Structure of the intersubband collective excitations in quasi-two-dimensional systems in a magnetic field

The spectrum of intersubband collective spin-and charge-density excitations is calculated for a system of quasi-two-dimensional electrons with $nu\le10$ ($\nu$ is the filling factor) in a magnetic field. The transitions both without changing the Landau level and with its change (Bernstein modes) are considered. All excitations are shown to have a multimode structure, the number of modes being determined by the filling factor. The dispersion and interaction of small-quasimomentum collective excitations are also considered. The possibility of observing the multimode structure is predicted.