Vortex-mediated microwave absorption in superclean layered superconductors

In the superclean case the spectrum of vortex core excitations in the presence of disorder is not random but consists of two series of equally spaced levels [Phys. Rev. B, 57, 5457 (1997)]. The $I-V$ characteristics of such superconductors displays many interesting phenomena. A series of resonances is predicted at frequencies commensurate with the spacing of the vortex excitations. These resonances reveal an even-odd anomaly. In the presence of one weak impurity the excitation levels can approach each other and almost cross. Absorption at low frequencies is identified with the resonances arising in this case. The results of such microscopic theory coincide up to the order of magnitude with both the theory employing kinetic equation [JETP Lett. 23, 578 (1976)] and the experiment [Phys. Rev. B 49, 4380 (1994)]. The nonlinear effects associated with Zener transitions in such crossings are studied. These phenomena can be used as a probe of vortex core excitations.