The effect of structural disorder on anomalous critical attenuation of ultrasound by solids

Abstract

A field-theory description of dynamic effects of acoustic wave propagation in solids under phase transition is performed. The effects of structure defects on critical attenuation of ultrasound in Ising-like three-dimensional systems is discussed. Calculations of the attenuation coefficient and scaling function are carried out in a two-loop approximation for regular and disordered systems.Advances in getting insight into the nature of critical phenomena are largely associated with the theoretical and experimental investigations of critical dynamics in condensed media. Until now, though, in the description of nonequilibrium behavior of systems there is a whole range of challenges. This is due to the fact that a study of dynamic properties of critical fluctuations characterized by anomalously large amplitudes faces a larger number of difficulties compared to equilibrium systems. From a qualitative standpoint, this results from the necessity to take into consideration interaction of the order parameter fluctuations with other long living excitations.Dynamics of phase transitions involves a number of physically important processes controlled by the behavior of a multi-spin correlation function, making them especially difficult for theoretical description. In particular, these are thermal processes occurring near the critical point for the liquid -gas system, attenuation of the electromagnetic field energy in the magnetic resonance phenomena, and anomalous attenuation and propagation of acoustic waves in the media under phase transitions. The latter are important due to the fact that they underlie the resonant and ultrasonic methods of investigation of critical dynamics.A unique feature of ultrasonic methods is an anomalously strong attenuation of ultrasound [1][2][3] in the vicinity of the temperatures of the phase transition of the second type, which is vividly revealed in the experiments. This anomalous behavior is due to the effects of interaction of low-frequency acoustic excitations with large-scale fluctuations of the order parameter. These fluctuations via a spin-phonon interaction of magnetostrictive origin generate a random force causing perturbation of normal acoustic modes.One of the most challenging and important tasks, both experimentally and theoretically, is investigation of the effects of structure defects on the characteristics of propagation of ultrasound in materials subjected to phase transitions. The structure disorder due to the presence of impurities or other structure defects and the presence of a few types of competing interactions in the effective Hamiltonian, which set the state of a complex system, very often play an important role in the behavior of real materials and physical systems. These factors might induce new phase transitions, prescribe new classes of universality of critical behavior, modify kinetic properties of systems and control low-frequency features of the system's dynamics. The most typical and worthy examples of such systems are disordered magnetic systems with impurities ...

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