Evidence for suppressed metallicity on the surface of La2-xSrxCuO4 and Nd2-xCexCuO4

TL;DR

This study uses hard X-ray PES to reveal suppressed metallic screening at the surface of La2-xSrxCuO4 and Nd2-xCexCuO4, showing differences in electronic states and doping effects with implications for surface vs. bulk properties.

Contribution

It demonstrates the surface suppression of metallic screening in cuprates and links core-level shifts to valence and conduction band modifications upon doping.

Findings

Metallic screening is strongly suppressed within 15 Å of the surface.

Presence of Zhang-Rice singlet in La2CuO4 but not in Nd2CuO4.

Small chemical potential shifts are consistent with band modifications upon doping.

Abstract

Hard X-ray Photoemission spectroscopy (PES) of copper core electronic states, with a probing depth of $\sim$60 \AA, is used to show that the Zhang-Rice singlet feature is present in La$_2$CuO$_4$ but is absent in Nd$_2$CuO$_4$. Hole- and electron doping in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO) result in new well-screened features which are missing in soft X-ray PES. Impurity Anderson model calculations establish metallic screening as its origin, which is strongly suppressed within 15 $\text{\AA}$ of the surface. Complemented with X-ray absorption spectroscopy, the small chemical-potential shift in core levels ($\sim0.2$ eV) are shown to be consistent with modifications of valence and conduction band states spanning the band gap ($\sim1$ eV) upon hole- and electron-doping in LSCO and NCCO.