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 parallel to 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 us to extrapolate the Lorenz ratio towards T = 0. At the critical field this yields L/L-0 = 0.92 +/- 0.03, suggesting a violation of the Wiedemann-Franz law due to strong inelastic scattering. DOI: 10.1103/PhysRevLett.110.176402