The interaction between strain and order parameter at the structural transition in SrTi03 is investigated by ultrasonic measurements of attenuation, velocity shifts, and stress dependence of the transition temperature T, . A method is described by which measurements of domain attenuation are used to correct the velocity data below T,. Also, biaxial pressure has been successfully used to suppress the domain structure in the tetragonal phase. By comparing the experimental results with the consequences of aphenomenological configurational potential the interaction parameters are deduced. The model is found to correlate the experimental results in a consistent way. The hydrostatic-pressure dependence of T, is predicted to be dT,/dp = 0. 5 x 10 K/dyn cm . Critical effects in sound velocity and attenuation are briefly described.

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The frequency dependence of the difference between the attenuation of longitudinal soundwaves in a perpendicular magnetic field and in zero field is used in an experimental analysis of three parameters in the electron-phoiion interaction. The parameters are the electron mean free path and the magnitude of the interaction in zero and infinite field. The first parameter is compared with the mean free path deduced from electrical size effects. The interaction parameter in zero field is found to be in fair agreement with theoretical estimates. I n high fields the interaction is not localized a t the €ermi surface, and it is found that symmetrically shaped orbits having an electron group velocity normal to the magnetic field, contribute most strongly to the attenuation.Die Frequenzabhgngigkeit der Differenz zwischen der Dampfung einer longitudinalen Schallwelle in einem unendlichen senkrechten Magnetfeld und im Feld Null wurde als Grundlage fur eine experimentelle Analyse der folgenden drei Parameter der Elektron-Phonon-Wechselwirkung benutzt : der mittleren freien Weglange der Elektronen und der GroBe der Wechselwirkung sowohl im Feld Null als auch im unendlichen Feld. Der erste Parameter wird mit der freien Weglange, bestimmt aus elektrischen Effekten, verglichen. Der Wechselwirkungsparameter im Feld Null wurde als in guter ifbereinstimmung mit dem a m der Theorie bestimmten gefunden. In hohen Feldern ist die Wechselwirkung nicht auf der Fermioberflache lokalisiert, und es wurde gefunden, da13 die Bahnen, die eine symmetrische Gestalt und eine Elektronengruppengeschwindigkeit normal zum magnetischen Feld haben, am starksten zur Dampfung beitragen.

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The frequency and temperature dependence of the electronic attenuation of longitudinal ultrasound in indium along [110] has been measured between 1.5 and 4.2 K in the frequency range 10-90 MHz. The zero-field results are well described by a modified free-electron Pippard expression. As expected from general theory the high-field attenuation was proportional to q2 and showed an anisotropy reflecting the symmetry of the electron distribution. The temperature dependence was found to depend on the direction of the magnetic field in the plane normal to q.