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

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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 Tc. Above Tc 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 Tc 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%.