Superconducting Nb-film LC resonator

Sputtered Nb thin-film LC resonators for low frequencies at 0.5 MHz have be en fabricated and tested in the temperature range 0.05-1 K in magnetic fiel ds up to 30 mT. Their Q value increases towards decreasing temperature as Q proportional to T-0.5 and reaches similar to 10(3) at 0.05 K. As a functio n 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 lay er, since the thin-film coil alone reaches a Q greater than or similar to 1 0(5) at 0.05 K. (C) 2001 American Institute of Physics.