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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.

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.

Evidence of Spin Density Waves in La₃Ni₂O₇−δ

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.

The recent discovery of high-temperature superconductivity in bilayer nickelate La3Ni2O7−δ single crystals under high pressure has sparked significant interest in the correlated electronic physics and unconventional pairing mechanisms within Ruddlesden-Popper phase nickelates, where the oxygen content plays a crucial role in both structure and superconductivity. Here we report the evolution of the structure and density wave order in La3Ni2O7−δ single crystals at ambient pressure under various oxygen annealing conditions. Structural analysis reveals that with the increase of annealing oxygen pressure, La3Ni2O7−δ undergoes a structural phase transition from an orthorhombic to a tetragonal phase. Magnetic torque measurements indicate that the density wave order, present in the orthorhombic phase, vanishes upon transition to the tetragonal structure, accompanied by an abrupt change in carrier concentration. A phase diagram illustrating the dependence of both the crystal structure and density wave order on the annealing oxygen pressure is mapped. Our findings suggest a promising pathway for studying the interplay between structure, density wave order, and superconductivity in bulk nickel-based materials.

This paper systematically measures the superfluid density of the infinite-layer nickel-based superconductor Nd1-xSrxNiO2 within the doping superconducting dome using the mutual inductance method. The results show that the superfluid stiffness is weak and exhibits an approximate square-root relationship with the superconducting transition temperature Tc. Additionally, a strong interaction between the Nd 4f magnetic moments and the superfluid is observed, leading to a significant suppression of the superfluid density at low temperatures, with an effect far beyond simple paramagnetic explanations, suggesting a coupling between magnetic order and the superconducting phase. These findings indicate that superconducting phase fluctuations play an important role in limiting Tc and reveal an unexpectedly strong coupling between rare-earth magnetic ions and the superfluid.

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