Source capture
Authors Yidi Liu, Yaoju Tarn, Lopa Bhatt, Puhua Zhang, Joonseo Song, Berit H. Goodge, David A. Muller, Harold Y. Hwang, Yijun Yu
arXiv / Publisher https://arxiv.org/abs/2610.27729
PDF https://arxiv.org/pdf/2610.27729
Relevance score 5.239
Primary category cond-mat.supr-con
Published 2026-10-21
Research paradigm Both
Sample form Thin Film

Summary

This study addresses bilayer nickelate films grown on SrLaAlO4(001) substrates (including La2PrNi2O7 and La2SmNi2O7) and explains the incomplete zero resistance and two-step superconducting transitions commonly observed in resistance measurements. Using circular-electrode devices, the azimuthal dependence of the transition was measured, revealing that the two-step feature is directly related to the substrate step–terrace orientation: when the current is parallel to the step edges, a single-step transition appears, whereas when it is perpendicular, the two-step transition is most pronounced; ADF-STEM also directly observed structural damage regions at the steps. On this basis, the films were modeled as intact regions with a higher transition temperature and step weak-link regions with a lower transition temperature, and simple models and finite-element simulations reproduced the experimental features; the lateral dimensions of the structural damage regions are approximately 1–5 nm, comparable to the Ginzburg-Landau in-plane coherence length of approximately 2–3 nm, supporting a Josephson-type weak-link picture. Transport measurements further show that the transition and even the occurrence of superconductivity depend strongly on the average terrace width: samples with average terrace widths of approximately 920 and 540 nm can reach zero resistance, those with approximately 250 nm do not reach zero resistance, and those with approximately 120 nm show no superconductivity. This work identifies step-induced weak links as the microscopic origin of the two-step transition in bilayer nickelate films and provides a framework for understanding anisotropic and percolative transport and low critical currents.

Materials

Methods

  • PLD
  • X-ray diffraction
  • ozone annealing
  • temperature-dependent resistivity measurements
  • van der Pauw transport measurements
  • circular-electrode azimuthal-angle-dependent transport measurements
  • atomic force microscopy
  • annular dark-field scanning transmission electron microscopy
  • focused ion beam lift-out
  • simple model calculations
  • finite-element simulations

Keywords

  • superconducting weak links
  • two step superconducting transition
  • substrate step terrace structures
  • step terrace orientation
  • anisotropic transport
  • percolative transport
  • josephson type weak links
  • terrace width dependence
  • incomplete zero resistance states
  • global superconducting percolation
  • low critical current densities
  • probe dependent superconducting temperatures
  • step terrace engineering

Highlights

  • The study establishes a direct correlation between azimuthal-angle-dependent two-step transitions and step-terrace orientation using circular-electrode devices.
  • Atomic-scale ADF-STEM directly visualizes structurally disrupted weak-link regions separating adjacent crystalline domains at step terraces.
  • Both a simple model and finite-element simulations reproduce the configuration-dependent two-step transition, supporting the weak-link interpretation.
  • The work reconciles widely varying two-step transitions in the literature by showing that measurement geometry strongly modulates the lower-temperature transition.
  • It rationalizes why mutual inductance onset temperatures are often lower than transport onset temperatures, because global diamagnetic screening requires coherence across the weak-link network.
  • It identifies step-terrace engineering as an essential consideration for optimizing superconductivity in metastable bilayer nickelates and ultrathin superconducting films.

Conclusions

  • Incomplete zero-resistance states and two-step superconducting transitions in bilayer nickelate films on SrLaAlO4 (001) arise from superconducting weak links at substrate step-terrace structures.
  • The azimuthal-angle-dependent two-step transition correlates directly with step-terrace orientation: current parallel to step edges gives a single-step transition, while current perpendicular to step terraces gives the most pronounced two-step transition.
  • ADF-STEM reveals structurally disrupted regions at step terraces, and their approximately 1–5 nm lateral size is comparable to the Ginzburg-Landau in-plane coherence length of approximately 2–3 nm, supporting a Josephson-type weak-link picture.
  • Modeling the films as pristine regions with higher transition temperature and step weak-link regions with lower transition temperature reproduces essential features of the two-step transition using both a simple model and finite-element simulations.
  • Transition characteristics and even the occurrence of superconductivity depend strongly on average terrace width; samples with approximately 920 and 540 nm terraces reach zero resistance, approximately 250 nm does not reach zero resistance, and approximately 120 nm shows no superconductivity.
  • Step-induced weak links are identified as the microscopic origin of two-step transitions in bilayer nickelate films and provide a framework for understanding anisotropic and percolative transport and low critical currents.
  • Substrate morphology, especially step-terrace geometry, is a key control parameter for optimizing superconductivity in metastable bilayer nickelates and ultrathin superconducting films.

Main claims

  • The commonly observed two-step superconducting transition and incomplete zero-resistance state in bilayer nickelate films arise from superconducting weak links at substrate step-terrace structures.
    • Evidence: Azimuthal-angle-dependent two-step transitions correlate directly with step-terrace orientation.,ADF-STEM directly visualizes structurally disrupted regions at step terraces.,Modeling the films as pristine and weak-link regions reproduces essential two-step features.
  • The two-step transition is orientation-dependent: current parallel to step edges produces a single-step transition, whereas current perpendicular to step terraces produces the most pronounced two-step transition.
    • Evidence: Circular-electrode device measurements showed angular dependence of the superconducting transition.,The normalized resistivity at 20 K approaches zero for current parallel to step terraces and reaches a maximum for current perpendicular to step terraces.
  • The films can be modeled as pristine regions with higher transition temperature and step weak-link regions with lower transition temperature, reproducing the anisotropic two-step transport response.
    • Evidence: A simple two-region model reproduced identical, anisotropic, and lower-transition-dominated van der Pauw responses.,Finite-element simulations showed qualitative agreement with experimental azimuthal-angle and electrode-separation data.
  • Average terrace width strongly controls the superconducting transition and even the occurrence of superconductivity.
    • Evidence: Samples with average terrace widths of approximately 920 and 540 nm reached zero resistance.,A sample with average terrace width of approximately 250 nm did not reach zero resistance.,A sample with average terrace width of approximately 120 nm showed no superconductivity.
  • The approximately 1-5 nm structurally disrupted regions are comparable to the approximately 2-3 nm Ginzburg-Landau in-plane coherence length, supporting a Josephson-type weak-link interpretation.
    • Evidence: The barrier thickness estimated from STEM was approximately 1-5 nm.,The Ginzburg-Landau in-plane coherence length of bilayer nickelate films on SLAO was approximately 2-3 nm.
  • The weak-link framework rationalizes low critical current densities and lower superconducting onset temperatures inferred from mutual inductance compared with transport.
    • Evidence: Superconducting weak links are described as limiting factors for global current flow.,Mutual inductance probes global diamagnetic screening, which can develop only after coherence is established across the weak-link network.

Workflow

  • sample_preparation — Bilayer nickelate films were grown and annealed on SLAO (001) substrates with different step-terrace geometries.
    • Materials: La2PrNi2O7 thin films; La2SmNi2O7 thin films; SrLaAlO4 (001) substrates
    • Methods: pulsed laser deposition; post-growth ozone annealing
    • Observations: bilayer nickelate films were grown on SLAO (001); samples had nominal average terrace widths of approximately 920, 540, 250, and 120 nm
  • structural_characterization — Substrate step-terrace orientation and terrace width were characterized and related to film structure.
    • Materials: bilayer nickelate films; SLAO (001) substrates
    • Methods: atomic force microscopy (AFM); two-dimensional autocorrelation mapping; X-ray diffraction (XRD)
    • Observations: AFM revealed step-terrace morphology and terrace orientation; XRD indicated comparable crystallinity for samples with different terrace widths
  • device_fabrication — Device geometries enabled probing of in-plane transport anisotropy relative to step-terrace orientation.
    • Materials: La2SmNi2O7 thin film; La2PrNi2O7 thin films
    • Methods: electrode deposition; circular-electrode device geometry; van der Pauw configurations
    • Observations: devices allowed transport measurements along multiple in-plane directions and in orthogonal van der Pauw configurations
  • transport_measurement — Superconducting transitions are anisotropic and correlate with step-terrace orientation and average terrace width.
    • Materials: patterned and unpatterned bilayer nickelate films
    • Methods: temperature-dependent resistivity measurements; van der Pauw transport measurements; azimuthal-angle-dependent transport measurements
    • Observations: a single-step transition appeared when current flowed predominantly parallel to step edges; the two-step transition was most pronounced when current flowed perpendicular to step terraces; zero resistance depended strongly on average terrace width
  • atomic_scale_imaging — Step terraces contain atomic-scale structurally disrupted regions consistent with weak links.
    • Materials: cross-sectional lamellae from bilayer nickelate films
    • Methods: focused ion beam lift-out; annular dark-field scanning transmission electron microscopy (ADF-STEM); Cs-corrected STEM at 300 kV
    • Observations: ADF-STEM showed structurally disrupted regions separating adjacent crystalline domains at step terraces; the lateral size of the disrupted regions was approximately 1-5 nm
  • modeling_and_simulation — A pristine/weak-link two-region model reproduces essential features of the two-step and anisotropic superconducting transitions.
    • Materials: model films with pristine region A and weak-link region B
    • Methods: minimal two-region model; finite-element simulations; calculation of two van der Pauw configurations
    • Observations: the model reproduced identical, anisotropic, and lower-transition-dominated van der Pauw responses; finite-element simulations qualitatively agreed with experimental azimuthal and electrode-separation data
  • interpretation — Substrate step-induced superconducting weak links are the microscopic origin of the two-step transition in bilayer nickelate films.
    • Methods: integration of transport, AFM, ADF-STEM, and simulation results
    • Observations: step-induced weak links explain incomplete zero resistance, two-step transitions, anisotropic and percolative transport, low critical currents, and probe-dependent superconducting temperatures