Correlation-driven metal-insulator transition in unconventional magnetic metal superoxides

Abstract

We have extensively studied the electronic and magnetic properties of alkali sodium superoxide (NaO2) in comparison with that of potassium superoxide (KO2) both at high and low temperatures, using first-principles electronic structure calculations. The unpaired electron donated by the alkali atoms Na and K to the O atoms, forming dimers in NaO2 and KO2 respectively, control their properties. In both cases, this unpaired electron is the cause of orbital fluctuations in the O-π* manifold. Both undergo several structural phase transitions to minimize orbital fluctuations. As a result, the O2− dimers undergo several rotational configurations. This leads to a complex linking of their orbital and spin degrees of freedom. Therefore, as the orientation of the O2− dimers changes, so do the magnetic properties that were found to be controlled by this unpaired electron. Simultaneously the alkali ion cages surrounding the O2− dimers change from square in the pyrite phase to rhombus and rectangle in the orthorhombic phase for NaO2 and square in the tetragonal phase to a parallelogram in the monoclinic phase for KO2 on the plane cutting through the dimers. The different degree of the lifting of degeneracy in the O-π* manifold is owing to an intrinsic difference in the electrostatic interaction between the K/Na cages and the O2− dimmers, as shown by the band structures of NaO2 in the low-temperature orthorhombic phase and that of KO2 in the monoclinic phase. This, along with electron correlation among the localized O-π* electrons, establishes complete orbital ordering (OO) and thereby metal-insulator transition (MIT) in both systems. Correlation-induced MIT also occurs upon K doping for Na in NaO2 but is predicted to take place at a temperature higher than that in pure NaO2. This makes MIT in Rb/Cs-doped NaO2 a possibility at higher temperatures.