Anomalous Expansion of the Copper-Apical Oxygen Distance in Superconducting La$_{2}$CuO$_{4}$ - La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ Bilayers

TL;DR

This study uses advanced X-ray phase-retrieval to precisely measure atomic structures in ultrathin superconducting bilayers, revealing significant surface-specific expansion of copper-apical oxygen distances that impact superconductivity.

Contribution

It introduces a rapid, high-precision phase-retrieval method for atomic-scale structural analysis of ultrathin films, highlighting surface-specific structural modifications.

Findings

Copper-apical oxygen distance increases by up to 0.45 Å near the surface.

Unit cell size remains constant near the surface.

Structural modifications are critical for understanding surface-sensitive superconductivity experiments.

Abstract

We have introduced an improved X-ray phase-retrieval method with unprecedented speed of convergence and precision, and used it to determine with sub-{\AA}ngstrom resolution the complete atomic structure of an ultrathin superconducting bilayer film, composed of La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ and La$_{2}$CuO$_{4}$ neither of which is superconducting by itself. The results show that phase-retrieval diffraction techniques enable accurate measurement of structural modifications in near-surface layers, which may be critically important for elucidation of surface-sensitive experiments. Specifically we find that close to the sample surface the unit cell size remains constant while the copper-apical oxygen distance shows a dramatic increase, by as much as 0.45 {\AA}. The apical oxygen displacement is known to have a profound effect on the superconducting transition temperature.