arXiv subjects

Chang-Jong Kang

Publications and source records attributed to Chang-Jong Kang.

At least 19 recordsLinked to original sources

arXiv2026

Higher-Order Topological Phase in the Two-Dimensional Type-IV Magnet MgCr$_2$O$_4$

Type-IV two-dimensional (2D) magnetism-a newly classified collinear magnetic phase featuring nonrelativistic spin degeneracy and spin-orbit-coupling-induced momentum-dependent spin splitting-extends the symmetry classification of collinear magnets, opening new opportunities for unconventional topological quantum states. Here, we reveal that the recently proposed two-dimensional type-IV 2D magnet MgCr$_2$O$_4$ hosts an intrinsic higher-order topological insulating phase, featuring $\mathcal{C}_{3z}$-protected corner states and a nontrivial rotational topological invariant of $χ^{(3)}$ = $\{-2,4\}$ with a quantized fractional corner charge of $4e/3$. Spin-orbit coupling breaks the spin-degeneracy-enforcing symmetry $[C_{2}||M_z]$ while preserving the crystalline $\mathcal{C}_{3z}$ rotational symmetry that protects the higher-order topological phase, thereby enabling spin splitting to coexist with the nontrivial topology. Furthermore, the higher-order topological phase remains intact throughout a wide range of biaxial strains without band-gap closing and topological phase transition, demonstrating the robustness of the symmetry-protected topological state against external perturbations. Our work establishes a direct connection between type-IV magnetic system and higher-order topology, providing a new route for symmetry-engineered magnetic topological quantum states.

physics.comp-ph↗

arXiv2026

Unconventional Mixed-Parity Magnetism in Rare-Earth Tetraborides

Altermagnetism has advanced the study of compensated magnets by revealing non-relativistic spin splitting, traditionally classified into strictly even- or odd-parity spin textures. Here, we unveil a fundamentally different regime: component-resolved mixed-parity spin splitting in a fully three-dimensional compensated magnet. Using first-principles calculations, tight-binding and $\mathbf{k} \cdot \mathbf{p}$ models, along with spin-group symmetry analysis, we demonstrate that the non-coplanar ground state of $\mathrm{TbB}_4$ enforces a unique momentum-space spin texture. The in-plane spin components exhibit odd-parity $p$- and $f$-wave-like textures, whereas the out-of-plane component retains an even-parity $d$-wave altermagnetic character. Crucially, the coexistence of the in-plane odd-parity textures is driven not by relativistic spin-orbit coupling, but by a staggered Berry phase arising from the inherent scalar spin chirality. This mixed-parity structure dictates distinct transport fingerprints, including bulk non-relativistic Edelstein and spin Hall responses, as well as a symmetry-allowed Berry curvature dipole. These results establish the rare-earth tetraborides as a robust platform for engineering complex spin-charge conversion phenomena.

cond-mat.str-el↗

arXiv2026

Floquet-Engineered Odd-Parity Altermagnetic Higher-Order Topology in a Two-Dimensional Antiferromagnet Cr$_2$CH$_2$

Periodic driving provides a platform to dynamically tailor quantum states of matter, yet its impact on symmetry-protected topological phases remains incompletely understood. Here, we demonstrate that periodic driving enables the realization of an odd-parity altermagnetic (AM) higher-order topological insulator (HOTI) phase in the Cr$_2$CH$_2$ monolayer. In equilibrium, Cr$_2$CH$_2$ is a 2D antiferromagnetic (AFM) HOTI protected by $\mathcal C_3$ rotational symmetry, characterized by a symmetry indicator $χ^{(3)}$ = $\{-2,1\}$ and robust corner states. Under circularly polarized light (CPL), the system develops a f-wave altermagnetic state governed by the symmetry $[C_{2}||\overline{3}_{001}]$ with odd-parity spin splitting. Despite substantial Floquet-induced band renormalization, the $\mathcal C_3$-protected corner states remain intact over a broad range of driving strengths, highlighting the altermagnetic higher-order topology under Floquet driving. As the light intensity increases, the system gradually evolves into an altermagnetic semimetallic state. These results establish a direct connection between magnetism and topology in a periodically driven AFM system, offering a route toward the control of coupled spin and topological transport.

physics.comp-ph↗

arXiv2026

Second-order topology in two-dimensional azulenoid kekulene carbon lattices

The discovery of higher-order topological insulator (HOTI) has established a new paradigm for understanding symmetry-constrained boundary electronic states. Here, based on first-principles calculations, we demonstrate the emergence of HOTI phase in organic lattices of two-dimensional azulenoid-kekulene-type carbon allotropes, namely AKC-[3,3] and AKC-[6,0]. Enabled by the $C_6$ rotational symmetry, the nontrivial bulk topology is confirmed through the topological invariant and fractionally quantized corner charge, giving $\{[M^{(I)}_{2}],[K^{(3)}_{2}]\}$ = $\{0,2\}$ and $Q_{\mathrm{corner}} = e/3$, respectively, accompanied by the emergence of exotic corner states in nanoflakes. Notably, the structural modifications are explored, revealing that in the derived structure PAK-[6,0], whose corner-localized states are preserved, highlighting the robustness of the higher-order topological phase. These findings highlight azulenoid-kekulene-based carbon allotropes as a promising platform to explore the interplay between structural design, crystalline symmetry, and higher-order topological boundary responses in two dimensional carbon systems.

cond-mat.mtrl-sci↗

arXiv2025

Role of On-site and Inter-site Coulomb Interactions in KV$_3$Sb$_5$: A first-principles DFT+$U$+$V$ study

Nonlocal Coulomb interactions play a crucial role in stabilizing distinct electronic phases in kagome materials. In this work, we systematically investigate the effects of on-site ($U$) and inter-site ($V$) Coulomb interactions on the electronic structure and stability of charge-density-wave (CDW) phases in the kagome metal KV$_3$Sb$_5$ using density functional theory (DFT+$U$+$V$) calculations. We demonstrate that $V$ promotes the formation and stability of CDW phases, whereas $U$ suppresses these phases, highlighting a fundamental competition between local and nonlocal Coulomb interactions. By directly comparing our theoretical results with angle-resolved photoemission spectroscopy (ARPES) data, we identify realistic values of $U$ and $V$ that accurately describe the electronic band structure of KV$_3$Sb$_5$. Our findings establish a detailed $U$-$V$ phase diagram for KV$_3$Sb$_5$, offering valuable insights into the correlated electronic states in kagome metals and serving as a foundation for future explorations of correlation-driven phenomena in related materials.

cond-mat.str-el↗

arXiv2024

Low-energy perspective on two-orbital Hund metals and the case of LaNiO2

The Hund-metal route to strong correlations continues to attract large interest in the condensed-matter community. The question arose to what extent it applies to the infinite-layer nickelates and, as a related question, to two-orbital systems in general. Here, we provide a low-energy perspective on this topic through a dynamical mean-field study using the numerical renormalization group (NRG) as a real-frequency impurity solver. We find that the RG flow from high to low energy is a uniquely adequate tool to reveal two-stage Kondo screening (2SKS), a fascinating mechanism for Hund physics. Further, we show that 2SKS takes place in a quarter-filled two-orbital system, but can be easily suppressed by a sufficiently large crystal-field splitting. We apply these insights to LaNiO2 using a recently proposed two-orbital model and show that it is indeed the crystal-field splitting that suppresses multiorbital phenomena in this scenario. Our general findings open the way for further explorations of 2SKS, and we propose a way of reviving low-energy Hund physics in LaNiO2 by counteracting the crystal field.

cond-mat.str-el↗

arXiv2024

Exploring the role of nonlocal Coulomb interactions in perovskite transition metal oxides

Employing the density functional theory incorporating on-site and inter-site Coulomb interactions (DFT+U+V), we have investigated the role of the nonlocal interactions on the electronic structures of the transition metal oxide perovskites. Using constrained random phase approximation calculations, we derived screened Coulomb interaction parameters and revealed a competition between localization and screening effects, which results in nonmonotonic behavior with d-orbital occupation. We highlight the significant role and nonlocality of inter-site Coulomb interactions, V, comparable in magnitude to the local interaction, U. Our DFT+U+V results exemplarily show the representative band renormalization, and deviations from ideal extended Hubbard models due to increased hybridization between transition metal d and oxygen p orbitals as occupation increases. We further demonstrate that the inclusion of the inter-site V is essential for accurately reproducing the experimental magnetic order in transition metal oxides.

cond-mat.str-el↗

arXiv2024

Connection between f-electron correlations and magnetic excitations in UTe2

The detailed anisotropy of the low-temperature, low-energy magnetic excitations of the candidate spin-triplet superconductor UTe$_2$ is revealed using inelastic neutron scattering. The magnetic excitations emerge from the Brillouin zone boundary at the high symmetry $Y$ and $T$ points and disperse along the crystallographic $\hat{b}$-axis. In applied magnetic fields to at least $μ_0 H=11$~T along the $\hat{c}-$axis, the magnetism is found to be field-independent in the $(hk0)$ plane. The scattering intensity is consistent with that expected from U$^{3+}$/U$^{4+}$ $f$-electron spins with preferential orientation along the crystallographic $\hat{a}$-axis, and a fluctuating magnetic moment of 2.3(7) $μ_B$. These characteristics indicate that the excitations are due to intraband spin excitons arising from $f$-electron hybridization.

cond-mat.str-el↗

arXiv2024

Weyl Fermion with various chiralities in a f-electron ferromagnetic system: PrB4

Rare-earth tetraborides ($R$B$_{4}$) have attracted a lot of recent attention due to their intriguing electronic, magnetic, and topological properties. We have theoretically investigated topological properties of PrB$_{4}$, which is unique among $R$B$_{4}$ family due to its ferromagnetic ground state. We have discovered that PrB$_{4}$ is an intrinsic magnetic Weyl system possessing multiple topological band crossings with various chiral charges. Density-functional-theory band calculations combined with tight-binding band analysis reveal large Fermi-arc surface states, which are characteristic fingerprints of Weyl fermions. Anomalous Hall conductivity is estimated to be very large, ranging from 500 to 1000 ($Ω\cdot$cm)$^{-1}$ near the Fermi level, which also demonstrates the topological Weyl character of ferromagnetic PrB$_{4}$. These findings suggest that PrB$_{4}$, being a potential candidate of magnetic Weyl system, would be a promising rare-earth topological system for applications to next-generation spintronic and photonic devices.

cond-mat.str-el↗

arXiv2023

Broken Kramers' degeneracy in altermagnetic MnTe

Altermagnetism is a newly identified fundamental class of magnetism with vanishing net magnetization and time-reversal symmetry broken electronic structure. Probing the unusual electronic structure with nonrelativistic spin splitting would be a direct experimental verification of altermagnetic phase. By combining high-quality film growth and $in~situ$ angle-resolved photoemission spectroscopy, we report the electronic structure of an altermagnetic candidate, $α$-MnTe. Temperature dependent study reveals the lifting of Kramers{\textquoteright} degeneracy accompanied by a magnetic phase transition at $T_N=267\text{ K}$ with spin splitting of up to $370\text{ meV}$, providing direct spectroscopic evidence for altermagnetism in MnTe.

cond-mat.mtrl-sci↗

arXiv2022

Mn(Pt$_{1-x}$Pd$_{x}$)$_5$P: Isovalent Tuning of Mn Sublattice Magnetic Order

We report the growth and characterization of MnPd$_5$P, a ferromagnet with T$_C$ $\approx$ 295 K, and conduct a substitutional study with its antiferromagnetic analogue MnPt$_5$P. We grow single crystals of MnPd$_5$P and Mn(Pt$_{1-x}$Pd$_x$)$_5$P by adding Mn into (Pt$_{1-x}$Pd$_{x}$)-P based melts. All compounds in the family adopt the layered anti-CeCoIn$_5$ structure with space group P4/mmm, and EDS and XRD results indicate that MnPt$_5$P and MnPd$_5$P form a solid solution. Based on magnetization and resistance data, we construct a T-x phase diagram for Mn(Pt$_{1-x}$Pd$_x$)$_5$P and demonstrate the antiferromagnetic order found in MnPt$_5$P is extraordinarily sensitive to Pd substitution. At low Pd fractions (x $<$ 0.010), the single antiferromagnetic transition in pure MnPt$_5$P splits into a higher temperature ferromagnetic transition followed on cooling by a lower temperature ferromagnetic to antiferromagnetic transition and then by a re-entrant antiferromagnetic to ferromagnetic transition at lower temperatures. The antiferromagnetic region makes up a bubble that persists to x $\approx$ 0.009 for T $\approx$ 150 K, with all samples x $<$ 0.009 recovering their initial ferromagnetic state with further cooling to base temperature. Over the same low x range we find a non-monotonic change in the room temperature unit cell volume, further suggesting that pure MnPt$_5$P is close to an instability. Once x $>$ 0.010, Mn(Pt$_{1-x}$Pd$_x$)$_5$P undergoes a single ferromagnetic transition. The Curie temperature increases rapidly with x, rising from T$_C$ $\approx$ 197 K at x = 0.013 to a maximum of T$_C$ $\approx$ 312 K for x $\approx$ 0.62, and then falls back to T$_C$ $\approx$ 295 K for pure MnPd$_5$P (x = 1). Given that Pt and Pd are isoelectronic, this work raises questions as to the origin of the extreme sensitivity of the magnetic ground state in MnPt$_5$P upon introducing Pd.

cond-mat.mtrl-sci↗

arXiv2022

Tunable electronic and magnetic phases in layered ruthenates: SrRuO3-SrTiO3 heterostructure upon strain

Layered ruthenates are a unique class of systems which manifests a variety of electronic and magnetic features emerged from competing energy scales. At the heart of such features lies the multi-orbital physics, especially, the orbital-selective behavior. Here, we propose that the SrRuO3-SrTiO3 heterostructure is a highly tunable platform to obtain the various emergent properties. Employing the density functional theory plus dynamical mean-field theory, we thoroughly investigate the orbital-dependent physics of the system and identify the competing magnetic fluctuations. We show that the epitaxial strain drives the system towards multi-orbital or orbital selective Mott phases from the Hund metal regime. At the same time, the two different types of static magnetism are stabilized, ferromagnetism and checkerboard antiferromagnetism, from the competition with the spin-density wave instability.

cond-mat.str-el↗

arXiv2022

Ferroelectricity driven by magnetism in quasi-one-dimensional Ba9Fe3Se15

The spin-induced ferroelectricity in quasi-1D spin chain system is little known, which could be fundamentally different from those in three-dimensional (3D) system. Here, we report the ferroelectricity driven by a tilted screw spin order and its exotic dynamic in the spin-chain compound Ba9Fe3Se15. It is found that the spin-induced polarization has already occurred and exhibits magnetoelectric coupling behavior far above the long-range spin order (LRSO) at TN = 14 K. The polarized entities grow and their dynamic responses slow down gradually with decreasing temperature and permeate the whole lattice to form 3D ferroelectricity at TN. Our results reveal that the short-range spin orders (SRSOs) in the decoupled chains play a key role for the exotic dynamic in this dimension reduced system. Ba9Fe3Se15 is the only example so far which exhibits electric polarization above LRSO temperature because of the formation of SRSOs.

cond-mat.str-el↗

arXiv2022

Preparation of large Cu3Sn single crystal by Czochralski method

Cu3Sn was recently predicted to host topological Dirac fermions, but related research is still in its infancy. The growth of large and high-quality Cu3Sn single crystals is, therefore, highly desired to investigate the possible topological properties. In this work, we report the single crystal growth of Cu3Sn by Czochralski (CZ) method. Crystal structure, chemical composition, and transport properties of Cu3Sn single crystals were analyzed to verify the crystal quality. Notably, compared to the mm-sized crystals from a molten Sn-flux, the cm-sized crystals obtained by the CZ method are free from contamination from flux materials, paving the way for the follow-up works.

cond-mat.mtrl-sci↗

arXiv2022

Competing spin-fluctuations in Sr$_2$RuO$_4$ and their tuning through epitaxial strain

In this study, we report the magnetic energy landscape of Sr2RuO4 employing the generalized Bloch approach within density functional theory. We identify the two dominant magnetic instabilities, ferromagnetic and spin-density-wave, together with other predominant instabilities. We show that epitaxial strain can change the overall magnetic tendency of the system, and tune the relative weight of the various magnetic instabilities in the system. Especially, the balance between spin-density wave and ferromagnetic instabilities can be controlled by the strain, and, eventually can lead to the new magnetic phases as well as superconducting phases with possibly altered pairing channels. Our findings are compared with previous theoretical models and experimental reports for the various magnetic features of the system and offer a first-principles explanation to them.

cond-mat.str-el↗

arXiv2022

Coexisting Kondo hybridization and itinerant f-electron ferromagnetism in UGe2

Kondo hybridization in partially filled f-electron systems conveys significant amount of electronic states sharply near the Fermi energy leading to various instabilities from superconductivity to exotic electronic orders. UGe2 is a 5f heavy fermion system, where the Kondo hybridization is interrupted by the formation of two ferromagnetic phases below a 2nd order transition Tc ~ 52 K and a crossover transition Tx ~ 32 K. These two ferromagnetic phases are concomitantly related to a spin-triplet superconductivity that only emerges and persists inside the magnetically ordered phase at high pressure. The origin of the two ferromagnetic phases and how they form within a Kondo-lattice remain ambiguous. Using scanning tunneling microscopy and spectroscopy, we probe the spatial electronic states in the UGe2 as a function of temperature. We find a Kondo resonance and sharp 5f-electron states near the chemical potential that form at high temperatures above Tc in accordance with our density functional theory (DFT) + Gutzwiller calculations. As temperature is lowered below Tc, the resonance narrows and eventually splits below Tx dumping itinerant f-electron spectral weight right at the Fermi energy. Our findings suggest a Stoner mechanism forming the highly polarized ferromagnetic phase below Tx that itself sets the stage for the emergence of unconventional superconductivity at high pressure.

cond-mat.str-el↗

arXiv2022

Dynamical mean-field theory study of a ferromagnetic CrI3 monolayer

We have employed one of the well-known many-body techniques, density functional theory plus dynamical mean-field theory (DFT + DMFT), to investigate the electronic structure of ferromagnetic monolayer CrI3 as a function of temperature and hole-doping concentration. The computed magnetic susceptibility follows the Curie's law, indicating that the ferromagnetism of monolayer CrI3 originates from localized magnetic moments of Cr atoms rather than Stoner-type itinerant ones. The DFT + DMFT calculations show a different coherent temperature for each spin component, demonstrating apparent strong spin-dependent electronic correlation effects in monolayer CrI3. Furthermore, we have explored the doping-dependent electronic structure of monolayer CrI3 and found that its electronic and magnetic properties are easily tunable by the hole-doping.

cond-mat.mtrl-sci↗

arXiv2022

Theory-guided investigation on magnetic evolution of MnPt$_{5-x}$Pd$_x$P and discovery of anti-CeCoIn$_5$-type ferromagnetic MnPd$_5$P

We report the magnetic changes from canted antiferromagnetic to ferromagnetic orderings in anti-115-type MnPt$_{5-x}$Pd$_x$P ($x$ = 1, 2, 2.5, 3, 4, and 5) and the discovery of a new rare-earth-free ferromagnet, MnPd$_5$P by both theoretical prediction and experimental investigation. The family compounds were synthesized using high temperature solid state method and characterized to crystalize in the anti-CeCoIn$_5$ type with the space group P4/mmm exhibiting a two-dimensional layered structural feature. The magnetic property measurements indicate that the compounds ordered from canted A-type antiferromagnet in MnPt$_5$P to ferromagnet above the room temperature with varying degrees of coercivity and magnetic moments in MnPd$_5$P by reducing the spin orbital coupling. The results of the MnPt$_{5-x}$Pd$_x$P have been analyzed in comparison to the other candidates of the 151 family of Mn(Pt/Pd)$_5$(P/As) to understand the complex structure-magnetism relationships.

cond-mat.mtrl-sci↗