Description
The extent to which electronic dimensionality influences magnetism and superconductivity in Ruddlesden–Popper (RP) nickelates remains unsettled. Here we report high-precision crystallographic-axis–resolved dc transport measurements on high-quality single crystals of bilayer and trilayer RP nickelates. Using a six-terminal geometry, we self-consistently determine the intrinsic in-plane ($\rho_\parallel$) and out-of-plane ($\rho_\bot$) resistivities on the same crystal, while minimizing uncertainties associated with current redistribution in highly anisotropic conductors. We uncover strong intrinsic electronic anisotropy in both bilayer and trilayer nickelates, in contrast to the weak anisotropy inferred from conventional four-probe measurements. Moreover, $\rho_\bot$ exhibits a nonmonotonic temperature dependence, revealing a universal coherent-to-incoherent crossover in interlayer transport. Across the RP nickelate series, the maximum superconducting transition temperature ($T_c$) observed under pressure is inversely correlated with the ambient-pressure resistivity anisotropy, suggesting that stronger interlayer electronic coherence is favorable for superconductivity. In addition, $\rho_\bot$ serves as an exceptionally sensitive and selective probe of magnetic and density-wave orders, exhibiting pronounced anomalies, whereas only weak signatures are observed in $\rho_\parallel$. Our results highlight interlayer coherence as a key organizing parameter that both tracks the relevant magnetic correlations and is closely tied to superconductivity, providing stringent constraints on microscopic theories of high-$T_c$ superconductivity in nickelates.
