Absence of Ni²/Ni³ charge disproportionation and possible roles of O 2p 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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In this study, bulk single crystals of the four-layer Ruddlesden–Popper nickelate La5Ni4O13 were grown by a high-pressure high-temperature method, and the synthesis of the five-layer La6Ni5O16 was reproduced; La5Ni4O13 single crystals were then topotactically reduced with CaH2 to yield the square-planar phase La5Ni4O10. This reduced phase behaves as a bulk superconductor at ambient pressure, with an onset transition temperature Tc^onset of approximately 18 K and a zero-field-cooled shielding fraction exceeding 60%, while a specific-heat jump confirms a bulk pairing transition; superconductivity coexists with density-wave or metal–insulator transitions in all crystals. The upper critical field anisotropy γ≈2.4 remains finite as T→Tc, indicating three-dimensional superconducting characteristics and significant coupling between the four-layer blocks. On this basis, the authors propose that square-planar nickelates R_{n+1}Ni_nO_{2n+2} can enter the superconducting regime solely by tuning the Ni valence through the layer number, without chemical substitution, applied pressure, or epitaxial constraint.
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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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