  The Drude model describes the free-electron conduction in simple metals,
governed by the freedom that the mobile electrons have within the material. In
strongly correlated systems, however, a significant deviation of the optical
conductivity from the simple metallic Drude behavior is observed. Here, we
investigate the optical conductivity of the heavy-fermion system
CeCu$_{\mathrm{6-x}}$Au$_{\mathrm{x}}$, using time-resolved, phase-sensitive
terahertz spectroscopy. 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
transitions between the heavy and light parts of the hybridized conduction band
that break the Kondo singlet. We find that the Kondo-singlet breaking interband
transitions do not create a Drude peak, while the Kondo-retaining intraband
excitations yield the expected Drude response; thus, making it possible to
separate these two fundamentally different correlated contributions to the
optical conductivity.
