Carbon-based single photon emitters in hexagonal boron nitride with triplet ground state

Most single photon emitters in hexagonal boron nitride has been identified as carbon-based defects. These defects, when forming a donor-acceptor pair have spin $S\leq \frac{1}{2}$. By means of density functional calculations, we show that two non-adjacent carbon substitutional defects of the same type (i.e. C$_\mathrm{B}$-C$_\mathrm{B}$, and C$_\mathrm{N}$-C$_\mathrm{N}$), can have a triplet ground state. 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.