Abstract
We have investigated the elementary excitations, the singlet and triplet excitons, of a two-dimensional electron gas with two filled Landau levels (of opposite spin) in the limit of large magnetic field. The ratio of the spin splitting ħω s to the cyclotron energy ℏhω c can be varied by changing the component of the magnetic field parallel to the layer. At a critical value of ħω s (smaller than ħω c) the excitation energy of the triplet exciton goes to zero, indicating a spin density wave (SDW) instability. However, for a simple single valley system a first order transition to a fully spin polarized state occurs at a larger value of ħω s, pre-emptying the SDW instability. We have extended the analysis to a many-valley system and find that inter-valley scattering tends to stabilize the SDW phase.
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