Multiple pre-edge structures in Cu K-edge of high Tc cuprates revealed by high resolution x-ray absorption spectroscopy

TL;DR

This study reveals multiple pre-edge structures in Cu K-edge X-ray absorption spectra of high-Tc cuprates, highlighting the importance of off-site transitions and many-body effects for accurate spectral interpretation.

Contribution

It demonstrates the presence of multiple pre-edge excitations in high-Tc cuprates and emphasizes the need for approaches beyond single-site models to understand their electronic structure.

Findings

Multiple pre-edge excitations depend on polarization and momentum.

Doubling of the main edge peak along c-axis suggests many-body charge-transfer effects.

Standard first-principles calculations do not reproduce the main edge doubling.

Abstract

Using high resolution x-ray absorption spectroscopy and state-of-the-art electronic structure calculations we demonstrate that the pre-edge region at the Cu K-edge of high T$_c$ cuprates is composed of several excitationsinvisible in standard X-ray absorption spectra. We consider in detail the case of Ca$_{2-x}$CuO$_2$Cl$_2$ and show that the many pre-edge excitations (two for c-axis polarization, four for in-plane polarization and out-of-plane incident X-ray momentum) are dominated by off-site transitions and intersite hybridization. This demonstrates the relevance of approaches beyond the single-site model for the description of the pre-edges of correlated materials. Finally, we show the occurrence of a doubling of the main edge peak that is most visible when the polarization is along the c-axis. This doubling, that has not been seen in any previous absorption data in cuprates, is not reproduced by first principles calculations. We suggest that this peak is due to many-body charge-transfer excitations, while all the other visible far-edge structures are single particle in origin. Our work indicates that previous interpretations of the Cu K-edge X-ray absorption spectra in high T$_c$ cuprates can be profitably reconsidered.