Experimentally Mapping the Phase Diagrams of Photoexcited Small Polarons

Abstract

Understanding the fundamental properties that dictate photoexcited polarons in materials is critical to tuning their properties. Theoretical models of polarons have only recently been extended to the excited state. Experimental measurements of polaron formation and transport have been widely undertaken across a range of materials, from photocatalysts and superconductors to soft conducting polymers. Here, we map thermalized excited state experimental measurements of quantities such as polaron strength onto phase diagrams of the Holstein, Hubbard-Holstein, and t-J-Holstein models. This work demonstrates that tuning electron-phonon coupling strength, electron localization, and spin exchange can be leveraged to suppress or control polarons in transition metal oxides. We find that the t-J-Holstein model best describes the measured iron oxides and could be generally applied to a wide range of systems that exhibit polaron formation in the excited state. This work combines experimental data with ground state models to provide a qualitative parameter space for informing photoexcited polaron design, under which excited state polaronic behavior can be classified within ground-state calculable models.