We present experimental inelastic x-ray scattering (IXS) and ab initio time-dependent density-functional-theory (TDDFT) studies of YBa2Cu3O7-{\delta}. The response of the low-lying Ba 5p and Y 4p core electrons is shown to interact strongly with the Cu 3d and O 2p excitations, with important consequences on screening. The agreement between IXS and TDDFT results is excellent, apart from a new type of excitations, mainly related to loosely bound Ba electrons and significantly affected by correlations. This points to correlation mechanisms not fully described by TDDFT that might have a role in giving rise to antiscreening.
@article{arxiv.1011.1436,
title = {Screening in YBCO at large wave vectors},
author = {Simo Huotari and J. Aleksi Soininen and György Vankó and Giulio Monaco and Valerio Olevano},
journal= {arXiv preprint arXiv:1011.1436},
year = {2010}
}