Millimeter-wave surface impedance of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin films

We have measured the millimeter wave (f=101.3 GHz) surface impedance of various ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ films deposited on various substrates. $R_s$(T) rapidly decreases below $T_c$ and is, at T=77, K orders of magnitude smaller than that found generally in ceramic and thin-film specimens prepared by other groups. A comparison is given with predictions of the two fluid model and the BCS theory.