We theoretically predict instability of a zero-dc-current state of thetwo-dimensional electron system formed on the surface of liquid helium inducedby the cyclotron resonance (CR). This conclusion follows from the theoreticalanalysis of the dc magnetoconductivity which takes into account thecontribution from radiation in an exact way. A many-electron model of thedynamic structure factor of the 2D Coulomb liquid is used to describe theinfluence of strong internal forces acting between electrons. For low electrondensities and high amplitudes of the microwave field, the dcmagnetoconductivity is shown to become negative in the vicinity of the CR whichcauses the instability. This effect is strongly suppressed by Coulomb forces inthe region of high densities.
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