High-Frequency Nonlinear Response of Superconducting Cavity-Grade Nb  Surfaces

Abstract

Nb superconducting radio-frequency (SRF) cavities are observed to break down and lose their high-Q superconducting properties at accelerating gradients below the limits imposed by theory. The microscopic origins of SRF cavity breakdown are still a matter of some debate. To investigate these microscopic issues, temperature-and power-dependent local third-harmonic response is measured on bulk Nb and Nb thin-film samples using a novel near-field magnetic microwave microscope between 2.9 and 10 K and 2 and 6 GHz. Both periodic and nonperiodic response as a function of applied rf field amplitude are observed. We attribute these features to extrinsic and intrinsic nonlinear responses of the sample. The rf-current-biased resistively shunted junction (RSJ) model can account for the periodic response and fits very well to the data using reasonable parameters. The nonperiodic response is consistent with vortex semiloops penetrating into the bulk of the sample once sufficiently high rf magnetic field is applied and the data can be fit to a time-dependent Ginzburg-Landau (TDGL) model of this process. The fact that these responses are measured on a wide variety of Nb samples suggests that we are capturing the generic nonlinear response of air-exposed Nb surfaces. arXiv:1904.07432v3 [cond-mat.supr-con]

A tunable high-Q millimeter wave cavity for hybrid circuit and cavity QED experiments

“…The nonlinear behavior appears at maximum simulated current densities of 100 A/cm 2 , which is much smaller than the Nb critical current density of 150 MA/cm 2 . This effect has been observed previously both in 2D 16,44,45 and in 3D 46 surface of granular superconductor have both superconducting and normal states, which drive the device into multiple metastable states: one with superconducting defects and one with normal metal defects at lower Q and different frequencies 16,44 . The magnetic flux penetration into the surface also plays an important role in this mechanism, and can enhance this effect making it more dramatic at high powers.…”

Time-dependent Ginzburg-Landau treatment of rf magnetic vortices in superconductors: Vortex semiloops in a spatially nonuniform magnetic field

“…Interestingly, Gor'kov and Eliashberg also studied the third-harmonic response of a thin film superconductor after deriving the TDGL equations [35]. In this paper, the treatment will be limited to bulk Nb samples generally adhering to the experimental conditions discussed in [26].…”

“…When the single domain simulation method was used, Eq. (26) was enforced at the inner boundary of the simulation (r = 5λ 0 ) and the vacuum domain was not utilized.…”

“…This phenomenon cannot be simulated unless the effects of the screening currents and fields on the superconducting order parameter are included self-consistently. This work is motivated by results for third-harmonic generation from a near-field microwave microscope utilized on Nb surfaces [20][21][22][23][24][25][26]. In this experiment, a magnetic writer probe from a conventional magnetic recording hard-disk drive is used to create a high-intensity, localized, and inhomogeneous rf magnetic field on the surface of a Nb superconducting sample.…”

“…In the experiment, the third-harmonic response and its dependence on the applied rf magnetic field amplitude and the temperature of the sample was studied. Preliminary results of TDGL modeling and comparison to experimental data was published in [26].…”