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 ~150).We observe a very sharp peak at TN = 72 mK with absolute values as high as C/T = 8 J/molK². A detailed analysis of the critical exponent α around T_N reveals α ≤ 0.38 which differs significantly from those of the conventional universality classes in the Ginzburg-Landau theory, where α ≤ 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.