Superconductor-insulator transitions in infinite-layer nickelates controlled via operando monitored reduction

Summary

B_y developing an in situ monitoring reduction (OMR) method, this study achieved continuous modulation of the Ni 3d orbital electron occupancy in infinite-layer nickelate superconductors over an ultra-wide range from approximately 3d⁷ to 3d⁹, thereby controllably driving the superconductor-insulator transition (SIT). Combining synchrotron X-ray absorption spectroscopy and scanning transmission electron microscopy analysis of oxygen atoms, the electron occupancy states were precisely calibrated, and the SIT was further modulated using ionic liquid gating and magnetic fields. Nernst effect measurements reveal that, unlike in cuprates, pairing initiates as soon as the resistance starts to drop, while the Meissner effect only appears in the zero-resistance state, marking the establishment of global phase coherence. Angle-dependent magnetotransport studies show that within the transition temperature range, superconductivity exhibits a mixture of two-dimensional and three-dimensional characteristics, indicating that the observed SIT deviates from the classical 2D model. These results provide a unique perspective for understanding the interplay between structural and electronic phase transitions in infinite-layer nickelates within the oxygen content–magnetic field–temperature parameter space.

Materials

• Nd₀.8Sr₀.2NiO₂

Methods

• Operando monitored reduction (OMR)
• X-ray absorption spectroscopy (XAS)
• STEM
• Electrical transport
• Mutual inductance
• Nernst effect
• Ionic liquid gating

Keywords

• superconductor insulator transition
• ni 3d orbital occupancy
• nernst effect
• phase coherence
• mixed dimensional superconductivity

Highlights

• Our results provide a unique perspective for understanding the interplay between structural and electronic phase transitions in the infinite-layer nickelates across the oxygen content–magnetic field–temperature parameter space.
• The OMR technique enables ultrawide-range continuous modulation of Ni 3d orbital electron occupancy from ≈3d7 to ≈3d9.

Conclusions

• Nernst effect measurements reveal that, unlike in cuprates, pairing initiates as soon as the resistance starts to drop, while the Meissner effect only appears in the zero-resistance state, marking the establishment of global phase coherence.
• Angle-dependent magnetotransport shows that within the transition temperature range, superconductivity exhibits a mixture of two-dimensional and three-dimensional characteristics.
• The observed SIT deviates from the canonical 2D model.

Main claims

• The operando monitored reduction (OMR) method enables continuous modulation of Ni 3d orbital electron occupancy over an ultrawide range from ≈3d7 to ≈3d9.
  • Evidence: Abstract: 'B_y developing an in situ monitoring reduction (OMR) method, this study achieved continuous modulation of the Ni 3d orbital electron occupancy… from approximately 3d7 to 3d9'
• Nernst effect measurements reveal that pairing initiates at the onset of the resistive drop, distinct from cuprates where fluctuations extend above T_c.
  • Evidence: Abstract: 'Nernst effect measurements reveal that, unlike in cuprates, pairing initiates as soon as the resistance starts to drop'

Workflow

• Sample preparation and operando reduction — OMR enables precise control of oxygen content and electron occupancy
  • Materials: Nd₀.8Sr₀.2NiO₃ thin films on LSAT (001)
  • Methods: Pulse laser deposition; Operando monitored reduction (OMR) with CaH₂
  • Observations: Continuous modulation of Ni 3d occupancy from ≈3d7 to ≈3d9
• Multi-probe characterization — Pairing initiates at the onset of resistive drop, distinct from cuprates
  • Materials: Reduced films at different stages
  • Methods: X-ray absorption spectroscopy (XAS); Scanning transmission electron microscopy (STEM); Transport, Nernst, angle-dependent magnetoresistance
  • Observations: SIT tuned by oxygen content, ionic liquid gating, and magnetic field; Nernst effect shows pairing begins at resistive drop, Meissner effect at zero resistance
• Dimensionality analysis — SIT deviates from canonical 2D model
  • Materials: Angle-dependent magnetoresistance data
  • Methods: Combined 2D Tinkham and 3D GL model
  • Observations: Mixed 2D and 3D superconducting characters