Large trigonal-field effect on spin-orbit coupled states in a pyrochlore iridate

Abstract

The half-filled topmost valence band of ${\mathrm{Ir}}^{4+}$ in several iridates such as ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4},\phantom{\rule{0.16em}{0ex}}{\mathrm{IrO}}_{2}$, and ${\mathrm{CaIrO}}_{3}$ has been proposed to originate mainly from the spin-orbit coupled ${J}_{\mathrm{eff}}=1/2$ states. In pyrochlore iridates ${R}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ ($R$: rare earth), some exotic electronic states are theoretically proposed by assuming ${J}_{\mathrm{eff}}=1/2$ states. However, the octahedral coordination around Ir is trigonally distorted, which may affect the energy level scheme of Ir $5d$ states. Here, we report spectra of resonant elastic and inelastic x-ray scattering in ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ at the Ir $L$ edges. A large suppression of the magnetic scattering signal at the Ir ${L}_{\mathrm{II}}$ edge supports the ${J}_{\mathrm{eff}}=1/2$ picture rather than the $S=1/2$ one. The inelastic scattering spectrum indicates that the magnitude of the trigonal field on the ${\mathrm{Ir}}^{4+}$ states is evaluated to be comparable to the spin-orbit interaction. The energy diagram of the $5d$ state is proposed based on the simple cluster model.