Ultrafast optical studies on heavy-fermion material CeCu₆

Heavy-fermion systems containing elements with partially filled 4f- or 5f-electron shells, represent some of the most intriguing materials in condensed matter physics. The localized f-states of rare-earth elements contribute to local magnetic moments that typically order magnetically at sufficiently low temperatures. The hybridization between f-electrons and conduction electrons leads to the Kondo effect, where local moments are screened via Kondo spin-singlet many-body states. We present findings from Fourier-transform infrared (FTIR) spectroscopy, time-resolved terahertz (THz) spectroscopy, and ultrafast pump-probe measurements on CeCu₆ single crystals. The reflectivity and real part of the optical conductivity display metallic behavior. A suppression and peak feature were observed below 15 K at 100 meV, arising from optical excitations across a hybridization gap. Ultrafast pump-probe measurements revealed a strong temperature dependence of transient reflectivity below 80 K, which is much higher than the Kondo coherence temperature of 13 K. Two relaxation times, t₁ and t₂, were obtained by fitting the data with an exponential function. The increase in t₁ corresponds to the enhancement of decay time attributed to the opening of the hybridization gap: the electron-hole pair recombination across this gap generates high-frequency phonons, which subsequently break additional electron-hole pairs. The second relaxation time, t₂, may be associated with spin-lattice relaxation.