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

We report measurements of the electrical and thermal transport on the hexagonal heavy-fermion metal YbAgGe for temperatures T≥40 mK and in magnetic fields H∥ab up to 14 T. This distorted kagome-lattice system displays a series of magnetic states and a quantum critical point at Hc=4.5 T. The Lorenz ratio L(T)/L₀ displays a marked reduction only close to Hc. 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/L₀=0.92±0.03, suggesting a violation of the Wiedemann-Franz law due to strong inelastic scattering.