Absence of Kondo effect in CeNiGe3 revealed by coherent phonon dynamics

Abstract

Cerium-based intermetallic ${\mathrm{CeNiGe}}_{3}$ has been generally believed to be a heavy-fermion material with Kondo behavior at low temperatures. Using femtosecond-resolved coherent phonon spectroscopy, we present a temperature-dependent dynamic investigation of the bosonic quasiparticles in ${\mathrm{CeNiGe}}_{3}$. Our data do not agree with the heavy-fermion expectation and instead show that the phonon stiffening from room temperature down to 5 K can be well explained by the anharmonic effect in the absence of a Kondo mechanism. Furthermore, the coherent lattice vibration located at \ensuremath{\sim}7.9 THz exhibits a mode splitting at ${T}^{+}\ensuremath{\approx}105\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is close to the energy scale of the first excited crystal field splitting of the Ce $4f$ level. We argue that an orbital crossover may account for this intriguing observation.