Critical slowing down of fermions near a magnetic quantum phase transition

TL;DR

This paper reports the first direct observation of critical slowing down in fermionic excitations near a magnetic quantum phase transition in a heavy-fermion compound, revealing fundamental insights into fermionic quasiparticle dynamics.

Contribution

It demonstrates fermionic critical slowing down in a heavy-fermion system, providing experimental evidence and a theoretical framework for fermionic quantum phase transitions.

Findings

Fermionic CSD observed in YbRh₂Si₂ near quantum critical point

Spectral weight buildup and quasiparticle rate increase below 10 K

Critical exponent characterizes fermionic breakdown at 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.