STM spectroscopy on bilayer nickelate ultrathin films reveals reproducible U-shaped spectra with nodeless gaps of ~14 and ~20 meV and flat zero-conductance regions.
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The discovery of superconductivity in Ruddlesden-Popper (RP) bilayer nickelate films under ambient pressure provides an unprecedented opportunity to directly investigate electronic energy scales of the superconducting state and the pairing mechanism. Here, we report angle-resolved photoemission spectroscopy measurements of superconducting (La,Pr,Sm)$_{3}$Ni$_2$O$_7$ thin films epitaxially grown on SrLaAlO$_4$ substrates by developing an ultra-high vacuum low-temperature quenching and transfer technique. A finite superconducting gap of ~18 meV with pronounced coherence peak is observed along the Brillouin zone diagonal direction. Remarkably, the finite superconducting gap persists across the entire Brillouin zone of the underlying Fermi surfaces, revealing the absence of gap nodes. An abrupt band renormalization, manifested as a kink in the energy-momentum dispersion at ~70 meV below the Fermi level, indicates an electron-boson coupling in the system. The simultaneous observation of a nodeless superconducting gap and electron-boson coupling provides crucial insights into the pairing symmetry and gluing mechanism in high-T$_c$ RP bilayer nickelates.
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Orbital-selective d-wave superconductivity arises exclusively from the itinerant orbital in the two-band t-J model, suppressed by local inter-orbital bound states from the quasi-localized orbital.
Observation of an energy-symmetric flat-bottom U-shaped gap with zero residual DOS in (La,Pr)3Ni2O7 thin films, showing unconventional temperature evolution and magnetic field suppression consistent with a nodeless superconducting gap.
Correlations in the nickelate drive the gamma band below the Fermi level and shift the dominant superconducting pairing to d_x2-y2 interlayer spin-singlet mediated by antiferromagnetism and Hund's coupling.
Compressive strain and oxygenation in (La,Pr)₃Ni₂O₇₋δ films delocalize Ni 3d_z² and O 2p_z orbitals, suppress long-range spin-density-wave order, and preserve short-range magnons as prerequisites for superconductivity.
Superconductivity in bilayer nickelates emerges only when coherent interlayer d_z2-p_z-d_z2 hybridization develops, suppressing static spin-density-wave order and damping spin excitations.
ARPES on (La,Pr,Sm)3Ni2O7 films reveals quasi-2D dx2-y2 bands, finite kz dispersion on dz2 bands, and a superconducting gap of ~18 meV with 2Δ/kBTc ~8 on the dz2-derived band.
A perpendicular electric field induces a transition from s±-wave to d-wave superconductivity in the bilayer Hubbard model for La3Ni2O7, with d-wave pairing exhibiting dome-like behavior.
Varying the Ni-Ni interlayer distance switches La3Ni2O7/LaAlO3 films between C-type and G-type spin density waves, with s± superconductivity emerging in between.
Terahertz spectroscopy on (La,Pr)3Ni2O7 films indicates disordered s±-wave pairing in the superconducting state and a distinct normal-state pseudogap above Tc onset.
A two-component scenario unifies key observations in bilayer nickelate superconductivity, predicting doping-dependent superconducting domes and normal-state behaviors that differ by interlayer coupling strength.
DMRG simulations on a 1D bilayer two-orbital model for La3Ni2O7 show superconducting correlations suppressed near half-filling of the 3d_z2 orbital, indicating its itinerancy favors pairing.
The two-step resistive transition in La2PrNi2O7-δ thin films arises from granular superconductivity involving two distinct grain phases coupled by a Josephson junction network.
A d_x2-y2 orbital bilayer t-J model with first-principles parameters unifies experimental Tc controls in La3Ni2O7 via particle-hole asymmetry and J_perp dependence, proposing electron doping to enhance Tc.
Constrained-path QMC simulations of a bilayer Hubbard model map a crossover from d-wave to s±-wave pairing driven by Hund's coupling and crystal field splitting in La3Ni2O7.
DFT-based tight-binding models and FRG calculations predict that reducing in-plane lattice constant or increasing out-of-plane constant in La3Ni2O7 films increases Fermi-level DOS and enhances Tc while preserving s± pairing.
Compressive strain enhances Jahn-Teller splitting Δ_JT in La3Ni2O7 films as the key microscopic tuning parameter for superconductivity, matching ARPES and Hall data on specific substrates.
Superconductivity in La3Ni2O7 arises from interlayer Cooper pairs of 3d_x2-y2 electrons driven by effective J_perp from Hund-assisted AFM exchange transfer, while localized 3d_z2 electrons form rung singlets that produce a pseudogap but no condensate.
Epitaxial strain enables ambient-pressure superconductivity in bilayer nickelate films, facilitating detailed studies of their properties and phase diagrams.
The review covers experimental and theoretical progress on superconductivity in Ruddlesden-Popper nickelates, emphasizing ambient-pressure thin-film results in La3Ni2O7.
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Atomically resolved intrinsic superconducting gap in (La,Pr)3Ni2O7 films
STM spectroscopy on bilayer nickelate ultrathin films reveals reproducible U-shaped spectra with nodeless gaps of ~14 and ~20 meV and flat zero-conductance regions.
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Observation of flat-bottom U-shaped energy gap in high-Tc nickelate (La,Pr)3Ni2O7 thin films
Observation of an energy-symmetric flat-bottom U-shaped gap with zero residual DOS in (La,Pr)3Ni2O7 thin films, showing unconventional temperature evolution and magnetic field suppression consistent with a nodeless superconducting gap.
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$3d_{z^2}$ orbital delocalization and magnetic collapse in superconducting (La,Pr)$_3$Ni$_2$O$_{7-\delta}$ films
Compressive strain and oxygenation in (La,Pr)₃Ni₂O₇₋δ films delocalize Ni 3d_z² and O 2p_z orbitals, suppress long-range spin-density-wave order, and preserve short-range magnons as prerequisites for superconductivity.
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Interlayer hybridization enables superconductivity in bilayer nickelates
Superconductivity in bilayer nickelates emerges only when coherent interlayer d_z2-p_z-d_z2 hybridization develops, suppressing static spin-density-wave order and damping spin excitations.
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Three-Dimensional Electronic Structures in Superconducting Ruddlesden-Popper Bilayer Nickelate Films
ARPES on (La,Pr,Sm)3Ni2O7 films reveals quasi-2D dx2-y2 bands, finite kz dispersion on dz2 bands, and a superconducting gap of ~18 meV with 2Δ/kBTc ~8 on the dz2-derived band.
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Perpendicular electric field induced $s^\pm$-wave to $d$-wave superconducting transition in thin film La$_3$Ni$_2$O$_7$
A perpendicular electric field induces a transition from s±-wave to d-wave superconductivity in the bilayer Hubbard model for La3Ni2O7, with d-wave pairing exhibiting dome-like behavior.
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Tunable superconductivity and spin density wave in La3Ni2O7/LaAlO3 thin films
Varying the Ni-Ni interlayer distance switches La3Ni2O7/LaAlO3 films between C-type and G-type spin density waves, with s± superconductivity emerging in between.
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Multimodal Terahertz Spectroscopy of the Pairing Symmetry and Normal-State Pseudogap in (La,Pr)$_3$Ni$_2$O$_7$ Films
Terahertz spectroscopy on (La,Pr)3Ni2O7 films indicates disordered s±-wave pairing in the superconducting state and a distinct normal-state pseudogap above Tc onset.
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A unified theory of thin film and bulk bilayer nickelates
A two-component scenario unifies key observations in bilayer nickelate superconductivity, predicting doping-dependent superconducting domes and normal-state behaviors that differ by interlayer coupling strength.
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Granular Superconductivity in La$_{2}$PrNi$_{2}$O$_{7-\delta}$ Thin Films
The two-step resistive transition in La2PrNi2O7-δ thin films arises from granular superconductivity involving two distinct grain phases coupled by a Josephson junction network.
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A Unified Understanding of the Experimental Controlling of the T$_\text{c}$ of La$_3$Ni$_2$O$_7$
A d_x2-y2 orbital bilayer t-J model with first-principles parameters unifies experimental Tc controls in La3Ni2O7 via particle-hole asymmetry and J_perp dependence, proposing electron doping to enhance Tc.
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Strain-Engineered Electronic Structure and Superconductivity in La$_3$Ni$_2$O$_7$ Thin Films
DFT-based tight-binding models and FRG calculations predict that reducing in-plane lattice constant or increasing out-of-plane constant in La3Ni2O7 films increases Fermi-level DOS and enhances Tc while preserving s± pairing.
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Jahn-Teller distortion on strained La$_3$Ni$_2$O$_7$ thin films
Compressive strain enhances Jahn-Teller splitting Δ_JT in La3Ni2O7 films as the key microscopic tuning parameter for superconductivity, matching ARPES and Hall data on specific substrates.
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Superconductivity in bilayer La$_3$Ni$_2$O$_7$: A review focusing on the strong-coupling Hund's rule assisted pairing mechanism
Superconductivity in La3Ni2O7 arises from interlayer Cooper pairs of 3d_x2-y2 electrons driven by effective J_perp from Hund-assisted AFM exchange transfer, while localized 3d_z2 electrons form rung singlets that produce a pseudogap but no condensate.
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Experimental Progress in Ambient-Pressure Superconducting Bilayer Nickelate Films
Epitaxial strain enables ambient-pressure superconductivity in bilayer nickelate films, facilitating detailed studies of their properties and phase diagrams.
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Superconductivity in Ruddlesden-Popper nickelates: a review of recent progress, focusing on thin films
The review covers experimental and theoretical progress on superconductivity in Ruddlesden-Popper nickelates, emphasizing ambient-pressure thin-film results in La3Ni2O7.
- Theoretical study on ambient pressure superconductivity in La$_3$Ni$_2$O$_7$ thin films : structural analysis, model construction, and robustness of $s\pm$-wave pairing