arXiv subjects

Young-Joon Song

Publications and source records attributed to Young-Joon Song.

13 recordsLinked to original sources

arXiv2026

Anomalous Behavior of the Ni$^{1+}$ moment and interstitial band in bi-infinite-layered La$_3$Ni$_2$O$_5$F

The discovery of superconductivity in hole-doped Ni$^{1+}$ systems with "infinite layer" NiO$_2$ square-lattices analogous to the Cu$^{2+}$ CaCuO$_2$ cuprate has renewed conflicting pictures of the Cu$^{2+}$$-$Ni$^{1+}$ similarity or distinction. Recent synthesis of formal Ni$^{1+}$ La$_3$Ni$_{2}$O$_{5}$F with two infinite NiO$_{2}$ layers per cell provides a novel member of this class. First principles density functional theory studies reveal an interstitial density derived single band $E^*$ in three layers unrelated to any atom, which provides self-doping to a Ni$^{1.09+}$ ion.The blocking La(O/F)La provides isolation of the NiO$_2$ bilayer and an interstitial $E^*$ density to strictly two-dimensional electronic and magnetic systems. Calculations of magnetic tendencies reveals behavior unlike previous nickelates, including vanishing susceptibility up to a large magnetic field. Two dimensional fluctuations and self-doping away from half-filling can account for the lack of observation of a magnetic transition.

cond-mat.supr-con↗

arXiv2026

Dichotomous electronic system in a bilayer Ni$^{1+}$ nickelate

"Infinite layer" nickelates (ILNs) ${\cal R}$NiO$_2$ (${\cal R}$=rare earth), having empty apical O sites, become superconducting upon hole doping, stimulating research into the related sequence Nd$_{n+1}$Ni$^{+p}_n$O$_{2n+2}$, formal charge state $p$=1+$\frac{1}{n}$, $n$=2,3,4,...., with the $n$=5 member being found to be superconducting. The two layer system La$_3$Ni$_2$O$_{7-\delta}$, with $\delta$=0,$\frac{1}{2}$,1 ($p$=2.5,2,1.5) approaches the peak in the nickelate superconducting dome but shows no superconductivity. Newly reported La$_3$Ni$_2$O$_5$F reaches the Ni$^{1+}$ goal while, as we show, introducing a partially occupied electron band $E^*$, based on an interstitial density that extends over the three open "apical" layers and leads to a single cylindrical electron Fermi surface giving self-doping. The commonly inert Ni $d_{xz},d_{yz}$ orbitals partner with interstitial $E^*$ to provide an incipient non-analytic Dirac point, with the critical point being reachable by pressure or further F insertion. The $E^*$ electron cylinder and the conventional Ni $dp\sigma$ hole carriers combine to provide a two-fluid dichotomy of hole and electron quasiparticles, affecting normal state properties that should verify the dichotomous aspect of transport.

cond-mat.supr-con↗

arXiv2025

Oxygen-vacancy-induced Raman softening in the catalyst Fe$_2$(MoO$_4$)$_3$

Iron molybdate (Fe$_2$(MoO$_4$)$_3$) is a widely used commercial catalyst for oxidative dehydrogenation. Recently, the possibility that bulk oxygen atoms participate in catalytic reactions has been proposed based on the experimentally observed significant reduction in Raman intensity during the catalytic process, which implies the formation of oxygen defects. In this work, we performed density functional theory (DFT) calculations to elucidate the microscopic mechanism of the experimentally observed Raman intensity variation. Our phonon analysis reveals that oxygen-dominated vibrational modes, with a small contribution from Mo, occur near 782cm$^{-1}$-- the same frequency region where the Raman intensity reduction was measured. To make the calculations computationally feasible for this large system, we introduced an effective frozen-phonon approach to mimic defect effects into the Raman intensity. Our results suggest that oxygen vibrations are primarily responsible for the decrease in the calculated Raman intensity. Moreover, structural relaxation of Fe$_2$(MoO$_4$)$_3$ containing an oxygen vacancy indicates that oxygen diffusion from the bulk to the surface may occur very rapidly, such that the local symmetry remains effectively unchanged. This interpretation is in line with the absence of measurable peak shifts or broadening in the experimental Raman spectra.

cond-mat.mtrl-sci↗

arXiv2024

Influence of graphene on the electronic and magnetic properties of an iron(III) porphyrin chloride complex

Although iron-based single atom catalysts are regarded as a promising alternative to precious metal catalysts, their precise electronic structures during catalysis still pose challenges for computational descriptions. A particularly urgent question is the influence of the environment on the electronic structure, and how to describe this properly with computational methods. Here, we study an iron porphyrin chloride complex adsorbed on a graphene sheet using density functional theory calculations to probe how much the electronic structure is influenced by the presence of a graphene layer. Our results indicate that weak interactions due to van der Waals forces dominate between the porphyrin complex and graphene, and only a small amount of charge is transferred between the two entities. Furthermore, the interplay of the ligand field environment, strong $p$ $-$ $d$ hybridization, and correlation effects within the complex are strongly involved in determining the spin state of the iron ion. By bridging molecular chemistry and solid state physics, this study provides first steps towards a joint analysis of the properties of iron-based catalysts from first principles.

cond-mat.mtrl-sci↗

arXiv2023

Influence of magnetism, strain and pressure on the band topology of EuCd$_2$As$_2$

Motivated by the wealth of proposals and realizations of nontrivial topological phases in EuCd$_2$As$_2$, such as a Weyl semimetallic state and the recently discussed semimetallic versus semiconductor behavior in this system, we analyze in this work the role of the delicate interplay of Eu magnetism, strain and pressure on the realization of such phases. For that we invoke a combination of a group theoretical analysis with ab initio density functional theory calculations and uncover a rich phase diagram with various non-trivial topological phases beyond a Weyl semimetallic state, such as axion and topological crystalline insulating phases, and discuss their realization.

cond-mat.str-el↗

arXiv2023

Microscopic analysis of the valence transition in tetragonal EuPd$_2$Si$_2$

Under temperature or pressure tuning, tetragonal EuPd$_2$Si$_2$ is known to undergo a valence transition from nearly divalent to nearly trivalent Eu accompanied by a volume reduction. Albeit intensive work, its microscopic origin is still being discussed. Here, we investigate the mechanism of the valence transition under volume compression by $ab~initio$ density functional theory (DFT) calculations. Our analysis of the electronic and magnetic properties of EuPd$_2$Si$_2$ when approaching the valence transition shows an enhanced $c$-$f$ hybridization between localized Eu 4$f$ states and itinerant conduction states (Eu 5$d$, Pd 4$d$, and Si 3$p$) where an electronic charge redistribution takes place. We observe that the change in the electronic structure is intimately related to the volume reduction where Eu-Pd(Si) bond lengths shorten and, for the transition to happen, we trace the delicate balance between electronic bandwidth, crystal field splitting, Coulomb repulsion, Hund's coupling and spin-orbit coupling. In a next step we compare and benchmark our DFT results to surface-sensitive photoemission data in which the mixed-valent properties of EuPd$_2$Si$_2$ are reflected in a simultaneous observation of divalent and trivalent signals from the Eu $4f$ shell. The study serves as well to explore the limits of density functional theory and the choice of exchange correlation functionals to describe such a phenomenon as a valence transition.

cond-mat.str-el↗

arXiv2020

Symmetry-protected Spinful Magnetic Weyl Nodal Loops and Multi-Weyl Nodes in $5d^n$ Cubic Double Perovskites $(n=1,2)$

Using both an effective three-band model and {\it ab initio} calculations, we have investigated various topological features in the cubic ferromagnetic $5d^{1,2}$ systems showing large spin-orbit coupling (SOC): Ba$_2$NaOsO$_6$, Sr$_2$SrOsO$_6$, and Ba$_2$$B$ReO$_6$ ($B$= Mg, Zn). In the presence of time-reversal symmetry (${\cal T}$), spinless Dirac nodal loops linked to each other at the $W$ points appear in the mirror planes. Remarkably, breaking ${\cal T}$ leads to spinful magnetic Weyl nodal loops (MWNLs) that are robust even at large SOC and correlation strength $U$ variation due to the combination of mirror symmetry and broken ${\cal T}$. Additionally, there are two types of magnetic Weyl points with chiral charges $|χ|=1, 2$ along the $C_{4v}$ symmetry line, and another type-II MWNL encircling the zone center, that are dependent on $U$. Furthermore, the ferromagnetic Ba$_2$ZnReO$_6$ is an ideal half semimetal with MWNLs and magnetic Weyl nodes at the Fermi level without the interference of topologically trivial bulk states. These systems give rise to a remarkably large anomalous Hall conductivity $σ_{xy}$ of up to 1160 ($Ω$cm)$^{-1}$. Our findings may apply widely for $t_{2g}$ systems with cubic (or slightly distorted) fcc-like structures.

cond-mat.mtrl-sci↗

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↗

arXiv2016

Tuning ferromagnetic BaFe$_2$(PO$_4$)$_2$ through a high Chern number topological phase

There is strong interest in discovering or designing wide gap Chern insulators. Here we follow a Chern insulator to trivial Mott insulator transition versus interaction strength U in a honeycomb lattice Fe-based transition metal oxide, discovering that a spin-orbit coupling energy scale $ξ$=40 meV can produce and maintain a topologically entangled Chern insulating state against large band structure changes arising from an interaction strength U up to 60 times as large. Within the Chern phase the minimum gap switches from the zone corner K to the zone center $Γ$ while maintaining the topological structure. At a critical strength $U_c$, the continuous evolution of the electronic structure encounters a gap closing then reopening, upon which the system reverts to a trivial Mott insulating phase. This Chern insulator phase of honeycomb lattice Fe$^{2+}$ BaFe$_2$(PO$_4$)$_2$ corresponds to a large Chern number C=-3 that will provide enhanced anomalous Hall conductivity due to the associated three edge states threading through the bulk gap of 80 meV.

cond-mat.str-el↗

arXiv2015

Large orbital moment and spin-orbit enabled Mott transition in the Ising Fe honeycomb lattice BaFe2(PO4)2

BaFe2(PO4)2 is an unusual Ising insulating ferromagnet based on the Fe$^{2+}$ spin $S$ = 2 ion, the susceptibility of which suggests a large orbital component to the Fe local moment. We apply density functional theory based methods to obtain a microscopic picture of the competing interactions and the critical role of spin-orbit coupling (SOC) in this honeycomb lattice system. The low-temperature ferromagnetic phase displays a half-semimetallic Dirac point pinning the Fermi level and preventing gap opening before consideration of SOC, presenting a case in which correlation effects modeled by a repulsive Hubbard $U$ fail to open a gap. Simultaneous inclusion of both correlation and SOC drives a large orbital moment in excess of 0.7 $μ_B$ (essentially $L$ = 1) for spin aligned along the $\hat{c}$ axis, with a gap comparable with the inferred experimental value. The large orbital moment accounts for the large Ising anisotropy, in spite of the small magnitude of the SOC strength on the 3$d$ (Fe) ion. Ultimately, the Mott-Hubbard gap is enabled by degeneracy lifting by SOC and the large Fe moments, rather than by standard Hubbard interactions alone. We suggest that competing orbital occupations are responsible for the structural transitions involved in the observed re-entrant rhombohedral-triclinic-rhombohedral sequence.

cond-mat.str-el↗

arXiv2014

Unquenched $e_g^1$ orbital moment in the Mott insulating antiferromagnet KOsO4

Applying the correlated electronic structure method based on density functional theory plus the Hubbard $U$ interaction, we have investigated the tetragonal scheelite structure Mott insulator KOsO$_4$, whose $e_g^1$ configuration should be affected only slightly by spin-orbit couping (SOC). The method reproduces the observed antiferromagnetic Mott insulating state, populating the Os $d_{z^2}$ majority orbital. The quarter-filled $e_g$ manifold is characterized by a symmetry breaking due to the tetragonal structure, and the Os ion shows a crystal field splitting $Δ_{cf}$ = 1.7 eV from the $t_{2g}$ complex, which is relatively small considering the high formal oxidation state Os$^{7+}$. The small magnetocrystalline anisotropy before including correlation (i.e., in the metallic state) is increased by more than an order of magnitude in the Mott-insulating state, a result of a strong interplay between large SOC and a strong correlation. In contrast to conventional wisdom that the $e_g$ complex will not support orbital magnetism, we find that for the easy axis [100] direction the substantial Os orbital moment $M_L\approx-0.2 μ_B$ compensates half of the Os spin moment $M_S$ = 0.4$μ_B$. The origin of the orbital moment is analyzed and understood in terms of additional spin-orbital lowering of symmetry, and beyond that due to structural distortion, for magnetization along [100]. Further interpretation is assisted by analysis of the spin density and the Wannier function with SOC included.

cond-mat.str-el↗

arXiv2013

Effects of Magnetovolume and Spin-orbit Coupling in the Ferromagnetic Cubic Perovskite BaRuO3

BaRuO3 having five different crystal structures has been synthesized by varying the pressure while sintering. Contrary to the other phases being nonmagnetic, the cubic perovskite phase synthesized recently shows an itinerant ferromagnetic character. We investigated this ferromagnetic BaRuO3 using first principles calculations. A few van Hove singularities appear around the Fermi energy, causing unusually high magnetovolume effects of $ΔM/Δa$ ~ 4.3 $μ_B$/Åas well as a Stoner instability [IN(0) ~ 1.2]. At the optimized lattice parameter a, the magnetic moment M is 1.01 $μ_B$ in the local spin density approximation. When spin-orbit coupling is included, the topologies of some Fermi surfaces are altered, and the net moment is reduced by 10% to a value very close to the experimentally observed value of ~ 0.8 $μ_B$. Our results indicate that this ferromagnetism is induced by the Stoner instability, but the combined effects of the p-d hybridization, the magnetovolume, and the spin-orbit coupling determine the net moment. In addition, we briefly discuss the results of the tight-binding Wannier function technique.

cond-mat.mtrl-sci↗

arXiv2013

Structural and Correlation Effects in the Itinerant Insulating Antiferromagnetic Perovskite NaOsO3

The orthorhombic perovskite NaOsO3 undergoes a continuous metal-insulator transition (MIT), accompanied by antiferromagnetic (AFM) order at T_N=410 K, suggested to be an example of the rare Slater (itinerant) MIT. We study this system using ab initio and related methods, focusing on the origin and nature of magnetic ordering and the MIT. The rotation and tilting of OsO6 octahedra in the GdFeO3 structure result in moderate narrowing the band width of the t_{2g} manifold, but sufficient to induce flattening of bands and AFM order within the local spin density approximation (LSDA), where it remains metallic but with a deep pseudogap. Including on-site Coulomb repulsion U, at U_c ~2 eV a MIT occurs only in the AFM state. Effects of spin-orbit coupling (SOC) on the band structure seem minor as expected for a half-filled $t_{2g}^{3}$ shell, but SOC doubles the critical value U_c necessary to open a gap and also leads to large magnetocrystalline energy differences in spite of normal orbital moments no greater than 0.1$μ_B$. Our results are consistent with a Slater MIT driven by magnetic order, induced by a combination of structurally-induced band narrowing and moderate Coulomb repulsion, with SOC necessary for a full picture. Strong p-d hybridization reduces the moment, and when bootstrapped by the reduced Hund's rule coupling (proportional to the moment) gives a calculated moment of ~1 $μ_B$, consistent with the observed moment and only a third of the formal $d^3$ value. We raise and discuss one important question: since this AFM ordering is at q=0 (in the 20 atom cell) where nesting is a moot issue, what is the microscopic driving force for ordering and the accompanying MIT?

cond-mat.str-el↗