Strong electron-phonon coupling and enhanced phonon Gr\"uneisen parameters in valence-fluctuating metal EuPd$_2$Si$_2$

TL;DR

This study reveals strong electron-phonon interactions and enhanced phonon Gr"uneisen parameters in EuPd$_2$Si$_2$, linked to valence fluctuations and lattice dynamics, using Raman spectroscopy and density functional theory.

Contribution

It provides new insights into the coupling between valence fluctuations and lattice vibrations in EuPd$_2$Si$_2$, highlighting the role of phonon anomalies and critical elasticity.

Findings

Asymmetric phonon lineshape indicating Fano interaction

Anomalies in phonon frequency and linewidth across valence crossover

Enhanced Gr"uneisen parameters near critical elasticity regime

Abstract

We study the valence crossover and strong electron-phonon coupling of EuPd$_2$Si$_2$ by polarization-resolved Raman spectroscopy. The fully-symmetric phonon mode shows strongly asymmetric lineshape at low temperature, indicating Fano-type interaction between this mode and a continuum of electron-hole excitations. Moreover, the frequency and linewidth of the phonon modes exhibit anomalies across the valence-crossover temperature, suggesting the coupling between valence fluctuations and lattice vibration. In particular, two phonon modes show significantly enhanced Gr\"uneisen parameter, suggesting proximity to a critical elasticity regime. The relative contribution of the structural change and valence change to the phonon anomalies is evaluated by density functional theory calculations.