  In strongly correlated systems such as heavy-fermion materials, the coherent
superposition of localized and mobile spin states leads to the formation of
Kondo resonant states, which on a dense, periodic array of Kondo ions develop
lattice coherence. Characteristically, these quantum-coherent superposition
states respond to a terahertz (THz) excitation by a delayed THz pulse on the
scale of the material's Kondo energy scale and, hence, independent of the
pump-light intensity. However, delayed response is also typical for
superradiance in an ensemble of excited atoms. In this case, quantum coherence
is established by the coupling to an external, electromagnetic mode and, hence,
dependent on the pump-light intensity. In the present work, we investigate the
physical origin of the delayed pulse, i.e., inherent, correlation-induced
versus light-induced coherence, in the prototypical heavy-fermion compound
CeCu_5.9Au_0.1. We study the delay, duration and amplitude of the THz pulse at
various temperatures in dependence on the electric-field strength of the
incident THz excitation, ranging from 0.3 to 15.2 kV/cm. We observe a robust
delayed response at approximately 6 ps with an amplitude proportional to the
amplitude of the incident THz wave. This is consistent with theoretical
expectation for the Kondo-like coherence and thus provides compelling evidence
for the dominance of condensed-matter versus optical coherence in the
heavy-fermion compound.
