Electronic and Magnetic Structure of LaCuO$_{2.5}$

TL;DR

This paper investigates the electronic and magnetic properties of LaCuO$_{2.5}$, revealing its three-dimensional character, proximity to a spin-liquid to antiferromagnetic transition, and the effects of electron interactions on its magnetic state.

Contribution

It provides a detailed theoretical analysis combining bandstructure fitting and mean-field theory to understand the magnetic phases and transitions in LaCuO$_{2.5}$, highlighting the role of interladder coupling and electron correlations.

Findings

LaCuO$_{2.5}$ is three-dimensional with significant interladder hopping.

The spin gap decreases with increasing interladder coupling, approaching a transition to magnetic order.

Introduction of on-site repulsion U induces a transition to an antiferromagnetic insulator.

Abstract

The recently-discovered ``ladder'' compound LaCuO$_{2.5}$ has been found to admit hole doping without altering its structure of coupled copper oxide ladders. While susceptibility measurements on the parent compound suggest a spin gap and a spin-liquid state, NMR results indicate magnetic order at low temperatures. These seemingly contradictory results may be reconciled if in fact the magnetic state is near the crossover from spin liquid to antiferromagnet, and we investigate this possibility. From a tight-binding fit to the valence LDA bandstructure, we deduce that the strength of the interladder hopping term is approximately half that of intraladder hopping, showing that the material is three-dimensional in character. A mean-field treatment of the insulating magnetic state gives a spin-liquid phase whose spin gap decreases with increasing interladder coupling, vanishing (signalling a transition to the ordered phase) at a value somewhat below that obtained for LaCuO$_{2.5}$. The introduction of an on-site repulsion term, $U$, to the band scheme causes a transition to an antiferromagnetic insulator for rather small but finite values of $U$, reflecting the predominance of (one-dimensional) ladder behavior, and an absence of any special nesting features.