In correlated insulators, where Coulomb interactions (U) drive the localization of charge carriers, the metal-insulator transition (MIT) can be described as either bandwidth (BC) or filling (FC) controlled . Recently, Sr2IrO4 has attracted a great deal of attention as a new type of spin-orbit assisted correlated insulator . In this material, the strong spin-orbit coupling (SOC) causes the t2g bands to rearrange into a filled jeff = 3/2 and a half-filled jeff = 1/2 manifold. The latter, with its reduced width, is susceptible to a moderate U=2 eV leading to the opening a correlation gap. Naturally, the question arises whether in this framework the MIT can be driven by SOC. Recent experiments in the group of A. Damascelli (QMI-UBC) have shown that an MIT can indeed be controlled by tuning spin-orbit coupling . By substituting Ir with Rh and (to a lesser extent) Ru, the effective SOC in the valence band is diluted, inducing metallic behavior for both substituted compounds.
The introduction of spin-orbit coupling as a third axis, next to bandwidth and filling, opens the possibility to investigate a rich collection of other properties in this new section of the phase diagram of correlated insulators . The comparable magnitudes of spin-orbit coupling, Coulomb repulsion, and bandwidth in a multi-orbital system make this a rich and fascinating playground for many-body physics. Building on the observation that the MIT can be driven by SOC, this project aims to address the evolution of charge and spin excitations – upon deliberate dilution of SOC – by the combined utilization of angle-resolved photoemission spectroscopy (ARPES), Raman scattering, and resonant inelastic x-ray scattering (RIXS). The results will be compared to those from other correlated systems, such as the high-Tc cuprates, to investigate the novel properties imbued by spin-orbit coupling.
The investigation will focus on resonant elastic- and inelastic x-ray scattering, at the Ir, Ru, and Rh edges. B. Keimer’s group at the MPI-Stuttgart has extensively investigated pure and electron-doped iridates by Raman scattering and Ir L-edge RIXS [5-8], and has recently completed a unique beamline for RIXS experiments at the L-edges of 4d transition metals including Ru and Rh . Complementary ARPES and spin-resolved ARPES studies of the electronic structure of the same compounds will be performed in the group of A. Damascelli (QMI-UBC), with theoretical support by I. Elfimov (QMI-UBC). High quality samples will be grown in the group of H. Takagi (MPI-Stuttgart).
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