TEMPERATURE-DEPENDENCE OF SURFACE IMPEDANCE OF PULSED-LASER-DEPOSITED YBCO FILMS

One promising application of HTS materials is as the reference resonator element for a stable frequency source. A high Q superconducting resonator operated at low reduced temperature (T/T-c much less than 1) should offer excellent frequency stability due to its low temperature coefficient of resonant frequency. HTS planar resonators appear to be particularly promising since a reduced temperature of 0.1 may be readily achieved with a closed-cycle cryocooler. We report here microwave measurements made on YBCO films grown by pulsed laser deposition (PLD), using both a Nb cavity and a parallel-plate resonator configuration, with particular emphasis on the variation of resonant frequency with temperature. We compare the data with simple theoretical models and find evidence from the low temperature data fora low energy gap parameter Delta(0)=6meV. Some predictions of attainable frequency stability of planar resonator structures are also made.