Abstract
We report high-sensitivity and high-temperature resolution experimental data for the temperature dependence of the optical birefringence of a nonpolar monolayer smectogen 4-butyloxyphenyl--decyloxybenzoate liquid crystal by using a rotating-analyzer technique. The birefringence data cover nematic and smectic- phases of the compound. The birefringence data are used to probe the temperature behavior of the nematic order parameter in the vicinity of both the nematic-isotropic and the nematic–smectic- () transitions. For the transition, from the data sufficiently far away from the smectic- phase, the average value of the critical exponent describing the limiting behavior of is found to be , which is in accordance with the so-called tricritical hypothesis, which predicts and excludes higher theoretical values. The critical behavior of at the transition is discussed in detail by comparing our results with the latest reports in the literature and we conclude that by comparing with the previously reported results, the isotropic internal field assumption by the Vuks-Chandrasekhar-Madhusudana model is adequate to extract the critical behavior of from the optical birefringence data. We observe that there is no discontinuous behavior in the optical birefringence, signaling the second-order nature of the transition. The effect of the coupling between the nematic and smectic- order parameters on the optical birefringence near the transition is also discussed. In a temperature range of about above and below the transition, the pretransitional evidence for the coupling have been detected. From the analysis of the optical birefringence data above and below the transition by means of various fitting expressions we test the validity of the scaling relation between the critical exponent describing the limiting behavior of the nematic order parameter and the specific heat capacity exponent . We then show that the temperature derivative of the nematic order parameter near exhibits the same power-law divergence as the specific heat capacity with an effective critical exponent of .
3 More- Received 28 June 2012
DOI:https://doi.org/10.1103/PhysRevE.86.041705
©2012 American Physical Society