Strong electron-phonon coupling in EuPd2Si2

Mixed-valence metal EuPd2Si2 exhibits a valence transition from Eu2+ to Eu3+ with the crossover temperature T𝑣 around 140K [Jpn. J. Appl. Phys. 50 (2011) 05FD03]. On cooling, the tetragonal crystal symmetry is unchanged, with the a-axis length decreasing and the caxis length essentially unchanging [arXiv:2203.05136]. We study the phonon modes of this material by Raman spectroscopy to explore the effect of valence transition and electron-phonon coupling. The Ramanactive 𝐴1𝑔 phonon mode shows Fano-type asymmetric lineshape, indicating interaction between the phonon mode and underlying continuum of electronic excitations. This mode also exhibits large frequency
hardening on cooling: the frequency at 25K is around 30% larger than that at 300 K. Such a large frequency change cannot be solely explained by the change of lattice parameters, which is only 2%, and points to the role played by electron-phonon interaction. Moreover, the frequency and linewidth of other Raman-active phonon modes show anomalies at T𝑣. We also present Raman spectra of EuPd2(Si0.94Ge0.06)2 for comparison.