Crossover from classical to random-field critical exponents in As-doped TbVO4

Using birefringence techniques we have measured the critical exponents beta , gamma, and delta in As-doped TbVO4, a structural realization of the rando m-field Ising model where random strain fields are introduced by V-As size mismatch. For pure TbVO4 we observe the expected classical critical exponen ts, while for a mixed sample with 15% As concentration our results are beta = 0.31 +/- 0.03, gamma = 1.22 +/- 0.07, and delta = 4.2 +/- 0.7. These val ues are consistent with the critical exponents for the short-range pure Isi ng model in three dimensions in agreement with a prediction by Toh. The sus ceptibility data showed a crossover with temperature from classical to rand om-field critical behavior.