Philipp Schütz (PhD thesis): Dimensionality-Driven Metal-Insulator Transition in Spin-Orbit-Coupled SrIrO3

Abstract

In the past decades correlated-electron physics due to strong Coulomb interactions and topological physics caused by band inversion often induced by strong spin-orbit coupling have been the workhorses of solid state research. While commonly considered as disparate phenomena, it was realized in the early 2010s that the interplay between the comparably strong Coulomb and spin-orbit interactions in the 5d transition metal oxides may result in hitherto unforeseen properties.
The layered perovskite Sr₂IrO₄ has attracted special attention due to the observation of an unconventional Mott-insulating phase and predictions of exotic superconductivity.
Less is known about its three-dimensional counterpart SrIrO₃, since rather than the cubic perovskite structure it adopts the thermodynamically stable hexagonal polymorph thereof. This thesis therefore sets out to establish the synthesis of epitaxially stabilized perovskite SrIrO₃ by pulsed laser deposition and to investigate its electronic and magnetic structure by state-of-the-art x-ray spectroscopy techniques.
In this endeavor the appropriate thermodynamic conditions for the growth of high-quality SrIrO₃ are identified with a focus on the prevention of cation off-stoichiometry and the sustainment of layer-by-layer growth. In the thus-optimized  films the cubic perovskite symmetry is broken by a tetragonal distortion due to epitaxial strain and additional cooperative rotations of the IrO₆ octahedra. As a consequence of the thermodynamic instability of the IrO₂ surface layer, the films unexpectedly undergo a conversion to a SrO termination during growth.
In an attempt to disentangle the interplay between spin-orbit and Coulomb interaction the three-dimensional electronic structure of perovskite SrIrO₃ is investigated in a combined experimental and theoretical approach using soft x-ray angle-resolved photoelectron spectroscopy and ab initio density functional theory calculations. The experimentally found metallic ground state hosts coherent quasiparticle peaks with a well-defined Fermi surface and is theoretically described by a single half-filled band with effective total angular momentum J_eff = 1/2 only upon incorporation of a sizeable local Coulomb repulsion and - to a lesser extent - the broken cubic crystal symmetry in the  film.
Upon reduction of the SrIrO₃ thickness below a threshold of four unit cells the scales are tipped in favor of a Mott-insulating phase as the on-site Coulomb repulsion surmounts the diminishing kinetic energy upon transition into the two-dimensional regime. Concomitantly, a structural transition occurs because the corner-shared octahedral network between substrate and film imposes constraints upon the IrO₆ octahedral rotations in the thin-film limit. The striking similarity between the quasi-two-dimensional spin-orbit-induced Mott insulator Sr₂IrO₄ and SrO-terminated SrIrO₃ in the monolayer limit underlines the importance of dimensionality for the metal-insulator transition and possibly opens a new avenue towards the realization of exotic superconductivity in iridate compounds.
Whether the analogy between SrIrO₃ in the two-dimensional limit and its Ruddlesden-Popper bulk counterparts extends to their complex magnetic properties ultimately remains an open question, although no indications for a remanent (anti)ferromagnetic order were found. The unprecedented observation of an x-ray magnetic circular dichroism at the O K absorption edge of iridium oxides in an external magnetic  field promises deeper insights into the intricate connection between the J_eff = 1/2 pseudospin state, its hybridization with the oxygen ligand states and the magnetic order found in the Ruddlesden-Popper iridates.

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