Realistic many-body models for manganese monoxide under pressure

In materials such as transition-metal oxides where electronic Coulomb correlations impede a description in terms of standard band theories, the application of genuine many-body techniques is inevitable. Interfacing the realism of density-functional-based methods with the virtues of Hubbard-type Hamiltonians, requires the joint ab initio construction of transfer integrals and interaction matrix elements (such as the Hubbard U) in a localized basis set. In this work, we employ the scheme of maximally localized Wannier functions and the constrained random-phase approximation to create effective low-energy models for manganese monoxide and track their evolution under external pressure. We find that in the low-pressure antiferromagnetic phase, the compression results in an increase in the bare Coulomb interaction for specific orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79, 235133 (2009)], this seemingly counterintuitive behavior is a consequence of the delocalization of the respective Wannier functions. The change in screening processes does not alter this tendency, and thus, the screened on-site component of the interaction, the Hubbard U of the effective low-energy system, increases with pressure as well. The orbital anisotropy of the effects originates from the orientation of the orbitals vis-agrave-vis the deformation of the unit cell. Within the high-pressure paramagnetic phase, on the other hand, we find the significant increase in the Hubbard U is insensitive to the orbital orientation and almost exclusively owing to a substantial weakening of screening channels upon compression.