Phonon anomalies in pyrochlore iridates studied by Raman spectroscopy

Abstract

We report polarization-resolved Raman scattering measurements on single crystals of the pyrochlore compound ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ which exhibits a ground state with noncollinear magnetic order, as well as its paramagnetic counterpart ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. The spectra reveal the six Raman-active optical phonons expected for the pyrochlore lattice symmetry. Combined with density functional calculations, polarization analysis of the Raman intensity allows us to assign all observed peaks to specific vibration patterns. Whereas most phonon profiles are weakly temperature dependent, an Ir-O-Ir bond-bending vibration in ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ exhibits a pronounced Fano asymmetry in the paramagnetic state, and marked softening and line-shape anomalies at the magnetic phase transition. These observations indicate strong electron-phonon interactions, which must be considered in models of the electronic properties and phase behavior of the pyrochlore iridates. In addition, a prominent feature appears at 210 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the Raman spectrum of ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ below the magnetic transition temperature, possibly due to a magnetic excitation.