Critical and Tricritical Behavior at the Nematic to Smectic-ATransition

High-resolution x-ray scattering measurements of the nematic to smectic-$A$ transitions in two different homologous series are reported; it is found that the correlation-length exponents ${\nu }_{\parallel }$ and ${\nu }_{\perp }$ and the susceptibility exponent $\gamma$ decrease continuously with decreasing nematic range; the transitions ultimately become first order. The exponents at the crossover point are close, but not identical, to mean-field tricritical values. For both series and all samples ${\nu }_{\parallel }\ne {\nu }_{\perp }$ while $\alpha +{\nu }_{\parallel }+2\mathrm{}{\nu }_{\perp }-2=0\mathrm{}$ to within the errors.