Multiple pre-edge structures in CuK-edge x-ray absorption spectra of high-Tccuprates revealed by high-resolution x-ray absorption spectroscopy

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 excitations invisible in standard x-ray absorption spectra. We consider in detail the case of ${\text{Ca}}_{2\ensuremath{-}x}{\text{CuO}}_{2}{\text{Cl}}_{2}$ 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.