Spin-flip scattering of critical quasiparticles and the phase diagram of YbRh2Si2

TL;DR

This paper explains unusual transport and thermodynamic behaviors in YbRh2Si2 near quantum criticality through a theory of critical quasiparticles, highlighting the role of spin-flip scattering and challenging the notion of Fermi surface breakdown.

Contribution

It introduces a new understanding of the T*(H) crossover line in YbRh2Si2 as due to spin-flip scattering effects within the critical quasiparticle framework, not Fermi surface collapse.

Findings

Step-like changes in Hall coefficient and magnetoresistivity as a function of magnetic field.

Quantitative agreement of the theory with observed transport anomalies.

Reinterpretation of the T*(H) line without invoking Fermi surface breakdown.

Abstract

Several observed transport and thermodynamic properties of the heavy-fermion compound YbRh2Si2 in the quantum critical regime are unusual and suggest that the fermionic quasiparticles are critical, characterized by a scale-dependent diverging effective mass. A theory based on the concept of critical quasiparticles (CQP) scattering off antiferromagnetic spin fluctuations in a strong-coupling regime has been shown to successfully explain the unusual existing data and to predict a number of so far unobserved properties. In this paper, we point out a new feature of a magnetic field-tuned quantum critical point of a heavy-fermion metal: anomalies in the transport and thermodynamic properties caused by the freezing out of spin-flip scattering of critical quasiparticles and the scattering off collective spin excitations. We show that a step-like behavior as a function of magnetic field of e.g. the Hall coefficient and magnetoresistivity results, which accounts quantitatively for the observed behavior of these quantities. That behavior has been described as a crossover line T*(H) in the T - H phase diagram of YbRh2Si2. Whereas some authors have interpreted this observation as signaling the breakdown of Kondo screening and an associated abrupt change of the Fermi surface, our results suggest that the T* line may be quantitatively understood within the picture of robust critical quasiparticles.