Spin Memory of Photocreated Carriers in Quantum Wells in High Magnetic Fields

Abstract

Spin relaxation of photocreated carriers in GaAs/GaAlAs quantum wells in high magnetic fields is investigated by luminescence experiments under selective optical excitation. The complete quantization of the two-dimensional energy structure in high magnetic fields implies a simultaneous exchange of energy and momentum for spin relaxation and makes spin conserving relaxation processes, both in thermalization and recombination, much faster than spin-flip processes. This results in a new spectroscopic tool to identify magneto excitonic states with the same electronic spin orientation. We apply this technique to study the zero field splitting of the heavy hole exciton ground state often observed in high quality GaAs/GaAlAs quantum wells. We find that each peak of this doublet splits in a magnetic field into two components, and using the selective relaxation we can attribute the conduction band spin character to each of these. It turns out that the ordering of the electron spins of the Zeeman splitting of the upper component of the doublet is inverted with respect to that of the lower component. This observation strongly suggests a magnetic mechanism, a possibility would be exchange, as the origin of the zero field splitting.