Antiferromagnetic and structural transitions in the superoxide KO2 from first principles: A 2p-electron system with spin-orbital-lattice coupling

TL;DR

This study uses first-principles calculations to explore the coupled magnetic, structural, and electronic transitions in KO2, revealing the roles of spin-orbit coupling, Coulomb interactions, and orbital ordering in its phase behavior.

Contribution

It provides a detailed first-principles analysis of the spin-orbital-lattice coupling mechanisms driving KO2's phase transitions, highlighting the importance of orbital ordering and electron correlations.

Findings

High-temperature phase is insulating due to spin-orbit coupling and Coulomb correlation.

Low-temperature phase's band gap and orbital order are influenced by crystal field and Coulomb effects.

Ferro-orbital ordering is crucial for the antiferromagnetic structure in KO2.

Abstract

KO2 exhibits concomitant antiferromagnetic (AFM) and structural transitions, both of which originate from the open-shell 2p electrons of O$_{2}^{-}$ molecules. The structural transition is accompanied by the coherent tilting of O$_{2}^{-}$ molecular axes. The interplay among the spin-orbital-lattice degrees of freedom in KO2 is investigated by employing the first-principles electronic structure theory and the kinetic-exchange interaction scheme. We have shown that the insulating nature of the high symmetry phase of KO2 at high temperature (T) arises from the combined effect of the spin-orbit coupling and the strong Coulomb correlation of O 2p electrons. In contrast, for the low symmetry phase of KO2 at low T with the tilted O$_{2}^{-}$ molecular axes, the band gap and the orbital ordering are driven by the combined effects of the crystal-field and the strong Coulomb correlation. We have verified that the emergence of the O 2p ferro-orbital ordering is essential to achieve the observed AFM structure for KO2.