Interplay between Hund’s Coupling and Spin–Orbit Interaction on Elementary Excitations in Sr₂IrO₄

Abstract

We study the elementary excitations in 5d transition metal oxide Sr2IrO4 by calculating the particle-hole Green's function within the random phase approximation on an antiferromagnetic ground state in the two-dimensional multi-orbital Hubbard model. The obtained magnetic excitations of bound states show a characteristic dispersion in consistent with the experiments. In addition, two new types of excitations are found due to the interplay between spin-orbit interaction and Hund's coupling: a magnetic excitation as a bound state, which has energy gap at the Γ point, and an exciton as a resonant mode in the continuum of electron-hole pair creation.Electron correlation effects on transition metals and their compounds have attracted much interest since the discovery of high-T C cuprate superconductors. In the 3d transition metal ions, the spin orbit interaction (SOI) plays a minor role on the electronic structures, since it is usually much smaller than the on-site Coulomb interaction and the crystal field splitting. In the 5d transition metal ions, however, the SOI is one order of magnitude larger than that of the 3d systems while the Coulomb interaction becomes weaker due to the extended nature of the 5d electrons. Accordingly, the interplay between the electron correlation and the SOI is expected to bring about new intriguing phenomena. For this reason, much attention has recently been paid to Ir oxides such as Sr 2 IrO 4 1-8 and Na 2 IrO 3 . [9][10][11][12] In particular, we focus on Sr 2 IrO 4 , which consists of two-dimensional IrO 2 layers showing structural similarity to the parent compound of high-T C cuprate La 2 CuO 4 , and exhibits a canted antiferromagnetic (AFM) order below 230 K. 1-3 The energy of the e g orbitals is estimated about 2 eV higher than that of the t 2g orbitals due to the large crystal field. Five 5d electrons are occupied per Ir atom, and one hole is sitting in the t 2g orbitals. Since the hole states have an effective orbital angular momentum ℓ equal to −1, the lowest-energy states on the localized electron picture are doubly degenerate with the effective total angular momentum j eff = 1 2 under the SOI: 13,14 |φ ± = 1 √ 3 (|yz, ∓σ ± i|zx, ∓σ ± |xy, ±σ ), where yz, zx, and xy designate t 2g orbitals, and spin component σ =↑. By introducing the isospin operators acting on these states, the system is mapped onto the Heisenberg model, from which an insulating AFM phase is derived, consistent with the experiments. 1-3 Furthermore, it has been pointed out that the small anisotropic terms emerge in addition to the isotropic term, when Hund's coupling is taken into account in the second-order process in the strong coupling expansion. 15,16 Recently, resonant inelastic x-ray scattering (RIXS) experiment at the Ir L edge has detected the excitation spectra, 17 whose low energy part follows the dispersion relation similar to the spin wave in the Heisenberg model on a square lattice. A notable point is that the excitation energy at the M point is nearly half of that at the X point, in contrast to t...

Analysis of resonant inelastic x-ray scattering fromSr2IrO4in an itinerant-electron approach

“…Figure 3 shows the dispersion relation of magnon thus determined, and the corresponding RIXS intensities at the L 3 edge. It is found that the magnon is split into two modes with slightly different energies, as already reported [32]. Although such split modes are not confirmed, their dispersion relation is in qualitative agreement with that derived from the RIXS experiment.…”

Theoretical studies of the osmium based perovskites AOsO3 (A=Ca, Sr and Ba)

“…The appreciable decrease occurs in susceptibilities of the these compounds as we go from temperature range of 0 K to 100 K, while this change goes further from 100 K to 250 K but is not as much as in the temperature range of 0 K to 100 K. The saturation in magnetic susceptibility will probably be observed beyond a temperature of 280 K. As far as the magnitude of the magnetic susceptibilities of the three compounds are concerned, χ for CaOsO 3 is 9.88 (10 -3 emu mol -1 ) at 25.47 K, for SrOsO 3 is 7.75 (10 -3 emu mol -1 ) at 24.62 K and for BaOsO 3 is 2.71(10 -3 emu mol -1 ) at 23.77 K. Our magnetic susceptibility curves are also consistent with the experiments 13 in which these materials show no significant magnetization at very low temperature. The inverse plot of the magnetic susceptibility shows that Weiss constant () is positive for CaOsO 3 and SrOsO 3 at about  = 25 K for both compounds, justifying Curie Weiss law of ferromagnetism. Furthermore, in the plot of BaOsO 3 , 1/χ versus T passes through the origin ( = 0 K), which confirms the paramagnetic nature of the compound.…”

“…The magnetic properties of CaOsO 3 , SrOsO 3 Table 1. It is observed that the energy of the ferromagnetic phase of CaOsO 3 and SrOsO 3 is lower than their paramagnetic energies, while in BaOsO 3 the paramagnetic phase has lower energy than the ferromagnetic phase.…”

“…Transition metals (TM) with 5d state compounds have recently attracted the focus of materials scientist due to their interesting behaviors of spin-orbit coupling (SOC) and electrons correlation effects, which strongly influences the electronic and magnetic properties of these compounds [1][2][3][4][5][6]. Magnetism in the 3d TM compounds is mainly originated from the spin moments of the delectrons rather than the orbitals moment which is quenched by the crystal fields [7].…”

“…The application of high pressure experimental techniques enabled researchers to synthesize and explore new perovskites, whose synthesis was not possible under ambient pressure [11] like CaRuO 3 , SrRuO 3 and BaRuO 3 . Magnetism in this series of compounds is a function of the ionic size of the Ca, Sr and Ba site cations [12].…”