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

TL;DR

This paper investigates the unconventional spin and orbital structure of a bulk nickelate with an infinite-layer structure, revealing a novel off-diagonal singlet state and proposing its relation to magnetic order and potential superconductivity.

Contribution

It uncovers a unique off-diagonal singlet state in a nickelate, demonstrating violation of Hund's rule and proposing a Kondo sieve model for its magnetic properties.

Findings

Identification of an unconventional low-spin off-diagonal singlet state.

Observation of a magnetic transition at 130 K without Curie-Weiss behavior.

Proposal of doping-induced superconductivity similar to cuprates.

Abstract

The infinite-layer structure nickelate Ba$_2$NiO$_2$(AgSe)$_2$ (BNOAS) with $d^8$ Ni ions and a peculiar susceptibility $\chi(T)$ is studied with correlated density functional methods. The overriding feature of the calculations is violation of Hund's rule coupled with complete but unconventional spin-orbital polarization, leading to an unexpected low spin $^1B_1$, "off-diagonal singlet" (ODS) textured by an internal orbital structure of compensating $d_{x^2-y^2}^{\uparrow}$ and $d_{z^2}^{\downarrow}$ spins. This unconventional configuration has 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}=$2.03 \AA, above which Ni becomes magnetic in square planar NiO$_2$ 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 invoke ordering of Ni $d^8$ moieties that are largely this generalized Kondo singlet. The underlying physics of this system is modeled by a Kondo sieve model (2D 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 previously unreported 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 similarity to the recently reported Ba$_2$CuO$_{3.2}$ that superconducts at 73 K.