Temperature and field dependence of the RF surface resistance of high Tc materials

The RF behavior of YBa₂Cu₃O_7-x is being evaluated for its potential as a material used to make microwave cavities for particle accelerators. Single crystals and thin films are under study at 1.5 and 6 GHz. The best results have been obtained using the crystals of Schneemeyer et al. (1989), which show a residual resistance below 5×10^-4 Ω at 6 GHz at 77 K. This is substantially below the resistance of 77 K copper. The resistance of these crystals at the superconducting transition drops more than two orders of magnitude within a few degrees of T_c. The sharpness of this drop is comparable to that of niobium. It is believed that these crystals are still far from ideal and that their properties deteriorate with time, but they are of much higher quality than the other forms of YBCO studied. Progress is being made towards producing thin films with useful RF properties. Compared with films of a year ago the sharpness of the superconducting transition has increased substantially although the residual losses are still high. The measurement techniques used are now sensitive to as little as 20 mm² of material with a resistance of 20 μΩ