Electrodynamics of superconductors is an intensively studied field due to the wealth of fundamental information and due to their technological importance. Microwave properties are also of great importance due to several key applications including sensing, microwave absorption, and energy conversion. We observe a significant microwave absorption in fine powders of superconductors (MgB2 and K3C60) when vortices are present, i.e. the magnetic field is slightly larger than Bc1 but significantly lower than Bc2. The absorption is larger than in the normal state and is  absent on large crystalline samples. Albeit surprising at first sight and it is a yet unobserved effect, we show that it is the direct consequence of the Ferrell-Glover-Tinkham sum rule, i.e. the conservation of the IR spectral weight. The collapse of spectral weight below the gap results in the DC conductivity (which is a Dirac function at omega=0), which can then be broadened toward finite frequencies when vortex motion is present.

Ref: Csősz et al. SciRep, 8, 11480 (2018)