Theoretical investigation of excitonic magnetism in LaSrCoO$_{4}$

TL;DR

This paper investigates the potential for excitonic magnetism in LaSrCoO4 using theoretical models, revealing a stable magnetic multipole order and conditions for exciton condensation, with differences noted compared to LaCoO3.

Contribution

It introduces a theoretical framework for excitonic magnetism in LaSrCoO4, including an effective model and analysis of exciton dispersion and condensation.

Findings

Stable magnetic multipole order identified

Exciton gap decreases and may vanish in layered LaSrCoO4

Comparison shows smaller excitonic gap in layered cobaltite

Abstract

We use the LDA+U approach to search for possible ordered ground states of LaSrCoO$_4$. We find a staggered arrangement of magnetic multipoles to be stable over a broad range of Co $3d$ interaction parameters. This ordered state can be described as a spin-denity-wave-type condensate of $d_{xy} \otimes d_{x^2-y^2}$ excitons carrying spin $S=1$. Further, we construct an effective strong-coupling model, calculate the exciton dispersion and investigate closing of the exciton gap, which marks the exciton condensation instability. Comparing the layered LaSrCoO$_4$ with its pseudo cubic analog LaCoO$_3$, we find that for the same interaction parameters the excitonic gap is smaller (possibly vanishing) in the layered cobaltite.