New phases of the 2D electron system in the ultra-quantum limit observed by cyclotron resonances

Cyclotron resonance on low density, 2D electron systems in the ultra-quantum limit show split and shifted resonances. This demonstrates the presence in the ground state of both spin orientation electrons, whose relative proportion is strongly temperature and occupancy dependent. A critical occupancy of {~}1/10 divides single-particle behavior in a gaseous phase from the quantum liquid where the resonance positions are temperature and carrier density dependent. The resonances suggest a mixed spin state for some regions of the liquid phase.