ABSORPTION OF ELECTROMAGNETIC-WAVES IN 2-DIMENSIONAL SYSTEMS UNDER A MAGNETIC-FIELD AND A PERIODIC POTENTIAL

The interaction of an electromagnetic wave with a two-dimensional elec tron gas subjected to a magnetic field and a weak periodic potential i s investigated. The electron-impurity interaction is included to the l owest order. The periodic modulation of the Landau states has a profou nd effect on the optical conductivity. Simultaneous excitation of an e lectron-hole pair contributes significantly to the absorption around a nd below the cyclotron frequency. This is in contrast to a uniform ele ctron gas under a perpendicular magnetic field where electron-hole pai r excitations are absent.