Investigation of the gap edge density of states at oxidized Niobium surfaces by RF measurements

Investigations of superconducting Nb cavities yield temperature and frequency dependences of the surface resistance, which deviate from the predictions of the BCS-theory for homogeneous Nb and strongly depend on the oxidation of the Nb surface. Increasing oxidation leads to an increase of the slope of the frequency dependence
R\propto f^{\alpha}
from α=1.55 to α=1.86 and to a decrease of the reduced energy gap Δ(O)/kT
c
from 1.92 to 1.76. The frequency dependence can be explained by a smeared out BCS density of states (DOS). The apparent width of the smearing increases with oxidation from 2εΔ(O)=0.05 meV to 0.19 meV. The energy gap reduction can partly be explained by smeared out DOS. In addition, the reduction of the energy gap by dissolved oxygen and the formation of oxygen clusters have to be taken into account. The rf results of differently oxidized cavities show a linear correlation between energy gap and width of the smearing of the DOS. Extrapolation to zero smearing yields an undistorted energy gap Δ(O)/kT
c
=1.975, which agrees well with the best result Δ(O) = 1.56 meV from tunneling investigations of polycristalline Nb.