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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.

Bosonic Phases across the Superconductor-Insulator Transitions in Infinite-Layer Samarium Nickelate

This study systematically investigates the superconductivity of compressively strained La₂LnNi₂O₇ thin films (Ln = lanthanide) under ambient and high pressure to clarify the relationship between ambient-pressure and high-pressure superconducting mechanisms. By suppressing the superconducting state with a 59 T magnetic field, the normal-state resistivity at ambient pressure tends toward T² behavior; after applying high pressure in a cubic anvil chamber, Tc increases from 41–42 K at ambient pressure to 67–73 K at 16 GPa. However, lattice compression induced by Ln substitution, although it may mimic pressure effects, lowers Tc. In both cases, Tc correlates with the evolution of normal-state transport between T² and T-linear behavior, indicating that lattice structural changes in bilayer nickelates have an important influence on superconductivity and providing clues for understanding the intrinsic connection between ambient-pressure strained films and high-pressure bulk superconducting states.

The Ruddlesden–Popper (R–P) bilayer nickelate, La3Ni2O7, was recently found to show signatures of high-temperature superconductivity (HTSC) at pressures above 14 GPa (ref. 1). Subsequent investigations achieved zero resistance in single-crystalline and polycrystalline samples under hydrostatic pressure conditions2–4. Yet, obvious diamagnetic signals, the other hallmark of superconductors, are still lacking owing to the filamentary nature with low superconducting volume fraction2,4,5. The presence of a new 1313 polymorph and competing R–P phases obscured proper identification of the phase for HTSC6–9. Thus, achieving bulk HTSC and identifying the phase at play are the most prominent tasks. Here we address these issues in the praseodymium (Pr)-doped La2PrNi2O7 polycrystalline samples. We find that substitutions of Pr for La effectively inhibit the intergrowth of different R–P phases, resulting in a nearly pure bilayer structure. For La2PrNi2O7, pressure-induced orthorhombic to tetragonal structural transition takes place at Pc ≈ 11 GPa, above which HTSC emerges gradually on further compression. The superconducting transition temperatures at 18–20 GPa reach $${T}_{{\rm{c}}}^{{\rm{onset}}}=82.5\,{\rm{K}}$$and $${T}_{{\rm{c}}}^{{\rm{zero}}}=60\,{\rm{K}}$$, which are the highest values, to our knowledge, among known nickelate superconductors. Importantly, bulk HTSC was testified by detecting clear diamagnetic signals below about 75 K with appreciable superconducting shielding volume fractions at a pressure of above 15 GPa. Our results not only resolve the existing controversies but also provide directions for exploring bulk HTSC in the bilayer nickelates.

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Recently, the Ruddlesden–Popper bilayer nickelate La3Ni2O7 has emerged as a superconductor with a transition temperature (Tc) of approximately 80 K above 14 GPa (refs. 1–3). Achieving a higher Tc in nickelate superconductors, along with the synthesis of reproducible high-quality single crystals without relying on high-oxygen-pressure growth conditions, remains a significant challenge4–7. Here we report superconductivity up to 96 K under high pressure in bilayer nickelate single crystals synthesized at ambient pressure. Energy-dispersive spectroscopy, single-crystal X-ray diffraction, nuclear quadrupole resonance and scanning transmission electron microscopy evidenced high crystal quality of the flux-grown La2SmNi2O7−δ single crystals. La2SmNi2O7 exhibits clear bulk superconductivity, including zero resistivity ( $${T}_{{\rm{c}},\max }^{{\rm{onset}}}$$ = 92 K and $${T}_{{\rm{c}},\max }^{{\rm{zero}}}$$ = 73 K at 21.6 GPa) and the Meissner effect (Tc = 60 K at 20.6 GPa). A low-temperature high-pressure structural study indicates that both monoclinic and tetragonal structures can support superconductivity in this bilayer nickelate. Furthermore, we established a correlation between higher Tc under high pressures and larger in-plane lattice distortion under ambient conditions, corroborated by observing even higher $${T}_{{\rm{c}}}^{{\rm{onset}}}$$of 96 K in La1.57Sm1.43Ni2O7−δ. This study overcomes key limitations in growing nickelate superconductor crystals, resolves the crystal structure in the superconducting state and demonstrates an effective pathway towards achieving higher Tc.

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