Field-dependent Microwave Surface Resistance Research Articles

Effect of interstitial impurities on the field dependent microwave surface resistance of niobium

Previous work has demonstrated that the radio frequency surface resistance of niobium resonators is dramatically reduced when nitrogen impurities are dissolved as interstitial in the material. This effect is attributed to the lowering of the Mattis-Bardeen surface resistance with increasing accelerating field; however, the microscopic origin of this phenomenon is poorly understood. Meanwhile, an enhancement of the sensitivity to trapped magnetic field is typically observed for such cavities. In this paper, we conduct a systematic study on these different components contributing to the total surface resistance as a function of different levels of dissolved nitrogen, in comparison with standard surface treatments for niobium resonators. Adding these results together, we are able to show which is the optimum surface treatment that maximizes the Q-factor of superconducting niobium resonators as a function of expected trapped magnetic field in the cavity walls. These results also provide insights on the physics behind the change in the field dependence of the Mattis-Bardeen surface resistance, and of the trapped magnetic vortex induced losses in superconducting niobium resonators.

Reduction in the field-dependent microwave surface resistance in YBa2Cu3O7−δ with submicrometric BaZrO3 inclusions as a function of BaZrO3 concentration

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa2Cu3O7−δ thin films, where submicrometric BaZrO3 particles have been incorporated. As a function of the BaZrO3 content, we observed that the absolute losses slightly decrease up to a BaZrO3 content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently in that they are not monotonic with increasing BaZrO3 concentration. At small concentration (2.5%) the field-induced losses increase, but large reduction in the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO3 concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance, we estimate vortex pinning-related parameters. We found that BaZrO3 inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO3 content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, leaving room for further improvements.

Strong reduction of the field-dependent microwave surface resistance in YBCO with BaZrO3 inclusions

We present measurements of the magnetic field dependent microwave surface resistance in laser-ablated YBa2Cu3O7-δ films on SrTiO3 substrates. BaZrO3 crystallites were included in the films using composite targets containing BaZrO3 inclusions with mean grain size smaller than 1 μm. X-ray diffraction showed single epitaxial relationship between BaZrO3 and YBa2Cu3O7-δ. The effective surface resistance was measured at 47.7 GHz for 60<T<90 K and 0<μ0H<0.8 T. The magnetic field had a very different effect on pristine YBa2Cu3O7-δ and YBa2Cu3O7-δ/BaZrO3, while for μ0H=0 only a reduction of Tc in the YBa2Cu3O7-δ/BaZrO3 film was observed, consistent with dc measurements. At low enough T, in moderate fields YBa2Cu3O7-δ/BaZrO3 exhibited an intrinsic thin film resistance lower than the pure film. The results clearly indicate that BaZrO3 inclusions determine a strong reduction of the field-dependent surface resistance. From the analysis of the data in the framework of simple models for the microwave surface impedance in the mixed state we argue that BaZrO3 inclusions determine very steep pinning potentials.