EXCITONS, SPIN-WAVES AND SKYRMIONS IN THE OPTICAL-SPECTRA OF A 2-DIMENSIONAL ELECTRON-GAS

We present a comprehensive study of photoluminescence (PL) mechanisms in high-quality GaAs/AlGaAs two dimensional electron gases (2DEGs) clo se to the integer quantum Hall state, v = 1. At this filling factor th e lowest-energy spin-split electron Landau level (e up arrow) is fully occupied and the higher spin-stale (e down arrow) empty. We demonstra te the Coulomb interaction between the 2DEG electrons and valence band (vb) hole has a crucial influence on the FL. This is elucidated by PL experiments on two different types of sample where we can vary the el ectron-hole separation. In single quantum well (QW) structures with fr ont and back depleting gates we are able to apply an electric field no rmal to the well, in order to polarise the hole wavefunction toward th e opposite face of the well from the 2DEG. In double QWs, we study eve n larger separations where the 2DEG and hole are confined in neighbour ing wells. Our results prove the existence of excitons formed between the hole and the charge excitations of the 2DEG, i.e. quasi-particles or Skyrmions. For small electron-hole separations the Coulomb interact ion is sufficiently strong to cause the spin reversal of a single 2DEG electron for v ii to form an exciton. Increasing the electron-hole se paration results in the v<1 ground state changing nature to be non-exc itonic. For the very large electron-hole separations found in the doub le QW, our results suggest extra spin-reversals take place to form a v >1 ground state consisting of a Skyrmion bound to the hole. The spectr a also show shake-up structure due to the formation of spin-waves afte r recombination of an electron in the full spin-state. (C) 1998 Elsevi er Science Ltd. All rights reserved.