High temperature cuprate superconductivity remains a defining problem in condensed matter physics.Among myriad approaches to addressing this problem has been the study of alternative transition metal oxides with similar structures and 3d electron count that are suggested as proxies for cuprate physics.None of these analogs has been superconducting, and few are even metallic. Here, we report that the lowvalent, quasi-two-dimensional trilayer compound, Pr4Ni3O8 avoids a charge-stripe ordered phase previously reported for La4Ni3O8, leading to a metallic ground state. By combining x-ray absorption spectroscopy and density functional theory calculations, we further find that metallic Pr4Ni3O8 exhibits a low-spin configuration and significant orbital polarization of the unoccupied eg states with pronounced dx 2 -y 2 character near the Fermi energy, both hallmarks of the cuprate superconductors. Belonging to a regime of 3d electron count found for hole-doped cuprates, Pr4Ni3O8 thus represents one of the closest analogies to cuprates yet reported and a singularly promising candidate for high-Tc superconductivity if appropriately doped.

The quasi-2D nickelate La 4 Ni 3 O 8 (La-438), consisting of trilayer networks of square planar Ni ions, is a member of the so-called T′ family, which is derived from the Ruddlesden-Popper (R-P) parent compound La 4 Ni 3 O 10−x by removing two oxygen atoms and rearranging the rock salt layers to fluorite-type layers. Although previous studies on polycrystalline samples have identified a 105-K phase transition with a pronounced electronic and magnetic response but weak lattice character, no consensus on the origin of this transition has been reached. Here, we show using synchrotron X-ray diffraction on high-pO 2 floating zone-grown single crystals that this transition is associated with a real space ordering of charge into a quasi-2D charge stripe ground state. The charge stripe superlattice propagation vector, q = (2/3, 0, 1), corresponds with that found in the related 1/3-hole doped single-layer R-P nickelate, La 5/3 Sr 1/3 NiO 4 (LSNO-1/3; Ni 2.33+ ), with orientation at 45°to the Ni-O bonds. The charge stripes in La-438 are weakly correlated along c to form a staggered ABAB stacking that reduces the Coulomb repulsion among the stripes. Surprisingly, however, we find that the charge stripes within each trilayer of La-438 are stacked in phase from one layer to the next, at odds with any simple Coulomb repulsion argument.charge stripe | charge order | nickelate | strongly correlated materials | transition metal oxides C ompetition between localized and itinerant electron behavior is an organizing construct in our understanding of correlated electron transition metal oxide (TMO) physics (1-4). Some of the most compelling phenomenology in these materials occurs in the mixed valent state for the transition metal, which is set by composition, doping, and anion coordination of the metal. Many mixed valent TMOs adopt insulating "charge ordered" states, in which an inhomogeneous but long-range ordered configuration of the charge density condenses from a uniform metallic state (5). The real space pattern of charge order varies by material (6-9), but a typically observed motif is some variety of charge stripes. Such stripes have been observed in cobaltites (10-12), cuprates (13-15), nickelates (16)(17)(18)(19), and manganites (20-22), albeit with highly materials-dependent configurations that hinge on a balance among Coulomb, lattice, and magnetic exchange energies. For instance, charge stripes in layered nickelates typically stagger themselves from layer to layer to reduce the collective electrostatic energy arising from the charge disproportionation (9, 18).Indeed, the case of nickelates plays a prominent role in charge stripe physics (6-9, 16-19, 23-28), because mixed valent Ni 2+ (d 8 ) and Ni 3+ (d 7 ) compounds, such as La 2 − x Sr x NiO 4 (LSNO), are structurally and electronically related to high T c superconductors and thus, have been targeted as potential alternatives to the cuprates. Instead of superconductivity, however, the ground state of such quasi-2D, octahedrally coordinated nickelates is marked by stati...

We report for the first time the magnetic structure of the high entropy oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O using neutron powder diffraction. This material exhibits a sluggish magnetic transition but possesses a long-range ordered antiferromagnetic ground state, as revealed by DC and AC magnetic susceptibility, elastic and inelastic neutron scattering measurements. The magnetic propagation wavevector is k=(½, ½, ½) based on the cubic structure Fm-3m, and the magnetic structure consists of ferromagnetic sheets in the (111) planes with spins antiparallel between two neighboring planes. Inelastic neutron scattering reveals strong magnetic excitations at 100 K that survive up to room temperature. This work demonstrates that entropy-stabilized oxides represent a unique platform to study long range magnetic order with extreme chemical disorder.

An ordinary Hall effect in a conductor arises due to the Lorentz force acting on the charge carriers. In ferromagnets, an additional contribution to the Hall effect, the anomalous Hall effect (AHE), appears proportional to the magnetization. While the AHE is not seen in a collinear antiferromagnet, with zero net magnetization, recently it has been shown that an intrinsic AHE can be non-zero in non-collinear antiferromagnets as well as in topological materials hosting Weyl nodes near the Fermi energy. Here we report a large anomalous Hall effect with Hall conductivity of 27 Ω−1 cm−1 in a chiral-lattice antiferromagnet, CoNb3S6 consisting of a small intrinsic ferromagnetic component (≈0.0013 μB per Co) along c-axis. This small moment alone cannot explain the observed size of the AHE. We attribute the AHE to either formation of a complex magnetic texture or the combined effect of the small intrinsic moment on the electronic band structure.

ABSTRAT:We report the first single crystal growth of the correlated metal LaNiO3 using a highpressure optical-image floating zone furnace. The crystals were studied using single crystal/powder x-ray diffraction, resistivity, specific heat, and magnetic susceptibility. The availability of bulk LaNiO3 crystals will (i) promote deep understanding in this correlated material, including the mechanism of enhanced paramagnetic susceptibility, and (ii) provide rich opportunities as a substrate for thin film growth such as important ferroelectric and/or multifer ro ic materials. This study demonstrates the power of high pO2 single crystal growth of nickelate perovskites and correlated electron oxides more generally.