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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. 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.

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Fig. 1: Heavy-fermion quasiparticles near a QCP.
Fig. 2: Time-resolved terahertz reflectivity of the heavy-fermion system CeCu6−xAux.
Fig. 3: Dynamics of heavy-fermion quasiparticle formation.
Fig. 4: Evolution of Kondo weight and Kondo temperature towards the QCP in the CeCu6−xAux system.


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The authors are grateful for financial support by the SNSF via project No. 200021-14708 (M.F., C.W.) and by the DFG via SFB/TR 185 (J.K).

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All authors contributed to the discussion and interpretation of the experiment and to the completion of the manuscript. C.W. and S.P. performed the experiment and the data analysis. O.S. and H.v.L. provided the CeCu6−xAux samples. K.K. and C.K. provided YbRh2Si2 samples for reference experiments. J.K. performed the theoretical analysis. J.K. and M.F. initiated the experiment and supervised the work.

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Correspondence to J. Kroha or M. Fiebig.

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Wetli, C., Pal, S., Kroha, J. et al. Time-resolved collapse and revival of the Kondo state near a quantum phase transition. Nature Phys 14, 1103–1107 (2018).

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