Collective oscillations in superconductors revisited

Abstract

In the recent paper Ohashi and Takada (OT) made statements that in the clean limit considered by us (AV) in 1975, weakly damped collective oscillations in superconductors do not exist due to the Landau damping and their spectrum differs from that obtained in AV. In this Comment we would like to note that these statements arise as a result of a misunderstanding of the term "clean" case. OT considered the limit of frequencies larger, than elastic scattering rate, meanwhile AV obtained weakly damped mode in the case when temperature is larger than scattering rate, the frequencies being smaller (!) than elastic scattering rate. All these problems were discussed in our review article in 1979 which was, presumably, unknown to OT.