arXiv subjects

Hyo-Sun Jin

Publications and source records attributed to Hyo-Sun Jin.

7 recordsLinked to original sources

arXiv2022

A $d^8$ anti-Hund's Singlet Insulator in an Infinite-layer Nickelate

The status of nickelate superconductors in relation to cuprate high temperature superconductors is one of the concepts being discussed in high temperature superconductivity in correlated transition metal oxides. New additions to the class of infinite layer nickelates can provide essential input relating to connections or distinctions. A recently synthesized compound \bnoas, which contains isolated `infinite layer' NiO$_2$ planes, may lead to new insights. Our investigations have discovered that, at density functional theory mean field level, the ground state consists of an unusual $e_g$ singlet on the Ni$^{2+}$ ion arising from large but separate Mott insulating gaps in both $e_g$ orbitals, but with different, anti-Hund's, spin directions of their moments. This textured singlet incorporates at the least new physics, and potentially a new platform for nickelate superconductivity, which might be of an unconventional form for transition metal oxides due to the unconventional undoped state. We include in this paper a comparison of electronic structure parameters of Ba$_2$NiO$_2$(AgSe)$_2$ with a better characterized infinite layer nickelate LaNiO$_2$. We provide more analysis of the $d^8$ anti-Hund's singlet that emerges in this compound, and consider a minimally correlated wavefunction for this singlet in an itinerant background, and begin discussion of excitations -- real or virtual -- that may figure into new electronic phases.

cond-mat.str-el↗

arXiv2021

Two-band Conduction and Nesting Instabilities in Superconducting Ba$_2$CuO$_{3+δ}$: a First Principles Study

First principles investigations of the high temperature superconducting system Ba$_2$CuO$_{3+δ}$, recently discovered at $δ\approx0.2$ at $T_c=70$ K, are applied to demonstrate the effects of oxygen ordering on the electronic and magnetic properties. The observed `highly over-doped' superconducting phase displays stretched Cu-planar oxygen O$_{\rm P}$ distances and anomalously shortened Cu-apical O$_{\rm A}$ separations compared with other cuprates. The stoichiometric system $δ=0$, with its strongly one-dimensional (1D) Cu-O$_{\rm P}$ chain structure, when nonmagnetic shows 1D Fermi surfaces that lead, within density functional theory, to antiferromagnetic Cu-O$_{\rm P}$ chains (a spin-Peierls instability). Accounting for 1D fluctuations and small interchain coupling according to the theory of Schulz indicates this system, like Sr$_2$CuO$_3$, is near the 1D Luttinger-liquid quantum critical phase. The unusual Cu-O bond lengths per se have limited effects on other properties for $δ$=0. We find that a `doubled bilayer' structure of alternating Cu-O$_{\rm P}$ chains and wide rung Cu$_3$O$_4$ ladders is the energetically preferred one of three possibilities where the additional oxygen ions bridge Cu-O$_{\rm P}$ chains in the superconducting phase $δ=1/4$. Nominal formal valences of the three Cu sites are discussed. The six-fold (octahedral) site is the most highly oxidized, accepting somewhat more holes in the $d_{z^2}$ orbital than in the $d_{x^2-y^2}$ orbital. The implication is that two-band physics is involved in the pairing mechanism and the superconducting carriers. The Fermi surfaces of this metallic bilayer structure show both 1D and 2D strong (incipient) nesting instabilities, possibly accounting for the lack of clean single-phase samples based on this structure and suggesting importance for the pairing mechanism.

cond-mat.supr-con↗

arXiv2020

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

The infinite-layer structure nickelate Ba$_2$NiO$_2$(AgSe)$_2$ (BNOAS) with $d^8$ Ni ions and a peculiar susceptibility $χ(T)$ 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}=$2.03 Å, above which Ni becomes magnetic in square planar NiO$_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 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 $d^{9-δ}$ configuration and possible superconductivity with similarity to the recently reported Ba$_2$CuO$_{3.2}$ that superconducts at 73 K.

cond-mat.str-el↗

arXiv2019

Noncentrosymmetric compensated half-metal hosting pure spin Weyl nodes, triple nodal points, nodal loops, and nexus fermions

Materials containing multiple topological characteristics become more exotic when combined with noncentrosymmetric crystal structures and unusual magnetic phases such as the compensated half-metal 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 Cr$_2$CoAl 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 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.

cond-mat.mtrl-sci↗

arXiv2018

Stability of Room Temperature Compensated Half-Metallicity in Cr-based Inverse-Heusler Compounds

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$_2$Co${\cal Z}$ (${\cal Z}$=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 ${\cal Z}$ ions. Additionally, in Cr$_2$CoAl, changing the volume by $\pm$ 5% or the ratio of $c/a$ by $\pm$ 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.

cond-mat.mtrl-sci↗

arXiv2014

Strain and Spin-Orbit Coupling Induced Orbital-Ordering in Mott Insulator BaCrO3

Using ab initio calculations, we have investigated an insulating tetragonally distorted perovskite BaCrO$_3$ with a formal $3d^2$ configuration, the volume of which is apparently substantially enhanced by a strain due to SrTiO$_3$ 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 $d_{xz}+id_{yz}$ and $d_{xz}-id_{yz}$ orbitals, which results in a large orbital moment $M_L$ ~ -0.78 $μ_B$ antialigned to the spin moment $M_S$ ~ $2|M_L|$ in Cr ions. Remarkably, this ordering also induces a considerable $M_L$ 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.

cond-mat.str-el↗

arXiv2011

Half-metallic Ferrimagnetism Driven by Coulomb Enhanced Spin-Orbit Coupling in PdCrO3

Recently, in the seemingly narrow gap insulating NiCrO$_3$ with the trigonally distorted (R-3c) 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 PdCrO$_3$, isostructural and isovalent to NiCrO$_3$. 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 NiCrO$_3$, 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 $μ_B$ 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 La$_2$PdCrO$_6$ is expected to be a half-metal ferromagnet with tiny orbital moments.

cond-mat.str-el↗