Superconductivity induced by structural reorganization in the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$

TL;DR

This study demonstrates that superconductivity in Nd$_{2-x}$Ce$_x$CuO$_4$ thin films can be induced by structural reorganization of oxygen ions during annealing, which alters the electronic properties and suppresses antiferromagnetic order.

Contribution

It reveals that oxygen migration to apical positions during annealing induces superconductivity, supported by experimental structural data and first-principles calculations, providing new insight into electron-doped cuprates.

Findings

Superconductivity occurs after annealing in oxygen-free environment.

Annealing increases interlayer distance between CuO$_2$ planes.

Oxygen migration to apical sites modulates electronic structure.

Abstract

Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is however only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content by only a tiny amount, but has a dramatic impact on the electronic properties of the sample. Here we report the occurrence of superconductivity in oxygen-deficient Nd$_{2-x}$Ce$_x$CuO$_4$ thin films grown in an oxygen-free environment, after annealing in pure argon flow. As verified by x-ray diffraction, annealing induces an increase of the interlayer distance between CuO$_2$ planes in the crystal structure. Since this distance is correlated to the concentration of oxygens in apical positions, and since oxygen content cannot substantially increase during annealing, our experiments indicate that the superconducting phase transition has to be ascribed to a migration of oxygen ions to apical positions during annealing. Moreover, as we confirm via first-principles density functional theory calculations, the changes in the structural and transport properties of the films can be theoretically described by a specific redistribution of the existing oxygen ions at apical positions with respect to CuO$_2$ planes, which remodulates the electronic band structure and suppresses the antiferromagnetic order, allowing the emergence of hole superconductivity.