STRAIN-RELATED FLUCTUATIONS NEAR TRICRITICAL POINTS - 1ST-ORDER TRANSITIONS FOR ANISOTROPIC FLUCTUATIONS AND COUPLING WITH A TRACER ORDER-PARAMETER

Fluctuations of order parameters Q near a tricritical point coupled wi th strain e(is-proportional-to eQ2) lead to fluctuation-induced first- order phase transitions. Anisotropic fluctuations with strong correlat ions along a 'hard' direction show a much reduced first-order behaviou r. In the case of improper ferroelastics the fluctuation-related renor malizations become immeasurably small and a classic tricritical behavi our is expected. Experimental observations of such fluctuations are fa cilitated if the fluctuating order parameter couples bi-quadratically with a second, non-critical order parameter which is used as tracer. T he fluctuation-induced renormalization of the tracer order parameter i s calculated.