Ultrasound as a probe of vortex matter in layered superconductors

Ultrasound is an effective technique to probe various states of the vortex matter (vortex crystal, vortex liquid, etc). An important feature of this method is that ultrasound penetrates into the whole bulk of a sample. Therefore surface phenomena do not complicate interpretation of experiments, as sometimes they do in the case of electromagnetic measurements. The paper presents a theory of ultrasound propagation in type-II superconductors for various directions of the magnetic field and the ultrasound wave vector with respect to the c-axis. Varying the propagation direction and the polarization of the wave one can measure all elastic moduli and therefore effectively study the phase diagram of the vortex matter. Increasing the frequency of ultrasound, experimentalists can measure the Josephson plasma frequency.