19
MAY
2016

Microwave suppression of surface resistance and dissipation limits in superconducting resonator cavities at strong RF fields.

Prof. Alexander Gurevich

Old Dominion University, Department of Physics, Norfolk, VA 23529, USA

Microwave suppression of surface resistance Rs(w,Ha)  in the Meissner state of  superconductors under strong electromagnetic fields H(t) = Hacos(wt) at low temperatures T << Tc is discussed. Solving the time-dependent Usadel equations in a non-equilibrium dirty limit shows that Rs(w,Ha) has a minimum as a function of the RF field amplitude Ha. The theory shows that microwave suppression of Rs(w,Ha) caused by a low frequency (hw << kBT) magnetic field mostly results from field-induced temporal oscillations of the quasiparticle density of states rather than the microwave stimulation of superconductivity [1]. The calculated field dependence Rs(w,Ha) is in good agreement with recent experiments on N-doper and Ti-doped Nb resonator cavities. Applying superimposed dc and ac fields, H(t) = H0 + Hacos(wt), can be used to reduce the rf dissipation in thin film nanostructures by tuning  Rs(w,Ha) with the dc field, as was observed in earlier experiments on thin films. I will also discuss ways of increasing the field onset for penetration of vortices by thin film S-I-S multilayers or by dirty layers deposited on the surface of a superconductor. Such structures can fully screen the applied magnetic field exceeding the superheating fields Hs of both the superconducting layers and the substrate, the maximum Meissner field is achieved at an optimum multilayer thickness [2]. The S-I-S layer structures also inhibit penetration of dendritic thermomagnetic vortex avalanches.

References

[1] A. Gurevich, PRL, 113, 087001 (2014).

[2] A. Gurevich, AIP Advances, 5, 017112 (2015)

Slides of this talk will be available soon.