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arxiv: 2605.15703 · v1 · pith:UVR2TNIInew · submitted 2026-05-15 · ❄️ cond-mat.supr-con · cond-mat.mes-hall· cond-mat.mtrl-sci· cond-mat.str-el

Observation of flat-bottom U-shaped energy gap in high-Tc nickelate (La,Pr)3Ni2O7 thin films

Zhen Liang , Tianheng Wei , Wei Ren , Haoran Ji , Zheyuan Xie , Yanzhao Liu , Ziqiang Wang , Jian Wang This is my paper

Pith reviewed 2026-05-19 19:36 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con cond-mat.mes-hallcond-mat.mtrl-scicond-mat.str-el
keywords high-Tc superconductivitynickelatesscanning tunneling microscopysuperconducting gapthin filmsnodeless gapRuddlesden-Popper phases
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The pith

An energy-symmetric flat-bottom U-shaped gap consistent with nodeless superconductivity is observed in high-Tc nickelate thin films at ambient pressure.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper reports the first observation of a flat-bottom U-shaped energy gap with zero residual density of states at the Fermi level in (La,Pr)3Ni2O7 thin films using ultra-low temperature scanning tunneling microscopy. Transport measurements on the same samples confirm superconducting behavior with onset Tc above 40 K and zero-resistance Tc above 20 K. The gap shows unconventional temperature evolution from U-shaped to V-shaped and is suppressed by a 14 T magnetic field. These features together indicate that the gap is likely a superconducting gap with a nodeless function at low temperatures, providing new insight into the mechanism of high-Tc superconductivity in nickelates without needing high pressure.

Core claim

In (La,Pr)3Ni2O7 thin films, ultra-low temperature STM/S reveals an energy-symmetric, flat-bottom U-shaped gap around the Fermi level with zero residual density of states. This gap, combined with its reduction under 14 T c-axis magnetic field and rapid filling to V-shaped with increasing temperature, matches the expected behavior of a superconducting gap, implying a nodeless gap function at ultra-low temperatures. Electrical transport on the same film shows zero resistance with Tc above 20 K, supporting the superconducting nature.

What carries the argument

The flat-bottom U-shaped tunneling spectrum with zero residual DOS, whose temperature and magnetic field dependence is used to identify it as the superconducting gap.

If this is right

  • The observation confirms the presence of superconductivity in the thin film samples at ambient pressure.
  • The gap function appears nodeless at ultra-low temperatures.
  • The STM method allows detailed study of the superconducting state in these materials.
  • Results suggest possibility of local superconductivity with Tc above 77 K in similar systems.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the surface-sensitive STM data truly captures the bulk gap, similar measurements could reveal the pairing symmetry in other nickelate compounds.
  • Connecting this to cuprate superconductors might highlight common mechanisms in layered oxides.
  • Further experiments varying film thickness or substrate could test if the gap is intrinsic or affected by interfaces.
  • Success here encourages ambient-pressure searches for higher-Tc nickelate superconductors.

Load-bearing premise

The tunneling spectra represent the intrinsic bulk superconducting density of states without dominant contributions from surface reconstruction, disorder, or non-superconducting phases.

What would settle it

If future measurements show that the U-shaped gap does not close at the superconducting critical temperature determined by transport or if it appears in non-superconducting regions of the film, the interpretation as a superconducting gap would be falsified.

read the original abstract

The discovery of high transition temperature (high-Tc) superconductivity in Ruddlesden-Popper (R-P) bilayer nickelates under high pressure has stimulated extensive work to understand the underlying mechanism and search for superconductors with higher Tc. The recent realization of superconductivity in R-P bilayer nickelate thin films with onset Tc above 40 K at ambient-pressure enables the use of a wide array of powerful experimental tools to investigate the unconventional high-Tc superconductivity in bilayer nickelates. Here, using ultra-low temperature scanning tunneling microscopy/spectroscopy (STM/S) and electrical transport study, we report the first successful observation of an energy-symmetric, flat-bottom U-shaped gap with zero residual density of states around the Fermi level in the high-Tc nickelate (La,Pr)3Ni2O7 thin film grown on SrLaAlO4 substrate. Before and after STM/S studies, transport measurements on the same sample reveal consistent superconducting behaviors showing zero resistance, with an onset Tc above 40 K and zero resistance Tc above 20 K. The tunneling spectra exhibit highly unconventional temperature evolution, characterized by a rapid filling of the U-shaped energy gap to a V-shaped gap as the temperature increases. Furthermore, the U-shaped energy gap is reduced under a c-axis magnetic field of 14 T. The energy-symmetric U-shaped gap, taken together with its dependence on magnetic field and temperature, is consistent with the behavior of a superconducting gap, suggesting a nodeless gap function at ultra-low temperatures. Our findings shed new lights on the nature of high-Tc superconductivity and provide an encouraging and thought-provoking hint for a local superconductivity with Tc above liquid nitrogen boiling temperature in nickelate superconductors at ambient or zero pressure.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports ultra-low temperature STM/S measurements on (La,Pr)3Ni2O7 thin films on SrLaAlO4 substrates, observing an energy-symmetric flat-bottom U-shaped gap with zero residual DOS at the Fermi level. Transport measurements on the same sample show consistent superconductivity with onset Tc above 40 K and zero-resistance Tc above 20 K. The gap fills rapidly into a V-shape with increasing temperature and is suppressed by a 14 T c-axis field; the authors interpret these features as evidence for a nodeless superconducting gap function.

Significance. If the STM spectra are shown to reflect the bulk superconducting DOS, the result would supply the first direct tunneling evidence for gap symmetry in ambient-pressure bilayer nickelate films. This could help discriminate among pairing mechanisms and support the possibility of local superconductivity above liquid-nitrogen temperature, thereby advancing the understanding of high-Tc nickelates.

major comments (2)
  1. [Results and Discussion (STM/S spectra and interpretation)] The central claim that the flat-bottom U-shaped gap indicates a nodeless bulk superconducting gap rests on the premise that the STM/S spectra map the intrinsic quasiparticle DOS. Because STM is confined to the topmost layers, the manuscript must address possible surface reconstruction, apical-oxygen loss, or substrate-induced strain that could produce analogous gaps unrelated to bulk superconductivity. No spatial correlation between STM locations and transport-defined superconducting regions, nor checks for phase separation or competing surface orders (CDW, magnetic), are described.
  2. [Temperature and magnetic-field dependence paragraphs] The temperature evolution (rapid filling of U-shape to V-shape) and 14 T field suppression are presented as consistent with superconductivity, yet the text provides no quantitative gap extraction, error bars, or model fits (e.g., to BCS or d-wave forms) that would allow readers to assess whether the data exclude small residual DOS or alternative gap-like features within experimental noise.
minor comments (2)
  1. The base temperature of the STM measurements and the numerical value of the gap edge (with uncertainty) should be stated explicitly in the abstract and main text.
  2. [Methods] Figure captions and methods should specify tunneling setpoint conditions, tip material, and any spatial homogeneity maps to aid reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. The comments highlight important aspects of data interpretation and analysis that we address point by point below. We will revise the manuscript to strengthen the presentation where feasible while maintaining scientific accuracy.

read point-by-point responses

  1. Referee: [Results and Discussion (STM/S spectra and interpretation)] The central claim that the flat-bottom U-shaped gap indicates a nodeless bulk superconducting gap rests on the premise that the STM/S spectra map the intrinsic quasiparticle DOS. Because STM is confined to the topmost layers, the manuscript must address possible surface reconstruction, apical-oxygen loss, or substrate-induced strain that could produce analogous gaps unrelated to bulk superconductivity. No spatial correlation between STM locations and transport-defined superconducting regions, nor checks for phase separation or competing surface orders (CDW, magnetic), are described.

    Authors: We agree that STM/S is inherently surface-sensitive and that the manuscript would benefit from a more explicit discussion of this point. Transport data were acquired on the same film before and after STM measurements, showing reproducible superconductivity with onset Tc above 40 K and zero-resistance Tc above 20 K; this macroscopic consistency provides indirect support that the gap is not an isolated surface artifact. In the revised manuscript we will add a dedicated paragraph addressing possible surface reconstruction, apical-oxygen loss, and substrate strain, referencing the structural characterization (XRD, AFM) already present in the methods. We note that the observed gap is energy-symmetric with strictly zero residual DOS and is suppressed by a 14 T c-axis field—features difficult to reconcile with typical CDW or magnetic surface orders, which usually produce asymmetric or gapped spectra with finite residual DOS. While we did not perform spatially resolved STM-transport correlations (transport being a global probe), spectra acquired at multiple locations on the film were reproducible; we will clarify this limitation and its implications. revision: partial

  2. Referee: [Temperature and magnetic-field dependence paragraphs] The temperature evolution (rapid filling of U-shape to V-shape) and 14 T field suppression are presented as consistent with superconductivity, yet the text provides no quantitative gap extraction, error bars, or model fits (e.g., to BCS or d-wave forms) that would allow readers to assess whether the data exclude small residual DOS or alternative gap-like features within experimental noise.

    Authors: We concur that quantitative fitting and error analysis would improve the ability of readers to evaluate the claims. In the revised version we will extract the gap magnitude versus temperature and magnetic field, report the associated uncertainties, and overlay BCS nodeless s-wave fits to the lowest-temperature spectra. These fits will be used to demonstrate that the residual DOS at the Fermi level is consistent with zero within experimental noise and that the temperature-induced evolution from U- to V-shape is inconsistent with a simple nodal gap. The 14 T field suppression will likewise be compared to the expected pair-breaking behavior for a nodeless gap. revision: yes

Circularity Check

0 steps flagged

No circularity: direct experimental observation with no derivation chain

full rationale

This is a purely observational experimental paper reporting STM/S tunneling spectra on (La,Pr)3Ni2O7 thin films. The flat-bottom U-shaped gap with zero residual DOS is extracted directly from raw differential conductance data, and its consistency with a nodeless superconducting gap is stated as an empirical interpretation based on observed temperature filling to V-shape and suppression under 14 T field. No equations, fitted parameters, predictions, or first-principles derivations are presented that could reduce to inputs by construction. No self-citations are invoked as load-bearing uniqueness theorems or ansatze. The central claim rests on experimental measurements and transport corroboration rather than any self-referential loop.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work is observational and relies on standard interpretations of STM spectra in superconductors without introducing new free parameters or postulated entities.

axioms (1)
  • domain assumption STM tunneling spectra reflect the local electronic density of states near the Fermi level.
    This standard assumption is used to interpret the measured gap as a superconducting gap.

pith-pipeline@v0.9.0 · 5892 in / 1200 out tokens · 51562 ms · 2026-05-19T19:36:09.718993+00:00 · methodology

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