Abstract-Time-resolved collapse and revival of the Kondo state near a quantum phase transition

C. Wetli, S. Pal, J. Kroha, K. Kliemt, C. Krellner, O. Stockert, H. v. Löhneysen,  M. Fiebig,

https://www.nature.com/articles/s41567-018-0228-3

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. Nevertheless, when a system’s ground state undergoes a qualitative change at a quantum critical point (QCP)1, the quasiparticles may disintegrate and give way to an exotic quantum-fluid state of matter. The nature of this breakdown is intensely debated2,3,4,5, because the emergent quantum fluid dominates material properties up to high temperatures and might even be related to the occurrence of superconductivity in some compounds6. Here we trace the dynamics of heavy-fermion quasiparticles in CeCu6−xAux and monitor their evolution towards the QCP in time-resolved experiments, supported by many-body calculations. A terahertz pulse disrupts the many-body heavy-fermion state. Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion state recovers, with a coherence time 100 times longer than typically associated with correlated metals7,8. The quasiparticle weight collapses towards the QCP, yet its formation temperature remains constant—phenomena believed to be mutually exclusive. Coexistence in the same experiment calls for revisions in our view on quantum criticality.

Abstract-Time-resolved collapse and revival of the Kondo state near a quantum phase transition

Ch. Wetli (1), J. Kroha (2,3), K. Kliemt (4), C. Krellner (4), O. Stockert (5), H. von Loehneysen (6), M. Fiebig (1) ((1) ETH Zurich, Switzerland, (2) Univ Bonn, Germany, (3) CCM Zhejiang Univ, Hangzhou, China, (4) Univ Frankfurt, Germany (5) MPI-CPfS Dresden, Germany (6) KIT Karlsruhe, Germany)

(Submitted on 13 Mar 2017)

https://arxiv.org/abs/1703.04443

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. Here we control the resurgence of heavy-fermion quasiparticles out of a photoexcited nonequilibrium state and monitor their dynamics towards the QCP in a time-resolved experiment, supported by many-body calculations. A terahertz pulse transforms heavy fermions in CeCu5.9Au0.1 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.