Exotic electronic states produced by strong spin-orbit coupling in complex Ir oxides

Date

Sep 28, 2015

Time

4:30 PM - 5:30 PM

Speaker

Hidenori Takagi

Affiliation

MPI für Festkörperforschung Stuttgart

Series

MPI-PKS Kolloquium

Language

en

Main Topic

Physik

Other Topics

Physik

Host

KORREL15 Workshop

Description

In 5d Iridium oxides, a large spin-orbit coupling of ~0.5 eV, inherent to heavy 5d elements, is not small as compared with other relevant electronic parameters, including Coulomb U, transfer t and crystal field splitting D, which gives rise to a variety of exotic magnetic ground states. In the layered perovskite $Sr_{2}IrO_{4}$, spin-orbital Mott state with Jeff=1/2 is realized due to the novel interplay of those energy scales [1-3]. Despite the strong entanglement of spin and orbital degrees of freedom, $J_{eff}$=1/2 iso-spins in $Sr_{2}IrO_{4}$ was found to be surprisingly isotropic, very likely due to a super-exchange coupling through almost 180° Ir-O-Ir bonds [4]. The temperature dependence of in-plane magnetic correlation length of $J_{eff}$=1/2 iso-spins, obtained from inelastic x-ray resonant magnetic scattering, was indeed well described by that expected for two-dimensional S=1/2 Heisenberg antiferromagnet [5]. The three-dimensional analog of $Sr_{2}IrO_{4}$, $SrIrO_{3}$ perovskite is very close a band insulator due to lattice distortion but a Dirac semimetal protected by crystalline symmetry [6]. Upon increasing effective Coulomb U, magnetism emerges and creates a gap at Dirac nodes, giving rise to a semimetal to magnetic insulator transition. This can be realized by controlling the dimensionality and hence the effective U in $(SrIrO_{3})_{m}/SrTiO_{3}$ (m: number of $SrIrO_{3}$ layer) super-lattice structure [7]. With reducing m, a transition to an insulator, accompanied with magnetism was clearly observed. At m=1, single layer, the transport remains insulating even above the magnetic ordering temperature, indicative of the increased Mott character. When Jeff=1/2 iso-spins interact with each other through 90° Ir-O-Ir bonds, very anisotropic bond dependent ferromagnetic coupling is expected, unique to strong SOC system. Complex Ir oxides with honeycomb and more recently identified hyper-honeycomb lattices [8], where x-, y- and z- 90° Ir-O-Ir bonds are realized, may be candidates for quantum spin liquid expected for the Kiatev model. Very likely due to the superposition of additional magnetic couplings not included in the Kitaev model [9], in reality, a long range magnetic ordering emerges at low temperatures in those compounds. Hyper-honeycomb $\beta-Li_{2}IrO_{3}$, though eventually show a marginal ordering, appears to be located at the critical vicinity to the Kitaev spin liquid. 1) B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008). 2) B. J. Kim et al., Science 323, 1329 (2009). 3) S. Fujiyama et al., Phys. Rev. Lett. 112, 016405 (2014). 4) G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009). 5) S. Fujiyama et al., Phys. Rev. Lett. 108, 247212 (2012). 6) Chen, Y. et al. ,Nat. Commun. 6:6593 doi: 10.1038/ncomms7593 (2015). 7) J. Matsuno et al., Phys. Rev. Lett. (2015). 8) T.Takayama, et al., s Phys. Rev. Lett.114, 077202 (2015). 9) A.Kitaev, Annals of Physics 312 2 (2006).

Last modified: Sep 28, 2015, 9:50:03 AM