Showing 1–10 of 10 results for author: Yeom, S

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

Authors: Jovin Ryan Joseph, Do Hoon Kiem, Sinchul Yeom, Mina Yoon

Abstract: 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 pro… ▽ More 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. △ Less

Submitted 28 November, 2025; originally announced December 2025.

Comments: 6 main figures, 9 SI figures

Controlling structural phases of Sn through lattice engineering

Authors: Chandima Kasun Edirisinghe, Anjali Rathore, Taegeon Lee, Daekwon Lee, An-Hsi Chen, Garrett Baucom, Eitan Hershkovitz, Anuradha Wijesinghe, Pradip Adhikari, Sinchul Yeom, Hong Seok Lee, Hyung-Kook Choi, Hyunsoo Kim, Mina Yoon, Honggyu Kim, Matthew Brahlek, Heesuk Rho, Joon Sue Lee

Abstract: Topology and superconductivity, two distinct phenomena offer unique insight into quantum properties and their applications in quantum technologies, spintronics, and sustainable energy technologies if system can be found where they coexist. Tin (Sn) plays a pivotal role here as an element due to its two structural phases, $α$-Sn and $β$-Sn, exhibiting topological characteristics ($α$-Sn) and superc… ▽ More Topology and superconductivity, two distinct phenomena offer unique insight into quantum properties and their applications in quantum technologies, spintronics, and sustainable energy technologies if system can be found where they coexist. Tin (Sn) plays a pivotal role here as an element due to its two structural phases, $α$-Sn and $β$-Sn, exhibiting topological characteristics ($α$-Sn) and superconductivity ($β$-Sn). In this study we show how precise control of $α$ and $β$ phases of Sn thin films can be achieved by using molecular beam epitaxy grown buffer layers with systematic control over the lattice parameter. The resulting Sn films showed either $β$-Sn or $α$-Sn phases as the lattice constant of the buffer layer was varied from 6.10 A to 6.48 A, covering the range between GaSb (closely matched to InAs) and InSb. The crystal structures of the $α$- and $β$-Sn films were characterized by x-ray diffraction and confirmed by Raman spectroscopy and scanning transmission electron microscopy. The smooth and continuous surface morphology of the Sn films was validated using atomic force microscopy. The characteristics of $α$- and $β$-Sn phases were further verified using electrical transport measurements by observing resistance drop near 3.7 K for superconductivity of the $β$-Sn phase and Shubnikov-de Haas oscillations for the $α$-Sn phase. Density functional theory calculations showed that the stability of the Sn phases is highly dependent on lattice strain, with $α$-Sn being more stable under tensile strain and $β$-Sn becoming favorable under compressive strain, which is in good agreement with experimental observations. Hence, this study sheds light on controlling Sn phases through lattice engineering, enabling innovative applications in quantum technologies and beyond. △ Less

Submitted 21 September, 2024; v1 submitted 24 July, 2024; originally announced July 2024.

Theoretical investigation of delafossite-Cu2ZnSnO4 as a promising photovoltaic absorber

Authors: Seoung-Hun Kang, Myeongjun Kang, Sang Woon Hwang, Sinchul Yeom, Mina Yoon, Jong Mok Ok, Sangmoon Yoon

Abstract: In the quest for efficient and cost-effective photovoltaic absorber materials beyond silicon, considerable attention has been directed toward exploring alternatives. One such material, zincblende-derived Cu2ZnSnS4 (CZTS), has shown promise due to its ideal band-gap size and high absorption coefficient. However, challenges such as structural defects and secondary phase formation have hindered its d… ▽ More In the quest for efficient and cost-effective photovoltaic absorber materials beyond silicon, considerable attention has been directed toward exploring alternatives. One such material, zincblende-derived Cu2ZnSnS4 (CZTS), has shown promise due to its ideal band-gap size and high absorption coefficient. However, challenges such as structural defects and secondary phase formation have hindered its development. In this study, we examine the potential of another compound Cu2ZnSnO4 (CZTO) with a similar composition to CZTS as a promising alternative. Employing ab initio density function theory (DFT) calculations in combination with an evolutionary structure prediction algorithm, we identify that the crystalline phase of the delafossite structure is the most stable among the 900 (meta)stable CZTO. Its thermodynamic stability at room temperature is also confirmed by the molecular dynamics study. Excitingly, this new phase of CZTO displays a direct band gap where the dipole-allowed transition occurs, making it a strong candidate for efficient light absorption. Furthermore, the estimation of spectroscopic limited maximum efficiency (SLME) directly demonstrates the high potential of delafossite-CZTO as a photovoltaic absorber. Our numerical results suggest that delafossite-CZTO holds another promise for future photovoltaic applications. △ Less

Submitted 17 October, 2023; originally announced October 2023.

The role of temperature on defect diffusion and nanoscale patterning in graphene

Authors: Ondrej Dyck, Sinchul Yeom, Sarah Dillender, Andrew R. Lupini, Mina Yoon, Stephen Jesse

Abstract: Graphene is of great scientific interest due to a variety of unique properties such as ballistic transport, spin selectivity, the quantum hall effect, and other quantum properties. Nanopatterning and atomic scale modifications of graphene are expected to enable further control over its intrinsic properties, providing ways to tune the electronic properties through geometric and strain effects, intr… ▽ More Graphene is of great scientific interest due to a variety of unique properties such as ballistic transport, spin selectivity, the quantum hall effect, and other quantum properties. Nanopatterning and atomic scale modifications of graphene are expected to enable further control over its intrinsic properties, providing ways to tune the electronic properties through geometric and strain effects, introduce edge states and other local or extended topological defects, and sculpt circuit paths. The focused beam of a scanning transmission electron microscope (STEM) can be used to remove atoms, enabling milling, doping, and deposition. Utilization of a STEM as an atomic scale fabrication platform is increasing; however, a detailed understanding of beam-induced processes and the subsequent cascade of aftereffects is lacking. Here, we examine the electron beam effects on atomically clean graphene at a variety of temperatures ranging from 400 to 1000 C. We find that temperature plays a significant role in the milling rate and moderates competing processes of carbon adatom coalescence, graphene healing, and the diffusion (and recombination) of defects. The results of this work can be applied to a wider range of 2D materials and introduce better understanding of defect evolution in graphite and other bulk layered materials. △ Less

Submitted 11 July, 2023; originally announced July 2023.

Structural Anisotropy in Sb Thin Films

Authors: Pradip Adhikari, Anuradha Wijesinghe, Anjali Rathore, Timothy Jinsoo Yoo, Gyehyeon Kim, Hyoungtaek Lee, Sinchul Yeom, Alessandro R. Mazza, Changhee Sohn, Hyeong-Ryeol Park, Mina Yoon, Matthew Brahlek, Honggyu Kim, Joon Sue Lee

Abstract: Sb thin films have attracted wide interests due to their tunable band structure, topological phases, and remarkable electronic properties. We successfully grow epitaxial Sb thin films on a closely lattice-matched GaSb(001) surface by molecular beam epitaxy. We find a novel anisotropic directional dependence of their structural, morphological, and electronic properties. The origin of the anisotropi… ▽ More Sb thin films have attracted wide interests due to their tunable band structure, topological phases, and remarkable electronic properties. We successfully grow epitaxial Sb thin films on a closely lattice-matched GaSb(001) surface by molecular beam epitaxy. We find a novel anisotropic directional dependence of their structural, morphological, and electronic properties. The origin of the anisotropic features is elucidated using first-principles density functional theory (DFT) calculations. The growth regime of crystalline and amorphous Sb thin films was determined by mapping the surface reconstruction phase diagram of the GaSb(001) surface under Sb$_2$ flux, with confirmation of structural characterizations. Crystalline Sb thin films show a rhombohedral crystal structure along the rhombohedral (104) surface orientation parallel to the cubic (001) surface orientation of the GaSb substrate. At this coherent interface, Sb atoms are aligned with the GaSb lattice along the [1-10] crystallographic direction but are not aligned well along the [110] crystallographic direction, which results in anisotropic features in reflection high-energy electron diffraction patterns, surface morphology, and transport properties. Our DFT calculations show that the anisotropic features originate from the GaSb surface, where Sb atoms align with the Ga and Sb atoms on the reconstructed surface. The formation energy calculations confirm that the stability of the experimentally observed structures. Our results provide optimal film growth conditions for further studies of novel properties of Bi$_{1-x}$Sb$_x$ thin films with similar lattice parameters and an identical crystal structure as well as functional heterostructures of them with III-V semiconductor layers along the (001) surface orientation, supported by a theoretical understanding of the anisotropic film orientation. △ Less

Submitted 11 May, 2023; originally announced May 2023.

Journal ref: APL Mater. 12, 011116 (2024)

A New Two-Dimensional Dirac Semimetal Based on the Alkaline Earth Metal, CaP$_3$

Authors: Seoung-Hun Kang, Wei Luo, Sinchul Yeom, Yaling Zheng, Mina Yoon

Abstract: Using an evolutionary algorithm in combination with first-principles density functional theory calculations, we identify two-dimensional (2D) CaP$_3$ monolayer as a new Dirac semimetal due to inversion and nonsymmorphic spatial symmetries of the structure. This new topological material, composed of light elements, exhibits high structural stability (higher than the phase known in the literature),… ▽ More Using an evolutionary algorithm in combination with first-principles density functional theory calculations, we identify two-dimensional (2D) CaP$_3$ monolayer as a new Dirac semimetal due to inversion and nonsymmorphic spatial symmetries of the structure. This new topological material, composed of light elements, exhibits high structural stability (higher than the phase known in the literature), which is confirmed by thermodynamic and kinetic stability analysis. Moreover, it satisfies the electron filling criteria, so that its Dirac state is located near the Fermi level. The existence of the Dirac state predicted by the theoretical symmetry analysis is also confirmed by first-principles electronic band structure calculations. We find that the energy position of the Dirac state can be tuned by strain, while the Dirac state is unstable against an external electric field since it breaks the spatial inversion symmetry. Our findings should be instrumental in the development of 2D Dirac fermions based on light elements for their application in nanoelectronic devices and topological electronics. △ Less

Submitted 2 March, 2023; originally announced March 2023.

Top-down fabrication of atomic patterns in twisted bilayer graphene

Authors: Ondrej Dyck, Sinchul Yeom, Andrew R. Lupini, Jacob L. Swett, Dale Hensley, Mina Yoon, Stephen Jesse

Abstract: Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic scale features scattered probabilistically throughout the material. In a top-down approach, different regions of the material are exposed to diffe… ▽ More Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic scale features scattered probabilistically throughout the material. In a top-down approach, different regions of the material are exposed to different parameters resulting in structural changes varying on the scale of the resolution. In this work, we combine the application of global and local parameters in an aberration corrected scanning transmission electron microscope (STEM) to demonstrate atomic scale precision patterning of atoms in twisted bilayer graphene. The focused electron beam is used to define attachment points for foreign atoms through the controlled ejection of carbon atoms from the graphene lattice. The sample environment is staged with nearby source materials, such that the sample temperature can induce migration of the source atoms across the sample surface. Under these conditions, the electron-beam (top-down) enables carbon atoms in the graphene to be replaced spontaneously by diffusing adatoms (bottom-up). Using image-based feedback-control, arbitrary patterns of atoms and atom clusters are attached to the twisted bilayer graphene with limited human interaction. The role of substrate temperature on adatom and vacancy diffusion is explored by first-principles simulations. △ Less

Submitted 4 January, 2023; originally announced January 2023.

Magnetic Frustration Driven by Itinerancy in Spinel CoV2O4

Authors: J. H. Lee, J. Ma, S. E. Hahn, H. B. Cao, Tao Hong, M. S. Yeom, S. Okamoto, H. D. Zhou, M. Matsuda, R. S. Fishman

Abstract: Localized spins and itinerant electrons rarely coexist in geometrically-frustrated spinel lattices. We show that the spinel CoV2O4 stands at the crossover from insulating to itinerant behavior and exhibits a complex interplay between localized spins and itinerant electrons. In contrast to the expected paramagnetism, localized spins supported by enhanced exchange couplings are frustrated by the eff… ▽ More Localized spins and itinerant electrons rarely coexist in geometrically-frustrated spinel lattices. We show that the spinel CoV2O4 stands at the crossover from insulating to itinerant behavior and exhibits a complex interplay between localized spins and itinerant electrons. In contrast to the expected paramagnetism, localized spins supported by enhanced exchange couplings are frustrated by the effects of delocalized electrons. This frustration produces a non-collinear spin state and may be responsible for macroscopic spin-glass behavior. Competing phases can be uncovered by external perturbations such as pressure or magnetic field, which enhance the frustration. △ Less

Submitted 9 February, 2017; originally announced February 2017.

Comments: 15 pages, 5 figures

Journal ref: Sci Rep. 7(1), 17129 (2017)

Strain-induced pseudo-magnetic fields and charging effects on CVD-grown graphene

Authors: N. -C. Yeh, M. L. Teague, S. Yeom, B. L. Standley, D. A. Boyd, M. W. Bockrath

Abstract: Atomically resolved imaging and spectroscopic characteristics of graphene grown by chemical vapor deposition (CVD) on copper are investigated by means of scanning tunneling microscopy and spectroscopy (STM/STS). For CVD-grown graphene remaining on the copper substrate, the monolayer carbon structures exhibit ripples and appear strongly strained, with different regions exhibiting different lattice… ▽ More Atomically resolved imaging and spectroscopic characteristics of graphene grown by chemical vapor deposition (CVD) on copper are investigated by means of scanning tunneling microscopy and spectroscopy (STM/STS). For CVD-grown graphene remaining on the copper substrate, the monolayer carbon structures exhibit ripples and appear strongly strained, with different regions exhibiting different lattice structures and electronic density of states (DOS). In particular, ridges appear along the boundaries of different lattice structures, which exhibit excess charging effects. Additionally, the large and non-uniform strain induces pseudo-magnetic field up to ~ 50 Tesla, as manifested by the integer and fractional pseudo-magnetic field quantum Hall effect (IQHE and FQHE) in the DOS of graphene. In contrast, for graphene transferred from copper to SiO2 substrates after the CVD growth, the average strain on the whole is reduced, so are the corresponding charging effects and pseudo-magnetic fields except for sample areas near topographical ridges. These findings suggest feasible "strain engineering" of the electronic states of graphene by proper design of the substrates and growth conditions. △ Less

Submitted 26 March, 2011; v1 submitted 31 August, 2010; originally announced September 2010.

Comments: 10 pages, 9 figures. Accepted for publication in a special issue of Surface Science: "Graphene Surfaces and Interfaces". Contac author: Nai-Chang Yeh (ncyeh@caltech.edu)

Journal ref: Surface Science 605, 1649-1656 (2011)

Structure and thermodynamics of associating rods solutions

Authors: Min Sun Yeom, Alexander V. Ermoshkin, Monica Olvera de la Cruz

Abstract: Thermoreversible sol-gel transitions in solutions of rod-like associating polymers are analyzed by computer simulations and by mean field models. The sol-gel transition is determined by the divergence of the cluster weight average. The analytically determined sol-gel transition is in good agreement with the simulation results. At low temperatures we observe a peak in the heat capacity, which max… ▽ More Thermoreversible sol-gel transitions in solutions of rod-like associating polymers are analyzed by computer simulations and by mean field models. The sol-gel transition is determined by the divergence of the cluster weight average. The analytically determined sol-gel transition is in good agreement with the simulation results. At low temperatures we observe a peak in the heat capacity, which maximum is associated with the precipitation transition. The gelation transition is sensitive to the number of associating groups per rod but nearly insensitive to the spatial distribution of associating groups around the rod. The precipitation is strongly dependent on both the number and distribution of associating groups per rod. We find negligible nematic orientational order at the gelation and precipitation transitions. △ Less

Submitted 21 November, 2002; originally announced November 2002.