Observation of magnetic excitons and spin waves in activation studies of a two-dimensional electron gas

Measurements of activated conductivity for well-resolved Landau levels at filling factors v=1, 2, and 3 reveal a large exchange contribution to the energy gap, both at v=1 and 3 (the familiar exchange-enhanced g factor) and for the first time demonstrate a similar contribution at v=2. These results provide a measure of the large-wave-vector limits of the spin-wave and magnetic-exciton dispersion relations. The surprisingly strong magnetic field dependences of the measured energies are caused by the disorder present in the system.