Evidence for strong $5d$ electron correlations in the pyrochlore ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ studied using high-resolution photoemission spectroscopy

We report an unusual behavior of highly extended $5d$ electrons in ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ belonging to the pyrochlore family of great current interest, using high-resolution photoemission spectroscopy. The experimental bulk spectra reveal signature of an intense lower Hubbard band in addition to the weak intensities in the vicinity of the Fermi level, ${ϵ}_F$. This provides direct evidence for strong electron correlation among the $5d$ electrons, which is in sharp contrast to the observation in other Ir compounds. The influence of electron correlation is also manifested by close to $\mid ϵ-{ϵ}_F{\mid }^2$ dependence of spectral line shape near ${ϵ}_F$ of the high-resolution spectrum at $300\mathrm{K}$. The spectral line shape evolves to a $\mid ϵ-{ϵ}_F{\mid }^{1.5}$ dependence below Curie temperature, emphasizing the importance of electron-magnon coupling in the magnetically ordered phase.

Figure 1

(Color online) Total density of states (TDOS), $\mathrm{Y}4d$ partial density of states (PDOS), $\mathrm{Ir}5d$ PDOS, $\mathrm{O}12p$ PDOS, and $\mathrm{O}22p$ PDOS obtained from LDA calculations including spin-orbit coupling (SOC). TDOS calculated without SOC is shown by the thick red line in the top panel. Zero in the energy axis represents ${ϵ}_F$.

Figure 2

(Color online) (a) $\mathrm{He}\mathrm{I}$, $\mathrm{He}\mathrm{II}$, and XP valence band spectra. Inset shows the magnetization data for the zero-field-cooled (ZFC) and field-cooled (FC) conditions, measured in $10\mathrm{kOe}$, demonstrating the onset of magnetic ordering at $150\mathrm{K}$, in agreement with Ref. 13. (b) Calculated TDOS shown in Fig. 1. The $\mathrm{O}2p$ part is shifted to match the experimental spectra.

Figure 3

(Color online) (a) $\mathrm{Ir}5d$ band at different photon energies. The line superimposed on open circles represents the resolution broadened $21.2\mathrm{eV}$ spectrum (solid line). (b) Extracted surface and bulk spectra at $300\mathrm{K}$ (symbols) and $20\mathrm{K}$ (solid line). (c) $\mathrm{Ir}5d$ PDOS calculated for different values of $U$ using LSDA+$U$ method. Symbols represent bulk spectrum at $300\mathrm{K}$.

Figure 4

(Color online) (a) High-resolution spectra near ${ϵ}_F$ as a function of temperature. (b) SDOS obtained by symmetrization (lines) and by dividing by the Fermi-Dirac function (symbols). In (c) and (d) symmetrized SDOS is plotted as a function of $\mid ϵ-{ϵ}_F{\mid }^{1.5}$ and $\mid ϵ-{ϵ}_F{\mid }^2$, respectively. Dashed lines are drawn to show the linear behavior.