Correlation of microwave nonlinearity and magnetic pinning in high-temperature superconductor thin film band-pass filters

Third-order intermodulation has been studied in two-pole X-band microstrip filters made of three high-temperature superconductors (HTS), including HgBa2CaCu2O6+delta (Hg-1212), Tl2Ba2CaCu2Oy (Tl-2212) and YBa2Cu3O7-delta (YBCO) at >= 77 K. In addition, the dc critical current density J(c) was investigated in these three types of HTS films in the same temperature range. Interestingly, the dc J(c) and the rf critical current density J(IP3) derived from the third-order intercept (IP3) have a similar dependence on the reduced temperature, suggesting that the magnetic vortex depinning in HTS materials dominates the microwave nonlinearity at elevated temperatures. This observation agrees with the recent theoretical discussion on the origin of the microwave nonlinearity. Disagreement between J(c) and J(IP3), however, has been observed in the trilayer YBCO/CeO2/YBCO filters. Although magnetic flux pinning and hence J(c) were improved by the insertion of a 20 nm thin CeO2 layer, the reduced J(IP3) is attributed to the high power loss from the extra interfaces between YBCO and CeO2.