NONLINEAR SCREENING OF A TOTALLY OCCUPIED LANDAU-LEVEL

The quadratic-response function of the particle density to a static harmonic potential is calculated for a totally occupied Landau level. Then, with use of a variational approach, it is shown that the linear component of the density response is dominant for any wave vector q of the external potential and for a large range of amplitudes, including those which remove the gap up to the next Landau level. The contribution of the nonlinear components does not exceed a few percent if q < l(B)-1 and may be statisfactorily described by the quadratic component if q < 2l(B)-1, where l(B) = (HBARc/eB)1/2 is the magnetic length.