arXiv:1805.03612v1 [cond-mat.str-el] 9 May 2018 Probing electronic excitations in iridates with resonant inelastic x-ray scattering and emission spectroscopy techniques Young-June Kim, * J. P. Clancy, and H. Gretarsson Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada G. Cao Department of Physics, University of Colorado Boulder, Boulder, Colorado 80309, USA Yogesh Singh Indian Institute of Science Education and Research Mohali, Sector 81, SAS Nagar, Manauli PO 140306, India Jungho Kim, M. H. Upton, D. Casa, and T. Gog Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA (Dated: May 10, 2018) We report a comprehensive resonant inelastic x-ray scattering (RIXS) study of various iridate materials focusing on core-level excitations and transitions between crystal-field split d-levels. The 2p core hole created at the Ir L3 absorption edge has a very short lifetime giving rise to a broad absorption width (∼ 5 eV). This absorption linewidth broadening can be overcome by studying the resonant x-ray emission spectroscopy (RXES) map, which is a two-dimensional intensity map of the Ir Lα2 emission obtained with high energy-resolution monochromator and analyzer. By limiting the emitted photon energy to a narrow range, one can obtain x-ray absorption spectra in the high energy-resolution fluorescence detection (HERFD) mode, while one can also simulate quasi-M4- edge absorption spectra by integrating over incident photon energies. Both methods improve the absorption line width significantly, allowing detailed studies of unoccupied electronic structure in iridates and other 5d transition metal compounds. On the other hand, the short lifetime of the 2p core hole benefits the study of excitations of valence electrons. We show that the incident energy dependence of the RIXS spectra for d − d transitions is simple to understand due to the short core- hole lifetime, which validates ultra-short core-hole lifetime approximation used widely in theoretical calculations. We compared d − d excitations in various iridates and found that the excitations between the t2g and eg states share many similarities among different materials. However, the RIXS spectra due to the transitions between the spin-orbit-split t2g levels vary widely depending on the oxidation state and electronic bandwidths. I. INTRODUCTION Resonant inelastic x-ray scattering (RIXS) has made significant contributions to understanding the physics of cuprates and iridates in the past decade. RIXS is a second-order scattering process which can be used to probe elementary excitations involving spin, orbital, charge, and lattice degrees of freedom 1–3 . When the in- cident photon energy is tuned to the relevant absorption edge, the scattering cross-section of these elementary ex- citations receives a large resonant enhancement. Past investigations have been mostly focused on core level ex- citations, which is often called resonant x-ray emission spectroscopy (RXES) to emphasize the “emission” na- ture of excitations 4,5 . However, in recent years, with im- proved energy resolution, RIXS has been widely used to refer to solid-state spectroscopic studies of elementary excitations such as magnons and orbital excitations 3 . In this article, we use RXES to denote core-level spec- troscopy, and RIXS to refer to spectroscopy focused on the behavior of valence electron systems near the Fermi level (see Fig. 1). One of the most important developments in RIXS was the recent experimental and theoretical realization of its sensitivity to spin-flip scattering processes, 6–10 including propagating magnon excitations 11–13 . There have been a large number of RIXS studies of various cuprate com- pounds carried out at the copper L 3 edge 14–20 . Partially motivated by these activities, similar L 3 edge RIXS ex- periments were carried out to study Sr 2 IrO 4 , in which crystal field excitations between the t 2g and e g elec- tronic states were observed to have a large resonance enhancement 21 . However, due to poor energy resolu- tion, it was not possible to resolve low energy elec- tronic or magnetic excitations in this early investigation. Thanks to the improved instrumentation, the first RIXS measurements of magnetic excitations in iridates were carried out at the Advanced Photon Source in 2010 22 , which showed spectacular dispersion of both magnons and a spin-orbit exciton mode, the latter arising from the strong spin-orbit coupled magnetic degree of free- dom. We note that iridium has a large neutron absorp- tion cross-section, and studying iridates with inelastic neutron scattering is difficult. Most of the iridate neu- tron scattering work so far have been limited to elastic scattering. 23–25 As a result, collective magnetic excita- tions in iridates have been largely elucidated by RIXS