The field-dependent surface resistance of doped niobium: new experimental and theoretical results∗

We present systematic work investigating how different doping and post-doping treatments affect the BCS surface resistance at 1.3 GHz and higher frequencies. We examine the field-dependent BCS resistance at many temperatures as well as the field-dependent residual resistance and use the results to reveal how impurity species and concentration levels affect the field-dependent RF properties. We further demonstrate the importance of thermal effects and their direct dependence on doping level. We use the tools of Density Functional Theory to work towards an ab initio model of electron overheating to theoretically confirm the impact of doping, create a full model that includes thermal effects to predict the field dependent resistance, and show that the predictions of the model agree with results from doped and non-doped cavities (e.g. the strength of the anti-Q-slope and the high-field Q slope). Finally, we use our experimental results to systematically assess and compare theories of the field-dependent BCS resistance, showing that the current theory on smearing of the density of states is incomplete.