Granular Superconductivity in La₂PrNi₂O₇-δ Thin Films

Summary

Research indicates that the two-step superconducting transition observed in La₂PrNi₂O_7-δ thin films originates from their granular superconducting nature, where two superconducting phases with distinct critical temperatures coexist and couple through a Josephson junction network. For films grown via pulsed laser deposition and subsequently ozone-annealed, transport measurements reveal a pronounced secondary low-temperature transition even when the residual resistance is minimal near 30 K, resulting in a zero-resistance temperature of only about 10 K. The hysteresis in magnetoresistance and the sensitive response to weak magnetic fields align with the effective field model of granular superconductors, ruling out the possibility of a spin-glass phase. Structural characterization identifies oxygen inhomogeneity and local structural disorder, such as monolayer phase intercalation, as the primary causes of the observed phase separation. These findings elucidate the microscopic mechanisms underlying the complex superconducting behavior in bilayer nickelate films and underscore that improving oxygen uniformity is crucial for achieving bulk superconductivity with higher zero-resistance temperatures, thereby providing a foundation for subsequent spectroscopic studies.

Materials

• La₂PrNi₂O₇-δ

Methods

• transport measurements
• X-ray diffraction (XRD)
• STEM

Keywords

• granular superconductivity
• two step transition
• josephson junction network
• oxygen inhomogeneity

Highlights

• Magnetoresistance hysteresis and sensitivity to weak magnetic fields align with the effective field model of granular superconductors.
• The secondary low-temperature transition persists even when residual resistance is minimal near 30 K, resulting in a zero-resistance temperature of only about 10 K.

Conclusions

• The two-step superconducting transition originates from granular superconductivity with two distinct superconducting grain phases coupled by Josephson junctions.
• Oxygen inhomogeneity and local structural disorder are the primary causes of the phase separation.

Main claims

• The two-step superconducting transition in La₂PrNi₂O₇ thin films arises from granular superconductivity with two distinct superconducting grain phases coupled by Josephson junctions
  • Evidence: Resistivity shows two distinct transitions; magnetoresistance hysteresis consistent with effective field model of granular superconductors; STEM reveals oxygen inhomogeneity and stacking faults

Workflow

• Thin film growth and annealing — Structural disorder correlates with two-step transition
  • Materials: La₂PrNi₂O₇ thin films on SLAO
  • Methods: Pulsed laser deposition (PLD); Ozone annealing
  • Observations: XRD peaks, STEM images show monolayer intergrowths
• Transport measurements — Granular superconductivity with two superconducting grain phases
  • Materials: PPMS system
  • Methods: Four-probe resistivity; Magnetoresistance
  • Observations: Two-step superconducting transition; Hysteretic magnetoresistance; Weak-field sensitivity