In this thesis I explore the dynamical behavior of electrons and excitons interacting with quantized nuclear vibrations. In the first chapter I begin by introducing the notion of vibronic models and discussing their utility for modeling dynamical phenomena in the condensed phase. In the second chapter, I continue to detail a collaborative effort for modeling photophysics and transport dynamics in aggregates of the organic dye molecule perylene diimide (PDI). There I discuss how the vibronic signatures in steady-state photoluminescence spectra may be used to decode the microscopic couplings that determine the hybrid H and J aggregate behavior in PDI crystals. I then show how interference between these couplings has a substantial effect on controlling ballistic and diffusive transport dynamics. In the third chapter I continue to address the challenge of describing finite temperature dynamics in the Holstein model in the thermodynamic limit. Toward this end, I present approximate solutions via the cumulant expansion and discuss in detail the successes and limitations of this method. Finally, in the interest of providing fully quantum mechanical solutions for vibronic models in the nonperturbative intermediate coupling regime, in the fourth chapter I discuss the application of the numerically exact reduced hierarchical equations of motion (HEOM) method. I expose how for models such as the Holstein model that incorporate a finite bath of undamped harmonic oscillators, temperature-dependent instabilities arise in HEOM which corrupt the long-time dynamics. Through a projection-based approach, I demonstrate how these instabilities may be removed, obviating the need for a costly and poorly-behaved convergence procedure with respect to the hierarchy depth. I also present a numerical iterative approach for accomplishing this projection, intended for use in cases where a diagonalization-based projection proves too costly. Overall, this thesis delves into applications as well as approximate and numerically exact solutions of vibronic models.

We numerically investigate a total transmission probability through QDs embedded in an AB ring. The QDs are formed by delta function-like double potential barriers and a magnetic flux is penetrated in the center of the ring. In particular, we study the coupled double-QDs in series and non-coupled double-QDs in parallel in an AB ring. In each model, we show the total transmission probability as a function of QD size and electron incident energy, and present the transmission amplitude on the complex-energy plane. Of interest is the change and progression of Fano resonances and corresponding zero-pole pairs on the complex-energy plane with magnetic flux in the center of the ring.To accomplish this, we analytically solve the scattering matrix at each junction and the transfer matrix through the arms of the ring using Schrodinger equation for the delta function barriers. Then, the total transmission probability is obtained as a function of electron energy and magnetic flux by cascading these matrices. Finally, the solutions of the analytical equations and the graphical output of the transmission characteristics in the system will be obtained numerically by using Mathematica programs run on desktop computers. / Department of Physics and Astronomy