Methods facet

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

Mon, 01 Jan 0001 00:00:00 +0000

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

Mon, 01 Jan 0001 00:00:00 +0000

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

Bulk high-temperature superconductivity in pressurized tetragonal La₂PrNi₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

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.

bulk superconductivity

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

Bulk superconductivity up to 96 K in pressurized nickelate single crystals

Mon, 01 Jan 0001 00:00:00 +0000

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 (

$${T}_{{\rm{c}},\max }^{{\rm{onset}}}$$

Mon, 01 Jan 0001 00:00:00 +0000

Trilayer Ruddlesden-Popper phase La4Ni3O10 has been observed with Tc of ∼30 K at high pressure in a recent experiment, which further expanded the family of nickelate superconductors. In this study, we explored the effects of electronic correlations in La4Ni3O10 using density functional theory plus dynamical mean-field theory at ambient and high pressures. Our derived spectral functions and Fermi surface of the ambient pressure phase are nicely consistent with the experimental results by angle-resolved photoemission spectroscopy, which emphasized the importance of electronic correlations in La4Ni3O10. We also found the electronic correlations in pressurized La4Ni3O10 are both orbital-dependent and layer-dependent due to the presence of Hund’s rule coupling. There is a competition between the Hund’s rule coupling and the crystal-field splitting, and therefore, the Ni–O layers with weaker crystal-field splitting energy would have stronger electronic correlations.

Electronic correlations and partial gap in the bilayer nickelate La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of superconductivity with a critical temperature of about 80 K in La3Ni2O7 single crystals under pressure has received enormous attention. La3Ni2O7 is not superconducting under ambient pressure but exhibits a transition at T ∗ ≃ 115 K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La3Ni2O7 features strong electronic correlations which significantly reduce the electron’s kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at T ∗ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni-

$${d}_{3{z}^{2}-{r}^{2}}$$

orbital is removed due to the transition at T ∗. Our experimental results provide pivotal information for understanding the transition at T ∗ and superconductivity in La3Ni2O7.

Electronic correlations, layer distinction, and electron doping in the alternating single-layer--trilayer La₃Ni₂O₇ polymorph

Mon, 01 Jan 0001 00:00:00 +0000

We employ a density-functional theory plus dynamical mean-field theory framework to investigate the correlated electronic structure of the alternating single-layer–trilayer (1313) polymorph of La3⁢Ni2⁢O7, which becomes superconducting under pressure. At ambient pressure, the single layer is in a Mott-insulating regime and the low-energy physics is dominated by the trilayer block. Under pressure, the gap in the single-layer block closes due to orbital-selective physics, enabling charge transfer into the trilayer block. This change in effective doping of the trilayer block could be linked to the higher 𝑇𝑐 obtained in La3⁢Ni2⁢O7−1313 (∼80 K) when compared to the nominal trilayer La4⁢Ni3⁢O10 compound (∼30 K). We conclude that correlation-driven layer differentiation is crucial in the La3⁢Ni2⁢O7−1313 polymorph and that its low-energy physics aligns closely with the trilayer La4⁢Ni3⁢O10 compound (in spite of the apparent differences in nominal filling) rather than with the conventional bilayer La3⁢Ni2⁢O7.

Electronic Nematicity Revealed by Polarized Ultrafast Spectroscopy in Bilayer La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

Using polarized ultrafast pump-probe spectroscopy, the researchers comparatively investigated the normal-state electronic dynamics of bilayer La₃Ni₂O₇ and trilayer La₄Ni₃O₁₀ single crystals under ambient pressure. Both materials exhibit a density-wave transition accompanied by the opening of a quasiparticle relaxation bottleneck, yet their electronic responses display markedly different symmetries: trilayer La₄Ni₃O₁₀ remains optically isotropic across the entire temperature range, whereas bilayer La₃Ni₂O₇ shows clear twofold (C₂) rotational symmetry breaking—i.e., electronic nematicity—at low temperatures. This nematicity manifests in the anisotropy of slow quasiparticle relaxation dynamics and effective gap scale, and below 115 K it competes with a secondary isotropic order, leading to a non-monotonic temperature dependence of the nematic signal. This work reveals the presence of electronic nematic fluctuations in bilayer nickelates, which are absent in the trilayer system, suggesting a close relationship between electronic nematicity and high-pressure superconducting pairing in La₃Ni₂O₇, thereby providing key insights into the microscopic mechanism of this class of nickel-based superconductors.

Electronic structure and correlation of La₄Co₂NiO₈Cl₂: a theoretical proposal for a La₄Ni₃O₁₀-like high-temperature superconductor

Mon, 01 Jan 0001 00:00:00 +0000

Building on the discovery of high-pressure superconductivity in trilayer nickelate La₄Ni₃O₁₀, this study employed density functional theory combined with dynamical mean-field theory (DFT+DMFT) to design and calculate the cobalt-based analogue La₄Co₂NiO₈Cl₂. By substituting the inner-layer Co in the high-pressure phase La₄Co₃O₁₀ with Ni and incorporating Cl to achieve electron doping, this compound acquires a crystal structure and strongly correlated electronic characteristics similar to those of superconducting La₄Ni₃O₁₀: the outer-layer Co orbitals exhibit strong effective mass enhancement and non-Fermi liquid behavior, while the inner-layer Ni behaves as a weakly correlated Fermi liquid; a flat band near the Fermi level originating from the outer-layer Co orbitals emerges around the M point; and there is pronounced orbital selectivity as well as local spin fluctuations mixing high-spin and low-spin states. These features are in close agreement with the key electronic states of La₄Ni₃O₁₀, indicating that La₄Co₂NiO₈Cl₂ is a promising candidate for realizing high-temperature superconductivity in cobalt-based layered compounds, providing a theoretical basis for subsequent experimental exploration.

Electronic structure and magnetic correlations in the trilayer nickelate superconductor La₄Ni₃O₁₀ under pressure

Mon, 01 Jan 0001 00:00:00 +0000

It has been recently shown that under pressure trilayer Ruddlesden-Popper nickelate La4⁢Ni3⁢O10 (LNO) becomes superconducting below a critical temperature ≈20 K, in addition to the infinite-layer and bilayer systems. Motivated by this observation, we explore the effects of electron correlations on its electronic structure and magnetic properties using the advanced density functional theory plus dynamical mean-field theory approach. Our results for the normal-state electronic structure and correlation effects in LNO show much in common with the infinite-layer and bilayer nickelates, with remarkable site- and orbital-dependent renormalizations of the Ni 3⁢𝑑 bands and notable incoherence of the Ni 𝑑3⁢𝑧2−𝑟2 states, caused by correlation effects. Our analysis of the Fermi surface and magnetic correlations suggests the emergence of competing spin and charge stripe states, implying the importance of in-plane spin fluctuations to explain superconductivity in this material.

Electronic structure and magnetic tendencies of trilayer La₄Ni₃O₁₀ under pressure: Structural transition, molecular orbitals, and layer differentiation

Mon, 01 Jan 0001 00:00:00 +0000

Motivated by the recent observation of superconductivity in the pressurized trilayer Ruddlesden-Popper (RP) nickelate La4⁢Ni3⁢O10, we explore its structural, electronic, and magnetic properties as a function of hydrostatic pressure from first-principles calculations. We find that an orthorhombic (monoclinic)-to-tetragonal transition under pressure takes place concomitantly with the onset of superconductivity. The electronic structure of La4⁢Ni3⁢O10 can be understood using a molecular trimer basis wherein 𝑛 molecular subbands arise as the 𝑑𝑧2 orbitals hybridize strongly along the 𝑐 axis within the trilayer. The magnetic tendencies indicate that the ground state at ambient pressure is formed by nonmagnetic inner planes and stripe-ordered outer planes that are antiferromagnetically coupled along the 𝑐 axis, resulting in an unusual ↑, 0, ↓ stacking that is consistent with the spin density wave model previously suggested by neutron diffraction. Such a state is destabilized at the pressure where superconductivity arises. Despite the presence of 𝑑𝑧2 states at the Fermi level, the 𝑑𝑥2−𝑦2 orbitals also play a key role in the electronic structure of La4⁢Ni3⁢O10. This active role of the 𝑑𝑥2−𝑦2 states in the low-energy physics of the trilayer RP nickelate, together with the distinct electronic behavior of the inner and outer planes, resembles the physics of multilayer cuprates.

Electronic structure of Ruddlesden-Popper nickelates: Strain to mimic the effects of pressure

Mon, 01 Jan 0001 00:00:00 +0000

Signatures of superconductivity under pressure have recently been reported in the bilayer La3⁢Ni2⁢O7 and trilayer La4⁢Ni3⁢O10 Ruddlesden-Popper (RP) nickelates with the general chemical formula La𝑛+1⁢Ni𝑛⁢O3⁢𝑛+1 (𝑛 = number of perovskite layers along the 𝑐-axis). The emergence of superconductivity is always concomitant with a structural transition in which the octahedral tilts are suppressed, bringing the apical Ni-O-Ni angle to 180∘ and causing an increase in the out-of-plane 𝑑𝑧2 orbital overlap. Due to this strong interlayer coupling, a flat band of pure 𝑑𝑧2 character crosses the Fermi level. Here, using first-principles calculations, we explore biaxial strain (both compressive and tensile) as a means to mimic the electronic structure characteristics of RP nickelates (up to 𝑛=5) under hydrostatic pressure. Our findings highlight that strain enables the decoupling of the structural and electronic structure effects obtained under hydrostatic pressure: While compressive strain brings the apical Ni-O-Ni angle closer to 180∘, it shifts the 𝑑𝑧2 flat bands away from the Fermi energy, giving rise to a more cupratelike electronic structure. In contrast, tensile strain reduces the apical Ni-O-Ni angle (to values of ∼160∘), but it recovers the flat 𝑑𝑧2 band at the Fermi level appearing in the bilayer and trilayer RP nickelates under pressure. Overall, strain represents a promising way to tune the electronic structure of RP nickelates and could be an alternative route to achieve superconductivity at ambient pressure in this family of materials.

Electronic structure trends in La₂RNi₂O₇ (R= Pr, Nd, Sm) from first-principles

Mon, 01 Jan 0001 00:00:00 +0000

Using first-principles DFT+U calculations, this work investigates the trends in crystal structure and electronic properties of La₃Ni₂O₇ doped with different rare-earth elements (Pr, Nd, Sm). The results show that dopant atoms preferentially occupy La sites in the rock-salt layer; as the ionic radius decreases from Pr to Sm, the chemical pressure effect leads to a monotonic reduction in unit-cell volume and a successive increase in the monoclinic-to-tetragonal structural transition pressure, with this transition largely coinciding with the emergence of superconductivity, in agreement with experimental observations. In the high-pressure tetragonal phase, the d_{z²} band flattens and crosses the Fermi level, producing a characteristic hole-type Fermi surface that is regarded as a key electronic hallmark of superconductivity. With decreasing rare-earth ion size, the in-plane hopping integral is enhanced, whereas the out-of-plane hopping integral is weakened due to the shortening of the apical Ni–O bond. These findings offer microscopic mechanistic insights into how rare-earth doping influences the electronic structure of bilayer Ruddlesden-Popper nickelates and its connection to the superconducting transition temperature.

Electronic structure, quasiparticle renormalizations, and magnetic correlations in the alternating single-layer bilayer nickelate La₅Ni₃O₁₁

Mon, 01 Jan 0001 00:00:00 +0000

This study systematically investigates the electronic structure and magnetic correlations in the normal state of the alternating monolayer-bilayer Ruddlesden-Popper nickelate La₅Ni₃O₁₁ (1212-LNO) using the DFT+DMFT method. The results reveal significant differences between structurally distinct monolayer and bilayer Ni ions: the e_g states of bilayer Ni ions form strongly renormalized quasiparticle bands, with effective mass enhancement factors of approximately 3.5 and 4.2 for the Ni x²-y² and 3z²-r² orbitals, respectively; while the e_g states of monolayer Ni ions exhibit an orbital-selective Mott insulating state, where the Ni 3z²-r² orbital possesses a narrow gap and the x²-y² orbital displays metallic but strongly incoherent (non-Fermi liquid) behavior. Magnetic correlation analysis indicates that intertwined spin and charge density wave stripes may form in the bilayer NiO₆ planes, with the primary instability corresponding to an “up-down-0” spin pattern at wave vector Q=(1/3,1/3) competing with a “up-up-down-down” double-stripe state at (1/4,1/4). The 3d electrons of monolayer Ni tend to form Néel-type magnetic order. Under pressure, 1212-LNO undergoes an orbital-selective Mott insulator-metal transition accompanied by the metallization of the monolayer Ni e_g states, which exhibit strongly incoherent non-Fermi liquid behavior near the Fermi level. Overall, correlation effects significantly restructure the magnetic correlations from DFT-predicted monolayer-dominated to bilayer-dominated behavior, emphasizing the critical roles of interlayer confinement and orbital-dependent correlations.

Electronic theory for scanning tunneling microscopy spectra in bilayer nickelate thin films

Mon, 01 Jan 0001 00:00:00 +0000

This paper theoretically analyzes scanning tunneling microscopy spectra of superconducting bilayer nickelate films using a two-orbital bilayer model based on first-principles Wannier functions and the continuous Green’s function method. The study finds that the multi-orbital character and the spatial anisotropy of Wannier functions render the local density of states highly sensitive to the tip position: as the tip height increases, the relative weights of coherence peaks from different bands change significantly, thereby enabling distance-dependent measurements to distinguish the orbital origins of the controversial γ-band and β-band coherence peaks. Furthermore, in impurity-containing systems, quasiparticle interference patterns can clearly resolve the symmetry of s-wave and d-wave superconducting order parameters. This work provides explicit theoretical guidance for experimentally identifying the band attribution of superconducting gaps and the pairing symmetry.

Emergence of Kugel-Khomskii physics in quarter-filled bilayer correlated systems

Mon, 01 Jan 0001 00:00:00 +0000

This study investigates a quarter-hole-filled two-orbital bilayer Hubbard model inspired by transition metal bilayer systems. By explicitly treating the strong interlayer bonding of the dz2 orbital in a molecular orbital basis and projecting out high-energy electronic states, we derive a low-energy effective Kugel-Khomskii Hamiltonian that describes the coupling between electron spins and layer pseudospins. Combining Weiss mean-field theory with generalized flavor-wave theory, we reveal a rich ground-state phase diagram, including ferromagnetic and antiferromagnetic phases accompanied by layer-staggered charge-density order, a layer-coherent phase with spontaneous interlayer quantum coherence, and a novel maximal spin-layer entangled phase. This entangled phase arises from an emergent O(4) symmetry that is spontaneously broken to O(3), and its excitation spectrum features three gapless Goldstone modes that are entangled. The results suggest a geometry-driven mechanism for realizing composite entanglement in strongly correlated bilayer systems, and provide a concrete theoretical framework for understanding bilayer nickelate superconductors and other multi-component correlated materials.

Emergent quantum phenomena via phase-coherence engineering in infinite-layer nickelate superconductors

Mon, 01 Jan 0001 00:00:00 +0000

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.

Emergent s+id Superconductivity from the Interplay between Electronic Correlations and Electron-Phonon Coupling in R₁₋ₓSrₓNiO₂

Mon, 01 Jan 0001 00:00:00 +0000

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.

energy-dispersive X-ray spectroscopy (EDS)

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

Enhanced s^±-wave superconductivity in electron-doped La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

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.

Enhancement of metallicity by Na doping in La₃Ni₂O₇+δ

Mon, 01 Jan 0001 00:00:00 +0000

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.

epitaxial strain

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Keywords facet

EuₓNd₁₋ₓNiO₂

Mon, 01 Jan 0001 00:00:00 +0000

Materials facet

EuₓPr₁₋ₓNiO₂

Mon, 01 Jan 0001 00:00:00 +0000

Materials facet

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

Mon, 01 Jan 0001 00:00:00 +0000

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

Evidence for Clean d-wave Superconductivity in Samarium Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

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 La₃Ni₂O_7-δ

Mon, 01 Jan 0001 00:00:00 +0000

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

Mon, 01 Jan 0001 00:00:00 +0000

Evidence of Spin Density Waves in La₃Ni₂O₇−δ

Evidence of universal spectral collapse at a marginal dynamical regime

Mon, 01 Jan 0001 00:00:00 +0000

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.

Evolution of structure and density wave order in La₃Ni₂O₇−δ single crystals at ambient pressure

Mon, 01 Jan 0001 00:00:00 +0000

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.

Evolution of the Superfluid Density in Infinite-Layer Nickelates

Mon, 01 Jan 0001 00:00:00 +0000

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.

Exact diagonalization

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Methods facet

Experimental evidence of Tc enhancement above 50 K and diode and paramagnetic-Meissner effects, in Nickelate films on highly reduced SrTiO₃

Mon, 01 Jan 0001 00:00:00 +0000

Oxygen-deficient nickelate thin films were fabricated on highly reduced and conductive SrTiO₃ substrates, and through Meissner effect and transport measurements, a superconducting onset temperature of 50–70 K was observed, with zero resistance achieved at 20–25 K, indicating the presence of superconductivity in island-like regions within the film. A giant paramagnetic Meissner effect peak appeared at approximately 48 K, further supporting the occurrence of a superconducting transition near this temperature. Additionally, a non-reciprocal, hysteresis-free superconducting diode effect was observed, with its polarity fully polarizable and reversible. The thin films comprise a mixture of various Ruddlesden–Popper phases, including the infinite-layer phase. These enhanced superconducting properties are attributed to the synergistic effect between the oxygen-deficient films and the highly reduced SrTiO₃ substrates.

Experimental Progress in Ambient-Pressure Superconducting Bilayer Nickelate Films

Mon, 01 Jan 0001 00:00:00 +0000

Bilayer Ruddlesden-Popper nickelates exhibit superconductivity near 80 K under high pressure, and recent work has stabilized RA₃Ni₂O₇ (RA = rare earth or alkaline earth element) superconducting thin films at ambient pressure via epitaxial strain, enabling transport, spectroscopic, microscopic, and device measurements. This review summarizes experimental progress on ambient-pressure superconducting bilayer nickelate thin films, covering synthesis routes, oxygen stoichiometry, substrate-induced strain, normal-state transport, superconducting properties, doping phase diagrams, and momentum-resolved electronic structure. Key unresolved issues include the reproducibility of phase-pure ultrathin films, the microscopic origin of the two-step superconducting transition, the roles of oxygen defects and substrate doping, the position of the Ni 3dz₂-derived γ band, and the pairing symmetry. The review concludes that future work must establish more quantitative links between crystal structure, orbital reconstruction, and superconductivity to deepen the understanding of this unconventional high-temperature superconducting system.

fermi surface nesting

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Keywords facet

fermi surface reconstruction

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Keywords facet

fermi surface topology

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Keywords facet

filamentary superconductivity

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Keywords facet

First-principles calculations

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Methods facet

First-principles calculations (DFT)

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Methods facet

First-principles DFT calculations

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Methods facet

First-Principles Evidence for Strongly Correlated Superconductivity Driven by Structural Variations in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

This study systematically analyzes the electronic correlation behavior of La₃Ni₂O₇ within the superconducting pressure range using first-principles simulations combined with constrained random phase approximation and ab initio molecular dynamics. The results show that, accompanying the structural phase transition, the effective on-site repulsion of Ni e_g orbitals is significantly enhanced, attributed to the dynamic balance between orbital localization and competing screening channels, particularly the spacer-layer La bands. This enhancement region aligns remarkably well with the experimentally observed right-triangular superconducting dome, reaching a peak correlation strength at 18 GPa that corresponds to the highest superconducting critical temperature. Finite-temperature simulations further clarify the boundaries of the structural phase diagram, while calculations on Ac₃Ni₂O₇ confirm the critical role of A-site cations in the pressure-driven evolution of electronic correlations. These findings directly reveal how structural changes drive unconventional superconductivity by modulating the strength of electronic correlations.

FLEX

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Methods facet

Flux method

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

From perovskite to infinite-layer nickelates: hole concentration from x-ray absorption

Mon, 01 Jan 0001 00:00:00 +0000

This study systematically investigated the evolution of PrNiOₓ thin films at various intermediate stages of topological reduction (x = 2–3) using soft X-ray absorption spectroscopy. By comparing Ni L-edge experimental spectra with single-cluster and double-cluster ligand-field model calculations, it was found that none of the samples exhibited a pure d⁹ electronic configuration. Quantitative analysis based on the charge sum rule revealed that even in the most reduced films, the average number of Ni 3d holes remained 1.35, while superconducting samples displayed higher hole counts, challenging previous assumptions regarding the hole doping limit. Concurrent changes in the O K-edge absorption spectra during reduction indicated the presence of O 2p holes even in the most reduced films. Collectively, these results suggest that a complex hole doping mechanism arises from the interplay between self-doping effects and oxygen non-stoichiometry.

gigantic-oxidative atomic-layer-by-layer epitaxy (GAE)

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

Granular Superconductivity in La₂PrNi₂O₇-δ Thin Films

Mon, 01 Jan 0001 00:00:00 +0000

Research indicates that the two-step superconducting transition observed in La₂PrNi₂O₇₋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.

H-linear magnetoresistance in the T₂ resistivity regime of overdoped infinite-layer nickelate La₁₋ₓSrₓNiO₂

Mon, 01 Jan 0001 00:00:00 +0000

We report systematic magnetotransport measurements on high-crystallinity overdoped infinite-layer nickelate La₁₋ₓSrₓNiO₂ thin films (x = 0.20–0.24), revealing two prominent normal-state features in pulsed magnetic fields up to 62 T: the magnetoresistance violates Kohler’s rule and exhibits H-linear behavior at high H/T limits, while the normal-state resistivity consistently follows a T² dependence below 30 K. These results demonstrate the coexistence of H-linear magnetoresistance and T² resistivity in this model unconventional superconductor, providing new insights into the transport properties of the normal ground state that hosts superconductivity in infinite-layer nickelates.

hall coefficient

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

hall coefficient sign reversal

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

Hall Coefficient Sign Reversal Driven by Orbital-Selective Oxygen-Vacancy Scattering in Nickelate Films

Mon, 01 Jan 0001 00:00:00 +0000

Combining a correlated multi-orbital quasiparticle model derived from DFT+CDMFT with the T-matrix method, this study treats oxygen vacancy scattering within a semiclassical Boltzmann transport framework and reveals the microscopic origin of the Hall coefficient sign reversal in bilayer nickelate thin films. Multiband compensation alone is insufficient to explain the phenomenon; in-plane oxygen vacancies strongly suppress the transport channel dominated by the d_{x^2-y^2} orbital through orbital-selective scattering, driving the Hall coefficient across zero to become positive, whereas apical oxygen vacancies tend to make the Hall coefficient more negative. This pocket-resolved and orbital-selective scattering mechanism demonstrates that oxygen vacancies act not only as electron doping sources but also as active scattering centers whose spatial distribution directly controls the normal-state transport behavior, providing a theoretical framework for a unified understanding of the diverse Hall responses observed experimentally as a function of oxygen stoichiometry.

Hall effect measurements

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

Hartree-Fock

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Methods facet

Heterostructuring as Gateway to Electron Doping of Nickelate Superconductors

Mon, 01 Jan 0001 00:00:00 +0000

This study proposes a novel route to electron doping in nickelate superconductors through heterostructuring. First-principles calculations reveal that upon inserting wide-bandgap insulating layers of LaXO₃ (X = Al, Ga, Sc) into La₂NiO₄, the additional (LaO)⁺ layers act as electron donors, releasing carriers into the Ni-3d orbitals and thereby achieving disorder-free electron doping of Ruddlesden–Popper nickelates. This doping naturally places La₂NiO₄:La₂AlO₄ in the optimal regime for d_{x^2−y^2}-wave superconductivity, with many-body methods—including dynamical vertex approximation, fluctuation exchange, and dynamical cluster approximation—predicting a superconducting critical temperature exceeding 50 K and reaching as high as 127 K. The strategy circumvents the disorder typically introduced by conventional chemical doping, and is equally applicable to other nickelates such as La₃Ni₂O₇, offering a viable scheme for comprehensively mapping the electron-doping phase diagram of nickelates and extending the approach to other transition metal oxide systems.

Hf-doped La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

Materials facet

Hierarchical structure of primary and hybridization-induced superconducting correlations in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

This study employs the variational Monte Carlo method to perform nonperturbative calculations on a bilayer two-orbital Hubbard model, revealing the hierarchical structure of superconducting pairing in the bilayer layered nickelate La₃Ni₂O₇. It is found that the primary pairing interaction originates from the bonding-anti-bonding splitting of the Ni 3d({z^2}) orbital, while orbital hybridization redistributes superconducting correlations into the 3d({x^2-y^2}) channel, despite its intrinsically weak pairing interaction. This distinction between the origin of pairing and the source of superconducting correlations explains why both orbital channels exhibit comparable long-range superconducting correlations, and the resulting s± state is robust against changes in Fermi surface topology, such as the disappearance of the α Fermi pocket. The results reconcile previously divergent theoretical perspectives on the pairing mechanism, indicating that the strength of superconducting correlations is primarily determined by the orbital character of the low-energy density of states, whereas the pairing interaction stems from orbital level splitting in the bilayer structure, highlighting the crucial role of orbital hybridization in stabilizing superconductivity in multilayer layered superconductors.

High oxygen pressure floating zone growth and crystal structure of the metallic nickelates R₄Ni₃O₁₀ ( R = La, Pr )

Mon, 01 Jan 0001 00:00:00 +0000

High oxygen pressure floating zone growth and crystal structure of the metallic nickelates R₄Ni₃O₁₀ ( R = La, Pr )

high tc superconductivity

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Keywords facet

high temperature superconductivity

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Keywords facet

High temperature transitions in Ruddlesden-Popper nickelates Lan+1NinO₃n+1

Mon, 01 Jan 0001 00:00:00 +0000

This study systematically investigates single-crystal and powder samples of Ruddlesden-Popper nickelates La_{n+1}Ni_nO_{3n+1} (n=1,2,3,∞) using a combination of powder and single-crystal X-ray diffraction, heat capacity, and differential scanning calorimetry measurements across a broad temperature range of 2–1000 K, revealing a previously overlooked high-temperature phase transition. For the n=2 and n=3 compounds, pronounced lattice-parameter anomalies are observed around 560 K: in the bilayer 2222 phase, the out-of-plane lattice constant exhibits a sudden increase while the in-plane parameter contracts, indicating an abrupt release of octahedral tilting, whereas the monolayer–trilayer 1313 polytype displays an isotropic volume collapse; in the trilayer n=3 phase, the monoclinic angle β shows a clear kink near this temperature, and heat-capacity and DSC data further confirm the thermodynamic character of the transition. This transition is entirely distinct from the known high-temperature tetragonal transition and the low-temperature density-wave transition, and the n=∞ perovskite LaNiO₃ shows no analogous behavior. The study establishes that this high-temperature phase transition is a universal feature of the nickelate RP series and emphasizes that, in the search for superconductivity, the potential influence of this high-temperature structural instability on low-temperature physical properties must be carefully considered.

High-energy electronic excitations in La₃Ni₂O₇ by time-resolved optical spectroscopy

Mon, 01 Jan 0001 00:00:00 +0000

This study employs time-resolved optical spectroscopy to investigate the ultrafast dynamics of high-energy electronic excitations in bilayer nickelate La₃Ni₂O₇ from 10 K to room temperature at ambient pressure. Two high-energy electronic excitations originating from distinct interband transitions are identified at approximately 1.8 eV and 2.4 eV, revealing different density wave (DW) gaps of about 54 meV and 67 meV, respectively. The relaxation dynamics of these two excited states are well described by the Rothwarf-Taylor model. Additionally, four coherent Raman-active phonon modes are observed, exhibiting varying coupling strengths to the different electronic excitations. The phonon softening upon heating from about 100 K to room temperature can be explained by a semi-quantitative model incorporating thermal expansion and anharmonic phonon-phonon coupling, while the deviation of measured phonon frequencies from the model fit at low temperatures suggests an additional contribution from electron-phonon coupling. This work directly demonstrates the complex gap structure and phonon dynamics in this material, providing key insights into its density wave mechanism and many-body effects.

High-field-stabilized reentrant superconductivity in infinite-layer nickelate thin films

Mon, 01 Jan 0001 00:00:00 +0000

This study reports the discovery of high-magnetic-field-stabilized reentrant superconductivity in (Sm,Eu,Ca,Sr)NiO₂ infinite-layer nickelate thin films. Through resistive and radio-frequency inductive measurements, in addition to the low-field superconducting state, another superconducting state characterized by a sharp resistivity drop was observed in the high-field region, with a transition temperature of approximately 9.6–11.7 K in low-Tc samples, while in high-Tc samples (up to 31.7 K) the low-field and high-field superconducting phases merge, and the upper critical field far exceeds the Pauli paramagnetic limit. The phase diagram can be accurately described by a Werthamer-Helfand-Hohenberg (WHH) model modified with an internal exchange field, indicating that this phenomenon originates from the Jaccarino-Peter compensation mechanism, where the internal exchange field generated by the Eu²⁺ magnetic moments counteracts the applied magnetic field. This finding realizes for the first time field-induced reentrant superconductivity in materials with relatively high superconducting transition temperatures, offering a new pathway toward developing superconducting magnets and devices capable of operating under magnetic fields of several tens of teslas.

High-pressure crystal growth and investigation of the metal-to-metal transition of Ruddlesden–Popper trilayer nickelates La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

Single crystals of Ruddlesden–Popper nickelates La4Ni3O10 were grown by means of the floating-zone technique at oxygen pressure of 20 bar. Our results reveal the effects of the annealing process under pressure on the crystal structure. We present the requirements for crystal growth and show how a reported ferromagnetic impurity phase can be avoided. The different growth and post-annealing processes result in two distinct phases 𝑃 21∕𝑎 and Bmab in which the metal-to-metal transitions occur at 152 K and 136 K, respectively.

High-pressure measurements

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

high-pressure transport

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

High-temperature superconductivity in Nd₀.85Sr₀.15NiO₂ membranes under pressure

Mon, 01 Jan 0001 00:00:00 +0000

Researchers have developed a technique to integrate free-standing infinite-layer Nd₀.₈₅Sr₀.₁₅NiO₂ thin films into diamond anvil cells, thereby overcoming the difficulties of measuring such films under high-pressure conditions. By applying pressures up to approximately 90 GPa to the films, they observed that the superconducting transition temperature (T_c) increased monotonically and linearly from about 17 K at ambient pressure to roughly 74.2 K, with an enhancement rate of approximately 0.65 K/GPa and no signs of saturation. This linear, non-saturating pressure dependence of T_c markedly differs from the pressure-induced overdoping that leads to T_c suppression in most copper oxide superconductors and bilayer nickelates, suggesting that the pairing strength in infinite-layer nickelates can be elevated to unexpectedly high levels. Furthermore, measurements of the upper critical field and coherence length confirm the pressure-induced enhancement of the superconducting state. This study provides a new pathway for continuously enhancing superconductivity through lattice compression, and the developed free-film high-pressure technique holds promise for broad application to other two-dimensional materials.

High-temperature superconductivity with zero resistance and strange-metal behaviour in La₃Ni₂O₇−δ

Mon, 01 Jan 0001 00:00:00 +0000

Recent experimental observations have showed some signatures of superconductivity close to 80 K in La3Ni2O7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La3Ni2O7−δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T-linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system.

hole doping

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

Hubbard-U-corrected electron-phonon interactions in strongly correlated materials via the finite-displacement method

Mon, 01 Jan 0001 00:00:00 +0000

This paper combines density functional theory with the Hubbard U correction (DFT+U) and the finite displacement method to achieve a full Hubbard-corrected calculation of phonon spectra and electron-phonon matrices in strongly correlated materials. The authors apply this method to two representative systems: infinite-layer nickelate LaNiO₂ and ruthenium dioxide RuO₂. The results show that in 20% hole-doped LaNiO₂, the Hubbard U correction weakly enhances the electron-phonon interaction, but the total coupling strength remains small and insufficient to explain the experimentally observed superconducting transition temperature of approximately 10–30 K; this contradicts recent predictions from the GW correction, with the discrepancy arising from differences in the Fermi surface topology obtained by DFT+U and GW methods. In RuO₂, the Hubbard U correction eliminates imaginary phonon modes under TiO₂ substrate strain and significantly reduces the electron-phonon coupling, alleviating the contradiction between the theoretically overestimated electron-phonon coupling and the experimentally observed low superconducting transition temperature. This work provides a computational scheme that fully incorporates the Hubbard U correction for electron-phonon properties and highlights the critical influence of Fermi surface shape and correlation effects on phonon spectra and electron-phonon matrices.

hund's coupling

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

hund's rule coupling

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

hybridization

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

hydrostatic pressure

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

Identification of superconductivity in bilayer nickelate La₃Ni₂O₇ under high pressure up to 100 GPa

Mon, 01 Jan 0001 00:00:00 +0000

Identification of superconductivity in the Ruddlesden-Popper phases of nickelates under high pressure remains challenging. Here, we report a comprehensive study of the crystal structure, electrical resistance, and Meissner effect in single crystals of bilayer nickelate La3Ni2O7 under hydrostatic pressures up to 104 GPa. Using high-pressure X-ray diffraction, we observe a structural transition from an orthorhombic to a tetragonal phase above 40 GPa. Superconductivity emerges with a maximum onset transition temperature Tconset of 83 K at 18.0 GPa, accompanied by zero resistance. The superconducting phase is gradually suppressed and vanishes above 80 GPa, forming a right-triangle-like superconducting region. Direct-current magnetic susceptibility measurements demonstrate the Meissner effect and reveal a superconducting volume fraction of ∼41% at 22.0 GPa and 20 K, confirming the bulk nature of superconductivity in La3Ni2O7. Our results highlight the intricate relationship between superconductivity, oxygen content, and structural transitions in this material.

Identifying the structure of La₃Ni₂O₇ in the pressurized superconducting state

Mon, 01 Jan 0001 00:00:00 +0000

Using high-pressure variable-temperature Raman spectroscopy and polarization analysis, this work systematically tracks the structural evolution of a La₃Ni₂O₇ single crystal down to 3 K and up to 32.7 GPa. Based on rigorous symmetry selection rules, the disappearance and renormalization of multiple phonon modes in the spectra indicate a first-order structural phase transition from the orthorhombic Amam phase to the orthorhombic Fmmm phase at approximately 14.5 GPa, precisely coinciding with the emergence of bulk superconductivity. Polarized Raman measurements further reveal that above 1.92 GPa the sample recovers its intrinsic D₂h symmetry through detwinning, and in the superconducting state (3 K, 19.45 GPa) phonon modes are still observed in polarization channels, directly ruling out the tetragonal I4/mmm phase. These results confirm that the intrinsic crystal structure of the pressurized superconducting state below 19.45 GPa is orthorhombic Fmmm, rather than the previously disputed tetragonal phase, and disclose that the 180° Ni–O–Ni bond angle along the c-axis is a key structural prerequisite for achieving a high superconducting transition temperature, thereby establishing a vital structural foundation for understanding the superconducting mechanism of bilayer nickelates.

Imaging stripe dynamics in trilayer nickelate La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

This study employed spin-polarized scanning tunneling microscopy to perform real-space imaging of the stripe order in the trilayer nickelate La₄Ni₃O₁₀, revealing its local magnetic and charge distributions. The experiments showed that the stripe order exhibits a four-unit-cell periodicity, highly reminiscent of the stripe order in cuprate high-temperature superconductors, and opens a nearly complete energy gap of approximately 66 meV near the Fermi level. More importantly, when the tunneling electron energy exceeds a threshold of about 20 meV, discrete phase slips can be triggered, enabling atomic-scale imaging of stripe dynamics. These results underscore the crucial role of correlated physics in driving stripe-like order in lanthanum nickelates and reveal striking similarities to cuprate superconductors, providing important clues for understanding the pairing mechanism in nickel-based superconductors.

Impact of multiband effects on non-Fermi-liquid transport phenomena in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

This study employs a multi-orbital tight-binding model to analyze non-Fermi liquid transport phenomena in the bilayer nickelate La₃Ni₂O₇, focusing on the influence of multiband effects on the Hall coefficient. Using the Green’s function method, a rigorous formula for the Hall coefficient incorporating the quasi-quantum metric (qQM) term is derived, revealing that the temperature dependence of this qQM term is crucial in strongly correlated multiband systems. Calculations show that spin fluctuations in the Ni d₂² orbital lead to stronger quasiparticle damping, while the Ni dₓ²₋ᵧ² orbital forms cold spots. The pronounced temperature dependence of the Hall coefficient in La₃Ni₂O₇ originates from the competition between the positive contribution of the hole band and the negative contribution of the electron band, with the qQM term enhancing the positive Hall coefficient at low temperatures and explaining the experimentally observed T-linear resistivity and the increase of the Hall coefficient upon cooling. Furthermore, the qQM term also plays a key role in describing the Nernst coefficient and other transport phenomena involving second derivatives of velocity. This study reveals the core mechanism of spin-fluctuation-induced orbital-selective renormalization in non-Fermi liquid transport, providing a theoretical framework for understanding the anomalous transport properties of this system.

Impact of pressure and apical oxygen vacancies on superconductivity in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The bilayer nickelate La3Ni2O7 under pressure has recently emerged as a promising system for high-Tc superconductivity. In this work, we investigate the fate of the superconducting properties in La3Ni2O7 under pressure, focusing on the effects of structural deformation and apical oxygen vacancies. Employing a low-energy effective t-J∥-J⊥ model for the

$$3{d}_{{x}^{2}-{y}^{2}}$$

orbitals within the slave-boson mean-field approach, we demonstrate that the pairing strength is significantly enhanced in the high-pressure tetragonal I4/mmm phase compared to the ambient pressure orthorhombic Amam phase. Furthermore, by simulating random configurations of apical oxygen vacancies, we show that oxygen vacancies suppress both pairing strength and superfluid density. These results underscore the critical role of pressure and oxygen stoichiometry in tuning the SC of La3Ni2O7, providing key insights into optimizing its high-Tc behavior.

in plane lattice distortion

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet

Incommensurate spin fluctuations and competing pairing symmetries in La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The recent discovery of superconductivity in the bilayer Ruddlesden-Popper nickelate

 La
3
Ni
2
O
7
under high pressure has generated much interest in the superconducting pairing mechanism of nickelates. Despite extensive work, the superconducting pairing symmetry in
La
3
Ni
2
O
7
remains unresolved, with conflicting results even for identical methods. We argue that different superconducting states in
La
3
Ni
2
O
7
are in close competition and highly sensitive to the choice of interaction parameters as well as pressure-induced changes in the electronic structure. Our study uses a multiorbital Hubbard model, incorporating all Ni
3
d
and O
2
p
states. We analyze the superconducting pairing mechanism of
La
3
Ni
2
O
7
within the random phase approximation and find a transition between
d
-wave and sign-changing
s
-wave pairing states as a function of pressure and interaction parameters, which is driven by spin fluctuations with different wave vectors. These spin fluctuations with incommensurate wave vectors cooperatively stabilize a superconducting order parameter with
d
x
2
−
y
2
symmetry for realistic model parameters. Simultaneously, their competition may be responsible for the absence of magnetic order in
La
3
Ni
2
O
7
, demonstrating that magnetic frustration and superconducting pairing can arise from the same set of incommensurate spin fluctuations.

This study investigates the superconductivity and magnetism of bilayer nickelates from an itinerant perspective. Based on tight-binding fitting of angle-resolved photoemission spectroscopy data from compressively strained films, the authors introduced standard on-site repulsive interactions (including intra-orbital U, inter-orbital U′, Hund’s coupling JH, and pair-hopping JP) and renormalized these bare interactions through the random phase approximation (RPA) by considering particle-hole fluctuations, thereby obtaining an effective pairing interaction. The results show that in the strong Hund’s coupling regime, s-wave superconductivity and (π/2, π/2) spin density wave (SDW) order are the dominant ground states, while under weak Hund’s coupling, d-wave pairing and (π, π) SDW become the leading ground states. These findings are qualitatively consistent with previous density matrix renormalization group (DMRG) studies, underscoring the critical role of Hund’s coupling in determining the superconducting pairing symmetry and magnetic type of the system.

Superconductivity and normal-state transport in compressively strained La₂PrNi₂O₇ thin films

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of superconductivity under high pressure in Ruddlesden–Popper phases of bulk nickelates has sparked great interest in stabilizing ambient-pressure superconductivity in the thin-film form using epitaxial strain. Recently, signs of superconductivity have been observed in compressively strained bilayer nickelate thin films with an onset temperature exceeding 40 K, although with broad, two-step-like transitions. Here we report the intrinsic superconductivity and normal-state transport properties in compressively strained La2PrNi2O7 thin films, achieved through a combination of isovalent Pr substitution, growth optimization and precision ozone annealing. The superconducting onset occurs above 48 K, with zero resistance reached above 30 K, and the critical current density at 1.4 K is 100-fold larger than previous reports. The normal-state resistivity exhibits quadratic temperature dependence indicative of Fermi liquid behaviour, and other phenomenological similarities to transport in overdoped cuprates suggest parallels in their emergent properties.

Superconductivity in an infinite-layer nickelate

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of unconventional superconductivity in (La,Ba)2CuO4 (ref. 1) has motivated the study of compounds with similar crystal and electronic structure, with the aim of finding additional superconductors and understanding the origins of copper oxide superconductivity. Isostructural examples include bulk superconducting Sr2RuO4 (ref. 2) and surface-electron-doped Sr2IrO4, which exhibits spectroscopic signatures consistent with a superconducting gap3,4, although a zero-resistance state has not yet been observed. This approach has also led to the theoretical investigation of nickelates5,6, as well as thin-film heterostructures designed to host superconductivity. One such structure is the LaAlO3/LaNiO3 superlattice7–9, which has been recently proposed for the creation of an artificially layered nickelate heterostructure with a singly occupied

$${d}_{{x}^{2}-{y}^{2}}$$

band. The absence of superconductivity observed in previous related experiments has been attributed, at least in part, to incomplete polarization of the eg orbitals10. Here we report the observation of superconductivity in an infinite-layer nickelate that is isostructural to infinite-layer copper oxides11–13. Using soft-chemistry topotactic reduction14–20, NdNiO2 and Nd0.8Sr0.2NiO2 single-crystal thin films are synthesized by reducing the perovskite precursor phase. Whereas NdNiO2 exhibits a resistive upturn at low temperature, measurements of the resistivity, critical current density and magnetic-field response of Nd0.8Sr0.2NiO2 indicate a superconducting transition temperature of about 9 to 15 kelvin. Because this compound is a member of a series of reduced layered nickelate crystal structures21–23, these results suggest the possibility of a family of nickelate superconductors analogous to copper oxides24 and pnictides25.

Superconductivity in bilayer La₃Ni₂O₇: A review focusing on the strong-coupling Hund's rule assisted pairing mechanism

Mon, 01 Jan 0001 00:00:00 +0000

The high-temperature superconductivity in bilayer La₃Ni₂O₇ originates from its unique two-orbital bilayer electronic structure, where the 3d_z² orbital is nearly half-filled and localized, generating strong interlayer antiferromagnetic exchange via the inner apical oxygen 2p_z orbital, while the 3d_x²-y² orbital is approximately quarter-filled and highly itinerant. Under strong coupling, Hund’s rule coupling aligns the spins of the two orbitals on the same nickel site, effectively transferring the interlayer antiferromagnetic exchange to the itinerant 3d_x²-y² orbital, forming an effective coupling J⊥. This mechanism can be simplified into a strong-coupling bilayer t-J-J⊥ model for the 3d_x²-y² band, where J⊥ drives electrons to form interlayer Cooper pairs, realizing extended s-wave pairing superconductivity with high critical temperature. Meanwhile, the strongly localized 3d_z² electrons tend to form interlayer ladder singlets; due to the lack of phase coherence, these singlets do not directly participate in the superconducting condensation but instead give rise to a pseudogap phase. This review systematically elaborates on this strong-coupling Hund’s rule assisted pairing theory, providing a unified framework for understanding the mechanism of high-temperature superconductivity in this system.

Superconductivity in doped symmetric mass generation insulator: a quantum Monte-Carlo study

Mon, 01 Jan 0001 00:00:00 +0000

This study employs sign-problem-free quantum Monte Carlo simulations to systematically investigate a bilayer fermionic model with strong interlayer antiferromagnetic exchange coupling and local Hubbard repulsion, which serves as a prototype for realizing the symmetric mass generation (SMG) insulator. The numerically exact results unambiguously demonstrate that robust superconducting pairing emerges upon doping the SMG insulator phase, and the Hubbard repulsion significantly enhances the superconducting order parameter. Given that this model may capture key features of the high-temperature superconductor La₃Ni₂O₇ under high pressure, this work establishes a new paradigm for achieving superconductivity starting from a doped SMG parent state, providing important theoretical guidance for future experimental exploration.

Superconductivity in monolayer-trilayer phase of La₃Ni₂O₇ under high pressure

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of 80 K superconductivity in pressurized bilayer Ruddlesden-Popper (RP) nickelate La$_3$Ni$_2$O$_7$ has established a new high-temperature superconductor family. The quest to understand the governing principles of RP nickelate superconductivity has become a central focus in condensed matter physics. Here, we report a critical advance by synthesizing and investigating a distinct structural polymorph of the same compound: the monolayer-trilayer (1313) hybrid phase of La$_3$Ni$_2$O$_7$. Under high pressure, synchrotron X-ray diffraction and Raman spectroscopy reveal a structural transition from the orthorhombic $Cmmm$ to the tetragonal $P4/mmm$ space group at 13~GPa. Above 19 GPa, the phase exhibits a clear superconducting transition, confirmed by a zero-resistance state, albeit at a significantly reduced temperature of 3.6 K. The stark contrast with the 80 K transition in the bilayer phase provides a uniquely clean experimental comparison. Our results demonstrate that the superconducting transition temperature is directly governed by the nature of the interlayer coupling, and the bilayer NiO$_6$ block as the essential structural motif for achieving high-$T_\text{c}$ superconductivity in the RP nickelates.

Superconductivity in pressurized trilayer La₄Ni₃O₁₀−δ single crystals

Mon, 01 Jan 0001 00:00:00 +0000

Superconductivity in pressurized trilayer La₄Ni₃O₁₀−δ single crystals

Superconductivity in Ruddlesden-Popper nickelates: a review of recent progress, focusing on thin films

Mon, 01 Jan 0001 00:00:00 +0000

In recent years, significant breakthroughs have been achieved in the study of Ruddlesden-Popper (RP) nickel oxide superconductors. This article systematically reviews the experimental and theoretical progress in this field, with a particular focus on thin-film systems. Key findings include the emergence of superconductivity in bilayer La₃Ni₂O₇ (T_c ~ 80 K) and trilayer La₄Ni₃O₁₀ under high pressure, and, critically, the realization of ambient-pressure superconductivity in ultra-thin films of La₃Ni₂O₇ grown on substrates providing compressive strain—a breakthrough that overcomes the high-pressure limitation and enables the use of experimental techniques previously inaccessible in the superconducting state, such as angle-resolved photoemission spectroscopy (ARPES). On the theoretical side, the system requires simultaneous consideration of both the Ni e_g and a_{1g} orbitals, as well as the strong interlayer coupling within bilayers that gives rise to a “dimer” picture, and exhibits strange metal behavior and strong correlation features reminiscent of cuprates. By comparing the similarities and differences among various RP nickel oxides, this article offers a new perspective on understanding the mechanism of high-temperature superconductivity in correlated electron systems and outlines future research directions.

Superconductivity onset above 60 K in ambient-pressure nickelate films

Mon, 01 Jan 0001 00:00:00 +0000

This study employed the enormous oxidation atomic layer epitaxy method to grow (La,Pr)3Ni2O7 thin films on SrLaAlO4 substrates under extreme non-equilibrium conditions, achieving a superconducting onset transition temperature of approximately 63 K at ambient pressure, with zero-resistance temperature reaching about 37 K and diamagnetic signal onset at around 23 K. This method overcomes the structural instability of the metastable superconducting phase through high-temperature and in-situ sufficient oxidation; X-ray diffraction and scanning transmission electron microscopy confirmed that the films possess large-scale crystalline purity. Transport measurements reveal a systematic evolution of the normal-state resistivity temperature power-law exponent α from Fermi liquid behavior (α≈2) in samples with low onset transition temperatures to strange metal behavior (α≈1) in samples with high onset transition temperatures, directly correlating enhanced superconductivity with non-Fermi liquid behavior. The vortex melting phase diagram constructed via mutual inductance technique indicates that the two-dimensional melting limit is suppressed to near zero, with interlayer coupling strength significantly stronger than that of bismuth-based cuprates. These results demonstrate that nickelates are strange metal high-temperature superconductors with strong interlayer coupling at ambient pressure.

superconductor insulator transition

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Keywords facet

Superconductor-insulator transitions in infinite-layer nickelates controlled via operando monitored reduction

Mon, 01 Jan 0001 00:00:00 +0000

By developing an in situ monitoring reduction (OMR) method, this study achieved continuous modulation of the Ni 3d orbital electron occupancy in infinite-layer nickelate superconductors over an ultra-wide range from approximately 3d⁷ to 3d⁹, thereby controllably driving the superconductor-insulator transition (SIT). Combining synchrotron X-ray absorption spectroscopy and scanning transmission electron microscopy analysis of oxygen atoms, the electron occupancy states were precisely calibrated, and the SIT was further modulated using ionic liquid gating and magnetic fields. Nernst effect measurements reveal that, unlike in cuprates, pairing initiates as soon as the resistance starts to drop, while the Meissner effect only appears in the zero-resistance state, marking the establishment of global phase coherence. Angle-dependent magnetotransport studies show that within the transition temperature range, superconductivity exhibits a mixture of two-dimensional and three-dimensional characteristics, indicating that the observed SIT deviates from the classical 2D model. These results provide a unique perspective for understanding the interplay between structural and electronic phase transitions in infinite-layer nickelates within the oxygen content–magnetic field–temperature parameter space.

superfluid density

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Keywords facet

Suppressed density wave and emergent negative magnetoresistance in Tb-doped La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The suppression of density wave in bilayer nickelate La3⁢Ni2⁢O7 under pressure has been identified as a critical factor enabling pressure-induced high-temperature superconductivity. However, this density wave state exhibits remarkable stability against most alternative tuning methods except the high-pressure technique. Herein through systematic investigations of Tb doping effects on electrical transport and magnetic properties, we observe a gradual suppression of density wave transition temperature with the increasing Tb concentration, accompanied by the emergence of negative magnetoresistance persisting up to 14 T. Magnetic susceptibility measurements further reveal the formation of a doping-induced spin-glass state, which likely accounts for the observed negative magnetoresistance phenomenon. This work establishes an effective chemical doping approach to manipulate the density wave state and correlated quantum state in La3⁢Ni2⁢O7, offering new insights into the mechanism of high-temperature superconductivity and potential pathways toward achieving ambient-pressure superconductivity in bulk nickelate crystals.

synchrotron X-ray diffraction

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Methods facet

t linear resistivity

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Keywords facet

The evolution of pairing correlation with 3d_z2 electron filling in a bilayer two-orbital model for La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

Using the density matrix renormalization group method, this work systematically investigates the evolution of superconducting pairing correlations with the filling of the Ni 3d_{z²} orbital in an effective bilayer two-orbital model of pressurized bilayer nickelate La₃Ni₂O₇ on a one-dimensional minimal geometric structure. By adjusting the orbital chemical potential difference to continuously tune the 3d_{z²} orbital from 1/12 doping to near half-filling, it is found that superconducting correlations are significantly suppressed near half-filling, indicating that the itinerancy of this orbital favors pairing. Moreover, pairing correlations are enhanced in regions with larger charge fluctuations, suggesting competition between charge order and superconductivity. These results support the existing theoretical picture that interlayer antiferromagnetic superexchange provides the pairing glue, while the itinerant 3d_{z²} orbital mediates pairing and establishes long-range coherence through hybridization. The study clarifies the crucial role of the itinerancy of the 3d_{z²} orbital in superconductivity, providing important evidence for understanding the unconventional pairing mechanism in this system.

theoretical analysis

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

Theoretical proposal of superconductivity in hole-doped reduced bilayer nickelate La₃Ni₂O₆: a manifestation of orbital-space bilayer model with incipient bands

Mon, 01 Jan 0001 00:00:00 +0000

This study establishes a correspondence between the multi-orbital Hubbard model and the bilayer Hubbard model, proposing an orbital-space bilayer model (OSBM) in which the orbital energy level difference ΔE plays a role analogous to interlayer hopping in a real-space bilayer model, and superconductivity is enhanced in the incipient-band regime. Based on this, the theory predicts that the reduced bilayer nickelate La₃Ni₂O₆, under appropriate hole doping, can serve as a candidate OSBM superconductor. A tight-binding model constructed from first principles reveals a large ΔE between the Ni d_{x²-y²} orbital and other d orbitals due to the absence of apical oxygen atoms. Using the fluctuation-exchange approximation, calculations show that in the incipient-band scenario, intersite interactions can drive s±-wave superconductivity, where the superconducting gap function changes sign between the d_{x²-y²} band and other d-orbital bands. The study also examines the energetic and dynamic stability of the crystal structure under atomic substitution and pressure. Although La₃Ni₂O₇ and La₃Ni₂O₆ share similar chemical formulas, this work suggests that the latter may realize a completely different pairing mechanism.

Theoretical study of superconductivity in freestanding infinite-layer nickelate membranes under pressure: mitigation of excess correlation enhances T_c

Mon, 01 Jan 0001 00:00:00 +0000

This study constructs a seven-orbital effective model based on first-principles calculations and employs the fluctuation exchange (FLEX) approximation to theoretically analyze the superconductivity of free-standing infinite-layer nickelate Nd₀.₈₅Sr₀.₁₅NiO₂ films under pressure. The results show that the superconducting transition temperature Tc increases monotonically with applied pressure, consistent with recent experimental observations. This enhancement is attributed to the mitigation of excessively strong electronic correlations arising from the extremely low valence state of Ni atoms, leading to a significant reduction in the effective electronic interaction parameter U, which in turn reduces quasiparticle damping and enhances spin-fluctuation-mediated d-wave pairing. Additionally, phonon calculations confirm that the crystal structure remains dynamically stable up to 90 GPa. By comparing models with different U values, the study demonstrates that only a relatively large U (approximately 5.1 eV) can reproduce the experimental trend, while smaller U values lead to premature saturation or even a dome-shaped Tc behavior, thereby supporting the mechanism whereby excessively strong correlations suppress superconductivity in infinite-layer nickelates, and pressure alleviates these correlations to enhance Tc.

Theoretical study on ambient pressure superconductivity in La₃Ni₂O₇ thin films: structural analysis, model construction, and robustness of s±-wave pairing

Mon, 01 Jan 0001 00:00:00 +0000

This study theoretically analyzes the superconductivity of La₃Ni₂O₇ thin films under ambient pressure. A model Hamiltonian is constructed based on first-principles structure optimization, with the in-plane lattice constants fixed to those of the experimental substrates (LSAT, LAO, and SLAO), and an additional model employing experimentally determined crystal structures is used. The linearized Eliashberg equation is solved using the fluctuation exchange approximation (FLEX) with full momentum- and frequency-dependent Green’s functions and pairing interactions. The results indicate that the electronic structure, including the presence or absence of the γ Fermi pocket, depends on the adopted crystal structure and whether +U corrections are included in the band calculations, yet the s±-wave pairing symmetry remains robust. This robustness primarily arises because pairing is mediated by finite-energy spin fluctuations, which are insensitive to details of the Fermi surface topology and yield a nearly momentum-independent interlayer d_{3z²−r²} pairing gap function in the orbital representation. On the other hand, the superconducting transition temperature of the thin films (approximately 40 K) is about half that of bulk material under pressure (approximately 80 K). Within the FLEX framework, this can only be understood by adopting a model with a small interlayer hopping parameter |t⊥| derived from experimentally determined crystal structures, although other contributing factors cannot be ruled out.

thermogravimetric analysis

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

Three-Dimensional Electronic Structures in Superconducting Ruddlesden-Popper Bilayer Nickelate Films

Mon, 01 Jan 0001 00:00:00 +0000

By employing a low-temperature ultrahigh-vacuum dark-box transfer technique to preserve sample surface quality, this study systematically resolves the three-dimensional electronic band structure of superconducting (La,Pr,Sm)₃Ni₂O₇/SrLaAlO₄ thin films using angle-resolved photoemission spectroscopy (ARPES) with multiple photon energies. The experiments reveal orbital-dependent dimensionality: the dx²-y²-dominated band exhibits quasi-two-dimensional character, while the dz²-dominated γ band displays clear kz dispersion. Finite gaps are observed along high-symmetry directions for all detected bands, with temperature-dependent analysis of the γ band indicating a superconducting gap of approximately 18 meV and a ratio 2Δ/kBTc ~ 8, far exceeding the weak-coupling BCS limit. Moreover, suppression of spectral weight near the Fermi level persists above the superconducting transition temperature, and ubiquitous waterfall-like spectral features are observed, indicating a significant influence of electron correlations. These findings underscore the critical role of the third dimension and the dz² orbital in the nickelate superconducting mechanism, imposing important constraints on theoretical models.

Threefold error in the reported zero-field cooled magnetic moment of single crystal La₂SmNi₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

This paper points out that Li et al. made three errors in calculating the superconducting volume fraction from zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements on single crystals of the high-pressure nickelate superconductor La₂SmNi₂O₇. First, due to the paramagnetic Meissner effect (Wohlleben effect), the magnetic moment in FC mode can be either positive or negative, and thus cannot be used to calculate the superconducting volume fraction. Second, reanalysis of Li et al.’s ZFC data reveals that, according to their own calculation method, the superconducting volume fraction should be 22.8%, not the reported 62.1%—a discrepancy of about a factor of three, primarily arising from differences in the demagnetization factor (Li et al. used 0.849, while the authors calculated 0.81548 using Brandt’s formula). Third, even if the ZFC magnetic moment value is correctly calculated, it cannot be directly used to determine the superconducting volume fraction, because there are infinitely many shapes and distributions of superconducting regions smaller than the actual sample size that can produce the same measured magnetic moment. The authors emphasize that they agree with Li et al.’s experimental confirmation of bulk superconductivity in pressurized nickelates, but argue that these calculation errors need to be corrected to avoid misleading future research.

Tight-binding model

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

tight-binding modeling

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

Time-reversal symmetry breaking superconductivity with electronic glass in nickelate (La, Pr, Sm)₃Ni₂O₇ films

Mon, 01 Jan 0001 00:00:00 +0000

The research team conducted electrical transport measurements on (La, Pr, Sm)₃Ni₂O₇ double-layer nickelate thin films and discovered a time-reversal symmetry breaking superconducting state accompanied by electronic glass behavior. This superconducting state emerges in the low-temperature regime near zero resistance, exhibiting three prominent features: first, an unconventional magnetoresistance hysteresis that directly evidences time-reversal symmetry breaking and remains robust under different magnetic field orientations, fundamentally distinct from vortex pinning or long-range magnetic order; continuous oxygen reduction simultaneously weakens both superconductivity and the hysteresis, revealing their connection to specific Ni 3d electronic orbitals. Second, the current–voltage response demonstrates magnetic history dependence and non-reciprocity under zero field, further confirming spontaneous intrinsic time-reversal symmetry breaking. Third, the resistance exhibits slow logarithmic relaxation after removing the magnetic field, a hallmark of glassy dynamics. These phenomena reveal for the first time in nickel-based superconductors a superconducting state that simultaneously possesses spontaneous time-reversal symmetry breaking and intrinsic glassy characteristics, providing significant phenomenological and conceptual breakthroughs for understanding the mechanism of high-temperature superconductivity.

Topochemical Oxidation of Ruddlesden–Popper Nickelates Reveals Distinct Structural Family: Oxygen-Intercalated Layered Perovskites

Mon, 01 Jan 0001 00:00:00 +0000

Layered perovskites─including the Dion–Jacobson, Ruddlesden–Popper, and Aurivillius families─exhibit a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here, we report a new family of layered perovskites realized through topochemical oxidation of Lan+1NinO3n+1+δ (n = 1–4) Ruddlesden–Popper nickelate thin films. Postgrowth ozone annealing induces a substantial c-axis expansion─17.8% for La2NiO4+δ (n = 1)─that monotonically decreases with increasing n. Surface synchrotron X-ray diffraction and coherent Bragg rod analysis (COBRA) reveal that this structural expansion arises from the intercalation of approximately δ ≈ 0.7–1.0 oxygen atoms into interstitial sites within the rock salt spacer layers, far exceeding the previous record of δ ≈ 0.3 for any Ruddlesden–Popper oxide. These oxygen-intercalated phases form a new class of layered perovskites with a spacer layer composition intermediate between the Ruddlesden–Popper and Aurivillius phases. Furthermore, oxygen intercalation induces metallicity, enhances nickel–oxygen hybridization, and suppresses oxygen octahedral rotations, a feature associated with high-temperature superconductivity in Ruddlesden–Popper nickelates. Our work establishes topochemical oxidation as a powerful approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to engineer electronic properties via intercalation chemistry.

topotactic reduction

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

Topotactical Hydrogen Induced Single-Band d -Wave Superconductivity in La₂ NiO 4

Mon, 01 Jan 0001 00:00:00 +0000

Topotactical Hydrogen Induced Single-Band d -Wave Superconductivity in La₂ NiO 4

Tracing the horizon of tetragonal-to-monoclinic distortion in pressurized trilayer nickelate La₄Ni₃O₁₀

Mon, 01 Jan 0001 00:00:00 +0000

This study employs pressure-temperature single-crystal X-ray diffraction and ab initio density functional theory calculations to reveal that the transition from the tetragonal phase (I4/mmm) to the monoclinic phase (P2₁/c) in flux-grown trilayer nickelate La₄Ni₃O₁₀ does not involve an intermediate orthorhombic Bmab phase; instead, it is a direct structural phase transition accompanied by the formation of a two-fold superstructure, manifested by the appearance of commensurate superlattice reflections. The transition temperature can be continuously suppressed from approximately 1030 K to 20 K under a pressure of 14 GPa, indicating that pressure effectively stabilizes the tetragonal phase. Furthermore, weak satellite reflections associated with incommensurate density wave ordering are detected for the first time in X-ray diffraction from flux-grown crystals, complementing previous results observed only in float-zone crystals, and Raman spectroscopy reveals additional phonon modes below 130 K, further corroborating this ordered state. The ab initio calculations are in good agreement with the experimental observations. This work clarifies the long-standing dispute over the structural symmetry of La₄Ni₃O₁₀, supports the emergence of superconductivity after the restoration of tetragonal symmetry under high pressure, and provides a critical crystallographic foundation for subsequent studies of electronic structure and superconducting mechanisms.

Transport measurements

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

transport measurements (R-T)

Mon, 01 Jan 0001 00:00:00 +0000

Methods facet

Transport, Magnetic and Thermal Properties of La₃Ni₂O₇-δ

Mon, 01 Jan 0001 00:00:00 +0000

The metal-insulator transition of La 3 Ni 2 O 7-δ with 2-dimensional electrons has been studied. Various physical properties can be understood by introducing a model of charge orderings in the NiO …

Triplon-mediated pairing and the superconducting gap structure in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

This study constructs a microscopic theoretical model for the superconducting gap structure of bilayer nickel oxides, where a conduction band with dx²-y² symmetry coexists with localized d3z²-r² spins. Strong interlayer coupling leads to a singlet ground state of local magnetic moments, whose virtual singlet-triplet excitations (i.e., “triplons”) mediate pairing interactions between conduction electrons, thereby generating interband s±-wave pairing with opposite signs of the order parameters on the two bands (α and β). The theoretical results naturally explain key experimental observations: despite the smaller density of states of the α band, its superconducting gap is larger, and the gap exhibits significant momentum-space anisotropy arising from nonlocal Kondo coupling. These findings strongly support the triplon-mediated pairing mechanism as the microscopic origin of superconductivity in bilayer nickel oxides.

Tunable superconductivity and spin density wave in La₃Ni₂O₇/LaAlO₃ thin films

Mon, 01 Jan 0001 00:00:00 +0000

Using first-principles calculations and the singular-mode functional renormalization group method, we systematically investigate the effect of interlayer nickel-nickel distance on the ground state in La₃Ni₂O₇/LaAlO₃ thin films. The results show that a smaller interlayer distance leads to a C-type spin density wave (interlayer ferromagnetic coupling), while a larger interlayer distance yields a G-type spin density wave (interlayer antiferromagnetic coupling). Between these two phases, an s±-wave superconducting state emerges, dominated by pairing in the Ni 3d₃z²⁻ᵣ² orbital. This finding explains the origin of superconductivity observed in the thin films under ambient pressure and predicts that applying pressure will suppress the superconducting transition temperature until the system enters the C-type spin density wave. If confirmed experimentally, this prediction will provide deep insight into the nature of electronic correlations in this system, as the C-type spin density wave naturally emerges within the itinerant electron picture, whereas it is difficult to realize within the local magnetic moment picture (where interlayer spins remain antiferromagnetically coupled).

Tunable Superconductivity in 1313-La₃Ni₂O₇: Suppressed under Compression and Possible s± Pairing under Tension

Mon, 01 Jan 0001 00:00:00 +0000

By combining density functional theory and random phase approximation, the effects of compressive and tensile strain on the superconductivity of 1313-La3Ni2O7 thin films are systematically investigated. A self-doping effect is found between the monolayer and trilayer blocks regardless of compressive or tensile strain, and it is most pronounced under tensile strain. Under compressive strain imposed by an LSAO substrate, even considering hole doping from strontium ion migration in the substrate, superconductivity is difficult to appear, consistent with experiments. However, under tensile strain from a KTO substrate, a band in the trilayer subsystem that originally did not cross the Fermi level shifts downward, giving rise to a small hole-type γ pocket at the M point, which is connected to a small electron-type σ pocket at the Γ point by a near-(π,π) wavevector. Random phase approximation calculations reveal that the trilayer subsystem can then form a stable s±-wave pairing state, with the order parameter reversing sign between these two pockets. Further analysis indicates that the size of the γ pocket is crucial for pairing, and an excessively large γ pocket suppresses superconductivity. This work predicts a strain-driven electronic structure reconstruction and proposes a design principle to realize superconductivity in 1313-La3Ni2O7 under ambient pressure through tensile strain engineering.

u shaped gap

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Keywords facet

Ultrafast Magneto-Pressure Spectroscopy and Control of Correlated Phases in a Trilayer Nickelate

Mon, 01 Jan 0001 00:00:00 +0000

This study developed an ultrafast magneto-pressure optical spectroscopy platform capable of operating simultaneously at pressures up to 40 GPa, magnetic fields up to 7 T, and temperatures as low as 5 K, and applied it to investigate the evolution of quasiparticle dynamics under magnetic pressure in the trilayer nickelate Pr₄Ni₃O₁₀. The experiments revealed a pronounced critical slowing down of quasiparticle relaxation near the charge density wave (CDW) transition, which disappears upon the application of pressure. At higher pressures, the low-temperature relaxation time instead becomes longer, consistent with initial superconducting correlation signatures. However, a magnetic field as high as 7 T hardly alters the relaxation behavior, and no vortex-induced pre-bottleneck dynamics—robustly observed in bulk superconducting control samples—was detected, suggesting that any superconducting state under the present pressure conditions is not bulk-like but rather filamentary or strongly inhomogeneous. This magneto-pressure ultrafast capability opens a new pathway for addressing unresolved issues of pressure-induced superconductivity and intertwined orders in correlated quantum materials.

Ultrafast optical evidence of coexisting density waves in bilayer nickelate La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

Ultrafast optical evidence of coexisting density waves in bilayer nickelate La₃Ni₂O₇

Unconventional Crystal Structure of the High-Pressure Superconductor La₃Ni₂O₇

Mon, 01 Jan 0001 00:00:00 +0000

The discovery of high-temperature superconductivity in La3⁢Ni2⁢O7 at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO6 octahedral bilayers stacked along the 𝑐-axis direction was consistently posited in initial studies on La3⁢Ni2⁢O7. Here, we reassess this structure in optical floating zone-grown La3⁢Ni2⁢O7 single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO6 octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La3⁢Ni2⁢O7, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.在高于 14 GPa 的压力下， La3⁢Ni2⁢O7 中高温超导性的发现激发了广泛的研究。然而，超导相的基本性质，包括其丝状特征的可能性，目前仍存在争议。相反，在对 La3⁢Ni2⁢O7 的早期研究中，人们一直假设其晶体结构为沿 𝑐 轴方向堆叠的 NiO6 八面体双层。本文中，我们重新评估了光学浮区法生长的 La3⁢Ni2⁢O7 单晶中的这种结构，这些单晶表现出丝状超导的迹象。利用扫描透射电子显微镜和高压下的单晶 X 射线衍射技术，我们观察到这些晶体中存在多种晶相，其中主要相由 NiO6 八面体的单层和三层交替构成，这表明其与先前提出的双层结构存在显著偏差。利用密度泛函理论，我们分离出单层和三层结构单元对 La3⁢Ni2⁢O7 电子能带结构的各个贡献，为高级理论建模和未来评估单层-三层结构承载超导性的潜力提供了坚实的基础。

unconventional superconductivity

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Keywords facet

Unconventional Superconductivity in La₃Ni₂O₇ from the Perspective of Symmetry

Mon, 01 Jan 0001 00:00:00 +0000

This study addresses the discrepancy between bulk La₃Ni₂O₇ with high transition temperatures under high pressure and its thin-film counterpart, which superconducts at ambient pressure but with a reduced Tc, by developing a symmetry-based phenomenological approach combined with DFT+U calculations and experimentally determined Tc and structural symmetries to analyze the superconducting gap structure. The results reveal that both systems exhibit s±-wave pairing symmetry and two-band superconductivity, but the dominant microscopic pairing configurations differ: in pressurized bulk, superconductivity is primarily governed by out-of-plane pairing of Ni-dz² orbitals, whereas in thin films, in-plane pairing of Ni-dx²-y² orbitals dominates. The reduction in Tc is attributed to a decreased ratio of interlayer to intralayer hopping in the film, which shifts the dominant pairing type from out-of-plane to in-plane. These findings highlight the crucial role of symmetry in unconventional superconductivity, and the developed methodology is expected to be extendable to other unconventional superconductors.

Uncovering origins of heterogeneous superconductivity in La₃Ni₂O₇ using quantum sensors

Mon, 01 Jan 0001 00:00:00 +0000

The family of nickelate superconductors have long been explored as analogs of the high temperature cuprates. Nonetheless, the recent discovery that certain stoichiometric nickelates superconduct up to high $T_c$ under pressure came as a surprise. The mechanisms underlying the superconducting state remain experimentally unclear. In addition to the practical challenges posed by working in a high pressure environment, typical samples exhibit anomalously weak diamagnetic responses, which have been conjectured to reflect inhomogeneous `filamentary’ superconducting states. We perform wide-field, high-pressure, optically detected magnetic resonance spectroscopy to image the local diamagnetic responses of as grown La$_3$Ni$_2$O$_7$ samples \emph{in situ}, using nitrogen vacancy quantum sensors embedded in the diamond anvil cell. These maps confirm significant inhomogeneity of the functional superconducting responses at the few micron scale. By spatially correlating the diamagnetic Meissner response with both the local tensorial stress environment, also imaged \emph{in situ}, and stoichiometric composition, we unravel the dominant mechanisms suppressing and enhancing superconductivity. Our wide-field technique simultaneously provides a broad view of sample behavior and excellent local sensitivity, enabling the rapid construction of multi-parameter phase diagrams from the local structure-function correlations observed at the sub-micron pixel scale.

Unified mechanism of charge-density-wave and high-T_c superconductivity protected from oxygen vacancies in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

Unconventional charge and spin density-wave states are commonly observed in bilayer nickelates, drawing considerable attention due to their proximity to high-$T_c$ superconductivity in various phase diagrams. However, the nature and mechanisms of charge and spin density-waves (DWs) in nickelates remain poorly understood. Numerous experiments have reported that the charge-density-wave (CDW) transition temperature $T_{ cdw}$ and the spin-density-wave (SDW) transition temperature $T_{sdw}$ are closely related but distinct. However, in contrast to these experiments, previous mean-field-type analyses have yielded only a simple SDW phase. To resolve this key problem, this paper demonstrates that sizable CDW instabilities emerge in proportion to the SDW instability in La$3$Ni$2$O$7$.This behavior is driven by the paramagnon-interference (PMI) mechanism, which captures important electron correlations beyond mean-field theory. Therefore, (i) experimental CDW + SDW coexisting state is naturally explained. In addition, (ii) the CDW + SDW fluctuations cooperatively drive high-$T_c$ superconductivity. Notably, the predicted $s$-wave SC state is robust against the inner apical O vacancies. Furthermore, (iii) the CDW instability is highly sensitive to the size of the $d_{z^2}$-orbital hole pocket, allowing for the realization of CDW quantum criticality through carrier-doping and pressure application. We find that the coexistence of charge and spin fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high-$T_c$ superconductivity.

Unified mechanism of charge-density-wave and high-Tc superconductivity protected from oxygen vacancies in bilayer nickelates

Mon, 01 Jan 0001 00:00:00 +0000

Unconventional charge- and spin-density-wave states are commonly observed in bilayer nickelates, drawing considerable attention due to their proximity to high-transition temperature (

$${T}_{{\rm{c}}}$$

) superconductivity. However, the nature and origin of these density waves remain poorly understood. Experiments show that the charge-density-wave and spin-density-wave transition temperatures are closely related but distinct, while mean-field-type analyses typically have yielded only a simple spin-density-wave phase. To resolve this key problem, this paper demonstrates that sizeable charge-density-wave instabilities emerge in proportion to spin-density-wave instabilities in La3Ni2O7 due to the paramagnon-interference mechanism, which captures electron correlations beyond mean-field theories. Therefore, (i) the experimental charge- and spin-density-wave coexisting state is naturally explained, and (ii) charge- and spin-density-wave fluctuations cooperatively drive high-

$${T}_{{\rm{c}}}$$

superconductivity. Furthermore, the predicted s-wave superconducting state is robust against the inner-apical oxygen vacancies. We find that the coexistence of charge- and spin-fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high-

Mon, 01 Jan 0001 00:00:00 +0000

Keywords facet