Interlayer coupling enhanced superconductivity near 100 K in La$_{3-x}$Nd$_x$Ni$_2$O$_7$

Abstract: Systematically controlling the superconducting transition temperature ($T_\text{c}$) in the bilayer Ruddlesden-Popper nickelate La$<em>3$Ni$_2$O$_7$ remains a significant challenge. Here, we address this by synthesizing high-quality polycrystalline La$</em>{3-x}$Nd$<em>x$Ni$_2$O$_7$ ($0 \leq x \leq 2.4$) with record-level rare-earth substitution. Nd doping compresses the lattice, particularly along the $c$ axis, enhances the spin density wave transition temperature, and elevates the pressure required for the orthorhombic-to-tetragonal structural transition. Superconductivity is observed across all doping levels under high pressures, with the onset $T</em>\text{c}$ rising to $\sim$93~K for $x = 2.1$ and $2.4$ from the electronic transport measurement. Using the radio-frequency transmission technique, newly applied to nickelate superconductors, we detect signatures of superconductivity at $98 \pm 2$~K in the $x=2.4$ compound, pushing the $T_\text{c}$ frontier further. We identify a universal linear relationship where $T_\text{c}$ decreases with the $c$-axis lattice parameter at a rate of approximately $-28$~K/\AA, demonstrating that enhanced interlayer magnetic exchange coupling is the dominant mechanism for superconducting pairing. Our work establishes the critical role of magnetism and provides a unified structural descriptor for elevating $T_\text{c}$ in bilayer nickelates.