Strong coupling theory of heavy fermion criticality

TL;DR

This paper develops a strong coupling theory for heavy fermion criticality, explaining how antiferromagnetic fluctuations induce diverging effective mass and non-Fermi liquid behavior, aligning well with experimental observations.

Contribution

It introduces a self-consistent strong coupling framework using critical quasiparticles to describe heavy fermion quantum critical points, extending beyond weak-coupling theories.

Findings

Diverging quasiparticle effective mass at the critical point.

Spin fluctuation spectrum exhibits omega/T scaling.

Theoretical results agree with experiments on YbRh2Si2 and CeCu(6-x)Au(x).

Abstract

We present a theory of the scaling behavior of the thermodynamic, transport and dynamical properties of a three-dimensional metal at an antiferromagnetic critical point. We show how the critical spin fluctuations at the AFM wavevector q=Q induce energy fluctuations at small q, giving rise to a diverging quasiparticle effective mass over the whole Fermi surface. The coupling of the fermionic and bosonic degrees of freedom leads to a self-consistent relation for the effective mass, which has a strong coupling solution in addition to the well-known weak-coupling, spin-density-wave solution. We thereby use the recently-introduced concept of critical quasiparticles, employing a scale-dependent effective mass ratio m*/m and quasiparticle weight factor Z. As a consequence of the diverging effective mass the Landau Fermi liquid interaction is found to diverge in all channels except the critical one, causing important vertex corrections. The ensuing spin fluctuation spectrum obeys omega/T scaling. Our results are in good agreement with experimental data on the heavy fermion compounds YbRh2Si2$ and CeCu(6-x)Au(x) assuming 3D and 2D spin fluctuations, respectively.