Acoustic Studies of High Temperature Superconductors

Acoustic studies of superconductors have a long tradition. The most notable of such studies was the early experimental confirmation of the temperature dependent ultrasonic attenuation in superconductors, predicted by the Bardeen, Cooper and Schrieffer (BCS) theory. This confirmation placed ultrasonics in the forefront of methods used for various features of superconductors. Among those, in particular, were measurements of the superconducting energy gap and of its anisotropy. Accordingly, the advent, three decades later, of high temperature superconductivity brought about a flurry of activity in acoustic measurements of this new state of solids. These studies include frequencies from less than 1 Hz to the GHz range, thus covering a much broader range than what is traditionally referred to as ultrasonics. In this paper a very brief review of the background is followed by a discussion of the highlights of low and high frequency stress wave propagation studies on high temperature superconductors. Emphasis is given to investigations of the dynamics of superconducting vortex line lattices