Collective nature of orbital excitations in layered cuprates in the absence of apical oxygens

TL;DR

This study reveals that in layered cuprates lacking apical oxygens, orbital excitations exhibit collective behavior with significant dispersion, driven by enhanced next-nearest-neighbor superexchange interactions.

Contribution

It demonstrates the impact of missing apical oxygens on orbital excitations, highlighting the role of NNN superexchange in orbiton dynamics in cuprates.

Findings

Orbital excitations in LCO are localized.

In CCO and NCO, orbitons disperse, indicating collectivity.

Large NNN superexchange is linked to absence of apical oxygens.

Abstract

We have investigated the 3d orbital excitations in CaCuO2 (CCO), Nd2CuO4 (NCO), and La2CuO4 (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the dxy orbital clearly disperse, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. We show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy.