Reduction of Microwave Loss by Mobile Fluxons in Grooved Nb Films

In the mixed state of type II superconductors penetrated by an external magnetic field in the form of a lattice of Abrikosov vortices, the dc resistance is known to increase with increasing velocity of the vortex lattice. Accordingly, vortex pinning sites impeding the vortex motion are widely used to preserve the low-dissipative response of the system. Here, while subjecting superconducting Nb films with nanogrooves to a high-frequency ac current stimulus and tuning the number of mobile and pinned vortices by varying the magnetic field around the so-called matching values, we observe a completely opposite effect. Namely, the vortex-related microwave excess loss for mobile vortices becomes smaller than for pinned vortices in a certain range of power levels at ac current frequencies above 100 MHz. Our findings appeal for the development of a theory of microwave-stimulated superconductivity in the vortex state.