Anomalous Reduction of the Lorenz Ratio at the Quantum Critical Point in YbAgGe

Abstract

We report measurements of the electrical and thermal transport on the hexagonal heavy-fermion metal YbAgGe for temperatures $T\ensuremath{\ge}40\text{ }\text{ }\mathrm{mK}$ and in magnetic fields $H\ensuremath{\parallel}ab$ up to 14 T. This distorted kagome-lattice system displays a series of magnetic states and a quantum critical point at ${H}_{c}=4.5\text{ }\text{ }\mathrm{T}$. 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 us to extrapolate the Lorenz ratio towards $T=0$. At the critical field this yields $L/{L}_{0}=0.92\ifmmode\pm\else\textpm\fi{}0.03$, suggesting a violation of the Wiedemann-Franz law due to strong inelastic scattering.