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This study investigates the pairing symmetry of the five-layer bulk superconductor La$_6$Ni$_5$O$_{12}$ by employing an effective five-layer Hubbard model constructed within density functional theory and variational Monte Carlo methods. Calculations indicate that the spin-singlet $d_{x^2-y^2}$-wave superconducting state is energetically dominant, whereas extended $s$-wave pairing is strongly suppressed. Based on the optimized state, the tunneling density of states is predicted to exhibit a V-shape, the superfluid density shows a linear low-temperature depletion, the Knight shift decreases, and the spin-lattice relaxation rate follows $1/T_1T\propto T^2$, characteristics that collectively point to a clean nodal superconductor. Furthermore, this framework reveals that infinite-layer LaNiO$_2$ possesses a robust $d$-wave tendency, while three-layer La$_4$Ni$_3$O$_8$ lies near the boundary between $d$-wave superconductivity and a non-superconducting state at nominal filling. These results establish La$_6$Ni$_5$O$_{12}$ as a platform for probing pairing symmetry in nickelates and suggest a prevalent $d_{x^2-y^2}$-wave pairing trend within the reduced Ruddlesden-Popper nickelate family.

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This study presents a detailed structural analysis of epitaxial La₃Ni₂O₇₋δ thin films using scanning transmission electron microscopy combined with electron energy loss spectroscopy. The films were prepared on SrLaAlO₄ substrates via pulsed laser deposition and exhibited significantly distinct superconducting properties after different ozone annealing treatments. It was found that the stabilization of the superconducting phase is closely related to oxygen stoichiometry uniformity, epitaxial strain, and specific stacking structural motifs such as bilayers and polytypes. By correlating the rich morphology of stacking polytypes with transport behavior, a theoretical framework for understanding metastable superconducting phases in bilayer nickelate thin films was established. The results reveal the critical roles of oxygen content, lattice strain, and structural ordering in achieving ambient-pressure superconductivity, providing a clear pathway for designing new nickel-based superconducting materials.

Through high-throughput first-principles simulations, this study systematically compares the thermodynamic stability of delafossite (D1), ordered rock salt variant (D2), and infinite-layer (IL) oxides at ABO₂ stoichiometry, constructing phase diagrams encompassing 2,346 elemental combinations. The results demonstrate that for nickelates, palladates, and platinate, the delafossite structure exhibits stability comparable to or even superior to the infinite-layer phase, with competition between these two phases and the perovskite phase. Electronic structure analysis reveals that delafossite compounds feature an inverted cation order, with the Fermi surface dominated by d_{z^2} orbital contributions, distinctly different from the d_{x^2-y^2} characteristics of the infinite-layer phase. Among all candidate systems, the La-Ni combination is the thermodynamically optimal choice for stabilizing the infinite-layer structure. Furthermore, hole doping via Ca, Sr, and Ba systematically enhances the relative stability of the infinite-layer phase across the three transition metal families. These findings elucidate the fundamental challenges in synthesizing substrate-free bulk infinite-layer oxides and provide guidance for the experimental exploration of novel superconducting compounds.

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5 linked papers