|
Very soon after publication of the famous BCS work explaining the puzzle of the superconductivity, N. N. Bogolyubov and coworkers proposed their version of the theory. One of the new results they obtained was the discovery of a collective mode -- an oscillation of the Cooper pair density with the energy smaller than 2(Delta) . P. W. Anderson has indicated that this collective mode cannot be found experimentally since the Coulomb forces neglected in the above mentioned work shift their energy to the plasma frequency, i.e., to the high- ultraviolet range in which the superconductivity is inessential. The interest for plasmons in superconductors revived a little in the search for the mechanism of the High-Tc superconductivity. However, a real surge of interest to this problem occurred after experimental observations of the plasma edge in the reflectivity of High-Tc superconductors La2-xSrxCuO4 and YBa2Cu3O8-y. It is worthwhile to mention that a theoretical prediction preceded the experiment. For external reasons, the paper was published only a long time after its completion. Here we present a brief review of the experiments and theoretical developments in the field. The theoretical works are presented in more detail, given the authors' specialization. The content of the review is as follows. In the second section we review the most general relationships for plasma frequency (PF) and dielectric function of the electron plasma in normal metals and their modification for layered normal metals. In the third section we review the experimental observations of the plasma edge in High-Tc superconductors (HTSC). In the fourth section a simple two-fluid model of the plasma oscillations is presented. The fifth section is devoted to the BCS theory of the same phenomenon. Peculiarities of Josephson-linked layered superconductors are considered in Section 6. The experimental observations of the resonance plasma attenuation are discussed in Section 7. Mechanisms of dissipation in the Josephson-linked superconductors are discussed in Section 8. A novel effect of selectional transparency is discussed in Section 9. The conclusions are collected in Section 10.
|
