Spin relaxation in optically excited quantum wells

Abstract Interband excitation of GaAs with circularly polarised light produces a spin-polarised population of photoexcited carriers. The equilibrium degree of spin polarisation under cw excitation depends on the balance of spin relaxation and recombination rates, and can be monitored directly through the degree of polarisation of recombination radiation. We have measured the polarisation at 2 K of recombination radiation following band edge excitation for a variety of GaAs/AlGaAs quantum well samples of carrier content ranging from ≈ 10 8 cm -2 up to ≈ 2×10 11 cm -2 in which values of recombination time have been measured, and have thereby determined the spin relaxation times. Two remarkable results emerge; (i) the spin relaxation time increases from 30 ps to > 8 ns as the carrier concentration is increased; and (ii) for degenerate n-type quantum wells the recombination radiation is close to 100% circularly polarised, there being strong evidence for a long spin memory in the electron Fermi sea. The first result may be explained in terms of a spin relaxation mechanism involving electron-hole exchange; the second has, at present, no clear theoretical interpretation.