Theory of Clean Type-II Superconductors in High Magnetic Fields with Particular Reference to the Ultrasonic Absorption Rate

It is shown that the method which Brandt, Pesch, and Tewordt used to solve the Gor'kov equations leads to a physically unacceptable result for the ultrasonic absorption rate in the clean limit. To account for this failure we examined the semiclassical approximation. We found that this approximation strongly influences the general structure of the Green's functions. In particular, it introduces an apparent quasiparticle damping, which can be considered the origin of the difficulties mentioned above. An alternative treatment of the Gor'kov equations including quantized Landau levels is outlined. From the general expression for the ultrasonic absorption rate in a superconductor in the vortex state the expression describing the giant quantum oscillations is re-derived.