Nonlinear interaction of low-frequency radio-waves in hard superconduc
tors is studied both theoretically and experimentally. It is shown tha
t this interaction gives rise to a new nonlinear phenomenon, namely, j
umps in the temporal dependence of the electric field at the surface o
f the sample. Heights and positions of the jumps are determined by the
amplitudes, frequencies, and phase difference of the interacting wave
s. Necessary conditions for the existence of the jumps are discussed.
The calculations are carried out in the framework of a critical-state
model with an arbitrary magnetic field dependence of the critical curr
ent density. Experimental results are found in good agreement with the
oretical ones.