Signatures of ambient pressure superconductivity in thin film La₃Ni₂O₇

Summary

Recently, the bilayer nickelate La₃Ni₂O₇ has been discovered as a new superconductor with transition temperature T_c near 80 K under high pressure1–3. Despite extensive theoretical and experimental work to understand the nature of its superconductivity4–29, the requirement of extreme pressure restricts the use of many experimental probes and limits its application potential. Here we present signatures of superconductivity in La₃Ni₂O₇ thin films at ambient pressure, facilitated by the application of epitaxial compressive strain. The onset T_c varies roughly from 26 to 42 K, with higher T_c values correlating with smaller in-plane lattice constants. We observed the co-existence of other Ruddlesden–Popper phases within the films and dependence of transport behaviour with ozone annealing, suggesting that the observed low zero resistance T_c of around 2 K can be attributed to stacking defects, grain boundaries and oxygen stoichiometry. This finding initiates numerous opportunities to stabilize and study superconductivity in bilayer nickelates at ambient pressure, and to facilitate the broad understanding of the ever-growing number of high temperature and unconventional superconductors in the transition metal oxides.

Materials

• La₃Ni₂O₇

Methods

• Epitaxial compressive strain
• Ozone annealing
• Transport measurements

Keywords

• ambient pressure superconductivity
• stacking defects
• grain boundaries
• oxygen stoichiometry

Highlights

• The coexistence of other Ruddlesden–Popper phases within the films is observed.
• This finding facilitates broad understanding of high temperature and unconventional superconductors in transition metal oxides.

Conclusions

• Signatures of superconductivity at ambient pressure are observed in La₃Ni₂O₇ thin films with onset T_c from 26 to 42 K.
• Low zero resistance T_c of around 2 K is attributed to stacking defects, grain boundaries, and oxygen stoichiometry.

Main claims

• Signatures of ambient pressure superconductivity observed in compressively strained La₃Ni₂O₇ thin films, with onset T_c up to 42 K.
  • Evidence: Abstract: 'we present signatures of superconductivity in La₃Ni₂O₇ thin films at ambient pressure',Abstract: 'onset T_c varies roughly from 26 to 42 K'
• Higher T_c correlates with smaller in-plane lattice constants (higher compressive strain).
  • Evidence: Abstract: 'higher T_c values correlating with smaller in-plane lattice constants',Fig. 5: T_c,onset vs in-plane lattice constant
• Zero resistance (T_c ≈2 K) is limited by structural defects, stacking faults, and oxygen stoichiometry variations.
  • Evidence: Abstract: 'suggesting that the observed low zero resistance T_c of around 2 K can be attributed to stacking defects, grain boundaries and oxygen stoichiometry'

Workflow

• film_growth — La₃Ni₂O₇ thin films with varying epitaxial strain fabricated.
  • Materials: La₃Ni₂O₇ target; SrTiO₃ capping layer; LSAT(001), LAO(001), SLAO(001) substrates
  • Methods: pulsed laser deposition
  • Observations: films grown on various substrates with different lattice mismatch
• ozone_annealing — Ozone annealing increases Ni oxidation state and induces metallicity.
  • Materials: as-grown films
  • Methods: ozone annealing at 300°C for ≈2h; O₃ concentration ≈0.4 wt%
  • Observations: as-grown films insulating become metallic after annealing; Ni valence changes from ≈2.45+ to ≈2.5+
• structural_characterization — Higher compressive strain and higher LNO₃₂₇ phase purity correlate with higher T_c.
  • Materials: annealed films
  • Methods: XRD θ-2θ scans; synchrotron reciprocal space mapping; STEM
  • Observations: films on SLAO have highest compressive strain; coexistence of LNO₂₁₄ and LNO₄₃₁₀ phases in some films; in-plane lattice constant correlated with T_c
• transport_measurements — Signatures of ambient pressure superconductivity in strained La₃Ni₂O₇ films.
  • Materials: films with Au electrodes
  • Methods: resistivity vs T; Hall effect; critical current; mutual inductance
  • Observations: superconducting onset T_c26-42K; zero resistance T_c ≈2K; two-step transitions; Jc ≈0.3 kA/cm2 at 150 mK; diamagnetic response below 2K
• valence_analysis — Increased Ni valence after ozone annealing supports oxygen stoichiometry change.
  • Materials: films
  • Methods: X-ray absorption spectroscopy (XAS) at Ni L₂-edge
  • Observations: Ni₂+/Ni₃+ ratio changes from 55:45 to 50:50 upon annealing