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

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