A previously unknown magnetic fluctuation origin is identified for the topological Hall effect in the kagome YMn ₆ Sn ₆ system.

Nickelates are a rich class of materials, ranging from insulating magnets to superconductors. But for stoichiometric materials, insulating behavior is the norm, as for most late transition metal oxides. Notable exceptions are the 3D perovskite LaNiO₃, an unconventional paramagnetic metal, and the layered Ruddlesden-Popper phases R₄Ni₃O₁₀, (R = La, Pr, Nd). The latter are particularly intriguing because they exhibit an unusual metal-to-metal transition. Here, we demonstrate that this transition results from an incommensurate density wave with both charge and magnetic character that lies closer in its behavior to the metallic density wave seen in chromium metal than the insulating stripes typically found in single-layer nickelates like La_2-xSr_xNiO₄. We identify these intertwined density waves as being Fermi surface-driven, revealing a novel ordering mechanism in this nickelate that reflects a coupling among charge, spin, and lattice degrees of freedom that differs not only from the single-layer materials, but from the 3D perovskites as well.

The recent reports of superconductivity in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ heterostructures have reinvigorated interest in potential superconductivity of low-oxidation state nickelates. Synthesis of ${\mathrm{Ni}}^{1+}$-containing compounds is notoriously difficult. In the current work, a combined sol-gel combustion and high-pressure annealing technique was employed to prepare polycrystalline perovskite ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ $(x=0, 0.1, \mathrm{and} 0.2)$. Metal nitrates and metal acetates were used as starting materials, and the latter were found to be superior to the former in terms of safety and reactivity. The ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ compounds were subsequently reduced to ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$ using calcium hydride in a sealed, evacuated quartz tube. To understand the synthesis pathway, the evolution from $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ to $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ was monitored using in situ synchrotron x-ray diffraction during the reduction process. Electrical transport properties were consistent with an insulator-metal transition occurring between $x=0$ and 0.1 for ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$. Superconductivity was not observed in our bulk samples of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$. Neutron diffraction experiments at 3 and 300 K were performed on ${\mathrm{Nd}}_{0.9}{\mathrm{Sr}}_{0.1}\mathrm{Ni}{\mathrm{O}}_2$, in which no magnetic Bragg reflections were observed, and the results of structural Rietveld refinement are provided.

Significance: Competition between localized and itinerant electrons in highly correlated materials can lead to myriad insulating ground states, including spatially inhomogeneous but ordered charge superlattices. In layered transition metal oxides, such charge order can take the form of stripes, which typically arrange themselves in staggered formations to reduce Coulomb repulsion. Having achieved single-crystal growth of the layered nickelate La₄Ni₃O₈, we show that its heretofore incompletely understood phase transition is associated with charge stripe ordering. We find that the stripes are stacked directly on top of one another within nickel oxide trilayers but staggered between successive trilayers. A unique, paradoxical ground state results, in which the electrostatic building principle is respected at long range but violated at short range.

Single crystals of the metallic Ruddlesden-Popper trilayer nickelates $R_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$) were successfully grown using an optical-image floating zone furnace under oxygen pressure $\left(p{\mathrm{O}}_2\right)$ of 20 bar for ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and 140 bar for ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. A combination of synchrotron and laboratory x-ray single-crystal diffraction, high-resolution synchrotron x-ray powder diffraction and measurements of physical properties revealed that $R_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$) crystallizes in the monoclinic $P{2}_1/a$ ($Z=2$) space group at room temperature, and that a metastable orthorhombic phase (Bmab) can be trapped by postgrowth rapid cooling. Both ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ crystals undergo a metal-to-metal transition (MMT) below room temperature. In the case of ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, the MMT is found at 157.6 K. For ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, the MMT depends on the lattice symmetry: 147.5 K for Bmab vs 138.6 K for $P{2}_1/a$. Lattice anomalies were found at the MMT that, when considered together with the pronounced dependence of the transition temperature on subtle structural differences between Bmab and $P{2}_1/a$ phases, demonstrate a not insignificant coupling between electronic and lattice degrees of freedom in these trilayer nickelates.