Evidence of a kondo destroying quantum critical point in YbRh₂Si₂

The heavy-fermion metal YbRh₂Si₂ is a weak antiferromagnet below T_N = 0.07 K. Application of a low magnetic field B_c = 0.06 T (⊥c) is sufficient to continuously suppress the antiferromagnetic (AF) order. Below T ≈ 10 K, the Sommerfeld coefficient of the electronic specific heat £(T ) exhibits a logarithmic divergence. At T < 0.3 K, γ( T ) ∼ T^-ε (ε: 0.3- 0.4), while the electrical resistivity γ(T ) = ρ₀ + aT (ρ₀: residual resistivity). Upon extrapolating finite-T data of transport and thermodynamic quantities to T = 0, one observes (i) a vanishing of the "Fermi surface crossover" scale T∗- (B), (ii) an abrupt jump of the initial Hall coefficient R_H(B) and (iii) a violation of the Wiedemann Franz law at B = B_c, the field-induced quantum critical point (QCP). These observations are interpreted as evidence of a critical destruction of the heavy quasiparticles, i.e., propagating Kondo singlets, at the QCP of this material.