Experimental Study Of The Electronic-Phonon Coupling In Low-Dimensional Materials

Abstract

In this thesis, we investigate the effects of electron-phonon coupling as manifested in exciton-polarons and Cooper pairs. The electron-phonon coupling imparts a level of protection on the electronic states in both cases, though in qualitatively different ways. This result is surprising considering the traditional scattering role of the electron-phonon interaction.We start with a theoretical description of excitons, polarons, and exciton-phonon coupling, all of which play a primary role in the subsequently introduced and focused on hybrid organic-inorganic perovskite materials class. Here we distinguish between bulk 3D and layered 2D hybrid perovskites, and highlight how polarons in these systems have been hypothesized to protect charge carriers from scattering. Next, we discuss time-resolved absorption and photoluminescence techniques that we employed to explore the exciton dynamics in 2D hybrid perovskites. These materials presented unique challenges for conventional time-resolved absorption methods, and we detail the development of a novel experimental scheme that permitted high-quality data collection and helped provide new insights into the exciton-phonon behavior. We follow up with an account of the excitonic fine structure in 2D hybrid perovskites, chronicling recent work suggesting a vibronic sideband origin, and then illustrating how the dynamical perspective provided by our measurements give strong evidence for the vibronic hypothesis. We close our discussion on 2D hybrid perovskites by presenting evidence for the formation dynamics of large exciton-polarons. Finally, we turn our attention to measurements of the superconducting proximity effect in low dimensional heterostructures of superconductors and antiferromagnets. Superconductivity is an example of the electron-phonon coupling mediating an electron-electron attraction, which leads to a scattering-protected Cooper pair. Here, we present results suggesting a singlet to triplet transformation in the Cooper pair, indicating the possibility for the protection of spin polarization via Cooper pairs. The appendix describes one of several experiments constructed in support of the Property Measurement Facility and the NSF funded MRSEC center at Penn. The work in this thesis has been supported by the Penn NSF MRSEC DMR-1120901 and DMR-1720530