Critical Suppression of Birefringence in a Smectic-ALiquid Crystal near the Smectic-CPhase

An unusual minimum in the optical birefringence has been observed at the smectic-A to smectic-C transition temperature of 4-n-pentylphenylthiol-4-n^'-octyloxybenzoate. The results are interpreted in terms of the reduction of the birefringence by short-range smectic-C order in the smectic-A phase. Data analysis yields a specific-heat critical exponent α of 0.32±{} 0.12.