Electronic Structure of Ce-Doped and -Undoped Nd$_2$CuO$_4$ Superconducting Thin Films Studied by Hard X-ray Photoemission and Soft X-ray Absorption Spectroscopy

TL;DR

This study investigates the electronic structure of Ce-doped and undoped Nd$_2$CuO$_4$ thin films using advanced spectroscopic techniques to understand the origin of superconductivity, revealing that undoped films can become superconducting through annealing-induced doping.

Contribution

It provides the first detailed spectroscopic analysis showing that superconducting undoped Nd$_2$CuO$_4$ films are electronically similar to Ce-doped counterparts, highlighting the role of annealing in doping.

Findings

Superconductivity in undoped films is induced by annealing, which alters electronic states.

Core-level spectra indicate increased electron doping after annealing.

Electronic states of undoped superconducting films resemble those of Ce-doped superconductors.

Abstract

In order to realize superconductivity in cuprates with the T'-type structure, not only chemical substitution (Ce doping) but also post-growth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T'-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T'-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopies on superconducting and non-superconducting Nd$_{2-x}$Ce$_x$CuO$_4$ ($x=$ 0, 0.15, and 0.19) thin films. By post-growth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the CuO$_2$ plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting Nd$_2$CuO$_4$ film is doped with electrons and that the electronic states are similar to those of Ce-doped superconductors.