Ultrafast Dynamics of Photo-Doped Mott Antiferromagnets

Citation

Mehio, Omar (2023) Ultrafast Dynamics of Photo-Doped Mott Antiferromagnets. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fsbz-pd46. https://resolver.caltech.edu/CaltechTHESIS:05202023-131846404

Abstract

Strong coupling between spin and charge degrees of freedom in two-dimensional spin-1/2 Mott antiferromagnets (AFMs) creates a rich platform to study quantum many-body physics. For decades, the consequences of these interactions have been intensely studied in thermal equilibrium, where the introduction of charge carriers through chemical doping has been shown to generate a vibrant phase diagram rich with unconventional types of charge, spin, and orbital ordering. In recent years, however, attention has grown to include the study of these materials as they are driven far from equilibrium using intense pulses of light produced by femtosecond laser sources. In addition to fundamental interest in the resultant dynamics, recent experimental and theoretical studies have suggested that driven Mott insulators can host states of matter that cannot be accessed in thermal equilibrium.

While many driving protocols have been developed---spanning from the selective excitation of bosonic modes to photon-dressing via coherent time-periodic driving---the simplest conceptual approach to engineering Mott insulators with light is known as photo-doping. In this procedure, the material is impulsively driven resonantly with a transition from a filled band to an empty band, transiently producing charge carriers. Given the impact of chemical doping in thermal equilibrium, photo-doping has garnered interest as an important tool in the study of driven Mott insulators. Early successes in the study of photo-doped Mott AFMs include the observation of ultrafast demagnetization and the prediction of non-thermal magnetic states, charge density waves, and superconductivity. Photo-doping thus holds promise to generate an out-of-equilibrium phase diagram that is equally rich to that found in equilibrium.

Yet, many open questions about the basic properties of photo-doped Mott insulators remain unresolved. Whether charge instabilities exist as a result of interactions between the photo-dopants has yet to be examined. Moreover, while theoretical studies have suggested that antiferromagnetic correlations can enhance attractive interactions between photo-dopants, evidence of the resultant bound states remain elusive. Even the light-matter interactions that generate the photo-dopants are in need of investigation, as the fate of a Mott insulator driven by strong electric fields remains a fundamental open theoretical and experimental problem.

In this thesis, I present a series of experiments designed to answer each of these questions. After describing the properties of Mott insulators in Chapter 1, I present the experimental details of the tools that enable these studies in Chapter 2. Taking a multi-messenger approach to ultrafast spectroscopy, a suite of ultrafast probes simultaneously track the spin and charge degrees of freedom to paint a holistic picture of the out-of-equilibrium state. In Chapter 3, I use ultrafast THz conductivity to establish the existence of an insulating photo-excited fluid of Hubbard excitons (HEs), which are bound states that are thought to form as a result of attractive spin-mediated interactions. This magnetic binding mechanism is studied in more detail in Chapter 4 by examining the properties of these HEs in the magnetic critical region of several materials that lie in different magnetic universality classes. In Chapter 5, I study the effects of HE formation on the ultrafast demagnetization that is known to occur following photo-doping. Finally, I turn my attention towards the photo-dopant generation mechanism in Chapter 6, exploring the effects of strong electric field driving in Mott insulators. I find signatures of the so-called Keldysh crossover from a multiphoton-absorption- to a quantum-tunneling-dominated pair production regime. Altogether, this work establishes photo-doped Mott insulators as a rich playground to engineer non-equilibrium phases of matter and study quantum many-body dynamics.

Thesis Files