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 Hc, 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 Hc, 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.