  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 (RRR 150). We observe a very sharp peak at T_N=72mK
with absolute values as high as C/T=8J/molK^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.
