Proposed ordering of textured spin singlets in a bulk infinite-layer nickelate

Abstract

The infinite-layer structure nickelate Ba2NiO2(AgSe)2 (BNOAS) with d8 Ni ions and a peculiar susceptibility χ(T), synthesized at high pressure, is studied with correlated density functional methods. The overriding feature of the calculations is a violation of Hund's rule coupled with complete but unconventional spin-orbital polarization, leading to an unexpected low-spin 1B1, “off-diagonal singlet” textured by an internal orbital structure of compensating dx2−y2↑ and dz2↓ spins. This unconventional configuration has a lower energy than conventional high-spin or low-spin alternatives. An electronic transition is obtained at a critical Ni-O separation dcNi-O=2.03Å, which corresponds closely to the observed critical value of 2.00–2.05 Å, above which Ni becomes magnetic in square planar NiO2 compounds. We propose scenarios for the signature of magnetic reconstruction in χ(T) at Tm=130 K without any Curie-Weiss background (no moment) that invokes ordering of Ni d8 moieties that are largely this generalized Kondo singlet. Because hole states are primarily Se 4p rather than O 2p, the usual issue of Mott insulator versus charge transfer insulator is supplanted by a character in which electrons and holes are separated in real space. The underlying physics of this system is modeled by a Kondo sieve spin model (two-dimensional Kondo necklace) of a “Kondo” dz2 spin on each site, coupled to a dx2−y2 spin that is itself strongly coupled to neighboring like-spins within the layer. The observed magnetic order places BNOAS below the quantum critical point of the Kondo sieve model, providing a realization of the long-range ordered near-singlet weak antiferromagnetic phase. We propose electron doping experiments that would drive the system toward a d9−δ configuration and possible superconductivity with a similarity to the recently reported hole-doped infinite-layer cuprate Ba2CuO3.2 that superconducts at 73 K.

Source-reported materials — not catalogue approval

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