Carbon substitutional spin defects in bulk hBN using r2SCAN

Quantum networks form the most important part of secure post-quantum communication and distributed quantum computing. Realizing long-distance quantum links requires efficient spin-photon interfaces that operate in the telecom range, which can be achieved through optically addressable paramagnetic defects. Hexagonal boron nitride (hBN) represents a very promising platform due to its wide bandgap and the ability to host many single-photon emitters. While intrinsic defects such as the negatively charged boron vacancy have been extensively explored as potential qubit candidates, different dopants are investigated, with carbon attracting considerable attention due to its ability to form complexes and interact with other known spin-defects. In this work, we investigate carbon substitutional occupying boron and nitrogen lattice sites in bulk hBN, using density functional theory with meta-GGA functionals. We compute the formation energies, electronic structure, and density of states across different charge configurations, and identify how substitutional doping influences the defect level in the bandgap. Furthermore, we study the excited state of these defects, which is formed by a defect-bound exciton capable of operating at low temperatures. The present findings highlight the importance of carbon defects and expand the range of candidate systems for quantum communication and sensing.