Quantum criticality in YbRh₂Si₂

We discuss recent low-temperature measurements on single-crystalline samples of the heavy fermion compounds YbRh₂(Si_1−xGe_x)₂ (T_K = 30 K) with x = 0 and 0.05 (nominal). Both show weak antiferromagnetic (AF) order at T_N = 70 and 20 mK respectively. At the field-induced quantum critical point (QCP) that occurs at B = B_c ≈ 0.06 T (B ⊥ c) for the pure compound, non-Fermi-liquid properties are observed down to the lowest accessible temperatures (≈10 mK). These are caused by quantum critical AF fluctuations predominating over the competing ferromagnetic ones. At B = 0 and for T_N < T < 0.3 K, both compounds show a magnetic susceptibility which follows a Curie–Weiss law with a surprisingly large effective moment of 1.4 μ_B, and a Sommerfeld coefficient of the electronic specific heat which deviates towards large values from the theoretically expected (− log T) dependence observed above T = 0.3 K. This observation is in striking contrast to the linear T dependence of the electrical resistivity found down to T ≈ 10 mK. It suggests a break up of the heavy fermion in the approach to the B = 0 QCP that occurs at a slightly larger Ge concentration.