Violation of Wiedemann-Franz law at the Kondo breakdown quantum critical point

TL;DR

This paper investigates the breakdown of the Wiedemann-Franz law at the quantum critical point in heavy-fermion systems, highlighting the role of spinon excitations and their impact on electrical and thermal transport.

Contribution

It demonstrates that the Kondo breakdown quantum critical point leads to a violation of the Wiedemann-Franz law due to additional entropy carriers, distinguishing it from other theoretical frameworks.

Findings

Wiedemann-Franz ratio exceeds the standard value near the QCP

Thermal conductivity includes contributions from localized fermions (spinons)

Electrical resistivity shows quasi-linear temperature dependence

Abstract

We study both the electrical and thermal transport near the heavy-fermion quantum critical point (QCP), identified with the breakdown of the Kondo effect as an orbital selective Mott transition. We show that the contribution to the electrical conductivity comes mainly from conduction electrons while the thermal conductivity is given by both conduction electrons and localized fermions (spinons), scattered with dynamical exponent $z = 3$. This scattering mechanism gives rise to a quasi-linear temperature dependence of the electrical and thermal resistivity. The characteristic feature of the Kondo breakdown scenario turns out to be emergence of additional entropy carriers, that is, spinon excitations. As a result, we find that the Wiedemann-Franz ratio should be larger than the standard value, a fact which enables to differentiate the Kondo breakdown scenario from the Hertz-Moriya-Millis framework.