MICROWAVE STUDY OF SURFACE ELECTRONS ON HELIUM FILMS IN A MAGNETIC-FIELD

We report preliminary results of a cyclotron resonance study of surfac e electrons (SE) on saturated helium films covering a PMMA substrate a t T > 1 K. The real and imaginary parts of the dielectric response eps ilon(k, omega) of the SE are measured at fixed k and omega in B-fields up to 10 T in a 12 GHz cavity. The cyclotron resonance of the SE is d etermined at different helium film thicknesses d(He) and at various el ectron densities. At small d(He) we find significant anomalies in the cyclotron resonance lineshape and position. As d(He) increases the lin eshape becomes progressively more symmetric and its peak moves towards the cyclotron field value expected for a free electron. To fit these data we have modified the classical Drude expression, introducing two different relaxation times for the low and high B-field regions. The p henomenological formulas fit the data quite well. A systematic theoret ical analysis of these results is in progress.