Superconducting Lanthanum Nickel Oxides with Bilayered and Trilayered Crystal Structures
In 2023, superconductivity in La$_3$Ni$_2$O$_7$ was discovered under high pressures above approximately 14 GPa. In addition to its high transition temperature ($T_{\mathrm{c}} \simeq 80$ K), the structural resemblance to high-$T_{\mathrm{c}}$ cuprates has strongly stimulated research, soon followed by the discovery of superconductivity in La$_4$Ni$_3$O$_{10}$. These compounds belong to the Ruddlesden--Popper phases, comprising double- and triple-layered NiO$_2$ square lattices separated by LaO rock-salt slabs. Research on these systems has rapidly developed along three major directions, as in other prominent families of superconductors such as the cuprates and iron arsenides: expanding the chemical variety of compounds, enhancing $T_{\mathrm{c}}$ through elemental substitution, and elucidating the superconducting mechanism. These challenges, being closely interconnected, continue to drive the field. The clarification of the pairing mechanism encounters a particular difficulty, since the key experiments must be performed under high pressures. This situation highlights the significance of developing nickel oxides that exhibit superconductivity at much lower pressures, ideally at ambient pressure, which would in turn broaden the scope of chemical tuning and detailed physical characterization. In this context, it is timely and meaningful to summarize the present state of knowledge. Here, we emphasize sample synthesis and characterization, which are already well established and often decisive for progress in unconventional superconductors, while providing a brief overview of the currently available electronic properties.