Spin-active single photon emitters in hexagonal boron nitride from carbon-based defects

Most single photon emitters in hexagonal boron nitride have been identified as carbon substitutional defects, forming donor-acceptor systems. Unlike the most studied bulk emitters (i.e. color centers in diamond), these defects have no net spin, or have a single unpaired spin. By means of density functional calculations, we show that two non-adjacent carbon substitutional defects of the same type (i.e. C-B-C-B, and C-N-C-N), can have a triplet groundstate. In particular, one of such defects has a zero phonon line energy of 2.5 eV, and its triplet state is nearly 0.5 eV more stable than its singlet. The mechanism behind the destabilization of the singlet state is related to a larger electrostatic repulsion of a symmetric wave function in a charged lattice.