Abstract When a system close to a continuous phase transition is subjected to perturbations, it takes an exceptionally long time to return to equilibrium. This critical slowing down is observed universally in the dynamics of bosonic excitations, such as order-parameter collective modes, but it is not generally expected to occur for fermionic excitations. Here using terahertz time-domain spectroscopy, we find evidence for fermionic critical slowing down in YbRh2Si2 close to a quantum phase transition between an antiferromagnetic phase and a heavy Fermi liquid. In the latter phase, the relevant quasiparticles are a quantum superposition of itinerant and localized electronic states with a strongly enhanced effective mass. As the temperature is lowered on the heavy-Fermi-liquid side of the transition, the heavy-fermion spectral weight builds up until the Kondo temperature TK ≈ 25 K, then decays towards the quantum phase transition and is, thereafter, followed by a logarithmic rise of the quasiparticle excitation rate below 10 K. A two-band heavy-Fermi-liquid theory shows that this is indicative of the fermionic critical slowing down associated with heavy-fermion breakdown near the quantum phase transition. The critical exponent of this breakdown could be used to classify this system among a wider family of fermionic quantum phase transitions that is yet to be fully explored.Abstract When a system close to a continuous phase transition is subjected to perturbations, it takes an exceptionally long time to return to equilibrium. This critical slowing down is observed universally in the dynamics of bosonic excitations, such as order-parameter collective modes, but it is not generally expected to occur for fermionic excitations. Here using terahertz time-domain spectroscopy, we find evidence for fermionic critical slowing down in YbRh2Si2 close to a quantum phase transition between an antiferromagnetic phase and a heavy Fermi liquid. In the latter phase, the relevant quasiparticles are a quantum superposition of itinerant and localized electronic states with a strongly enhanced effective mass. As the temperature is lowered on the heavy-Fermi-liquid side of the transition, the heavy-fermion spectral weight builds up until the Kondo temperature TK ≈ 25 K, then decays towards the quantum phase transition and is, thereafter, followed by a logarithmic rise of the quasiparticle excitation rate below 10 K. A two-band heavy-Fermi-liquid theory shows that this is indicative of the fermionic critical slowing down associated with heavy-fermion breakdown near the quantum phase transition. The critical exponent of this breakdown could be used to classify this system among a wider family of fermionic quantum phase transitions that is yet to be fully explored.

arXiv:2208.05932Nature PhysicsJul 31, 202314 citations

Perturbation theory for many fermion systems

Joel Feldman, Eugene Trubowitz B

Dec 15, 198929 citations

Perturbation theory of a many-fermion system

Robert William Jones, Franz Mohling

Nuclear Physics AAug 1, 197021 citations

Casimir interaction among heavy fermions in the BCS-BEC crossover

Yusuke Nishida

We investigate a two-species Fermi gas with a large mass ratio interacting by an interspecies short-range interaction. Using the Born-Oppenheimer approximation, we determine the interaction energy of two heavy fermions immersed in the Fermi sea of light fermions as a function of the $s$ -wave scattering length. In the BCS limit, we recover the perturbative calculation of the effective interaction between heavy fermions. The $p$ -wave projection of the effective interaction is attractive in the BCS limit while it turns out to be repulsive near the unitarity limit. We find that the $p$ -wave attraction reaches its maximum between the BCS and unitarity limits, where the maximal $p$ -wave pairing of heavy minority fermions is expected. We also investigate the case where the heavy fermions are confined in two dimensions and the $p$ -wave attraction between them is found to be stronger than that in three dimensions.We investigate a two-species Fermi gas with a large mass ratio interacting by an interspecies short-range interaction. Using the Born-Oppenheimer approximation, we determine the interaction energy of two heavy fermions immersed in the Fermi sea of light fermions as a function of the $s$-wave scattering length. In the BCS limit, we recover the perturbative calculation of the effective interaction between heavy fermions. The $p$-wave projection of the effective interaction is attractive in the BCS limit while it turns out to be repulsive near the unitarity limit. We find that the $p$-wave attraction reaches its maximum between the BCS and unitarity limits, where the maximal $p$-wave pairing of heavy minority fermions is expected. We also investigate the case where the heavy fermions are confined in two dimensions and the $p$-wave attraction between them is found to be stronger than that in three dimensions.

arXiv:0811.1615Physical Review AJan 29, 200938 citations

s

-wave scattering length. In the BCS limit, we recover the perturbative calculation of the effective interaction between heavy fermions. The

p

-wave projection of the effective interaction is attractive in the BCS limit while it turns out to be repulsive near the unitarity limit. We find that the

p

-wave attraction reaches its maximum between the BCS and unitarity limits, where the maximal

p

-wave pairing of heavy minority fermions is expected. We also investigate the case where the heavy fermions are confined in two dimensions and the

p

-wave attraction between them is found to be stronger than that in three dimensions.