Quantum nickelate films reveal atomic-scale superconducting gap

Atomic‑Scale View of Superconductivity in Nickelate Films

A team at the State Key Laboratory of Quantum Functional Materials, Southern University of Science and Technology, has reported the first atomically resolved superconducting gap in bilayer nickelate (La,Pr)₃Ni₂O₇ films. By combining a precise “gigantic‑oxidative” atomic‑layer‑by‑layer growth method with ultra‑fast cryogenic transfer to a scanning tunneling microscope (STM), the researchers distinguished two distinct gap shapes—U‑shaped spectra that signal a fully opened, nodeless gap, and V‑shaped spectra that arise when oxygen is lost during handling. The work clarifies how oxygen stoichiometry controls the local electronic signature of superconductivity in these materials.

Precise Growth and Transfer Preserve the Intrinsic Gap

The films were grown as 1.5‑unit‑cell layers on SrLaAlO₄ substrates using a layer‑by‑layer epitaxy that yields atomically flat terraces and a reproducible √2 × √2 surface reconstruction. Rapid cooling below 150 K followed by transfer in an ultrahigh‑vacuum cryogenic suitcase (under five minutes) kept the oxygen content intact. Under these conditions the STM spectra displayed a homogeneous U‑shaped double gap with outer and inner gap energies of roughly 20 meV and 12–15 meV, respectively. Longer transfer times (>10 min) allowed oxygen loss, producing V‑shaped spectra with a reduced gap (~16 meV) and a broad background dip linked to density‑wave features.

Spectroscopic Signatures Distinguish Intrinsic Superconductivity

The U‑shaped spectra are consistent with a nodeless, multigap superconducting state, contradicting earlier interpretations of V‑shaped gaps as evidence for d‑wave pairing. Magnetic‑field‑dependent measurements (2.2 T perpendicular field) showed only minor changes, confirming the robustness of the gap under oxygen‑sufficient conditions. Importantly, the √2 × √2 surface reconstruction persisted even when the gap degraded, indicating that the lattice order is more resilient than the superconducting order parameter.

Implications for Nickelate Research

These findings highlight two practical lessons for the community:

1. Transfer Protocol Matters – Reporting the exact handling timeline is essential for reproducible STM studies of oxygen‑sensitive superconductors.
2. Gap Shape Is Not a Direct Proxy for Pairing Symmetry – V‑shaped tunnelling spectra can arise from oxygen deficiency rather than unconventional pairing, urging caution when inferring symmetry from local spectroscopy alone.

By establishing a clear link between oxygen stoichiometry and the observed gap shape, the work provides a framework for reconciling local spectroscopic data with bulk transport measurements (which still show a transition near 40–50 K even after oxygen loss). This deeper understanding will aid the design of future experiments aimed at uncovering the pairing mechanism in nickelate superconductors.

--- The article is based on the authors’ preprint (DOI: http://link.aps.org/doi/10.1103/rkn7-f5rs) and the accompanying press summary.