Paper

By fabricating periodic nanohole arrays to construct infinite-layer nickelate superconducting thin films (Nd₀.₈Sr₀.₂NiO₂) into Josephson junction arrays, researchers systematically enhanced the phase fluctuations of the system. In the nanopatterned films, the weakening of macroscopic phase coherence drives the superconducting transition to exhibit a two-stage characteristic and ultimately tends toward an anomalous metallic ground state with saturated resistance. The emergence of charge-2e quantum oscillations indicates inter-array coherence, while the anomalous zero-field magnetoresistance peak marks the persistence of extreme quantum phase fluctuations down to very low temperatures. Notably, through the synergistic enhancement of nanopatterning and magnetic fields, a reversal of superconducting anisotropy is observed in Nd-nickelates, where the in-plane critical field becomes lower than the out-of-plane critical field. The evolution of this anisotropy may reveal an intrinsic exchange Zeeman field coupled to collective electronic states. These results elucidate how superconductivity evolves in response to phase fluctuations and establish nanopatterning as an effective paradigm for unveiling hidden intertwined orders in strongly correlated systems.

Combining first-principles calculations with fluctuation exchange-Migdal-Eliashberg theory, this study investigates the interplay between electron correlations and electron-phonon coupling in infinite-layer nickelate superconductors. The results show that spin fluctuations drive robust d-wave superconductivity in the Ni d_{x^2-y^2} orbital, while electron-phonon coupling induces s-wave pairing in interstitial orbitals, and their synergy gives rise to a mixed s+id superconducting state. The emergence of the s-wave component strongly depends on carrier density: a moderate electron-phonon coupling strength (λ=0.4) stabilizes the mixed state only at an electron density n=0.9, but not at n=0.8. In the thermodynamic limit, the critical coupling required to stabilize the s-wave component is about 0.6, but it can be reduced to 0.4 in finite-size systems. These results reveal that local oxygen defects, by modulating the local electron density, can form finite-size domains with distinct pairing symmetries, thereby providing a microscopic explanation for the spatially inhomogeneous superconducting gaps observed experimentally, and highlight the crucial influence of the cooperative effect of electron correlations and electron-phonon coupling on the pairing symmetry in nickelate superconductors.

Researchers have prepared freestanding La₀.₈Sr₀.₂NiO₂ thin films and performed electrical resistance measurements using a diamond anvil cell at ultrahigh pressures up to 210 GPa. The experiments reveal a dome-shaped evolution of the superconducting onset transition temperature: beginning at approximately 16 K at ambient pressure, it rises monotonically with increasing pressure, reaches a peak of 74.5 K at 146 GPa, and then slowly decreases while still retaining 57.4 K at 210 GPa. Through derivative analysis and magnetic field suppression experiments, it is confirmed that this enhancement in transition temperature stems from an intrinsic strengthening of superconductivity, rather than a simple broadening of the transition width. A comparison with Nd₀.₈₅Sr₀.₁₅NiO₂ films shows that both exhibit a linear increase with pressure below 60 GPa with similar slopes, though their behaviors diverge at higher pressures; nonetheless, the maximum onset temperature for both is approximately 75 K, indicating that the NiO₂ planes dominate the superconductivity and that the magnetic moments of rare-earth f orbitals have minimal influence. This work represents the first observation of robust superconductivity in oxide superconductors over such a wide pressure range, demonstrating that the pairing strength in infinite-layer nickelates remains unsuppressed under extreme compression, and provides important clues for understanding the mechanism of unconventional superconductivity.

Using first-principles calculations and large-scale dynamic cluster quantum Monte Carlo simulations, this work systematically investigates the effect of electron doping on the superconducting properties of two-orbital bilayer models for three representative systems: bulk La₃Ni₂O₇ under ambient pressure and at 15 GPa, as well as the La₃Ni₂O₇:La₃Al₂O₇ heterostructure. The results show that electron doping universally enhances s±-wave pairing superconductivity, with the heterostructure exhibiting the highest superconducting transition temperature in the underdoped region, even exceeding that of bulk samples under 15 GPa pressure. Further analysis reveals an inter-orbital synergistic mechanism: pairing on the d_{z²} orbital induces pairing on the d_{x²-y²} orbital, which gradually dominates at low temperatures, forming a two-orbital collaborative superconducting instability. This conclusion is validated by simulations with two different cluster sizes. This study provides a theoretical prediction for enhanced superconductivity in electron-doped Ruddlesden-Popper phase nickelates and proposes the heterostructure as a feasible experimental pathway, awaiting future experimental verification.

Polycrystalline samples of La₃₋ₓNaₓNi₂O₇₊δ with various sodium doping concentrations were synthesized via a solid-state method, and their structural, thermal, magnetic, and electrical transport properties were systematically investigated using X-ray diffraction, thermogravimetric analysis, and measurements of magnetic susceptibility and electrical resistivity. X-ray diffraction analysis revealed that when the sodium doping level x ≥ 0.075, the samples undergo a structural transition from the ‘327’ Amam phase to the ‘4310’ Bmab phase, accompanied by gradual lattice expansion. Resistivity measurements indicated that sodium doping significantly enhances metallicity while slightly suppressing the density wave transition temperature; applying external pressure further suppresses the density wave transition, yet the low-temperature insulating behavior remains insensitive to pressure. These findings demonstrate that hole doping introduced by substituting sodium for lanthanum effectively modulates competing electronic phases in layered nickelates, providing crucial experimental evidence for understanding the roles of elemental substitution and carrier doping in stabilizing high-pressure superconducting phases.

Evidence for charge and spin density waves in single crystals of La₃Ni₂O₇ and La₃Ni₂O₆

Using ultrafast terahertz spectroscopy, we performed optical-pump terahertz-probe experiments on an infinite-layer samarium nickel oxide thin film with Tc = 20 K to measure the temperature-dependent photoconductivity. Under weak excitation, the photoinduced destruction of the superfluid density is proportional to the equilibrium superfluid density and decreases linearly with increasing temperature, consistent with clean-limit d-wave pairing. From this linear relationship, the superconducting gap was extracted to be 2.5 meV, yielding 2Δ/kTc ≈ 3, indicating the system is in the weak coupling regime. Furthermore, independent estimates of the ratio of the mean free path to the coherence length (l/ξ) give approximately 1.5, further confirming clean-limit behavior. These results demonstrate that nickel oxide superconductors can realize a clean superconducting state and reveal a close similarity in pairing mechanism to cuprate high-temperature superconductors.

Evidence of Spin Density Waves in La₃Ni₂O₇₋δ

The recently discovered superconductivity with critical temperature 𝑇𝑐 up to 80 K in the double-layer Nickelate La3⁢Ni2⁢O7−𝛿 under pressure has drawn great attention. Here, we report the positive muon spin relaxation (𝜇+⁢SR) study of polycrystalline La3⁢Ni2⁢O6.92 under ambient pressure. Zero-field 𝜇+⁢SR experiments reveal the existence of magnetic order in La3⁢Ni2⁢O6.92 with 𝑇𝑁=154 K. The weak transverse field 𝜇+⁢SR measurements reveal the bulk nature of magnetism. In addition, a small quantity of oxygen deficiencies can greatly broaden the internal magnetic field distribution sensed by muons.

This study proposes that incoherent electronic states in strongly correlated materials arise not from disorder or material-specific mechanisms, but from self-generated dynamical disorder induced by competing fluctuations. In this marginal dynamical regime, electron dynamics naturally couple with time-dependent scattering, yielding the spectral function form ρ(z)=exp(-z²/4)D_ν(z), where z is the scaled energy, D_ν is the parabolic cylinder function, and ν=-1/2 is fixed. By independently scaling the angle-resolved photoemission spectroscopy (ARPES) energy distribution curves of the cuprates Nd₂₋ₓCeₓCuO₄ and Bi₂Sr₂CaCu₂O₈₊δ, the Kagome metal CsCr₃Sb₅, and the bilayer nickelate La₃Ni₂O₇, all datasets collapse onto a single universal curve, with only the amplitude and energy scale varying among materials. This spectral collapse indicates that microscopic details such as lattice geometry, band structure, and chemical composition become irrelevant in the low-energy regime, exhibiting fixed-point-like dynamical behavior. The result establishes a unified quantitative framework for the continuously dominant ARPES spectra across diverse strongly correlated materials.