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Revealing nanoscale structural phase separation in La$_{3}$Ni$_{2}$O$_{7-\delta}$ single crystal via scanning near-field optical microscopy
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abstract
The discovery of superconductivity in La3Ni2O7-${\delta}$ under high pressure,with an onset critical temperature around 80 K, has sparked significant interest in the superconducting phases of Ruddlesden-Popper nickelates, Lan+1NinO3n+1. While La4Ni3O10 exhibits nearly 100% superconductivity with Tc~30 K under high pressure, magnetic susceptibility studies on La3Ni2O7-${\delta}$, however, reveal a more complex picture, indicating either filamentary superconductivity or that approximately 50% of crystal phase becomes superconducting in polycrystalline samples. In this study, we employed scattering-type scanning near-field optical microscopy to visualize nanoscale structural phase separation in La3Ni2O7-${\delta}$, identifying enhanced optical conductivity with stripes approximately 183 nm wide. These stripes run diagonally with respect to the Ni-O-Ni bond directions in the a-b plane, ruling out the possibility that they arise from impurity phases, like the '1313', '214' or '4310' structures. The dark regions and bright stripes exhibit optical conductivities ~ 22% and 29% of gold's, respectively. Additionally, we find that the bright stripes constitute about 38% of the total field of view, while the remainder consists of dark regions and the transitional region between dark regions and bright stripes. Our results suggest that optical conductivity stripes originate from nanoscale structural phase separation. In contrast, La4Ni3O10 exhibits uniform and higher optical conductivity with no observable evidence of phase separation. Thus, our study represents a pioneering effort to directly image nanoscale phase separation in Lan+1NinO3n+1 nickelates. This observation could provide crucial insights into the factors that limit the superconducting volume fraction of La3Ni2O7-${\delta}$, highlighting SNOM as a powerful probe for exploring nanoscale low-energy physics in correlated quantum materials.
Forward citations
Cited by 7 Pith papers
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Angle-resolved photoemission spectroscopy of superconducting (La,Pr)3Ni2O7/SrLaAlO4 heterostructures
The superconducting state of atomically thin (La,Pr)3Ni2O7 films is hole-doped with both Ni dx2-y2 and dz2 bands at the Fermi level, with conduction confined to the interface layer.
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Resolving Structural Origins for Superconductivity in Strain-Engineered La$_3$Ni$_2$O$_7$ Thin Films
Direct atomic imaging of strained La3Ni2O7 films shows compressive strain lifts oxygen octahedral symmetry and that in-plane, rather than c-axis, lattice matching tracks superconductivity.
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Electronic Nematicity Revealed by Polarized Ultrafast Spectroscopy in Bilayer La$_3$Ni$_2$O$_7$
Bilayer nickelate La3Ni2O7 shows spontaneous two-fold electronic anisotropy (nematicity) below the spin-density-wave transition, whereas trilayer La4Ni3O10 remains isotropic.
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Direct Visualization of an Incommensurate Unidirectional Charge Density Wave in La$_4$Ni$_3$O$_{10}$
STM/STS directly images an incommensurate unidirectional charge density wave with qCDW ≈ 0.76 qb and a Fermi-level gap of 2Δ ≈ 71 meV in La4Ni3O10.
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Low volume fraction of high-Tc superconductivity in La3Ni2O7 at 80 K and ambient pressure
Oxygen annealing of La3Ni2O7 single crystals produces a 0.1-0.2% volume-fraction diamagnetic signal at 80 K and ambient pressure, attributed to filamentary superconductivity.
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Origin of the Diagonal Double-Stripe Spin-Density-Wave and Potential Superconductivity in Bulk La$_3$Ni$_2$O$_{7}$ at Ambient Pressure
An RPA analysis of an eight-band tight-binding model for ambient-pressure La3Ni2O7 reproduces the experimentally observed unidirectional diagonal double-stripe spin-density-wave and predicts enhanced pairing under hol...
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Low-temperature mean valence of nickel ions in pressurized La$_3$Ni$_2$O$_7$
Nickel valence in La3Ni2O7 stays close to 2.5+ from ambient pressure to 40 GPa at 20 K, so pressure-induced superconductivity is tied to a structural transition rather than a change in nickel charge.
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