Exchange Enhancement of a Spin-polarized 2D Electron Gas Determined by Optical Absorption Spectroscopy.

A large exchange enhancement of the spin-splitting of a two- dimensional electron gas has been determined from optical spectroscopy. At high magnetic fields a striking temperature dependence has been found in interband optical absorption transitions associated with the lowest Landau level of an n-GaAs quantum well. When the Fermi energy is located between the spin-split electron states, the temperature dependence of transitions to both spin levels monitors changes in the distribution function, yielding a direct measure of the occupation. Analysis reveals a large spin-splitting attributed to an exchange enhancement of the electron g-factor of ~ 9.