Niobium shielded sapphire resonator for field-dependent surface resistancemeasurements of superconducting films

For successful realization of planar superconducting microwave devices (e.g . high-power filters in communication systems) large area films (1 " less t han or equal to empty set less than or equal to 3 ") on dielectric substrat es with low surface resistance R-s up to high microwave field amplitudes B- s are required. Therefore, we have developed a very sensitive dielectric re sonator technique to investigate the temperature and field dependence, R-s( T, B-s), of both low-T-c (Nb, Nb3Sn) and high-T-c unpatterned films (YBa2Cu 3O7-delta, Tl2Ba2Ca1(2)Cu-2(3)O-x). The measurement system is based on a lo w-loss sapphire rod (empty set = 7 mm, h = 3.5 mm, tan delta less than or e qual to 3 x 10(-8)) resonant at f(0) = 19 GHz in the TE011-mode. This sapph ire is shielded on one side by an open niobium cavity and on the other side by the film under test which is thermally isolated and can therefore be he ated up to T-c separately. Adjustable coupling antennas allow an ill situ v ariation of the coupling strength. Q(0)-values above 3 x 10(7) for a niobiu m film at T = 1.8 K reflect low parasitic losses equivalent to R-s = 20 mu Omega. Maximum B-s-values of about 50 mT have been obtained for both low- a nd high-T-c films in pulsed power measurements at 4.2 K.