Quasi-two-dimensional Ising critical behavior of de Vries liquid crystals observed in the heat capacity and dielectric response.

Heat capacity and dielectric measurements have been made on a liquid crystal compound exhibiting de Vries type of smectic-A{*} (Sm-A{*}) phase. Heat capacity shows a significant anomaly which is almost symmetric for above and below the Sm-A{*}-Sm-C{*} transition temperature. The transition was found to be very weakly first order. The critical exponent gamma determined from the dielectric data lies 1.8+/-0.2. The present heat capacity data as well as former data for another compound of de Vries type have been analyzed in detail. It was found that the heat capacity data for both compounds are fitted well with a logarithmic divergence except in the immediate vicinity of the transition. These results agree with an expectation that de Vries Sm-A{*}-Sm-C-{*} transition can exhibit quasi-two-dimensional Ising critical behavior.