Superconducting phase diagram of multi-layer square-planar nickelates

Summary

This study systematically constructs the superconducting phase diagram of multilayer square-planar nickelates Ndn+1Ni_nO₂n+2 (n = 4–8), revealing that compounds with n = 4 to 7 exhibit signs of superconductivity, with a maximum onset critical temperature of 12.9 K (n = 6), while n = 8 shows only weak superconducting correlations. As the layer number n decreases, the superconducting anisotropy undergoes a reversal due to the effect of 4f electrons at the neodymium sites—the electronic structure approaches that of cuprates, and magnetic fluctuations persist in both the superconducting region and the overdoped nonsuperconducting region. Notably, this superconducting region overlaps with that of chemically doped infinite-layer nickelates, highlighting commonalities and differences among different structural realizations in square-planar nickelates. This work establishes a general template for synthesizing novel nickel-based superconductors through atomic-precision layered design.

Materials

• Nd₄Ni₃O₈
• Nd₅Ni₄O₁₀
• Nd₆Ni₅O₁₂
• Nd₇Ni₆O₁₄
• Nd₈Ni₇O₁₆
• Nd₉Ni₈O₁₈

Methods

• MBE
• topotactic reduction
• XRD
• STEM
• transport measurements
• RIXS

Keywords

• square planar nickelates
• superconducting dome
• magnetic fluctuations
• cuprate like electronic structure

Highlights

• Constructs the first comprehensive phase diagram of multi-layer square-planar nickelates.
• Shows that structural tuning via layering provides a unique knob to access the overdoped regime.
• Observes that magnetic excitations are robust and persist even after superconductivity is destroyed.

Conclusions

• Superconductivity is observed in Nd_n+1Ni_nO₂n+2 compounds for n=4-8, with maximal T_c up to 12.9 K for n=6.
• The superconducting regime overlaps with chemically-doped infinite-layer nickelates, showing a universal doping region near 3d9.
• Decreasing n enhances cuprate-like electronic structure (increased O-Ni hybridization, reduced Nd-Ni hybridization).
• Magnetic fluctuations persist from the superconducting regime into the overdoped non-superconducting regime (n=3).
• The 4f moments of Nd affect the superconducting anisotropy, reversing the expected behavior with decreasing n.

Main claims

• Superconductivity observed in Ndn+1Ni_nO₂n+2 for n=4-8 with maximal T_c12.9 K at n=6
  • Evidence: From abstract: 'construct a phase diagram of multi-layer square-planar Ndn+1Ni_nO₂n+2 compounds and discover signatures of superconductivity for n = 4 - 8',From abstract: 'The maximal observed T_c,onset occurs at 12.9 K in n=6'
• Superconducting regime overlaps with chemically-doped infinite-layer nickelates
  • Evidence: From abstract: 'superconducting regime overlaps with that of chemically-doped infinite-layer nickelates'
• Magnetic excitations persist into overdoped non-superconducting regime
  • Evidence: From abstract: 'magnetic fluctuations persist from within the superconducting regime and into the over-doped, non-superconducting regime'

Workflow

• thin_film_growth — Entire series of Ndn+1Ni_nO₂n+2 compounds synthesized
  • Materials: Ndn+1Ni_nO₃n+1 precursors
  • Methods: oxide molecular beam epitaxy; topotactic reduction with CaH₂
  • Observations: epitaxial multi-layer square-planar nickelate films
• structural_characterization — Atomically precise layered structure with local lattice expansion near fluorite layers
  • Materials: Ndn+1Ni_nO₂n+2 films
  • Methods: X-ray diffraction; scanning transmission electron microscopy
  • Observations: crystal structure; lattice constants; local lattice expansion
• transport_measurements — Superconductivity observed for n=4-7 with maximal T_c12.9 K at n=6
  • Materials: Ndn+1Ni_nO₂n+2 films
  • Methods: four-probe resistivity; angle-dependent magnetoresistance
  • Observations: superconducting transitions; anisotropy
• resonant_inelastic_x-ray_scattering — Magnetic excitations persist from superconducting n=5 to overdoped non-superconducting n=3
  • Materials: Ndn+1Ni_nO₂n+2 films
  • Methods: RIXS at Ni L₃ edge
  • Observations: orbital excitations; magnetic excitations