Nonlinear effects in high temperature superconductors: 3rd order intercept from harmonic generation

The nonlinearity or microwave surface resistance of high temperature superconductors (HTS) is of considerable interest. We report the first measurement of the 3rd order intercept (TOI) from harmonic generation for HTS. The nonlinearities were measured for a series of coplanar transmission lines composed of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ or YBa/sub 2/Cu/sub 3/O/sub 7/. At a fundamental frequency of 1.4 GHz, the TOI for a 28 /spl mu/m wide by 5 mm long Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ 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 handled up to 4 Watts of cw power at 80 K which corresponds to an average rf current density of 2.2/spl times/10/sup 6/ A/cm/sup 2/. The 3rd harmonic exhibited the expected slope 3 on a log log scale at both low and high powers and a shift of the TOI was observed at some intermediate power. The scaling rules for power handling from changes of a Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ coplanar transmission line width, length and temperature were also derived. These data suggest a redistribution of the rf current within the transmission line across its width at high powers. A pronounced TOI shift was not observed for YBa/sub 2/Cu/sub 3/O/sub 7/ coplanar transmission lines.<>