Time-Resolved Collapse and Revival of the Kondo State near a Quantum Phase Transition Time-Resolved Collapse and Revival of the Kondo State near a Quantum Phase Transition

One of the most successful paradigms of many-body physics is the concept of quasiparticles: excitations in strongly interacting matter behaving like weakly interacting particles in free space. Quasiparticles in metals are very robust objects. Yet, when a system's ground state undergoes a qualitative change at a quantum critical point (QCP), these quasiparticles can disintegrate and give way to an exotic quantum-fluid state of matter where the very notion of particles comprising the system breaks down. The nature of this breakdown is intensely debated, because the emergent quantum fluid dominates the material properties up to high temperature and might even be related to the occurence of superconductivity in some compounds. In collaboration with the experimental group of Manfred Fiebig at the ETH Zürich, supported by groups at KIT Karlsruhe, MPI-CPfS Dresden and U Frankfurt, the resurgence of heavy-fermion quasiparticles out of a photoexcited nonequilibrium state and their dynamics towards the QCP are monitored in a time-resolved experiment and analyzed by our many-body calculations. A terahertz pulse transforms heavy fermions in CeCu_{5.9}Au_{0.1} into light electrons. Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion state recovers, with a memory time 100 times longer than the coherence time typically associated with metals. The quasiparticle weight collapses towards the QCP, yet its formation temperature remains almost constant. This suggests a revised view of quantum criticality in between disintegration and preservation of the quasiparticle picture.