Birefringence in the smectic and hexatic chiral phases of CE-8

The temperature dependence of the birefringence in the S-A, S-C* and S-I* phases of 4-(2'-methylbutyl) phenyl 4'-n-octylbiphenyl-4-carboxylate (CE-8) has been measured. Whereas in the S-A phase the birefringence is nearly temperature independent, we observe a smooth decrease in the S-C* and a small and discontinuous increase at the S-C*-S-I* phase transition. The observed temperature dependence of the birefringence is in quantitative agreement with a first order approximation to the wave equation in birefringent and spatially modulated media. The tilt angle, as determined from our measurements, shows a mean-field temperature dependence beta = 0.48 +/- 0.02 close to T-AC* and a crossover to the value beta = 0.33 +/- 0.02 for T < T-AC* - 1 K.