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Leveraging the light-matter interaction in angle-resolved photoemission spectroscopy

University of British Columbia

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, selection rules impose constraints on those electronic orbits to which one 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 also viable. We present here a numerical scheme within which such information can be garnered, with specific application to several experiments on candidate materials. The Fe-based superconductors are an ideal platform for application of this methodology. The electronic structure is characterized by a large number of closely spaced, moderately correlated states. The competition and cooperation between several competing energy scales pose a considerable challenge for both theory and experiment. The unique sensitivity to both spin and orbital degrees of freedom which photoemission provides therefore allow for a comprehensive exploration of the various energy scales in these compounds. Taking advantage of this sensitivity, we have mapped the momentum and energy dependence of spin-orbit entanglement in representative compounds, FeSe and LiFeAs. Despite the surface sensitivity which inhibits 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, a contentious claim which 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 valence-band 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 Fe-superconductor 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.

Text

2020-08-12

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/75505

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/