Electron spin resonance of the two-dimensional electron system in AlxGa1-xAs/GaAs at subunity filling factors

The electron spin resonance (ESR) of a two-dimensional electron system (2DE S) in AlxGa1-xAs/GaAs in the regime of fractional filling (nu<1) of the low est Landau level is investigated by a photoconductivity technique at millim eter wave frequencies. By performing the experiments in a standing wave the ESR is shown to be a magnetic-dipole transition. This is in agreement with a calculation based on an 8-band model and kp theory. The (single-electron ) theory also yields excellent agreement for the experimental and theoretic al mag magnetic field dependences of the ESR transition energy, i.e., singl e-electron transition energies are measured and Kohn's theorem is not viola ted. Nevertheless, due to electron-electron interaction the ground state of the 2DES can be a many-particle state with similar amplitudes of spin-up a nd spin-down contributions. Evidence for such a reduced spin polarization i s found in a striking reduction of the ESR strength when decreasing the tem perature from 1.6 to 0.3 K.