Wiedemann-Franz law and nonvanishing temperature scale across the field-tuned quantum critical point of YbRh₂Si₂

The in-plane thermal conductivity κ and electrical resistivity ρ of the heavy-fermion metal YbRh₂Si₂ were measured down to 50 mK for magnetic fields H parallel and perpendicular to the tetragonal c axis, through the field-tuned quantum critical point H_c, at which antiferromagnetic order ends. The thermal and electrical resistivities, w≡L₀T/κ and ρ, show a linear temperature dependence below 1 K, typical of the non-Fermi-liquid behavior found near antiferromagnetic quantum critical points, but this dependence does not persist down to T = 0. Below a characteristic temperature T^∗ ≃ 0.35 K, which depends weakly on H, w(T) and ρ(T) both deviate downward and converge as T → 0. We propose that T* marks the onset of short-range magnetic correlations, persisting beyond H-c. By comparing samples of different purity, we conclude that the Wiedemann-Franz law holds in YbRh₂Si₂, even at H_c, implying that no fundamental breakdown of quasiparticle behavior occurs in this material. The overall phenomenology of heat and charge transport in YbRh₂Si₂ is similar to that observed in the heavy-fermion metal CeCoIn₅, near its own field-tuned quantum critical point.