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

Abstract

The infinite-layer structure nickelate Ba2NiO2(AgSe)(2) (BNOAS) with d(8) Ni ions and a peculiar susceptibility chi(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 B-1(1), "off-diagonal singlet" textured by an internal orbital structure of compensating d(x2-y2)(up arrow) and d(z2)(down arrow) 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) c = 2.03 angstrom, which corresponds closely to the observed critical value of 2.00-2.05 angstrom, above which Ni becomes magnetic in square planar NiO2 compounds. We propose scenarios for the signature of magnetic reconstruction in chi(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(z2) spin on each site, coupled to a d(x2-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 d(9-delta) configuration and possible superconductivity with a similarity to the recently reported hole-doped infinite-layer cuprate Ba2CuO3.2 that superconducts at 73 K.