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

Abstract

The infinite-layer structure nickelate Ba_{2}NiO_{2}(AgSe)_{2} (BNOAS) with d^{8} 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 ^{1}B_{1}, “off-diagonal singlet” textured by an internal orbital structure of compensating d_{x^{2}−y^{2}}^{↑} and d_{z^{2}}^{↓} 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 d_{c}^{Ni-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 NiO_{2} compounds. We propose scenarios for the signature of magnetic reconstruction in χ(T) at T_{m}=130 K without any Curie-Weiss background (no moment) that invokes ordering of Ni d^{8} 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” d_{z^{2}} spin on each site, coupled to a d_{x^{2}−y^{2}} 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 d^{9−δ} configuration and possible superconductivity with a similarity to the recently reported hole-doped infinite-layer cuprate Ba_{2}CuO_{3.2} that superconducts at 73 K.