Giant permanent dipole moment of 2D excitons bound to a single stacking fault

We investigate the magneto-optical properties of excitons bound to single stacking faults in high-purity GaAs. We find that the two-dimensional stacking fault potential binds an exciton composed of an electron and a heavy-hole, and confirm a vanishing in-plane hole $g$-factor, consistent with the atomic-scale symmetry of the system. The unprecedented homogeneity of the stacking-fault potential leads to ultra-narrow photoluminescence emission lines (with full-width at half maximum ${\lesssim 80~\mu \text{eV} }$) and reveals a large magnetic non-reciprocity effect that originates from the magneto-Stark effect for mobile excitons. These measurements unambiguously determine the direction and magnitude of the giant electric dipole moment (${\gtrsim e \cdot 10~\text{nm}}$) of the stacking-fault exciton, making stacking faults a promising new platform to study interacting excitonic gases.