“Leveraging the Light-Matter Interaction in Angle-Resolved Photoemission Spectroscopy”

Departmental Oral Examination

Abstract:
The light-matter interaction is central to the photoemission process, with an ultraviolet photon providing the necessary impulse required to eject those electrons which we collect in an effort to understand the electronic structure of matter. As such, the selection rules associated with this interaction impose strict constraints on those electronic orbits to which the technique is sensitive. Photoemission-based techniques then present an opportunity to access information beyond spectroscopic characterization of a material’s level structure; an orbital description of the underlying wavefunctions is viable under consideration of the photoemission mechanism. We present here a numerical scheme within which such information about the photoemission experiment can be garnered, with specific application to several experiments on candidate materials. In particular, this methodology allows for new insights in the problem of the Fe-based superconductors. These materials are an ideal platform to apply this methodology. The low energy electronic structure is characterized by a large number of closely spaced, moderately correlated states. Their phenomenology is dictated by a delicate balance of several interactions with similar energy scales, the competition and cooperation amongst which pose a considerable challenge for both theory and experiment. The kinetic Hamiltonian, in addition to interactions involving Coulomb and Hund’s coupling, as well as nematic order and spin-orbit coupling are all closely related to the orbital structure of the electronic states. The unique sensitivity to both spin and orbital degrees of freedom which photoemission provides therefore allow for a comprehensive exploration of such energy scales in these compounds. Taking advantage of this senstivity, we have mapped the momentum and enegy dependence of spin-orbit entanglement in candidate materials FeSe and LiFeAs.

Despite the remarkable surface sensitivity which limits access to the crystal bulk in photoemission, there is a strong inclination to assert a correspondence between the bulk electronic structure, and that measured experimentally. Such
a connection is by no means guaranteed, and is frequently the cause of misinterpretation. We explore the surface issue in detail, and discover an interference mechanism which provides justification for the unanticipated success of valenceband photoemission in quasi two-dimensional materials. The surface issue is of specific relevance to the Fe-superconductors, where certain orbitals exhibit significant dispersion perpendicular to the surface. We examine the canonical Fesuperconductor LiFeAs, wherein a confluence of three-dimensional dispersion, spin-orbit coupling, and surface states have conspired to preclude identification of the low-energy electronic structure. We combine detailed photon-energy dependent measurements with results from a slab-projected model to unambiguously identify the three-dimensional Fermi surface of this material.