Superconducting Nb-film LC resonator

Sputtered Nb thin-film LC resonators for low frequencies at 0.5 MHz have been fabricated and tested in the temperature range 0.05–1 K in magnetic fields up to 30 mT. Their Q value increases towards decreasing temperature as Q∝T−0.5 and reaches ∼103 at 0.05 K. As a function of magnetic field Q is unstable and displays variations up to 50%, which are reproducible from one field sweep to the next. These instabilities are attributed to dielectric losses in the plasma deposited SiO2 insulation layer, since the thin-film coil alone reaches a Q≳105 at 0.05 K.