Violation of Critical Universality at the Antiferromagnetic Phase Transition of YbRh2Si2

We report on precise low-temperature specific-heat measurements, C(T), of YbRh2Si2 in the vicinity of the antiferromagnetic phase transition on a single crystal of superior quality (residual resistivity ratio of similar to 150). We observe a very sharp peak at T-N=72 mK with absolute values as high as C/T=8 J/mol K-2. A detailed analysis of the critical exponent alpha around T-N reveals alpha=0.38 which differs significantly from those of the conventional universality classes in the Ginzburg-Landau theory, where alpha <= 0.11. Thermal-expansion measurements corroborate this large positive critical exponent. These results provide insight into the nature of the critical magnetic fluctuations at a temperature-driven phase transition close to a quantum critical point.