Bosonic phases across the superconductor-insulator transition in infinite-layer samarium nickelate

Summary

This study realized a superconductor-insulator transition by fabricating infinite-layer samarium nickel oxide superconducting thin films into a spatially periodic network structure, thereby modulating the phase coherence of Cooper pairs. The observation of magnetoresistance oscillations with a period of h/2e in the experiments directly confirmed the existence of 2e Cooper pairs in nickel oxides. The transition was primarily driven by enhanced superconducting fluctuations, with Cooper pairs participating in charge transport throughout the entire transition process. Two anomalous metallic states were also identified: one emerging under finite magnetic fields and the other appearing even at zero magnetic field; both states could be characterized by bosonic excitations, suggesting the dynamic role of vortices in the ground state. This work establishes nickel oxides as a key platform for studying the rich bosonic phases arising from the modulation of Cooper pair phase coherence.

Materials

• Sm₀.95-xEu_xCa₀.05NiO₂

Methods

• PLD
• topotactic reduction
• RIE
• transport measurements

Keywords

• superconductor insulator transition
• cooper pairs
• anomalous metal
• bosonic strange metal
• quantum oscillations

Highlights

• First observation of h/2e quantum oscillations in nickelate superconductors, confirming Cooper pairing.
• Discovery of a bosonic strange metal state in a nanopatterned nickelate film, analogous to cuprate networks.
• Establishes nickelates as a platform for studying bosonic phases via phase coherence engineering.

Conclusions

• Magnetoresistance oscillations with period h/2e provide direct evidence of 2e Cooper pairing in nickelates.
• The SIT is driven by enhanced superconducting fluctuations, with Cooper pairs involved in transport across the transition.
• Two types of anomalous metallic phases are observed: one at finite magnetic fields and one down to zero field, characterized by bosonic excitations.
• A bosonic strange metal state with linear temperature-dependent resistance is observed below the superconducting onset.

Main claims

• Magnetoresistance oscillations with period h/2e provide direct evidence of 2e Cooper pairing in nickelates
  • Evidence: From abstract: 'Magnetoresistance oscillations with a periodicity of h/2e provide direct evidence of 2e Cooper pairing in nickelates'
• Two types of anomalous metallic phases observed: at finite field and at zero field, characterized by bosonic excitations
  • Evidence: From abstract: 'we observe two types of anomalous metallic phases, emerging respectively at finite magnetic fields and down to zero magnetic field. They can be characterized by bosonic excitations, suggesting the dynamic roles of vortices in the ground states'

Workflow

• film_growth_and_reduction — High-quality infinite-layer nickelate films
  • Materials: Sm₀.95-xEu_xCa₀.05NiO₃; LaAlO₃ substrate
  • Methods: pulsed-laser deposition; topotactic reduction with CaH₂
  • Observations: superconducting Sm₀.95-xEu_xCa₀.05NiO₂ films
• nanopatterning — Periodic network structure with 50 nm holes
  • Materials: Sm₀.95-xEu_xCa₀.05NiO₂ film; AAO mask
  • Methods: reactive ion etching
  • Observations: honeycomb network patterns
• low_temperature_transport_measurements — h/2e oscillations prove Cooper pairing; anomalous metallic states observed
  • Materials: Nanopatterned film
  • Methods: four-probe resistivity; lock-in technique
  • Observations: resistance vs temperature and field; magnetoresistance oscillations