Terahertz conductivity of heavy-fermion systems from time-resolved spectroscopy

The Drude model is often used to describe the movement of free-electrons in simple metals. Throughout their path, electrons undergo different types of interactions, ranging from charge to spin scattering. In strongly correlated systems, however, a significant deviation of the transport properties is observed. Electrons in heavy-fermion systems, for instance, scatter from the localized magnetic moments of the rare-earth ions and form a hybridized state. As a result, their effective masses are drastically enhanced. Ever since the beginning of correlated-electron research, it has not been easy to disentangle this heavy-fermion formation from the more conventional interactions within the conduction band. Both excitations produce similar low-energy signatures in response experiments. In this contribution, we demonstrate a way to perform this disentanglement by using terahertz (THz) time-resolved spectroscopy and investigate the individual behaviour of strongly and weakly correlated electrons in the CeCu$_{6-x}$Au$_{x}$ system [1].

Ultrafast THz spectroscopy has been used as a tool to coherently probe the quasiparticle dynamics across the quantum phase transition in heavy-fermion compounds [2,3]. The incident THz pulse with a spectral range of 0.1--3 THz creates collective intraband excitations within the heavy band, as well as resonant interband transitions between the hybridizing heavy and light parts of the conduction band. The former leaves the heavy quasiparticle intact, while the latter breaks the Kondo-singlet and leads to a time-delayed echo-like response. Thus, by analysing the signals within different time windows, we can separately investigate the effect of strong correlations between the localized and itinerant electrons leading to the heavy-fermion formation, and weak correlations stemming from conventional charge and static impurity scattering. In our study, we find that for heavy-fermion sample (CeCu$_{6}$) and quantum-critical sample (CeCu$_{5.9}$Au$_{0.1}$), the instantaneous pulses originating from intraband transitions show simple metallic Drude-like low-frequency behaviours at high temperature that develop into heavy-Fermi-liquid or non-Fermi-liquid behavior below the Kondo lattice temperature. In contrast, the delayed echo-like pulses originating from the interband transitions do not yield any Drude responses throughout the entire temperature range.

[1] C.-J.~Yang {\emph{et al.}}, PRR~{\bf 2},~033296~(2020).
[2] C.~Wetli {\it et al.}, Nat.~Phys.~{\bf 14},~1103~(2018).
[3] S.~Pal {\it et al.}, PRL~{\bf 122},~096401~(2019).