Electronic structure of ternary palladates and effect of hole doping: a valence band photoemission spectroscopic study

Abstract

We investigate the electronic structure of ternary palladates APd3O4 (A = Sr, Ca) using valence band photoemission spectroscopy and band structure calculations. Energy positions of various features and overall width of the experimental valence band spectra are well captured by band structure calculations using hybrid functional. Band structure calculations within local density approximations lead to metallic ground state while the calculations using hybrid functional provide band gap of 0.25 eV and 0.22 eV for CaPd3O4 and SrPd3O4 respectively, suggesting moderate to strong electron correlation strength in these narrow band gap semiconducting palladates. High resolution spectra reveal negligibly small intensity at Fermi level, E F, for parent compounds, while hole doped SrPd3O4 (by 15% Li substitution at Sr site) exhibits a Fermi cut-off suggesting metallic character in contrast to semiconducting transport. These observations reveal the importance of localization of electrons in case where the Fermi edge falls in the mobility edge.