Non-Fermi-liquid signatures of quadratic band touching and phonon anomalies in metallic Pr2Ir2O7

A single Fermi node in ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ protected by time reversal and inversion symmetry resulting from an interplay between strong electronic correlations and spin-orbit coupling (SOC) has attracted significant attention. Here, we report a detailed temperature-dependent Raman study of the ``quadratic band touching Luttinger semimetal'' ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. In addition to phonons, we observe an underlying flat electronic continuum due to electronic Raman scattering (ERS) that is dominant below a characteristic temperature ${T}_{Q}$. The observed ERS is understood using the Raman susceptibility of a non-Fermi liquid expected in a quadratic band touching system. The phonon frequencies and linewidths show anomalous temperature dependence below ${T}_{Q}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}150$ K attributed to strong electron-phonon coupling (EPC) in a quadratic band touching (QBT) state. The Ir-O-Ir bending ${E}_{g}$ mode shows pronounced Fano asymmetry as well as anomalous linewidth broadening below ${T}_{Q}$. Further, a Raman line at $\ensuremath{\sim}460\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ involving crystal field transition couples strongly to the ${T}_{2g}$ phonon at $\ensuremath{\sim}350\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Our results establish the enhanced scale of electron-electron correlations and electron-phonon coupling in the QBT state of metallic ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$.