Electron spin resonance of the two-dimensional electron system in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ at subunity filling factors

The electron spin resonance (ESR) of a two-dimensional electron system (2DES) in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ in the regime of fractional filling (ν<1) of the lowest Landau level is investigated by a photoconductivity technique at millimeter 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 k⋅p theory. The (single-electron) theory also yields excellent agreement for the experimental and theoretical magnetic field dependences of the ESR transition energy, i.e., single-electron transition energies are measured and Kohn’s theorem is not violated. Nevertheless, due to electron-electron interaction the ground state of the 2DES can be a many-particle state with similar amplitudes of spin-up and spin-down contributions. Evidence for such a reduced spin polarization is found in a striking reduction of the ESR strength when decreasing the temperature from 1.6 to 0.3 K.