Superconductivity in a quintuple-layer square-planar nickelate

Abstract: Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials. One prime materials platform has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd${0.8}$Sr${0.2}$NiO$2$. Undoped NdNiO$_2$ belongs to a series of layered square-planar nickelates with chemical formula Nd${n+1}$Ni$n$O${2n+2}$ and is known as the 'infinite-layer' ($n = \infty$) nickelate. Here, we report the synthesis of the quintuple-layer ($n = 5$) member of this series, Nd$6$Ni$_5$O${12}$, in which optimal cuprate-like electron filling ($d^{8.8}$) is achieved without chemical doping. We observe a superconducting transition beginning at $\sim$13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd$6$Ni$_5$O${12}$ interpolates between cuprate-like and infinite-layer nickelate-like behavior. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors which can be tuned via both doping and dimensionality.

Superconductivity in Quintuple-Layer Square-Planar Nickelate

This research paper presents the synthesis and characterization of a quintuple-layer nickelate material, Nd$6$Ni$_5$O${12}$, which exhibits superconductivity at temperatures around 13 K. Nickelates have long been considered potential alternatives to cuprates due to their similar electronic structures, particularly the presence of Ni$^{1+}$ with a similar $d^9$ electron count as Cu$^{2+}$. Previous investigations focused on infinite-layer nickelates, particularly doped NdNiO$_2$, which have demonstrated superconducting properties. This study extends the realm of nickelate superconductivity by introducing novel dimensionality in the form of a quintuple-layer structure that achieves optimal electron filling without the need for chemical doping.

The synthesis utilized reactive-oxide molecular beam epitaxy (MBE) followed by topotactic reduction involving CaH$_2$ to convert the Ruddlesden-Popper phase into the targeted square-planar phase. The transformation was confirmed through a variety of structural and electronic characterization methods, including high angle annular dark field (HAADF) and annular bright field (ABF) scanning transmission electron microscopy (STEM), x-ray diffraction (XRD), electron energy loss spectroscopy (EELS), and x-ray absorption spectroscopy (XAS). These analyses verified the removal of apical oxygen and the compression of the lattice constants characteristic of the new square-planar configuration.

Electronic characterization illuminated distinctive properties in Nd$6$Ni$_5$O${12}$ compared to its infinite-layer and trilayer counterparts. Resistivity measurements indicated a superconducting transition around 13 K, suppressed by magnetic fields up to 9 T. Notably, the quintuple-layer nickelate exhibited positive Hall coefficients at all temperatures, contrasting with negative coefficients in doped infinite-layer nickelates. This suggests a single-band character akin to cuprates rather than the multi-band nature of infinite-layer nickelates, attributed in part to lesser involvement of Nd-5d bands.

Electronic structure calculations and quantitative assessments of Ni-O hybridization reveal notable differences. The quintuple-layer nickelates demonstrate a more two-dimensional electronic environment, with hybridization parameters closer to those observed in cuprates. Charge transfer energy comparisons showed smaller values for the layered nickelates compared to infinite-layer compounds, suggesting enhanced covalency and a possibly stronger mediating role in superconductivity.

The synthesis of Nd$6$Ni$_5$O${12}$ opens paths for further exploration and manipulation of nickelate superconductivity through manipulating dimensionality and electron filling. There is potential for extending these findings to other nickelate configurations and investigating their superconducting phase diagrams further. This study contributes to the understanding of nickelates as a distinct superconductor family and calls for future research into combined chemical doping and dimensional tuning to optimize superconductivity across this promising new class of materials.