Collective excitations in Na$_2$IrO$_3$
| Autor: | Igarashi, Jun-ichi, Nagao, Tatsuya |
|---|---|
| Rok vydání: | 2015 |
| Předmět: | |
| Zdroj: | J. Phys.:Condense. Matter, 28 (2016) 026006 |
| Druh dokumentu: | Working Paper |
| DOI: | 10.1088/0953-8984/28/2/026006 |
| Popis: | We study the collective excitations of Na$_2$IrO$_3$ in an itinerant electron approach. We consider a multi-orbital tight-binding model with the electron transfer between the Ir $5d$ states mediated via oxygen $2p$ states and the direct $d$-$d$ transfer on a honeycomb lattice. The one-electron energy as well as the ground state energy are investigated within the Hartree-Fock approximation. When the direct $d$-$d$ transfer is weak, we obtain nearly flat energy bands due to the formation of quasimolecular orbitals, and the ground state exhibits the zigzag spin order. The evaluation of the density-density correlation function within the random phase approximation shows that the collective excitations emerge as bound states. For an appropriate value of the direct $d$-$d$ transfer, some of them are concentrated in the energy region $\omega <$ 50 meV (magnetic excitations) while the others lie in the energy region $\omega >$ 350 meV (excitonic excitations). This behaviour is consistent with the resonant inelastic x-ray scattering spectra. We also show that the larger values of the direct $d$-$d$ transfer are unfavourable in order to explain the observed aspects of Na$_2$IrO$_3$ such as the ordering pattern of the ground state and the excitation spectrum. These findings may indicate that the direct $d$-$d$ transfer is suppressed by the structural distortions in the view of excitation spectroscopy, as having been pointed out in the \it{ab initio} calculation. Comment: 8 pages, 6 figures, Latex, Figures 5 and 6 have been replaced by those obtained for more appropriate parameter values. The concomitant descriptions have been also modified. To be published in Journal of Physics: Condensed Matter |
| Databáze: | arXiv |
| Externí odkaz: |
|