NONLINEAR EFFECTS IN HIGH-TEMPERATURE SUPERCONDUCTORS - 3RD-ORDER INTERCEPT FROM HARMONIC-GENERATION

The nonlinearity of microwave surface resistance of high temperature s uperconductors (HTS) is of considerable interest. We report the first measurements of the 3(rd) order intercept (TOI) from harmonic generati on for HTS. The nonlinearities were measured for a series of coplanar transmission lines composed or Ti2Ba2CaCu2O8 or YBa2Cu3O7. At a fundam ental frequency of 1.4 GHz, the TOI for a 28 mu m wide by 5 mm long Tl 2Ba2CaCu2O8 transmission line was: +52 dBm at 100 K; +68 dBm at 90 K; +72 dBm at 80 K; and >+80 dBm at 50 K. This narrow transmission line h andled up to 4 Watts of cw power at 80 K which corresponds to an avera ge rf current density of 22 x 10(6) A/cm(2). The 3(rd) harmonic exhibi ted the expected slope 3 on a log log scale at both low and high power s and a shift of the TOI was observed at some intermediate power. The scaling rules for power handling from changes of a Tl2Ba2CaCu2O8 copla nar transmission line width, length and temperature were also derived. These data suggest redistribution of the rf current within the transm ission line across its width at high powers. A pronounced TOI shift wa s not observed for YBa2Cu3 coplanar transmission lines.