  Measurement of harmonic power on a suspended stripline microwave resonator
patterned out of thin film YBa_2Cu_3O_7 (YBCO) has been carried out as a
function of temperature and microwave power. The third harmonic power P_3 shows
a subtle nonlinear dependence on the fundamental power P_1 on a log-log scale.
Fit to a straight line yields slopes of P_3 vs. P_1 between 1.5 to 3, contrary
to elementary calculations that predict a slope of exactly 3. It is shown that
third harmonic power generated from higher order terms in the nonlinear
impedance could account for the discrepancy quantitatively without resorting to
any additional ad-hoc assumptions. Results of measurement of fifth harmonic
power is presented as a confirmation. A small second harmonic power is also
observed that does not show anomalous power dependence within the experimental
accuracy.
