Temperature dependence of intermodulation distortion in YBCO

Intermodulation distortion (IMD) in high-T _c superconductor devices is an important limitation in practical devices. We have measured the third-order IMD versus power as a function of temperature from 1.7 K to T _c, for a variety of YBCO films in a stripline resonator. At low powers, the results show an increase in IMD at low temperatures for most films, and the IMD diverges nearly as 1/T ² for temperatures below 5 K. At higher input powers, the IMD becomes nearly independent of temperature. The temperature and power dependence of the IMD are consistent with the nonlinear Meissner effect, which has been predicted for superconductors with d-wave symmetry but has not been previously observed. The results are compared with a many-body calculation that yields the lowest order correction to the nonlinear penetration depth. The calculation reproduces the observed low-temperature divergence and power dependence of the IMD.