Self-doping processes between planes and chains in the metal-to-superconductor transition of YBa2Cu3O6.9

TL;DR

This study reveals how a reconstruction of interplanar orbitals during the metal-superconductor transition in YBCO causes self-doping, transferring holes from chains to planes, which is crucial for understanding high-temperature superconductivity.

Contribution

It provides new insights into the self-doping process involving interplanar orbital reconstruction during the MST of YBCO, combining spectroscopy and ionic modeling.

Findings

Reconstruction of apical O(4)-derived orbitals during MST.

Substantial hole transfer from chains to planes in YBCO.

Suppression of divalent charge-transfer configurations in the superconducting state.

Abstract

The interplay between the quasi 1-dimensional CuO-chains and the 2-dimensional CuO2 planes of YBa2Cu3O6+x (YBCO) has been in focus for a long time. Although the CuO-chains are known to be important as charge reservoirs that enable superconductivity for a range of oxygen doping levels in YBCO, the understanding of the dynamics of its temperature-driven metal-superconductor transition (MST) remains a challenge. We present a combined study using x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) revealing how a reconstruction of the apical O(4)-derived interplanar orbitals during the MST of optimally doped YBCO leads to substantial hole-transfer from the chains into the planes, i.e. self-doping. Our ionic model calculations show that localized divalent charge-transfer configurations are expected to be abundant in the chains of YBCO. While these indeed appear in the RIXS spectra from YBCO in the normal, metallic, state, they are largely suppressed in the superconducting state and, instead, signatures of Cu trivalent charge-transfer configurations in the planes become enhanced. In the quest for understanding the fundamental mechanism for high-Tc-superconductivity (HTSC) in perovskite cuprate materials, the observation of such an interplanar self-doping process in YBCO opens a unique novel channel for studying the dynamics of HTSC.