A nonlinear effect in the electrodynamics of hard superconductors is predicted and theoretically studied. It is shown, that a low-frequency (LF) electromagnetic wave can pass through a superconducting plate, due to its interaction with another wave propagating in the opposite direction, even at small amplitudes of the LF wave when it cannot reach the opposite boundary of the sample without assistance. The dependence of the LF surface impedance on the amplitude of the high-frequency (hf) wave is studied within the framework of the critical state model. The LF surface impedance is found to be drastically increased by the overlapping of the regions of wave penetration.
The nonlinear interaction of DC current flowing in a thin metal film with an external low-frequency AC electromagnetic field is studied theoretically. The nonlinearity is related to the influence of the magnetic field of the DC current and the magnetic field of the wave on the form of electron trajectories. This magnetodynamic mechanism of nonlinearity is the most typical for pure metals at low temperatures. We find that such interaction causes sharp kinks in the temporal dependence of the AC electric field of the wave on surface of the sample. The phenomenon of amplification of the electromagnetic signal on the metal surface is predicted. We also calculate the nonlinear surface impedance and show that it turns out to be imaginary-valued and its modulus decreases drastically with the increase of the wave amplitude.
The nonlinear interaction of a dc current flowing in a thin metal film with an electromagnetic wave illuminating the sample in a symmetric manner with respect to the magnetic field is investigated theoretically. The nonlinearity is due to the most typical mechanism for pure metals at low temperatures — the magnetodynamic mechanism, involving the effect of the magnetic field of the current and wave field on the electron trajectories. It is shown that this interaction leads to sharp kinks in the time dependence of the electric field of the wave at the surface of the sample and to a peculiar electromagnetic signal amplification effect.
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