Spectrum for nonmagnetic mott insulators from power functional within reduced density matrix functional theory.

Area of Science:

• Solid-state physics
• Quantum chemistry
• Materials science

Background:

• Transition metal oxides like NiO and MnO exhibit complex electronic properties.
• Accurate theoretical modeling of these materials is challenging due to electron correlation effects.
• Previous methods struggled to capture the insulating nature and spectroscopic signatures.

Purpose of the Study:

• To evaluate the efficacy of reduced density matrix functional theory (RDMFT) with a power functional for NiO and MnO.
• To investigate the role of long-range spin order in achieving quantitative agreement with experimental data.
• To analyze the nature of natural orbitals and their deviation from Kohn-Sham orbitals.

Main Methods:

• Application of reduced density matrix functional theory (RDMFT) coupled with the power functional.
• Calculation of electronic band structures and single-particle spectra.
• Comparison of theoretical predictions with experimental X-ray photoemission spectroscopy (XPS) and Bremsstrahlung isochromat spectroscopy (BIS) data.
• Analysis of natural orbital localization and comparison with Kohn-Sham orbitals.

Main Results:

• RDMFT successfully predicts a gapped single-particle spectrum for nonmagnetic NiO and MnO.
• Qualitative agreement with experimental data is achieved without long-range spin order.
• Quantitative agreement with XPS and BIS data necessitates the inclusion of long-range spin order.
• Natural orbitals in NiO and MnO show significant Hubbard localization, differing greatly from Kohn-Sham orbitals.
• In contrast, Kohn-Sham orbitals closely resemble RDMFT natural orbitals for the band insulator silicon.

Conclusions:

• RDMFT with a power functional is a viable method for studying correlated insulators.
• Long-range spin order is essential for quantitative spectroscopic predictions in NiO and MnO.
• The significant difference between natural and Kohn-Sham orbitals in these transition metal oxides highlights strong electron correlation effects.