Violation of the Wiedemann-Franz law at the quantum critical point of YbRh2Si2

The prototype heavy-fermion system YbRh2Si2 appears to be situated extremely close to an antiferromagnetic (AF) quantum critical point (QCP). Besides the disappearance of the AF ordering (TN=70 mK) at a tiny critical magnetic field, Bc∼60 mT, applied perpendicular to the tetragonal c axis, the breakdown of the Kondo scale accompanied by an abrupt Fermi surface reconstruction is assumed to take place at the very same field. Beyond Bc a heavy Landau Fermi-liquid (FL) phase emerges. We have studied the thermal conductivity κ(T,B) as well as the electrical resistivity ρ(T,B) in a wide temperature and field range (T≥30 mK, B≤ 1 T) using the same YbRh2Si2 single crystalline sample for both measurements. The comparison of both quantities evidences that, in the limit T=0, the Wiedemann-Franz law, κρ=L0T, is fulfilled deep inside the AF phase as well as in the FL regime of YbRh2Si2. The universal relation between heat and charge conduction is, however, distinctly violated upon approaching the field-induced QCP from either side, caused by the emergence of a novel type of inelastic small-angle scattering processes.