Anomalous reduction of the Lorenz ratio at the quantum critical point in YbAgGe

TL;DR

This study investigates electrical and thermal transport in YbAgGe near its quantum critical point, revealing a significant reduction in the Lorenz ratio indicative of Wiedemann-Franz law violation due to strong inelastic scattering.

Contribution

It provides the first detailed measurement of the Lorenz ratio at a quantum critical point in a heavy-fermion system, highlighting anomalous transport behavior.

Findings

Lorenz ratio decreases near the critical magnetic field

Evidence of violation of the Wiedemann-Franz law at the QCP

Presence of a T-linear thermal conductivity contribution

Abstract

We report measurements of the electrical and thermal transport on the hexagonal heavy-fermion metal YbAgGe for temperatures $T\geq$ 40 mK and in magnetic fields $H\parallel ab$ up to 14 T. This distorted Kagome-lattice system displays a series of magnetic states and a quantum critical point at $H_c =4.5$ T. The Lorenz ratio $L(T)/L_0$ displays a marked reduction only close to $H_c$. A $T$-linear contribution below 120 mK, present at all different fields, allows to extrapolate the Lorenz ratio towards T=0. At the critical field this yields $L/L_0=0.92\pm 0.03$, suggesting a violation of the Wiedemann-Franz law due to strong inelastic scattering.