Nonlinear surface resistance and reduction of dissipation in superconductors in the Meissner state under strong RF fields

Recent technological advances in superconducting Nb cavities have resulted in the accelerating electric fields up to 50 MV/m at quality factors Q(H) up to 10¹¹ @ 2K and 1-2 GHz. The resonator cavity geometry offers an opportunity to probe such fundamental issues as the lower limits of RF dissipation and surface resistance, and the physics of nonequilibrium superconductivity under strong RF current densities close the depairing limit at which the Meissner state becomes unstable with respect to penetration of vortices. In this talk I will discuss mechanisms of reduction of surface resistance R_s by pairbreaking effects which broaden the gap peaks in the density of states, for instance, by a small density of magnetic impurities or a metallic oxide layer at the surface. Pairbreaking effects caused by strong RF or microwave fields can also result is a significant reduction of the nonlinear surface resistance with the field amplitude, which explains the anomalous increase of Q(H) which has been observed on Ti or N-treated Nb cavities. The theory also shows that the surface resistance can be optimized and even reduced below its value for an ideal surface by engineering an optimum local density of states at the surface by impurity management and by surface nanostructuring of the Nb resonators with a thin metallic layer of multilayers of Nb₃Sn, MgB₂ or iron pnictides. Addressing the fundamental physics and materials science of a 100 nm thick surface layer of RF field penetration can bring new ways of decreasing the surface resistance by optimizing materials disorder and peaks in the quasiparticle density of states while tuning the properties of materials defects which can turn their behavior from beneficial to benign and to deadly. These issues can also be essential for reducing losses in superconducting microresonators used for quantum computing.