Absence of Ni₂/Ni₃ charge disproportionation and possible roles of O₂ p holes in La₃Ni₂O₇−δ revealed by hard x-ray photoemission spectroscopy
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Recently, the Ruddlesden-Popper bilayer nickelate $La_3Ni_2O_7$ has emerged as a superconductor with a transition temperature ($T_c$) of ~ 80 K above 14 GPa$^{[1-4]}$. Efforts to search for nickelate superconductors with higher $T_c$$^{[5,6]}$, to grow reproducible high-quality single crystals$^{[2,7-10]}$, and to eliminate reliance on demanding high gas pressure synthesis conditions$^{[11]}$, remain significant challenges. Here we report superconductivity up to 92 K under high pressure in single crystals of bilayer nickelates synthesized at ambient pressure using flux methods. High quality $La_2SmNi_2O_{7-δ}$ single crystals with dimensions up to 220 μm on edge were successfully grown. At ~ 15 GPa, these crystals exhibit superconductivity with an onset transition temperature ($T_c^{onset}$) of 68 K and zero-resistance temperature ($T_c^{zero}$) of 47 K. Increasing pressure further enhances both transition temperatures, reaching record values for nickelates: $T_{c,max}^{onset}$ = 92 K and $T_{c,max}^{zero}$ = 73 K @ 21 GPa. Notably, higher $T_c$ correlates with larger in-plane lattice distortion at ambient conditions for bilayer nickelates. Furthermore, we observed a structural transition from monoclinic $P2_1/a$ to tetragonal $I4/mmm$ at ~ 18 GPa, indicating that tetragonal structure is not a prerequisite for superconductivity to appear in this bilayer nickelate. This study provides an easy-to-access method for growing reproducible high-quality bilayer nickelate single crystals and offers new insights into achieving higher Tc superconductivity.
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Using polarization-resolved electronic Raman scattering measurements on high-quality La₃Ni₂O₇ single crystals, we observe a pronounced, symmetry-dependent spectral weight redistribution across the density-wave transition below 150 K: the B₁g channel exhibits an asymmetric peak, while the B₂g channel shows a symmetric broad peak, corresponding to electronic excitations near the X/Y points of the Brillouin zone and along the diagonal directions, respectively. Quantitative analysis extracts two sets of SDW gap values, with the B₁g channel gap approximately 37.5–40.4 meV (2Δ/kBTc ≈ 5.5–5.9) and the B₂g channel gap about 23.0 meV (2Δ/kBTc ≈ 3.4), indicating intermediate-to-strong coupling for the former and weak coupling for the latter. This momentum-selective anisotropic coupling strength cannot be explained by simple weak-coupling nesting theory, revealing that the unconventional SDW originates from anisotropic electronic correlations. The temperature dependence of the gap is significantly weaker than mean-field expectations, and the isotropy of the B₂g channel along with the weak anisotropy of the B₁g channel further support the coexistence of two distinct coupling mechanisms. This work establishes the electronic characteristics of the SDW in La₃Ni₂O₇, providing a microscopic foundation for understanding the emergence of high-temperature superconductivity in nickelates under pressure.
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This study employs first-principles density functional theory (GGA and GGA+U) to investigate the electronic and magnetic properties of La3Ni2O5F, which features a bilayer NiO2 infinite-layer structure where La(O/F)La blocking layers achieve strict isolation of the NiO2 bilayers, forming a purely two-dimensional electronic and magnetic system. Calculations reveal an E* single band composed of electron density in the interstitial region, which is not associated with any atomic orbital, dips below the Fermi level along the M-A direction, and provides a self-doping of 0.09 holes per Ni, resulting in an actual Ni valence of +1.09; the Fermi surface of this E* band is cylindrical, occupying 9% of the Brillouin zone area. The dpσ band is nearly half-filled but is shifted near a Van Hove singularity due to the self-doping, and the magnetic response exhibits anomalous characteristics distinct from previous nickelates, with the magnetic susceptibility tending to vanish under a large magnetic field. The absence of a magnetic phase transition can be attributed to strong two-dimensional spin fluctuations and the self-doping effect away from half-filling, revealing the unique behavior of Ni¹⁺ ions in this system and the critical influence of the interfacial blocking layer on the interstitial band morphology.