Hysteresis effect in microwave power transmission of high temperature superconducting coplanar transmission lines

We have observed an unusual hysteresis loop in rf power dependence of high-temperature superconductor (HTS) coplanar transmission lines. At 77 K and 1 GHz, the signal transmission was linear (insertion loss <0.1 dB) until the input power (P_in) reached a critical value of P_c=0.4 W. An abrupt drop by ∼37 dB in the transmitted power (P_out) was observed at P_in=P_c followed by a slow increase (P _out∝P_in^0.62) for P_in> P _c. As P_m was then decreased, it retraced the previous curve only until P_in=P_c. However, as P_in was reduced further P_out did not rise until P_in=158 mW, at which P_out started to increase rapidly and returned to the linear regime for P_in<71 mW. Such a sharp transition and a strong hysteresis effect were unique to the HTS line whereas a gold line with an identical structure did not exhibit these characteristics. This behavior may be utilized for potential applications of new HTS microwave power (current) limiters and switches. Several possible origins for the hysteresis effect are proposed and discussed.