Two-dimensional ising model criticality in a three-dimensional uniaxial relaxor ferroelectric with frozen polar nanoregions

The charge-disordered three-dimensional uniaxial relaxor ferroelectric Sr0.61Ba0.39Nb2O6 splits up into metastable polar nanoregions and paraelectric interfaces upon cooling from above T-c. The frozen polar nanoregions are verified by piezoresponse force microscopy, respond domainlike to dynamic light scattering and dielectric excitation, reveal nonergodicity at T > T-c via global aging, and coalesce into polar nanodomains below T-c. Contrastingly, the percolating system of unperturbed interfaces becomes ferroelectric with two-dimensional Ising-model-like critical exponents alpha=0, beta=1/8, and gamma=7/4, as corroborated by ac calorimetry, second harmonic generation, and susceptometry, respectively.