The effect of vacancies on the microwave surface resistance of niobium revealed by positron annihilation spectroscopy

Using variable-energy positron annihilation spectroscopy, we demonstrate that a different near-surface vacancy concentration accompanies drastic differences in surface resistance of superconducting niobium cavities for particle acceleration. Our data suggest that vacuum baking at 120 degrees C leads to the doping of a near-surface layer with vacancy-hydrogen complexes, and that higher vacancy-type defect concentration distinguishes electropolished from chemically etched cavities. Our findings may help to explain a strong dependence of cavity performance on heat and chemical treatments, and may be of interest to other physics fields including cavity quantum electrodynamics (QED), microresonators, and single photon detectors. (C) 2013 AIP Publishing LLC.