Observation of strong electron-phonon coupling effects inYbPO4

The electronic and vibrational Raman spectra of a single crystal of ${\mathrm{YbPO}}_4$ have been investigated. The ${\mathit{E}}_{\mathit{g}}$ phonon at 310 ${\mathrm{cm}}^{\mathrm{-}1}$ is found to have an anomalously large Raman linewidth of 50 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature. Particularly striking was the observation that this transition is split into several separate transitions that move apart in energy as the temperature is lowered. The magnitude of these effects is very large. These observations are interpreted on the basis of electron-phonon coupling, and phenomenological calculations are presented and compared with the experimental data.