Electronic ground state properties of Iridate oxides from x-ray absorption spectroscopy*

Element (Ir)- and orbital (5d)-specific L$_{2,3}$ edge x-ray absorption and magnetic circular dichroism measurements are used to probe the nature of the electronic ground state in magnetic insulators BaIrO$_3$ [1] and Sr$_2$IrO$_4$ [2,3]. A spin-only description of the magnetic ground state is directly ruled out by the measurements. Instead, the measurements show spin-orbit entanglement in 5$d$ states resulting in a larger orbital ($L_z$) than spin ($S_z$) contribution to the magnetic moment, even in the presence of strong crystal field and band effects. Measured x-ray absorption cross sections at spin-orbit split L$_{2,3}$ edges impose constraints on the nature of the ground state [3]. Experiments under chemical- (doping) and applied-pressure conditions provide evidence for a delicate interplay between electronic bandwidth and Coulomb interactions leading to the gapped, spin-orbit coupled ground state of these complex oxides. \\[4pt] [1] M. A. Laguna Marco et al., Phys. Rev. Lett. 105, 216407 (2010).\\[0pt] [2] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008).\\[0pt] [3] L. C. Chapon and S. W. Lovesey, J. Phys. Condens. Matter 23, 252201 (2011).