Session C1: Invited Session: Spin-Orbit-Controlled Ground States in Single-Crystal Iridates

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).   

Twisted Hubbard Model for Sr$_2$IrO$_4$: Magnetism and Possible High Temperature Superconductivity

Sr$_2$IrO$_4$ has been suggested as a Mott insulator from a single $J_{eff}=1/2$ band, similar to the cuprates. However this picture is complicated by the measured large magnetic anisotropy and ferromagnetism. Based on a careful mapping to the $J_{eff}=1/2$ (pseudospin-1/2) space, we propose that the low energy electronic structure of Sr$_2$IrO$_4$ can indeed be described by a SU(2) invariant pseudospin-1/2 Hubbard model very similar to that of the cuprates, but with a ``twisted'' coupling to external magnetic field (a g-tensor with a staggered antisymmetric component). This perspective naturally explains the magnetic properties of Sr$_2$IrO$_4$. We also derive several simple facts based on this mapping and the known results about the Hubbard model and the cuprates, which may be tested in future experiments on Sr$_2$IrO$_4$. In particular we propose that (electron-)doping Sr$_2$IrO$_4$ can potentially realize high-temperature superconductivity.   

Exotic Physics from Doping a Strongly Spin-Orbit Coupled Mott Insulator

Doping a Mott insulator, as in the case of high T$_{c}$ cuprates, has given rise to many exotic physics in the doping diagram, such as the pseudogap, Fermi arc and vortex phase. An important topic in these strongly correlated systems is to distinguish the properties that are intrinsic to the Mott physics from those that are materials specific. Recent studies of Sr$_{2}$IrO$_{4}$, whose Mottness requires strong spin orbit coupling, provide a new venue to look into the topic, where the spin, orbital, charge and lattice degrees of freedom interact. Using ARPES we studied the evolution of the electronic structure of Sr$_{2}$IrO$_{4}$ with both Rh and La doping. We show that the Rh substitution acts as immobile effective local holes, without a strong renormalization of the overall band structure, while La acts as an electron dopant. Particularly interesting is the lightly hole-doped regime, which showcases some of the same exotic physics as seen in the cuprates, including pseudogaps and Fermi arcs. By observing the scattering rate evolution as a function of energy and temperature, we confirm the non-Fermi liquid nature of the Fermi arc.   

Magnetic and crystal structures of the honeycomb lattice Na$_{2}$IrO$_{3}$ and single layer Sr$_{2}$IrO$_{4}$

5$d$ based iridates have recently attracted great attention due to the large spin-orbit coupling (SOC). It is now recognized that the SOC that competes with other relevant energies, particularly the on-site Coulomb interaction U, and have driven novel electronic and magnetic phases [1-3]. Combining single crystal neutron and x-ray diffractions, we have investigated the magnetic and crystal structures of the honeycomb lattice Na$_{2}$IrO$_{3}$ [4]. The system orders magnetically below 18.1 K with Ir$^{4+}$ ions forming zigzag spin chains within the layered honeycomb network with ordered moment of 0.22 $\mu$B /Ir site. Such a configuration sharply contrasts the Neel or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form nearly ideal 2D honeycomb lattice while the IrO$_{6}$ octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much-needed experimental insights into the magnetic and crystal structure crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5$d$-electron based honeycomb lattice. Neutron diffraction experiments are also performed to investigate the magnetic and crystal structure of the single layer iridate Sr$_{2}$IrO$_{4}$, where new structural information and spin order are obtained that is not available from previous neutron powder diffraction measurement.\\[4pt] [1] B. J. Kim \textit{et al.}, Phys. Rev. Lett. 101, 076402 (2008).\\[0pt] [2] B. J. Kim \textit{et al.}, Science 323, 1329 (2009).\\[0pt] [3] A. Shitade \textit{et al.}, Phys. Rev. Lett. 102, 256403 (2009).\\[0pt] [4] F. Ye, \textit{et al.}, Phys. Rev. B 85, 180403(R) (2012)