  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.
