Quantum criticality in YbRh2Si2

Abstract

We discuss recent low-temperature measurements on single-crystalline samples of theheavy fermion compounds YbRh2(Si1−xGex)2(TK = 30 K)with x = 0and 0.05 (nominal). Both show weak antiferromagnetic (AF) order at TN = 70and 20 mK respectively. At the field-induced quantum critical point (QCP)that occurs at B = Bc ≈ 0.06 T (B ⊥ c)for the pure compound, non-Fermi-liquid properties are observed down tothe lowest accessible temperatures (≈10 mK). These arecaused by quantum critical AF fluctuations predominating over the competing ferromagneticones. At B = 0and for TN < T < 0.3 K,both compounds show a magnetic susceptibility which follows a Curie–Weiss lawwith a surprisingly large effective moment of 1.4 μB,and a Sommerfeld coefficient of the electronic specific heat whichdeviates towards large values from the theoretically expected (− log T)dependence observed above T = 0.3 K.This observation is in striking contrast to the linearTdependence of the electrical resistivity found down to T ≈ 10 mK.It suggests a break up of the heavy fermion in the approach to theB = 0QCP that occurs at a slightly larger Ge concentration.