Decoupled Equations (DECIDE), for calculating steady-state heat currents in the nonequilibrium spin-boson (NESB) model - a model molecular junction consisting of a two-level system coupled to two harmonic oscillator baths at different temperatures. Furthermore, we investigated the importance of quantizing the initial thermal bath distributions in calculating the time-dependent heats and heat currents across a wide range of bath parameter regimes, viz., temperatures, temperature gaps, reorganization energies, and cutoff frequencies. Our results show that DECIDE performs quite well and captures the expected trends in the steady-state heat current. In addition, our findings underscore the importance of performing quantum sampling of the bath coordinates across a wide range of bath parameter regimes. For direction (ii), using adiabatic mixed quantum-classical dynamics, we studied the influence of lattice vibrations on the transfer of vibrational excitation energy in a modified Su-Schrieffer-Heeger (SSH) dimer chain model. We found that the incorporation of lattice vibrations greatly enhances the rate of long-range population transfer compared to the static chain. Finally, we investigated the effects of coupling the end sites of the SSH chain to thermal baths at different temperatures on the vibrational excitation energy transfer. Regardless of the temperature gap, we found that the long-range population transfer is enhanced in the static chain compared to the chain without the baths. However, in the case of the non-static chain, the presence of the temperature baths does not significantly alter the transfer, pointing to the robustness of the process. Together, these studies shed light on the microscopic mechanisms of quantum energy transport in molecular junctions and polymer chains, how to control the rates/mechanisms of the energy transport, and the validity of using mixed quantum-classical dynamics for simulating quantum energy transport. Ultimately, our methodology and findings may guide the experimental design of novel nanophononic devices.

• Subjects / Keywords

  • energy transfer
  • mixed quantum-classical dynamics
  • molecular junctions

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-a7fe-5p65

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.