An experimental study of the critical fluctuations in a two-dimensional Ising model

Abstract

Neutron scattering measurements have been made of the critical fluctuations in K₂CoF₄ at the Brookhaven High Flux Beam reactor. The critical fluctuations were studied for both temperatures above and below T_c. Above T_c the scattering is well described by the Ornstein-Zernike form and the exponents and amplitude of kappa (the inverse correlation length) are in excellent accordance with theory. Below T_c the Ornstein-Zernike form gives a reasonable description of the results, but the resulting kappa are very different from those expected theoretically. The approximate form developed by Tarko and Fisher (1975) gives a better description of the results and the resulting parameters are in accordance with theoretical expectations. The authors conclude that the critical fluctuations in the d=2 Ising model are not described by the Ornstein-Zernike form. The amplitude ratios have been measured for the correlation length, the susceptibility, and the ratio connecting the Bragg scattering to the susceptibility by using two-scale-factor universality. The results in each case agree with theory to within the experimental error which varies between 10 and 15%.