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 Ni-O c = 2.03 Å, 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.

Using three correlated band approaches, namely the conventional band approach plus on-site Coulomb repulsion U , the modified Becke-Johnson functional, and hybrid functional, we have investigated inverse-Heusler ferrimagnets Cr 2 CoZ (Z=Al, Ga, In). These approaches commonly indicate that the Cr 2 CoAl synthesized recently is a precise compensated half-metal (CHM), whereas Cr 2 CoGa and Cr 2 CoIn are ferrimagnets with a small moment. This is also confirmed by the fixed spin moment approach. Analysis of the Bader charge decomposition and the radial charge densities indicates that this contrast is due to chemical differences among the Z ions. Additionally, in Cr 2 CoAl, changing the volume by ± 5% or the ratio of c/a by ± 2% does not alter the CHM state, suggesting that this state is robust even under application of moderate pressure or strain. Considering the observed high Curie temperature of 750 K, our results suggest that Cr 2 CoAl is a promising candidate for robust high T C CHMs. Furthermore, the electronic structure of the CHM Cr 2 CoAl is discussed. Figure 1: Inverse Heusler XA (or Xα) structure of Cr 2 CoAl, consisting of Cr1-Al 4 and Co-Al 4 tetrahedra, and Cr2-Al 6 octahedra. The Cr1-Cr2-Co-Al sequence appears along the diagonal direction. Here, the two tetrahedra are edge-shared with each other, and face-shared with the octahedra. In the ground state, the spin moment of Cr1 ion is antialigned to that of both Cr2 and Co ions.

Using ab initio calculations, we have investigated an insulating tetragonally distorted perovskite BaCrO3 with a formal 3d 2 configuration, the volume of which is apparently substantially enhanced by a strain due to SrTiO3 substrate. Inclusion of both correlation and spin-orbit coupling (SOC) effects leads to a metal-insulator transition and in-plane zigzag orbital-ordering (OO) of alternating singly filled dxz +idyz and dxz −idyz orbitals, which results in a large orbital moment ML ≈ −0.78µB antialigned to the spin moment MS ≈ 2|ML| in Cr ions. Remarkably, this ordering also induces a considerable ML for apical oxygens. Our findings show metal-insulator and OO transitions, driven by an interplay among strain, correlation, and SOC, which is uncommon in 3d systems.

Materials containing multiple topological characteristics become more exotic when combined with noncentrosymmetric crystal structures and unusual magnetic phases such as the compensated halfmetal state, which is gapped in one spin direction and conducting in the other. First principles calculations reveal these multiple topological features in the compensated half-metal Cr2CoAl having neither time-reversal nor inversion symmetries. In the absence of (minor) spin-orbit coupling (SOC), there are (1) a total of twelve pairs of magnetic Weyl points, (2) three distinct sets of triple nodal points (TNPs) near the Fermi level that are (3) interconnected with six symmetry related nodal lines. This combination gives rise to fully spin polarized nexus fermions, in a system with broken time-reversal symmetry but negligible macroscopic magnetic field. The observed high Curie temperature of 750 K and calculated SOC hybridization mixing of several meV should make these nexus fermions readily measurable. Unlike topological features discussed for other Heuslers which emphasize their strong ferromagnetism, this compensated half-metal is impervious to typical magnetic fields, thus providing a complementary set of experimental phenomena. Making use of the soft calculated magnetic state, large magnetic fields can be used to rotate the direction of magnetism, during which certain topological features will evolve. Our results suggest that these features may be common in inverse-Heusler systems, particularly the isostructural and isovalent Ga and In analogs.

Recently, in the seemingly narrow gap insulating NiCrO3 with the trigonally distorted (R3c) perovskite-like structure, a compensated half-metal (CHM) is predicted, as applying a modest pressure. Using ab initio calculations including both Coulomb correlations and spin-orbit coupling (SOC), we investigate the as-yet-unsynthesized PdCrO3, isostructural and isovalent to NiCrO3. Upon applying the on-site Coulomb repulsion U to both Pd and Cr ions, the Cr spin moment is precisely compensated with the antialigned spin moments of Pd and oxygens. Coincidentally only one spin channel remains metallic due to the twice larger width of the Pd 4d bands than the Ni 3d bands in NiCrO3, indicating CHM in ambient pressure. Inclusion of SOC as well as correlation effects (LDA+U+SOC) produces a SOC constant enhanced twice over the value of LDA+SOC, leading to unusually large orbital moment of -0.25 µB on Pd. However, the half-metallicity still survives, so that a transition of CHM to a half-metallic ferrimagnet occurs due to Coulomb enhanced SOC. On the other hand, an isovalent, but presumed cubic double perovskite La2PdCrO6 is expected to be a half-metal ferromagnet with tiny orbital moments.