SPIN-LATTICE RELAXATION EFFECTS IN CRITICAL SOUND-PROPAGATION

The nonasymptotic critical properties of sound propagation are studied in compressible Ising system above T-C. In the present paper we analy se a model where in addition to the coupling to two order-parameter fl uctuations the sound mode couples linearly to the fluctuations of spin -energy an lattice-energy densities. Both subsystems exchange energy w ith the rate determined by the bare spin-lattice relaxation time. The total energy may be conserved or not. The crossover between insulator- like behaviour similar to t(-2 alpha) and metal-like behaviour similar to t(-(z nu+alpha)) in ultrasonic attenuation is investigated accordi ng to the value of ultrasonic frequency, the reduced temperature t, ba re relaxation times and various coupling constants.