Effective on-site Coulomb interaction based on linear response theory for transition-metal monoxide thin films

摘要

Density Functional Theory (DFT) has been commonly regarded as one of the most important tools to study the electronic structure of materials. However, DFT is insufficient to adequately describe the electronic structure of transition-metal oxides (MTOs) as it is impossible to fully treat the strong interaction caused by the localized electrons. As an alternative, DFT+Umethod has become one of effective means to deal with this problem. This method describes the electronic states by introducing the screened on-site Coulomb interaction in terms of Hubbard model parameter,Ueff. TheUeff values are commonly chosen to match experimental observations or calculated directly from first-principles calculations. However, various values for theUeffhave been reported even for the same ionic state and the same material. Since material properties are susceptiole to theUeff values, it is important to choose reliable ones. Previously, we have investigated the on-site Coulomb interaction in bulk 3d TMOs, based on constraint DFT approach within alinear response theory.[1]We here extend our approach to determine theUeff values for thin films of 3d TMOs. Calculations were carried out based on the generalized gradient approximation[2] by using full-potential linearized augmented plane wave method. [3] With a model of a freestanding (001) monolayer, the in-plane lattice constants were fixed at experimental values of bulk TMOs and a ferromagnetic structure was assumed for simplicity. In the FeO monolayer, for example, the value ofひeff result is 1.74 eV when a unit cell including one TM atom is included. For a 4x4 supercell with sixteen TM atoms, it is converged to 9.13 eV. Furthermore, we confirmed the insulating characters obtained from DFT+Udue to the localized 3d electrons. Details of the discussion, as well as those for other thin films of CoO and NiO, will be presented.