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

## TL;DR

This study investigates the dynamics of heavy-fermion quasiparticles near a quantum critical point in CeCu$_{6-x}$Au$_{x}$, revealing long-lived coherence and unexpected behavior of quasiparticle formation during a quantum phase transition.

## Contribution

It provides time-resolved experimental evidence and theoretical insights into quasiparticle collapse and revival near a quantum critical point, challenging existing theories.

## Key findings

- Heavy-fermion state recovers with a coherence time 100 times longer than typical metals.
- Quasiparticle weight collapses near QCP while formation temperature remains constant.
- Coexistence of quasiparticle collapse and formation challenges current quantum criticality models.

## Abstract

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), the quasiparticles may disintegrate and give way to an exotic quantum-fluid state of matter. 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 trace the dynamics of heavy-fermion quasiparticles in CeCu$_{6-x}$Au$_{x}$ 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 metals. 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.