Knowledge

Spin-charge-orbital order in nickelate superconductors

High-pressure studies have revealed superconductivity in La4⁢Ni3⁢O10, sparking interest in its ambient-pressure properties and the underlying electronic correlations. Motivated by experimental observations of an incommensurate spin-density wave (SDW) at ambient pressure, we investigate the SDW characteristics and possible superconductivity in La4⁢Ni3⁢O10 using a multiorbital random-phase approximation (RPA). Starting with a 12-orbital tight-binding model derived from density functional theory (DFT) calculations, we include Hubbard interactions to explore the interplay between electronic correlations and magnetic instabilities. Our analysis reveals a stripe-like SDW with a wave vector 𝐐≈(±0.7⁢𝜋,0), suggesting a possible density wave instability in agreement with experiments. This configuration is driven by nesting between the 𝛼1 pocket, primarily contributed by the outer-layer Ni 𝑑𝑧2 orbitals, and the 𝛽1 pocket, contributed by both the 𝑑𝑧2 and 𝑑𝑥2−𝑦2 orbitals of the outer layer. It exhibits interlayer antiferromagnetic ordering between the top and bottom NiO layers, with the magnetic moment of the middle layer being nearly zero. We demonstrate that the Hund coupling 𝐽𝐻 is the primary driver of the observed SDW and determine the specific criterion: 𝐽𝐻>0.16⁢𝑈. Building upon our findings on the SDW mechanism, we further demonstrate that hole doping (𝛿=−0.4) enhances Fermi surface nesting, leading to the emergence of a superconducting state with a gap structure similar to that of the high-pressure phase.

This study successfully synthesized high-quality, long-range ordered hybrid 1313-type La₃Ni₂O₇ single crystals (with an alternating monolayer-trilayer structure) and systematically characterized their physical properties. At ambient pressure, the material exhibits typical semiconducting behavior and displays distinct anomalies in resistivity, magnetic susceptibility, and specific heat at 170 K. ¹³⁹La nuclear magnetic resonance spectroscopy unambiguously confirms that these anomalies originate from a spin-density wave (SDW) transition. High-pressure electrical transport measurements indicate that the application of pressure can induce metallization; however, no superconductivity is observed up to 65 GPa. These findings establish hybrid 1313-type La₃Ni₂O₇ as a new member of the Ruddlesden-Popper nickelate family featuring a unique SDW transition, offering a platform for investigating the interplay among crystal structure, electronic order, and superconductivity in hybrid nickelates.

Employing density functional theory combined with cluster dynamical mean-field theory, this study investigates bilayer Ruddlesden-Popper nickelates and finds that their physical properties are primarily governed by interlayer “dynamical singlets” formed by single electrons in the 3z²−r² orbital, which hybridize with itinerant x²−y² planar orbitals. This hybridization responds distinctly to hydrostatic pressure and in-plane compressive strain: strain enhances interlayer correlations, leading to an orbitally selective singlet-pairing Mott mechanism, whereas hydrostatic pressure mainly promotes in-plane itinerancy. This difference explains experimental discrepancies between bulk and strained thin films observed in angle-resolved photoemission spectroscopy and transport measurements. The theoretical framework views this low-energy state as a hybridized system of dynamical singlets and itinerant orbitals, offering a new perspective for understanding superconductivity.

Stabilizing and tuning superconductivity in La₃Ni₂O₇−δ films: Oxygen recycling protocol reveals hole-doping analogue

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After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctua…