Debye mechanism of giant microwave absorption in superconductors

We discuss a novel mechanism of microwave absorption in superconductors, which is similar to the Debye absorption mechanism in molecular gases. The contribution of this mechanism to the ac conductivity is proportional to the inelastic quasiparticle relaxation time ${\tau }_{\mathrm{in}}$ rather than the elastic one ${\tau }_{\mathrm{el}}$ and therefore it can be much larger than the conventional one. The Debye contribution to the linear conductivity arises only in the presence of a dc supercurrent in the system and its magnitude depends strongly on the orientation of the microwave field relative to the supercurrent. The Debye contribution to the nonlinear conductivity exists even in the absence of dc supercurrent, and it is proportional to ${\tau }_{\mathrm{in}}$. Therefore the nonlinear threshold is anomalously low. Microwave absorption measurements may provide direct information about ${\tau }_{\mathrm{in}}$ in superconductors.

Figure 1

Schematic plots of $\nu (\epsilon ,p_s)$ at: $p_s=0$, dashed green line, in the diffusive regime $\mathrm{\Delta }{v}_F{p}_s{\tau }_{\mathrm{el}}^2\ll 1$ - blue line, and in the ballistic regime $\mathrm{\Delta }{v}_F{p}_s{\tau }_{\mathrm{el}}^2\gg 1$, orange line.