Decoding Superconductivity in La$_3$Ni$_2$O$_{7-\delta}$ Thin Films via Ozone-Driven Structure and Oxidation Tuning

TL;DR

This study investigates the structural and electronic factors influencing superconductivity in La$_3$Ni$_2$O$_{7-eta}$ thin films, highlighting the importance of oxygen content, strain, and structure for stabilizing superconductivity at ambient conditions.

Contribution

It provides a detailed structural analysis linking polymorphs and transport properties, offering insights into stabilizing superconductivity in metastable nickelate thin films.

Findings

Superconductivity varies with post-annealing conditions.

Structural polymorphs correlate with transport behavior.

Oxygen homogeneity and strain are critical for superconductivity.

Abstract

The discovery of superconductivity in bulk Ruddlesden-Popper La$_3$Ni$_2$O$_7$(LNO327) under high hydrostatic pressure has redefined the recent experimental consensus that nickelate superconductivity is restricted to systems with a $3d^9$ electronic configuration and square-planar coordination. However, the structural and electronic prerequisites for stabilizing superconductivity, whether under pressure or at ambient conditions in the case of thin films, remain poorly understood, largely due to the metastable nature of the LNO327 phase. Here, we present a detailed structural study of epitaxial La$_3$Ni$_2$O$_{7-\delta}$ thin films by using scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). Grown via pulsed laser deposition onto SrLaAlO$_4$ substrates, those films exhibit distinct superconducting properties as a function of the different post-annealing conditions used. By correlating the rich landscape of stacking polymorphs with transport behavior, this work establishes a framework for understanding the metastable superconducting phase in bilayer nickelate thin films. Our findings underscore the critical role of homogeneity in oxygen stoichiometry, epitaxial strain and structural motif in stabilizing superconductivity, offering a clear pathway for designing ambient-pressure superconducting nickelates.