Tuning the Spin-Orbit Coupled Ground State of Iridates with Pressure

The electronic ground state of the novel magnetic insulators BaIrO$_{3}$ [1] and Sr$_{2}$IrO$_{4}$ [2] is probed at ambient and high-pressure conditions using x-ray absorption and magnetic circular dichroism measurements. A spin-only description of the magnetic ground state is ruled out, spin-orbit entanglement in 5$d $states resulting in comparable orbital ($L_{z})$ and spin ($S_{z})$ contributions to the localized magnetic moments despite the presence of strong crystal fields and band effects in Ir \textit{5d} states. Pressures of $\sim$ 5 GPa and 20 GPa quench the ``weak'' ferromagnetic ordering in BaIrO$_3$ and Sr$_{2}$IrO$_{4}$, respectively, despite robust local moments and insulating behavior remaining at these pressures, confirming the Mott character of the insulating gap. The expectation value of the angular part of the S-O interaction, \textless \textbf{L}\textbullet \textbf{S}\textgreater , extrapolates to zero at 80--90 GPa in Sr$_{2}$IrO$_{4}$ where an increased bandwidth strongly mixes J$_{\mathrm{eff}}=$1/2, 3/2 states and S-O interactions no longer dominate the electronic ground state. The likely appearance of a single, metallic band at a pressure of $\sim$ 1 Mbar (100 GPa) provides an exciting backdrop for searches of superconductivity at high pressures [3]. Work at Argonne is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC-02-06CH11357. \\[4pt] [1] M. A. Laguna Marco \textit{et al}., \textit{Phys. Rev. Lett.} \textbf{105}, 216407 (2010).\\[0pt] [2] D. Haskel \textit{et al., Phys. Rev. Lett.} \textbf{109}, 027204 (2012).\\[0pt] [3] F. Wang and T. Senthil, \textit{Phys. Rev. Lett}. \textbf{106}, 136402 (2011).