GAPLESS SPIN EXCITONS IN PERIODICALLY MODULATED 2-DIMENSIONAL ELECTRON-GAS

The spin-exciton band structure in a periodically modulated two-dimens ional electron gas, under strong magnetic fields, is shown to be unusu ally rich due to the peculiar nature of the free exciton dispersion la w. It is found that the periodic modulation potential can distort the single spin excitonic state so strongly that the energy gap in the spe ctrum disappears. It is argued that these gapless spin excitons radica lly affect the nuclear spin transport in the heterojunction via the hy perfine interaction and may condense into a collective spin density wa ve ground state.