Israel Bar Joseph | Searcharxiv

arXiv2024

Giant Hyperfine Interaction between a Dark Exciton Condensate and Nuclei

We study the interaction of a dark exciton Bose-Einstein condensate with the nuclei in GaAs/AlGaAs coupled quantum wells and find clear evidence for nuclear polarization buildup that accompanies the appearance of the condensate. We show that the nuclei are polarized throughout the mesa area, extending to regions which are far away from the photoexcitation area, and persisting for seconds after the excitation is switched off. Photoluminescence measurements in the presence of RF radiation reveal that the hyperfine interaction between the nuclear and electron spins is enhanced by two orders of magnitude. We suggest that this large enhancement manifests the collective nature of the N-excitons condensate, which amplifies the interaction by a factor of sqrt{N}.

cond-mat.mes-hall↗

arXiv2022

The Role of Spin-Flip Collisions in a Dark Exciton Condensate

We show that a Bose-Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under in-plane magnetic field allows us to preferentially modify the bright excitons density, and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate equations model, which well-reproduces the observed temperature, power, and magnetic field dependence of the exciton density.

cond-mat.mes-hall↗