Mmtm. Dierichs et al., EVALUATION OF NIOBIUM TRANSMISSION-LINES UP TO THE SUPERCONDUCTING GAP FREQUENCY, Applied physics letters, 63(2), 1993, pp. 249-251
The frequency dependence of the coupling of niobium superconducting tr
ansmission lines is measured for frequencies up to and above the super
conducting gap frequency. For this purpose superconducting microstrips
are integrated in log-periodic planar antennas and the spectral respo
nse is measured by means of a Fourier transform spectrometer with 1 mu
m2 Nb/Al2O3/Nb SIS junctions as detectors. Resonances of microstrips a
re observed up to 650 GHz, the superconducting gap frequency of niobiu
m, but above that frequency no resonances have been detected. The meas
ured resonance frequencies are in good agreement with a dispersive mod
el for superconducting transmission lines based on the Mattis-Bardeen
theory. The radiation coupling is lower than calculated, indicating si
gnificant dielectric losses.