Critical slowing down of fermions near a magnetic quantum phase transition

Abstract

A universal phenomenon in phase transitions is critical slowing down (CSD) - systems, after an initial perturbation, take an exceptionally long time to return to equilibrium. It is universally observed in the dynamics of bosonic excitations, like order-parameter collective modes, but it is not generally expected to occur for fermionic excitations because of the half-integer nature of the fermionic spin. Direct observation of CSD in fermionic excitations or quasiparticles would therefore be of fundamental significance. Here, we observe fermionic CSD in the heavy-fermion (HF) compound YbRh${}_2$Si${}_2$ by terahertz time-domain spectroscopy. HFs are compound objects with a strongly enhanced effective mass, composed of itinerant and localized electronic states. We see that near the quantum phase transition in YbRh${}_2$Si${}_2$ the build-up of spectral weight of the HFs towards the Kondo temperature $T_K\approx 25$ K is followed by a logarithmic rise of the quasiparticle excitation rate on the heavy-Fermi-liquid side of the quantum phase transition below $10$ K. A critical two-band HF liquid theory shows that this is indicative of fermionic CSD. This CSD is a clear indication that the HF quasiparticles experience a breakdown near the quantum phase transition, and the critical exponent of this breakdown introduces a classification of fermionic quantum phase transitions analogous to thermodynamic phase transitions - solution to a long-standing problem.

6 pages, 4 figures. Nature Phys. (2023)

References in corpus (5)

• Fermi-liquid instabilities at magnetic quantum phase transitions
• Multiple energy scales at a quantum critical point
• Kondo volume collapse, Kondo breakdown, and Fermi surface transitions in heavy-fermion metals
• Kondo Destruction in RKKY-Coupled Kondo Lattice and Multi-Impurity Systems
• Terahertz Conductivity of Heavy-fermion Systems from Time-resolved Spectroscopy

Cited by in corpus (6)

• Quantum critical metals and loss of quasiparticles
• Strong terahertz third-harmonic generation by kinetic heavy quasiparticles in CaRuO${}_3$
• Nonequilibrium Dynamics of Dirac Quantum Criticality in Imaginary Time
• Missing spectral weight in a heavy-fermion system far above Néel temperature
• Kondo coherence versus superradiance in THz radiation-driven heavy-fermion systems
• Amplified response of cavity-coupled quantum-critical systems