Divergence of the Heavy Quasiparticle Mass at the Antiferromagnetic Quantum Critical Point in YbRh₂Si₂

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We report low temperature specific heat, C, magnetization, M, susceptibility, chi, and electrical resistivity, rho, measurements on high-quality single crystals of the heavy-fermion system YbRh₂(Si₁₋ₓ Geₓ)₂ (x=0 and 0.05). The undoped compound shows weak antiferromagnetic (AF) order at Tₙ=70 mK which is suppressed to below 10 mK by a tiny volume expansion in the x=0.05 system. In the latter pronounced deviations from Landau Fermi liquid (LFL) behavior occur, e.g. Delta rho T over three decades in T. Both thermodynamic and magnetic properties show a crossover at about 0.3 K: At 0.3 K <= T <= 10 K we observe C/T log(T₀/T) and a "non-Curie" behavior chi⁻¹ Tᵃlpha with alpha<1 similar to what was found for the prototypical system CeCu₅.₉ Au₀.₁. Below 0.3 K, chi turns into a Curie-Weiss dependence chi⁻¹ (T-Theta) indicating large unscreened Yb³⁺ moments whereas in C(T)/T a pronounced upturn occurs. In the undoped compound the AF order is suppressed continuously by critical fields Bc₀ = 0.06 T and 0.7 T applied perpendicular and parallel to the c-axis, respectively. For B>Bc₀ a LFL state with Delta rho = A(B)T² and C(T)/T = gamma₀(B) is induced, that fulfills the Kadowaki-Woods scaling A gamma₀². Upon reducing the magnetic field to Bc₀ a 1/(B-Bc₀) dependence of A(B) and gamma₀²(B) indicates singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass.