Hole doping in high temperature superconductors using the XANES technique

Abstract

Superconducting and physical properties of F-doped HgPb-1223 and Ce-doped Tl-1223systems were considerably improved through adjusting the hole content of the two systems.In this study, we have used the x-ray absorption near-edge structure (XANES)technique to investigate the electronic structure of the two systems by probing theunoccupied electronic states. For the F-doped Hg-1223 system, the O K-edge, CaL2,3 andCu L2,3-edge structures were thoroughly investigated. The pre-edge features of O K-edge spectra,as a function of doping, reveal important information about the projected local density ofunoccupied states on the O sites in the region close to the absorption edge, whichis a measure of O 2p hole concentration in the valence band. In the originallyunder-doped Hg-1223, the results indicate that the number of O 2p holes in theCuO2 planesincreases as fluorine was introduced up to an optimal value, after which it decreases. Furthermore, theCu L2,3 absorption edge provides useful information about the valencestate of Cu which is also related to the hole density in theCuO2 planes and confirms the same previous conclusion. The CaL2,3-edge shows the presence crystal field splitting inHgPb1223/Fx which issimilar to CaF2 and CaO in addition to the spin–orbit splitting of the Ca 2p core levelelectrons. These results ensure that fluorine goes into the structure ofHgPb-1223/Fx and it occupies the vacant interstitial oxygen site in the Hg–O plane,as was expected. In Ce-substituted Tl-1223, similar measurements wereperformed for samples with different Ce content. The pre-edge feature of theO K-edge spectra shows clearly the drastic decrease of the hole content inCuO2 planes of this originally over-doped system with increasing Ce content. This result is alsoconfirmed from the chemical state of Ce in the structure as obtained from the CeM4,5-edge spectra.