Singlet Polaron Theory of Low-Energy Optical Excitations in NiPS3

We develop a theory that explains the low-energy optical excitations near 1.5 eV observed by optical experiments in NiPS₃. Using ab initio methods, we construct a two-band Hubbard model for two effective Ni orbitals. The dominant effective hopping corresponds to third-nearest neighbors. This model exhibits triplet-singlet excitations of energy near 2 times the Hund exchange. We derive an effective model for the movement of two singlets in an antiferromagnetic background, that we solve using a generalized self-consistent Born approximation, disentangling the nature of these novel excitations, which move coherently as "singlet polarons".