Controlling the Band Filling and the Band Width in Nickelate Superconductors

Summary

This study employs high-pressure synthesis and hydrostatic high-pressure transport techniques to systematically modulate the bandwidth and band filling in the bilayer nickelate La₃Ni₂O₇ family, aiming to investigate their effects on superconductivity and non-superconducting state properties. B_y partially substituting La with smaller Nd (which increases NiO₆ octahedral tilting and reduces bandwidth), the pressure required for the superconducting phase is significantly elevated; conversely, co-introducing Sr for hole doping reverses this trend, lowering the onset pressure of superconductivity. In the non-superconducting state, up to three characteristic resistance anomalies are observed, evolving with pressure, likely corresponding to charge density wave and spin density wave orders that compete with superconductivity. A comprehensive comparison of phase diagrams across samples with different compositions indicates that independent control of bandwidth and filling is key to unraveling the mechanism of unconventional superconductivity and its competing orders in this system.

Materials

• La₃Ni₂O₇
• La₂NdNi₂O₇
• La₁.9NdSr₀.1Ni₂O₇
• La₂.9Sr₀.1Ni₂O₇
• La₁.8NdSr₀.2Ni₂O₇

Methods

• high-pressure synthesis
• transport measurements
• X-ray diffraction
• scanning electron microscopy with energy-dispersive X-ray (SEM-EDX)
• STEM
• electron energy loss spectroscopy (EELS)

Keywords

• bandwidth
• band filling
• density wave anomalies
• phase diagram

Highlights

• Independent control of bandwidth and filling is key to unraveling the mechanism of unconventional superconductivity and its competing orders.
• The anomalies exhibit opposite pressure dependences, in contrast to cuprates.

Conclusions

• Partial substitution of La with Nd increases the pressure required for superconductivity, while co-introducing Sr lowers it.
• Up to three characteristic resistance anomalies are observed in the nonsuperconducting state, likely corresponding to CDW and SDW orders that compete with superconductivity.

Main claims

• Partial substitution of La with smaller Nd increases the pressure required to induce superconductivity in bilayer nickelates, while co-introducing Sr reverses this trend
  • Evidence: Transport measurements under pressure show that La₂NdNi₂O₇ requires ≈20 GPa for superconductivity, compared to ≈10 GPa for La₃Ni₂O₇; La₁.9NdSr₀.1Ni₂O₇ has onset at ≈14 GPa

Workflow

• Sample synthesis — High-quality samples with controlled band width and filling
  • Materials: La_3-xNd_xNi₂O₇ and Sr-doped variants
  • Methods: High-pressure synthesis; KClO₄ as oxidizing agent
  • Observations: XRD, SEM-EDX, STEM-EELS
• High-pressure transport measurements — Nd substitution shifts superconductivity to higher pressure, while Sr doping lowers it
  • Materials: Cubic anvil press
  • Methods: Resistivity under pressure up to 20 GPa
  • Observations: Resistance anomalies at various temperatures; Superconducting onset and zero resistance