Debye mechanism of giant microwave absorption in superconductors*

We present a theory of microwave absorption in conventional superconductors and show that in the presence of a superfluid velocity V_s the microwave absorption coefficient can be orders of magnitude larger than at V_s = 0. The reason for this is that the V_s-dependent contribution is proportional to the inelastic scattering time rather than the elastic scattering time. The mechanism responsible for this larger contribution is similar to the Debye mechanism for microwave absorption in molecules. Further, the V_s dependance of the absorption coefficient is non-analytical, and measurement of this contribution to the absorption coefficient will give direct information about the inelastic scattering time in superconductors which in general has been difficult to directly measure. In the absence of the superfluid velocity we identify a new mechanism of non-linear conductivity with anomalously low threshold.