Anisotropy of critical X-ray scattering in the vicinity of smectic B-nematic phase transition in EPPCB

The high-resolution X-ray scattering has been used to study the anisotropy of the density correlation function in the vicinity of S_B-N phase transition in the recently developed material EPPCB. The effective critical exponents estimated in the temperature interval 10^?4 <t< 10^?2 (t = T/T_C ? 1) are: γ = 1.34 ± 0.09, ν_⊥ = 0.56 ± 0.08, ν_| = 0.70 ± 0.06 and fulfill, within experimental errors, the relation γ = ν_⊥ + ν_|.The novel feature is that the iso-intensity contours in the scattering plane containing the direction normal to the smectic layers have been found to be of four-winged shaped. A phenomenological (mean-field) theoretical approach explaining this shape in terms of the independence of fluctuations in the longitudinal (perpendicular to the smectic layers) and transversal (in the smectic planes) directions has been proposed.