ACOUSTIC ATTENUATION IN A TYPE-II SUPERCONDUCTOR AT HIGH MAGNETIC-FIELDS

We have calculated the longitudinal acoustic attenuation in a type-II superconductor in high magnetic fields within a mean-field BCS theory. We predict two additional features in the corresponding attenuation s ignal as compared to that of the Meissner state. Our analytical calcul ations predict the existence of oscillations in the attenuation as the external magnetic field is varied-this effect is associated with the Landau-level structure of the electron states and is analogous to the well-known de Haas-van Alphen oscillations in the mixed state. The att enuation directly probes the quasiparticle energies; the presence of g apless points in the quasiparticle spectrum, which is characteristic o f type-II superconductors at high magnetic fields, shows up in the fre quency omega and temperature T dependence of the attenuation in the li mit of low omega and and low T, respectively. At low T there is no ana log to the discontinuity in the attenuation observed in the Meissner s tate when (h) over bar omega=2 Delta, where Delta is the quasiparticle energy gap. This result opens up the possiblity of experimentally det ermining the existence and nature of the gapless points in the quasipa rticle spectrum of a type-II superconductor in high magnetic fields.