Reduction of microwave loss by mobile fluxons in grooved Nb films

TL;DR

This study demonstrates that in nanogrooved Nb superconducting films, mobile vortices can reduce microwave loss compared to pinned vortices at high frequencies, challenging conventional vortex pinning strategies.

Contribution

It reports an unexpected reduction in microwave loss due to mobile vortices in nanogrooved Nb films, contrasting with traditional vortex pinning effects.

Findings

Mobile vortices reduce microwave excess loss at frequencies above 100 MHz.

The effect is observed at specific power levels and magnetic field matching conditions.

The phenomenon suggests a competition between quasiparticle cooling and Joule heating.

Abstract

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 combination of dc and ac current stimuli 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. While a theoretical description of the observed effect is yet to be elaborated we interpret our findings in terms of a competition of the effective cooling of the system by the quasiparticles leaving the vortex cores with the conventional Joule heating caused by the current flow.