In materials showing a high level of electron electron correlation, local character and collective excitations both contribute in shaping the emergent phenomena peculiar of these systems. Among others, orbital dd excitations are widely believed to carry a strongly local atomic-like character in cuprates insulating parent compounds, due to the strong Coulomb repulsion that suppresses electron mobility.
In the last decades Resonant Inelastic X-ray Scattering (RIXS) has been widely demonstrated to be the ideal technique to study orbital dd excitations, thanks to its resonant character that enhances the dd cross section.
In this letter we highlight that dd excitations show clear dispersing behaviour in the fully undoped insulating cuprates CaCuO2 (CCO) and Nd2CuO4 (NCO), thus calling for a collective nature of such excitations in terms of orbitons. Indeed, extremely high resolution RIXS momentum maps, shown in Fig. 1, clearly demonstrate a dispersing behaviour, mostly of the dxy peak.
The peculiarity of CCO and NCO is the absence of apical oxygens above and below copper-oxygen planes, which in instead a characteristic of most of cuprates families. We demonstrated that this enhances the next-nearest-neighbor orbital superexchange, which in turn we identify as the key ingredient that allows for delocalized orbitons in such families of cuprates.
Martinelli et al., Physical Review Letters 132.6 (2024): 066004
Figure 1
RIXS momentum maps for CCO and NCO along high symmetry directions.
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