Deviations from linear theory for fluctuations below the supercritical primary bifurcation to electroconvection

We report measurements of thermally-induced mean-square director-angle fluctuations <theta^2> below primary supercritical bifurcations to electroconvection of the nematic liquid crystals I52 and "Merck Phase V". For epsilon_mf = V^2/V_c,mf^2 - 1 < -0.1 (V is the amplitude of the applied alternating voltage) we find <theta^2> proportional to epsilon_mf^(-gamma) with gamma given by linear theory (LT). Closer to the bifurcation there are deviations from LT with a smaller gamma and with V_c^2 > V_c,mf^2. For I52 measurements as a function of the conductivity sigma of <theta^2> above V_c^2 suggest a tricritical bifurcation at sigma_t = 4.0 x 10^(-9) Omega^(-1)m^(-1), and (V_c^2 - V_c,mf^2)/V_c^2 increases strongly as sigma_t is approached from above.