title: Test of Halperin-Lubensky-Ma crossover function at the N-Sm-A transition in liquid crystal binary mixtures via high-resolution birefringence measurements.
abstract: We report optical birefringence data for a series of mixtures of the liquid crystals octylcyanobiphenyl (8CB) and decylcyanobiphenyl (10CB). Nematic order parameter S data in the nematic and smectic A phases have been derived from phase angle changes obtained in temperature scans with a rotating analyzer method. These S values have been used to arrive at values for possible entropy discontinuities at the smectic A to nematic phase transition temperature T_{NA}. The 10CB mole fraction dependence of the obtained entropy discontinuities could be well fitted with a crossover function consistent with the mean-field free-energy expression with a nonzero cubic term arising from the coupling between the smectic-A order parameter and the orientational order parameter director fluctuations in the Halperin-Lubensky-Ma theory. The obtained results are in good agreement with existing results from adiabatic scanning calorimetry. By exploiting the fact that the temperature derivative of the order parameter S(T) near T_{NA} exhibits the same power law divergence as the specific heat capacity, we have extracted the effective critical exponent α values for the compositions under study. The critical exponent α has been observed to reach the tricritical value α_{TCP}=0.5 for the 10CB mole fraction of x=0.330.
authors.affiliation: Department of Physics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey.
authors.forename: Sevtap
authors.initials: S
authors.lastname: Yıldız
authors.affiliation: Department of Physics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey.
authors.forename: Mehmet Can
authors.initials: MC
authors.lastname: Çetinkaya
authors.affiliation: Department of Physics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey.
authors.forename: Şenay
authors.initials: Ş
authors.lastname: Üstünel
authors.affiliation: Department of Physics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey.
authors.forename: Haluk
authors.initials: H
authors.lastname: Özbek
authors.affiliation: Soft Matter and Biophysics, Department of Physics and Astronomy, KU Leuven, 3001 Leuven, Belgium.
authors.forename: Jan
authors.initials: J
authors.lastname: Thoen