Transport in superconducting niobium films for radio frequency applications

Nb films have been used for many years in superconducting electronics and in rf cavity applications. Recent interest in high rf power TM cavities raises new issues. It is well known that in unprotected Nb films oxidation deteriorates external surfaces strongly and proceeds rapidly along growth island boundaries, resulting in networks of strong and weak links. In superconducting electronics high dc transport current densities Jc are needed. The surface resistance increases exponentially with temperature T and with frequency like ω2, R(T⩽Tc∕2,ω<Δ∕30ℏ)∝ω2exp(−Δ∕kT), with the gap frequency 2Δ∕h=0.7THz, and this is the regime exploited for rf application. But whereas in dc the best current path defines Jc, in rf even the weakest links are sampled, which yield exceptionally strong increases of Rhys∝ω(B∕Jc)n (n=1,2) at rather low fields. This makes the homogeneity of Nb films and localized heating critical issues that must be addressed for rf applications. In this paper we discuss the effects of oxidation at external surfaces or along growth boundaries, especially the consequences of weak/strong link networks on Jc(T,B) and on R(T,B,ω) at low and high rf power, as well as heat transfer, and other factors and mechanisms that affect cavity performances.