Spin excitations in two-dimensional electron gas, their relaxation, photoexcitation, and detection methods, and the role of Coulomb correlations

We discuss spin excitations in a degenerate 2D electron gas in a perpendicular quantizing magnetic field: spin-wave and ‘Goldstone’ excitons in a quantum Hall ferromagnetic (filling factor $\nu {=1}$), and spin-cyclotron excitons in a quantum Hall insulator ($\nu {=2}$). The latter exhibit record-setting long lifetimes, up to 1 ms, owing to which a transition to a basically new collective state, a magnetofermionic condensate, is observable at temperatures $T$<1 K. The condensate's properties may be explained in terms of a coherent state being formed due to the emergence of a dense ensemble of photoexcited long-lived spin-cyclotron excitons obeying Bose statistics in a nonequilibrium system of 2D fermions.