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 [1]. Recently, Sr₂IrO₄ has attracted a great deal of attention as a new type of spin-orbit assisted correlated insulator [2]. In this material, the strong spin-orbit coupling (SOC) causes the t_2g bands to rearrange into a filled j_eff = 3/2 and a half-filled j_eff = 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 [3]. 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 [4]. 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 [9]. 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).

 

 

[1] M. Imada, A. Fujimori, and Y. Tokura, Rev. Mod. Phys. 70, 1039 (1998).

[2] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008).

[3] B. Zwartsenberg et al., arXiv:1903.00484 (2019).

[4] N. Kaushal et al., Phys. Rev. B 96, 155111 (2017).

[5] H. Gretarsson et al., Phys. Rev. Lett. 116, 13640 (2016)

[6] H. Gretarsson et al., Phys. Rev. B 96, 115138 (2017)

[7] H. Gretarsson et al., Phys. Rev. Lett. 117, 107001 (2016)

[8]  J. Porras et al., Phys. Rev. B 99, 085125 (2019)

[9] H. Suzuki et al., Nature Materials doi: 10.1038/s41563-019-0327-2 (2019)