Exciton spin relaxation in resonantly excited CdTe/ZnTe self-assembled quantum dots

TL;DR

This study investigates how exciton spin relaxation in CdTe/ZnTe quantum dots depends on degeneracy and external magnetic fields, revealing that non-degenerate states preserve spin polarization much longer.

Contribution

It demonstrates that exciton spin relaxation time significantly increases when degeneracy is lifted, providing insights into spin dynamics in quantum dots under different conditions.

Findings

Spin polarization is rapidly lost in degenerate states at zero magnetic field.

Applying a magnetic field extends the exciton spin polarization lifetime.

Shape asymmetry in quantum dots preserves spin polarization similar to magnetic field effects.

Abstract

We study the exciton spin relaxation in CdTe self-assembled quantum dots by using polarized photoluminescence spectroscopy in magnetic field. The experiments on single CdTe quantum dots and on large quantum dot ensembles show that by combining phonon-assisted absorption with circularly polarized resonant excitation the spin-polarized excitons are photo-excited directly into the ground states of quantum dots. We find that for single symmetric quantum dots at B=0 T, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by applying external magnetic field, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. We also observe that the exciton spin polarization is conserved when the splitting between exciton states is caused by quantum dot shape asymmetry. Similar behavior is found in a large ensemble of CdTe quantum dots. These results show that while exciton spins scatter rapidly between degenerate states, the spin relaxation time increases by orders of magnitude as the exciton spin states in a quantum dot become non-degenerate. Finally, due to strong electronic confinement in CdTe quantum dots, the large spin polarization of excitons shows no dependence on the number of phonons emitted between the virtual state and the exciton ground state during the excitation.