Three-wave-mixing scanning RF-SQUID magnetometer: low-noise and wide-bandwidth imaging of thin NbN films

In this talk we present high-sensitivity, low-back-action magnetic imaging with a scanning RF SQUID with a tunable resonance frequency from 2–3.2 GHz. We demonstrate optimized flux sensitivity by tuning the flux-to-voltage transduction by two control knobs: (i) increasing probe power for increased quality factor and (ii) three-wave mixing providing phase-sensitive parametric gain of 20 dB at 3 K; the details are presented in the part 1 of this talk. We achieve a flux noise of ~600 nΦ₀/√Hz at 3 K, about 200 electron-spin sensitivity 𝜇B/√Hz for a 1.8 µm diameter SQUID loop, with ~100 MHz bandwidth. We use this improved flux sensitivity to image diamagnetic susceptibility and vortices in sub-20 nm NbN films of varying thickness to study the superconductor–insulator transition and place a lower bound on the measurable local superfluid density. Using the large bandwidth, we aim to study vortex dynamics and magnetic noise from vortex binding/unbinding across the Berezinskii–Kosterlitz–Thouless (BKT) transition in these films.