Triplet and Singlet Electronic States of a Local Neutral Defect in Hexagonal AlN with Excitation Profile Favorable for Solid-State Spin Qubit Applications: An Ab initio Study*

By applying an educated guess, we designed a local neutral defect in hexagonal AlN as a prospect for having a triplet ground state. Density functional theory and linear response GW calculations confirm that the system has indeed a triplet electronic ground state. A low energy optical excitation leads to a charge density redistribution that decreases the symmetry of the system and hints a structural transformation. We model such structural transformation and achieve a singlet ground state, which is at a local energy minimum in the coordinate space. Analysis of the GW electronic states and the Bethe-Salpeter equation eigenstates related to the relevant optical excitations, and the phonon spectrum of the considered defected system in both the triplet and singlet states let us construct a many-body scheme similar to that of the NV^— center in diamond: 1) an optical excitation in the triplet state reduces the symmetry of the system, 2) the excited triplet state can undergo a phonon-assisted transition to an excited singlet state, 3) an optical emission brings the system to a local ground singlet state, 4) a non-radiative transition may bring the system back to the symmetric triplet ground state.