  We have investigated the anomalous nonlinear microwave response of epitaxial
electron-beam coevaporated YBa2Cu3O7-x films on MgO. The power and temperature
dependent surface impedance and two-tone intermodulation distortion were
measured in stripline resonators at several frequencies between 2.3 and 11.2
GHz and at temperatures between 1.7 K and Tc. All of the eight films measured
to date show a decrease of the surface resistance Rs of up to one order of
magnitude as the microwave current is increased up to approximately 1 mA for T
< 20 K. The surface reactance Xs showed only a weak increase in the same
region. The usual nonlinear increase of Rs and Xs was observed at high currents
in the 100-mA range. The minimum of Rs correlates with a pronounced plateau in
the third-order intermodulation signal. We have developed a phenomenological
two-fluid model, incorporating a microwave current-dependent quasiparticle
scattering rate g and normal fraction fn. The model contains only three
adjustable parameters. We find good agreement with the measured data for
constant fn and a current-dependent scattering rate g whose magnitude increases
almost linearly with temperature and frequency. Obvious mechanisms leading to
nonlinear microwave response like Josephson coupling across grain boundaries,
nonlocal or nonequilibrium effects cannot explain the data. The anomalies could
rather reflect the specific charge and spin ordering and transport phenomena in
the low-dimensional cuprate superconductors.
