Entangled orbital, spin, and ferroelectric orders in $p$-electron magnet CsO$_2$

TL;DR

This paper uncovers the coexistence and entanglement of orbital, spin, and ferroelectric orders in CsO$_2$, revealing its unconventional magnetoelectric properties driven by complex quantum interactions.

Contribution

It demonstrates the presence of highly entangled orbital, spin, and ferroelectric orders in CsO$_2$, a p-electron magnet, using combined theoretical and experimental approaches.

Findings

CsO$_2$ has a canted antiferromagnetic ground state.

A spin-flop transition induces ferroelectricity.

Orbital order stabilizes magnetic structure and induces polarization.

Abstract

Alkali superoxides differ from conventional transition metal magnets, exhibit magnetism from partially occupied oxygen molecular $\pi^*$-orbitals. Among them, CsO$_2$ stands out for its potential to exhibit novel quantum collective phenomena, such as an orbital order induced Tomonaga-Luttinger liquid state. Using ab-initio Hubbard models, superexchange theory, and experimental spin wave measurements, we propose that CsO$_2$ exhibits unconventional magnetoelectric characteristics at low temperature. Our analysis confirms a canted antiferromagnetic ground state and a spin-flop transition, with ferroelectricity is induced by breaking inversion and time-reversal symmetry in the spin-flop phase. Consequently, our analysis reveals a strong interplay not only between exchange interactions but also among magnetically-induced polarization and orbital order. The magnetic structure, stabilized by orbital order, induces magnetically-induced polarization through an antisymmetric mechanism. Overall, our results reveal the coexistence of three highly entangled orders in CsO$_2$, namely, orbital, spin and ferroelectricity.