arXiv subjects

Do Hoon Kiem

Publications and source records attributed to Do Hoon Kiem.

7 recordsLinked to original sources

arXiv2025

Interpretable Graph Neural Networks for Classifying Structure and Magnetism in Delafossite Compounds

Delafossites (ABC2, where A and B are metals and C is a chalcogen) are a versatile family of quantum materials and layered oxides/chalcogenides whose properties are highly sensitive to atomic composition and stacking geometry. Their broad chemical tunability makes them an ideal platform for large-scale combinatorial exploration and high-throughput computational screening with desirable quantum properties. In this work, we employ a Concept Whitening Graph Neural Network, a gray-box AI model, to classify delafossite structures by stacking sequence and magnetic states. By aligning learned representations with human-interpretable physical concepts, this gray-box approach enables both accurate prediction and insight into the structural and chemical features driving magnetic behavior. The magnetic-ordering models achieved validation accuracies exceeding 80 percent, with a further slight uptick observed in the model incorporating the largest number of concepts. Concept alignment analysis revealed measurable learning across nine physically meaningful descriptors, with coefficients of determination ranging from approximately 0.6 for the d-shell valence-electron concepts to 0.4-0.5 for the magnetic coupling parameters. Furthermore, we mapped the concept importances onto the material graph representation, elucidating interpretable physical trends and the progression of stable concept-aligned regions across training. These results demonstrate the potential of interpretable graph-based learning to capture the underlying physics of complex materials systems and provide an interpretable framework for accelerating the discovery and understanding of delafossites and related crystalline materials.

cond-mat.mtrl-sci↗

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↗

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

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↗

arXiv2021

Complete mapping of magnetic anisotropy for prototype Ising van der Waals FePS$_3$

Several Ising-type magnetic van der Waals (vdW) materials exhibit stable magnetic ground states. Despite these clear experimental demonstrations, a complete theoretical and microscopic understanding of their magnetic anisotropy is still lacking. In particular, the validity limit of identifying their one-dimensional (1-D) Ising nature has remained uninvestigated in a quantitative way. Here we performed the complete mapping of magnetic anisotropy for a prototypical Ising vdW magnet FePS$_3$ for the first time. Combining torque magnetometry measurements with their magnetostatic model analysis and the relativistic density functional total energy calculations, we successfully constructed the three-dimensional (3-D) mappings of the magnetic anisotropy in terms of magnetic torque and energy. The results not only quantitatively confirm that the easy axis is perpendicular to the $ab$ plane, but also reveal the anisotropies within the $ab$, $ac$, and $bc$ planes. Our approach can be applied to the detailed quantitative study of magnetism in vdW materials.

cond-mat.mtrl-sci↗

arXiv2021

Hund's physics and the magnetic ground state of CrOX (X = Cl, Br)

To understand the magnetic property of layered van der Waals materials CrOX (X = Cl, Br), we performed the detailed first-principles calculations for both bulk and monolayer. We found that the charge-only density functional theory combined with the explicit on-site interaction terms (so-called cDFT$+U$) well reproduces the experimental magnetic ground state of bulk CrOX, which is not the case for the use of spin-dependent density functional (so-called sDFT$+U$). Unlike some of the previous studies, our results show that CrOX monolayers are antiferromagnetic as in the bulk. It is also consistent with our magnetic force linear response calculation of exchange couplings $J_{\rm ex}$. The result of orbital-decomposed $J_{\rm ex}$ calculations shows that the Cr $t_\textrm{2g}$-$t_\textrm{2g}$ component mainly contributes to the antiferromagnetic order in both bulk and monolayer. Our result and analysis show that taking the correct Hund's physics into account is of key importance to construct the magnetic phase diagram and to describe the electronic structure.

cond-mat.mtrl-sci↗

arXiv2020

First-principles-based calculation of branching ratio for 5$\boldsymbol{d}$, 4$\boldsymbol{d}$, and 3$\boldsymbol{d}$ transition metal systems

A new first-principles computation scheme to calculate `branching ratio' has been applied to various $5d$, $4d$, and $3d$ transition metal elements and compounds. This recently suggested method is based on a theory which assumes the atomic core hole interacting barely with valence electrons. While it provides an efficient way to calculate the experimentally measurable quantity without generating spectrum itself, its reliability and applicability should be carefully examined especially for the light transition metal systems. Here we select 36 different materials and compare the calculation results with experimental data. It is found that our scheme well describes 5$d$ and 4$d$ transition metal systems whereas, for 3$d$ materials, the difference between the calculation and experiment is quite significant. It is attributed to the neglect of core-valence interaction whose energy scale is comparable with the spin-orbit coupling of core $p$ orbitals.

cond-mat.str-el↗