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This study systematically investigates the bilayer Kondo lattice model using single-site dynamical mean-field theory (DMFT) to explore the phase diagram of the normal state of bilayer nickelates. In the absence of interlayer tunneling, a non-Fermi-liquid critical point tuned by interlayer spin coupling or hole doping is identified, separating the standard Fermi liquid in the overdoped region from a pseudogap metal in the underdoped region. This pseudogap phase, termed the “second Fermi liquid,” is characterized by small hole pockets and violates the perturbative Luttinger theorem, yet exhibits no symmetry breaking or fractionalization; its behavior resembles that of a heavy Fermi liquid with small quasiparticle residues and large effective masses. Furthermore, an intuitive analytical description of the pseudogap and ground-state wavefunction is provided within the ancilla fermion framework, where the ancilla fermion is interpreted as a spin polaron, and the Kondo resonance peak of this composite fermion is directly shown in DMFT calculations. Extending the analysis to finite interlayer tunneling, the study applies the results to the bilayer nickelate La₃Ni₂O₇, proposing that current experimental samples (x≈0.5) lie in the overdoped Fermi liquid region, while electron doping may drive the system into the pseudogap phase and the non-Fermi-liquid critical regime, offering theoretical predictions for understanding anomalous metallic behavior in such materials.

Infinite-layer nickelate superconductors provide a new platform to study the quantum critical origin of strange metals, yet their thin-film form forbids conventional calorimetry. Using the Seebeck coefficient as a proxy for low-temperature specific heat, we find at the critical doping x* that the high-temperature Seebeck response quantitatively matches the band structure measured by ARPES, indicating well-defined quasiparticles, while below 60 K, S/T develops a logarithmic divergence that persists to the lowest temperatures (after suppressing superconductivity with a magnetic field), marking x* as a quantum critical point that terminates the underdoped phase. Moreover, the Ni-d_{x^2-y^2} carrier density drops abruptly from 1+x above x* to x below x*, reproducing the hallmark signature of the pseudogap phase in cuprates. These results indicate that the underdoped region of infinite-layer nickelates is a pseudogap-like state, at whose end emergent strange metal behavior with T-linear resistivity provides strong evidence for a quantum critical origin of strange metals.

5 linked papers

Using a first-principles six-band three-layer multiorbital model, the authors treat electronic correlations within the FLEX approximation and investigate charge density wave instability in La4Ni3O10 using density-wave equations with vertex corrections, while analyzing superconducting pairing by combining FLEX and the Bethe–Salpeter method. They find that short-range spin density wave fluctuations near Qsdw on the outer NiO2 layers generate, through quantum interference, an inter-outer-layer bond order with Qcdw ≈ 2Qsdw; this bond order further induces an inner-layer-centered orbital density wave, which anti-phase modulates the occupancies of Ni d3z2-r2 and dx2-y2 orbitals, producing strong orbital polarization but only weak total charge modulation. This intertwined bond and orbital order explains the layer-selective electronic reconstruction inferred from NMR/NQR and is consistent with Raman spectroscopy and scanning tunneling microscopy results. Because the charge density wave has even mirror parity and the spin density wave has odd mirror parity, their fluctuations can cooperatively stabilize s±-wave superconductivity through Mz mirror-parity selection rules. This work provides a unified microscopic framework for intertwined density-wave orders and high-temperature superconductivity in multilayer nickelates.

1 linked paper

This study systematically analyzes the Raman response of superconducting multi-orbital systems using electronic Raman scattering methods, with nickelates as the application target. For three models—a single-layer and a bilayer two-orbital model involving d_{x^2-y^2} and d_{z^2} orbitals, and a bilayer single-orbital model with only d_{x^2-y^2} orbitals—multiple pairing symmetries including d-wave, s±-wave, and s-wave are considered, and the response characteristics under various Raman symmetries (A1g, B1g, B2g) are calculated. In the two-orbital models, a full multi-orbital approach is employed, incorporating both intra-orbital and inter-orbital scattering, and compared with the additive approximation that simply sums the Raman responses of individual bands. The results reveal distinct fingerprint features in the Raman spectra for different pairing symmetries and model structures, with the full multi-orbital calculations uncovering inter-orbital mixing effects that the additive approximation may overlook. These findings help clarify the minimal model for nickelate superconductivity, determine the magnitude and symmetry of their superconducting gaps, and provide a general theoretical framework for Raman experimental analysis of other multi-orbital superconductors, such as iron-based superconductors.

12 linked papers

5 linked papers

This review summarizes recent advances in nickelate superconductors, covering infinite-layer, bilayer, and trilayer systems, their superconducting properti

By systematically tuning the oxygen content of La₃Ni₂O₇₊δ samples, this study synthesized materials with varying phase compositions, including pure bilayer phase, a mixed phase of bilayer and monolayer–bilayer hybrid, and a predominant bilayer phase containing trilayer intergrowths. High-pressure transport measurements revealed that these phases correspond to distinct superconducting transition temperatures (T_c), with the bilayer phase exhibiting superconductivity at approximately 80 K, while the hybrid and trilayer-intergrowth phases show lower T_c values. Oxygen content not only influences phase purity but also directly modulates the upper critical field (H_c2) of the bilayer superconductivity, with the pure bilayer phase displaying a higher H_c2. By constructing a phase diagram of T_c and H_c2 as functions of oxygen content, this study achieves precise control over oxygen stoichiometry in Ruddlesden–Popper nickelates, providing critical experimental insights for understanding the high-pressure superconducting mechanism.