arXiv subjects

Myung Joon Han

Publications and source records attributed to Myung Joon Han.

At least 19 recordsLinked to original sources

arXiv2026

Plaid-Like Spin Splitting and Chirality of Magnon Bands in Antiferromagnetic MnTe$_2$

Altermagnets constitute an emerging class of magnetic materials that combine compensated antiferromagnetic order with spin-split excitations arising from crystalline symmetries. Despite strong theoretical interest, their experimental identification remains challenging. Here, we demonstrate that helicity- and angle-resolved Raman scattering measurements reveal reduced rotational symmetries of magnons and a pronounced imbalance between left- and right-circular polarization channels, indicating momentum-dependent magnon handedness. First-principles DFT+$U$ calculations combined with linear spin-wave theory uncover a characteristic plaid-like spin-splitting structure in momentum space. The resulting magnon spin textures are dictated by the unconventional sublattice symmetries of MnTe$_2$ and closely emulate those of altermagnetic electronic bands. Our work provides evidence of chiral spin-wave excitations unique to this non-coplanar antiferromagnet.

cond-mat.str-el↗

arXiv2026

Many-body electronic structure, self-doped double-exchange, and Hund metallicity in 1T-CrTe2 bulk and monolayer

The van der Waals (vdW) ferromagnet 1T-CrTe2 is an emerging spintronics platform, notable for its high Curie temperature (Tc) and intriguing transport properties. However, the fundamental interplay between the electron correlations and magnetism underlying its high Tc still remains elusive. Here, using density functional theory plus dynamical mean-field theory (DFT+DMFT), we identify 1T-CrTe2 as a self-doped double-exchange ferromagnet with pronounced Hund metallicity. This identification is grounded in the first detailed analysis of its many-body electronic structure, which reveals a dual electronic nature of Cr-d orbitals where itinerant eg electrons coexist with localized t2g moments. The interaction between these orbitals, mediated by Hund's coupling, drives the double-exchange ferromagnetism, establishing 1T-CrTe2 as a Hund metal reminiscent of orbital-selective Mott systems. In the monolayer limit, while this physical picture persists, structural deformation, rather than reduced dimensionality, notably reduces Tc.Our findings offer a new perspective on the high-Tc ferromagnetism in 1T-CrTe2, a mechanism potentially pivotal for other correlated two-dimensional vdW metallic magnets.

cond-mat.str-el↗

arXiv2026

Ab initio screening of quantum frustrated materials with kagome and triangular geometries

Geometrical frustration is a powerful route to realize exotic phases such as quantum spin liquids. Despite extensive efforts, systematic searches targeting specific frustration motifs and their potential to host unconventional magnetic ground states remain rare, thus highlighting the need for a more focused and predictive materials discovery approach. Here we present a new strategy combining high-throughput first-principles calculations, magnetic force theory, and spin Hamiltonian analysis. Starting from the 150,000 material database, we catalogue candidate materials that may host competing exchange interactions and new types of magnetic states with the focus on kagome or triangular lattices. Our workflow not only reproduces the majority of known frustrated magnetic materials, validating our approach, but also predicts novel candidate compounds with targeted frustration profiles that have not yet been experimentally synthesized. Among these, we identify six promising new materials: one triangular lattice compound, KMgNiIO6, and five kagome lattice compounds; Li4Fe3WO8, Li2V3F8, Li5VP2(O4F)2, and Li2MgCo3O8 (P2/m and C2/m). For each candidate, we identify detailed magnetic properties and further propose their potential magnetic ground states, revealing that some of them may host entirely new magnetic phases driven by their distinct frustration characteristics.

cond-mat.str-el↗

arXiv2025

Fully Generalized Spin Models with Strain Effects of Kitaev Spin Liquid Candidate Materials

The $KJΓΓ'$ spin model-originally derived for an ideal $P\bar{3}1m$ symmetric geometry-has long served as a central framework for understanding candidate Kitaev materials. In realistic crystals, however, this ideal geometry is seldom realized, either at low temperatures or under external perturbations, limiting the model's quantitative applicability. Here we introduce a fully generalized spin model, denoted $ε$-$KJΓΓ'$, that explicitly incorporates arbitrary lattice deformations $ε$. All spin-exchange interactions and their strain-dependent coefficients are obtained from density-functional theory (DFT) calculations and a microscopic derivation of coupling constants for materials based on $d^5$ transition-metal ions. For $α$-RuCl$_3$ under a strain of $3\%$, new emergent exchange channels acquire magnitudes comparable to their unstrained counterparts. Building on these parameters, we investigate strain-driven quantum phase transitions between competing magnetic states-including the zigzag order and the Kitaev quantum spin liquid (KQSL)-and identify a strain-induced topological transition within the KQSL states that offers a practical diagnostic of Kitaev physics. Furthermore, our symmetry analysis of the $ε$-$KJΓΓ'$ model is applicable to both $d^{5}$ ions, such as $α$-RuCl$_3$, and $d^{7}$ systems, including cobalt-based compounds.

cond-mat.str-el↗

arXiv2025

Polymorphic spin ordering in a single-crystalline cobalt-doped Fe3GaTe2

A single crystalline system typically stabilizes a unique state for spin ordering below a critical temperature. Certain materials exhibit multiple magnetic states, driven by structural phase transitions under varying thermodynamic conditions. Recently, van der Waals magnets have demonstrated subtle interlayer exchange interactions, offering a promising approach to electrically control spin states without structural transformation. Here, we report the emergence of three distinct magnetic states, ferromagnetic ordering and both collinear and non-collinear antiferromagnetic orderings, in a layered single crystalline magnet, cobalt-doped Fe3GaTe2 ((Co, Fe)3GaTe2). These three magnetic phases occur without structural phase transitions, a phenomenon we designate as polymorphic spin ordering in the material. The introduction of 16% Co-doping in Fe3GaTe2 modulates the interlayer magnetic interaction, enabling multiple spin orderings within the same lattice system with three critical temperatures: a Curie temperature for a ferromagnetic state (Tc=210 K) and two Neel temperatures for the collinear (TN1=110 K) and non-collinear (TN2=30 K) antiferromagnetic states. Our findings are supported by magnetic force microscopy, first-principles calculations, and circular dichroism angular photoemission spectroscopy, which reveals varying spin ordering and changes in the topological band structure and Berry curvature at different temperatures within the single-crystalline (Co, Fe)3GaTe2.

cond-mat.mtrl-sci↗

arXiv2024

Layer-dependent magnetic property in a superconducting quintuple-layer nickelate La6Ni5O12

To investigate the detailed magnetic properties of a recently discovered superconducting nickelate Nd6Ni5O12, we performed the first-principles electronic structure calculation based on density functional theory. The band dispersion, electronic charge distribution and the magnetic moment are computed with La substituted for Nd, and compared with another structural type of nickel-based superconducting material, namely, RNiO2 (R: rare-earth elements). In particular, we estimated the magnetic exchange interaction strength based on magnetic force theory. Our results show that the inter-atomic magnetic couplings are notably reduced by intrinsic hole doping from the blocking fluorite slab which validates the conventional view of regarding Nd6Ni5O12 as a doped case of its infinite-layer counterpart. At the same time, however, the interactions are markedly layer-dependent. The outer most NiO2 layer adjacent to the blocking fluorite roughly corresponds to the 20% chemical doping in the infinite-layer material whereas the inner layers have stronger couplings. The long-range nature and the out-of-plane interactions are also presented. Our results provide useful information to understand this new superconducting nickelate whose intrinsic layer structure is obviously distinctive.

cond-mat.supr-con↗

arXiv2024

Inversion and Tunability of Van Hove Singularities in $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, and Cs) kagome metals

To understand the alkali-metal-dependent material properties of recently discovered $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, and Cs), we conducted a detailed electronic structure analysis based on first-principles density functional theory calculations. Contrary to the case of $A$ = K and Rb, the energetic positions of the low-lying Van Hove singularities are reversed in CsV$_{3}$Sb$_{5}$, and the characteristic higher-order Van Hove point gets closer to the Fermi level. We found that this notable difference can be attributed to the chemical effect, apart from structural differences. Due to their different orbital compositions, Van Hove points show qualitatively different responses to the structure changes. A previously unnoticed highest lying point can be lowered, locating close to or even below the other ones in response to a reasonable range of bi- and uni-axial strain. Our results can be useful in better understanding the material-dependent features reported in this family and in realizing experimental control of exotic quantum phases.

cond-mat.supr-con↗

arXiv2023

Singular Hall response from a correlated ferromagnetic flat nodal-line semimetal

Topological quantum phases have been largely understood in weakly correlated systems, which have identified various quantum phenomena such as spin Hall effect, protected transport of helical fermions, and topological superconductivity. Robust ferromagnetic order in correlated topological materials particularly attracts attention, as it can provide a versatile platform for novel quantum devices. Here, we report singular Hall response arising from a unique band structure of flat topological nodal lines in combination with electron correlation in an itinerant, van der Waals ferromagnetic semimetal, Fe3GaTe2, with a high Curie temperature of Tc=360 K. High anomalous Hall conductivity violating the conventional scaling, resistivity upturn at low temperature, and a large Sommerfeld coefficient are observed in Fe3GaTe2, which implies heavy fermion features in this ferromagnetic topological material. Our circular dichroism in angle-resolved photoemission spectroscopy and theoretical calculations support the original electronic features in the material. Thus, low-dimensional Fe3GaTe2 with electronic correlation, topology, and room-temperature ferromagnetic order appears to be a promising candidate for robust quantum devices.

cond-mat.str-el↗

arXiv2023

Tetrahedral triple-Q magnetic ordering and large spontaneous Hall conductivity in the metallic triangular antiferromagnet Co1/3TaS2

The triangular lattice antiferromagnet (TLAF) has been the standard paradigm of frustrated magnetism for several decades. The most common magnetic ordering in insulating TLAFs is the 120 structure. However, a new triple-Q chiral ordering can emerge in metallic TLAFs, representing the short wavelength limit of magnetic skyrmion crystals. We report the metallic TLAF Co1/3TaS2 as the first example of tetrahedral triple-Q magnetic ordering with the associated topological Hall effect (non-zero σ_{xy}(H=0)). We also present a theoretical framework that describes the emergence of this magnetic ground state, which is further supported by the electronic structure measured by angle-resolved photoemission spectroscopy. Additionally, our measurements of the inelastic neutron scattering cross section are consistent with the calculated dynamical structure factor of the tetrahedral triple-Q state.

cond-mat.str-el↗

arXiv2023

Fingerprints for anisotropic Kondo lattice behavior in the quasiparticle dynamics of the kagome metal Ni$_3$In

We present a temperature- and polarization-resolved phononic and electronic Raman scattering study in combination with the first-principles calculations on the kagome metal Ni$_3$In with anisotropic transport properties and non-Fermi liquid behavior. At temperatures below 50 K and down to 2 K, several Raman phonon modes, including particularly an interlayer shear mode, exhibit appreciable frequency and linewidth renormalization, reminiscent of the onset of the Kondo screening without an accompanying structural or magnetic phase transition. In addition, a low-energy electronic continuum observed in polarization perpendicular to the kagome planes reveals strong temperature dependence below 50 K, implying thermal depletion of incoherent quasiparticles, while the in-plane continuum remains invariant. These concomitant electronic and phononic Raman signatures suggest that Ni$_3$In undergoes an anisotropic electronic crossover from an incoherent to a coherent Kondo lattice regime below 50 K. We discuss the origin of the anisotropic incoherent-coherent crossover in association with the possible anisotropic Kondo hybridization involving localized Ni-$3d_{xz}$ flat-band electrons.

cond-mat.str-el↗

arXiv2023

Frozen spin ratio and the detection of Hund correlations

We propose a way to identify strongly Hund-correlated materials by unveiling a key signature of Hund correlations at the two-particle level. The defining feature is the {\it sign} of the response of the {\it frozen spin ratio} (the long-time local spin-spin correlation function divided by the instantaneous value) under variation of electron density. The underlying physical reason is that the sign is closely related to the strength of charge fluctuations between the dominant atomic multiplets and higher-spin ones in a neighboring charge subspace. It is the predominance of these fluctuations that promotes Hund metallicity. The temperature dependence of the frozen spin ratio can further reveal a non-Fermi-liquid behavior and thus the Hund metal states. We analyze both degenerate and non-degenerate multiorbital Hubbard models and corroborate our argument by taking doped La$_2$CuO$_4$ and LaFeAsO as representative material examples, respectively, of Mott and Hund metals. Our proposal should be applicable to systems with non-half-filled integer electron fillings and their doped cases provided the doping drove the electron density toward the half filling.

cond-mat.str-el↗

arXiv2023

Epitaxially strained ultrathin LaNiO$_3$/LaAlO$_3$ and LaNiO$_3$/SrTiO$_3$ superlattices: a density functional theory + $U$ study

By employing first-principles electronic structure calculations we investigate nickelate superlattices [LaNiO$_3$]$_1$/[LaAlO$_3$]$_1$ and [LaNiO$_3$]$_1$/[SrTiO$_3$]$_1$ with (001) orientation under epitaxial tensile strain. Within density functional theory augmented by mean-field treatement of on-site electronic correlations, the ground states show remarkable dependence on the correlation strength and the strain. In the weakly and intermediately correlated regimes with small epitaxial strain, the charge-disproportionated insulating states with antiferromagneitc order is favored over the other orbital and spin ordered phases. On the other hand, in the strongly correlated regime or under the large tensile strain, ferromagnetic spin states with Jahn-Teller orbital order become most stable. The effect from polar interfaces in LaNiO$_3$]$_1$/[SrTiO$_3$]$_1$ is found to be noticeable in our single-layered geometry. Detailed discussion is presented in comparison with previous experimental and theoretical studies.

cond-mat.str-el↗

arXiv2022

$\mathrm{Fe_3GeTe_2}$: A site-differentiated Hund metal

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has lately attracted considerable attention from the point of view of both fundamental science and device applications. Obviously, establishing the detailed and solid understanding of their magnetism is the key first step toward various applications. Although $\mathrm{Fe_3GeTe_2}$ is a representative ferromagnetic (FM) metal in this family, many aspects of its magnetic and electronic behaviors still remain elusive. Here, we report our new finding that $\mathrm{Fe_3GeTe_2}$ is a special type of correlated metal known as 'Hund metal'. Furthermore, we demonstrate that Hund metallicity in this material is quite unique by exhibiting remarkable site-dependence of Hund correlation strength, hereby dubbed 'site-differentiated Hund metal'. Within this new picture, many of previous experiments can be clearly understood including the ones that were seemingly contradictory to one another.

cond-mat.str-el↗

arXiv2022

Intertwining orbital current order and superconductivity in Kagome metal

The nature of superconductivity in newly discovered Kagome materials, $\text{AV}_3\text{Sb}_5$ (A=K, Rb, Cs), has been a subject of intense debate. Recent experiments suggest the presence of orbital current order on top of the charge density wave (CDW) and superconductivity. Since the orbital current order breaks time-reversal symmetry, it may fundamentally affect possible superconducting states. In this work, we investigate the mutual influence between the orbital current order and superconductivity in Kagome metal with characteristic van Hove singularity (vHS). By explicitly deriving the Landau-Ginzburg theory, we classify possible orbital current order and superconductivity. It turns out that distinct unconventional superconductivities are expected, depending on the orbital current ordering types. Thus, this information can be used to infer the superconducting order parameter when the orbital current order is identified and vice versa. We also discuss possible experiments that may distinguish such superconducting states coexisting with the orbital current order.

cond-mat.supr-con↗

arXiv2022

Strain engineering and the hidden role of magnetism in monolayer VTe$_2$

Two-dimensional transition metal dichalcogenides have attracted great attention recently. Motivated by a recent study of crystalline bulk VTe$_2$, we theoretically investigated the spin-charge-lattice interplay in monolayer VTe$_2$. To understand the controversial experimental reports on several different charge density wave ground states, we paid special attention to the 'hidden' role of antiferromagnetism as its direct experimental detection may be challenging. Our first-principles calculations show that the 4$\times$1 charge density wave and the corresponding lattice deformation are accompanied by the 'double-stripe' antiferromagnetic spin order in its ground state. This phase has not only the lowest total energy but also the dynamical phonon stability, which supports a group of previous experiments. Interestingly enough, this ground state is stabilized only by assuming the underlying spin order. By noticing this intriguing and previously unknown interplay between magnetism and other degrees of freedom, we further suggest a possible strain engineering. By applying tensile strain, monolayer VTe$_2$ exhibits phase transition first to a different charge density wave phase and then eventually to a ferromagnetically ordered one.

cond-mat.mtrl-sci↗

arXiv2022

Non-local Coulomb interaction and correlated electronic structure of TaS$_2$: A GW+EDMFT study

By means of $ab~initio$ computation schemes, we examine the low-energy electronic structure of monolayer TaS$_2$ in its low-temperature commensurate charge-density-wave structure. We estimate and take into account both local and non-local Coulomb correlations within cRPA (constrained random phase approximation) and GW+EDMFT (GW plus extended dynamical mean-field theory) method. Mott nature of its insulating phase is clearly identified. By increasing the level of nonlocal approximation from DMFT ($V=0$) to EDMFT and GW+EDMFT, a systematic change of charge screening effects is clearly observed while its quantitative effect on the electronic structure is small in the realistic Mott state.

cond-mat.str-el↗

arXiv2022

Theory of Moire Magnets and Topological Magnons: Applications to Twisted Bilayer CrI3

We develop a comprehensive theory of twisted bilayer magnetism. Starting from the first-principles calculations of two-dimensional honeycomb magnet CrI3, we construct the generic spin models that represent a broad class of twisted bilayer magnetic systems. Using Monte-Carlo method, we discover a variety of non-collinear magnetic orders and topological magnons that have been overlooked in the previous theoretical and experimental studies. As a function of the twist angle, the collinear magnetic order undergoes the phase transitions to the non-collinear order and the magnetic domain phase. In the magnetic domain phase, we find that the spatially varying interlayer coupling produces the magnetic skyrmions even in the absence of the Dzyaloshinskii-Moriya interactions. In addition, we describe the critical phenomena of the magnetic phase transitions by constructing the field theoretical model of the moire magnet. Our continuum model well-explains the nature of the phase transitions observed in the numerical simulations. Finally, we classify the topological properties of the magnon excitations. The magnons in each phases are characterized by the distinct mass gaps with different physical origins. In the collinear ferromagnetic order, the higher-order topological magnonic insulator phase occurs. It serves as a unique example of the higher-order topological phase in magnonic system, since it does not require non-collinear order or asymmetric form of the interactions. In the magnetic domain phases, the magnons are localized along the domain wall and form one-dimensional topological edge mode. As the closed domain walls deform to a open network, the confined edge mode extends to form a network model of the topological magnons.

cond-mat.str-el↗

arXiv2022

Crucial role of out-of-plane Sb-$p$ orbitals in Van Hove singularity formation and electronic correlation for superconducting Kagome metal CsV$_3$Sb$_5$

First-principles density functional theory calculations are performed to understand the electronic structure and interaction parameters for recently discovered superconducting Kagome metal CsV$_3$Sb$_5$. A systematic analysis of the tight-binding parameters based on maximally localized Wannier function method demonstrates that the out-of-plane Sb$^{\rm out}$-$p$ orbital is a key element in complete description of the three Van Hove singularity structures known in this material at $M$ point near the Fermi level. Further, the correlation strengths are also largely determined by Sb$^{\rm out}$-$p$ states. Based on constrained random phase approximation, we find that on-site and inter-site interaction parameter are both significantly affected by the screening effect of Sb$^{\rm out}$-$p$ orbitals. As the role of this previously unnoticed orbital state can be tuned or controlled by out-of-plane lattice parameters, we examine the electronic structure and particularly the evolution of Van Hove singularity points as a function of strain and pressure, which can serve as useful knobs to control the material properties.

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