Paper

Using DFT combined with dynamical mean-field theory, we investigate the effects of electronic correlations and epitaxial strain on the normal-state electronic structure, quasiparticle renormalization, Fermi surface, and magnetic correlations of the bilayer Ruddlesden-Popper nickelate La3Ni2O7. The results show that the Ni 3d bands exhibit pronounced orbital-selective renormalization and strong incoherence, with the Ni x2−y2 and 3z2−r2 states approaching orbital-selective localization. The electronic properties are highly sensitive to in-plane strain: tensile strain and moderate compressive strain up to about −2% significantly enhance magnetic correlations, whereas at a large compressive strain of about −4% the system undergoes a Lifshitz transition in which the γ Fermi surface sheet disappears. This sheet is associated with an almost fully occupied, shallow bonding Ni 3z2−r2 flat band and its disappearance leads to a sharp decrease in magnetic correlations, implying that superconductivity is suppressed. Overall, the results support the picture that spin/charge density wave stripe instability is driven by Fermi surface nesting and indicate that pressure and strain can effectively tune spin-charge density wave order, thereby enhancing spin fluctuations.

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.

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.

Using density functional theory (DFT) and random phase approximation (RPA) calculations, we systematically investigated the electronic structures of the four-layer Ruddlesden–Popper nickelate La5Ni4O13 under ambient pressure, 5% isotropic compressive strain, and 4% c-axis uniaxial strain, and constructed a four-layer two-orbital model based on Wannier downfolding. DFT shows that isotropic strain broadens the Ni-e_g bands and induces charge transfer from O-p to Ni-d, whereas c-axis uniaxial strain selectively shifts the d_z2-derived bonding1 band upward while leaving the d_x2-y2 dispersion almost unchanged. The two-orbital model reproduces the low-energy Ni-e_g bands. Under ambient pressure and 5% isotropic strain, the Fermi surface consists of two electron pockets (α, δ) and three hole pockets (β, β′, β″); under uniaxial strain, a γ hole pocket with d_z2 orbital character emerges. RPA calculations show that the dominant spin response changes from q≈(2π/3,2π/3) at ambient pressure to q≈(π,π) under both strains, and is enhanced under c-axis compression. The authors therefore suggest that c-axis compression may provide four-layer nickelates with a favorable superconducting pathway similar to those of bilayer and trilayer nickelates.

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.

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.

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.

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.

This study systematically investigates the electronic role of the intercalated La₂NiO₄ (214) layer in the superconducting material La₅Ni₃O₁₁ by combining density functional theory, Wannier tight-binding modeling, and rotationally invariant slave-boson calculations. A two-orbital model is constructed for the embedded 214 layer using realistic electronic parameters extracted from PBE and HSE06 functionals, and its correlated ground-state phase diagram is computed via the slave-boson approach. The results reveal that within realistic ranges of crystal-field splitting and Coulomb interaction, this layer tends to reside in an antiferromagnetic insulating state or a band insulating state rather than a paramagnetic metallic state, with its low-energy spectral weight completely suppressed. Further analysis of a coupled model incorporating the La₃Ni₂O₇ (327) block and the 214 layer shows that interlayer hybridization does not restore any effective density of states originating from the 214 layer near the Fermi level. Consequently, the low-energy electronic structure of La₅Ni₃O₁₁ is dominated by the 327 block, while the intercalated 214 layer remains electronically inert and does not contribute to the formation of the Fermi surface. This finding clarifies the minimal low-energy effective model for this mixed-layer nickelate and offers a unified picture for understanding superconductivity in intercalated Ruddlesden-Popper nickelates.

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.