Ambient-pressure superconductivity onset above 40 K in bilayer nickelate ultrathin films

TL;DR

This paper reports the discovery of superconductivity above 40 K at ambient pressure in bilayer nickelate ultrathin films, achieved through epitaxial growth and strain engineering, with evidence of BKT-like transition and Meissner effect.

Contribution

It demonstrates ambient-pressure superconductivity in bilayer nickelates with transition temperature above 40 K, using a novel epitaxial growth method and strain control.

Findings

Superconductivity onset at 45 K in bilayer nickelate films.

Observation of BKT-like transition with TBKT = 9 K.

Detection of Meissner diamagnetism at 8.5 K.

Abstract

The discovery of bilayer nickelate superconductors under high pressure has opened a new chapter in high-transition temperature (high-TC) superconductivity. Here, we report ambient-pressure superconductivity onset above the McMillan limit (40 K) in bilayer nickelate epitaxial ultrathin films. Three-unit-cell (3UC) thick La2.85Pr0.15Ni2O7 single-phase-crystalline films are grown using the gigantic-oxidative atomic-layer-by-layer epitaxy (GOALL-Epitaxy) on SrLaAlO4 substrates. Resistivity measurements and magnetic-field responses indicate onset TC = 45 K. The transition to zero resistance exhibits characteristics consistent with a Berezinskii-Kosterlitz-Thouless (BKT)-like behavior, with TBKT = 9 K. Meissner diamagnetic effect is observed at TM = 8.5 K via a mutual inductance setup, in agreement with the BKT-like transition. In-plane and out-of-plane critical magnetic fields exhibit anisotropy. Scanning transmission electron microscopy (STEM) images and X-ray reciprocal space mappings (RSMs) show that the films maintain a tetragonal phase with coherent epitaxial compressive strain ~2% in the NiO2 planes relative to the bulk. Our findings pave the way for comprehensive investigations of nickelate superconductors under ambient pressure conditions and for exploring superconductivity at higher transition temperature through strain engineering in heterostructures.