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

The Drude model describes the free-electron conduction in simple metals, governed by the freedom thatthe mobile electrons have within the material. In strongly correlated systems, however, a significant deviationof the optical conductivity from the simple metallic Drude behavior is observed. Here, we investigate theoptical conductivity of the heavy-fermion system CeCu6−xAux, using time-resolved, phase-sensitive terahertzspectroscopy. The terahertz electric field creates two types of excitations in heavy-fermion materials: First,the intraband excitations that leave the heavy quasiparticles intact. Second, the resonant interband transitionsbetween the heavy and light parts of the hybridized conduction band that break the Kondo singlet. We find that theKondo-singlet-breaking interband transitions do not create a Drude peak, while the Kondo-retaining intrabandexcitations yield the expected Drude response. This makes it possible to separate these two fundamentallydifferent correlated contributions to the optical conductivity.