Critical behavior of a noncritical field: destruction of smectic order at the smectic A-C tricritical point and implications for de Vries behavior.

Critical behavior near the smectic A-C tricritical point is studied using renormalization group techniques. Critical fluctuations induce a singular softening of the smectic bulk modulus in the A phase. At the tricritical point, the quasi-long-range positional order of the smectic layers is destroyed. Despite this loss of order, dislocations remain bound so that smectic elasticity is retained but becomes anomalous, i.e., length scale dependent. The critically induced large layer fluctuations lead to negative thermal expansion of the layers in the A phase and may explain the origin of de Vries behavior. Experimental predictions are given for the temperature dependence of the smectic bulk modulus, x-ray structure factor, and layer spacing in the A phase.