Showing 1–28 of 28 results for author: Jeong, W

Bridging Atomistic Simulation and Experimental Processing Timescales with Goal-Directed Deep Reinforcement Learning

Authors: Wonseok Jeong, Francesca Tavazza, Brian DeCost

Abstract: Atomic-scale modeling has advanced rapidly through integration of machine learning, yet a key bottleneck remains. Even with an accurate potential energy surface and a clear target material, we still lack a practical atomistic dynamics framework that can simulate how materials form under realistic synthesis and processing conditions. Many processing transformations are governed by rare events in no… ▽ More Atomic-scale modeling has advanced rapidly through integration of machine learning, yet a key bottleneck remains. Even with an accurate potential energy surface and a clear target material, we still lack a practical atomistic dynamics framework that can simulate how materials form under realistic synthesis and processing conditions. Many processing transformations are governed by rare events in non-idealized evolving environments, while direct molecular dynamics is limited by femtosecond timesteps and short accessible trajectories. Existing acceleration methods often require prior mechanistic knowledge, including reaction coordinates, collective variables, event tables, or pathway guesses, which is rarely available in real experiments. Here we present an E(3)-equivariant deep reinforcement learning framework that enables goal-directed pathway discovery without hand-crafted reaction coordinates. The framework introduces a complementary operating mode for atomistic simulation in which realistic, non-idealized environments can be addressed directly while retaining kinetic plausibility through barrier-aware rewards. As a challenging benchmark, we target silicon dry oxidation, where rare-event pathways in amorphous SiO2 are effectively inaccessible to conventional atomistic methods. We treat an O2 molecule as an agent that performs continuous rigid-body translations and rotations in a Si/a-SiO2 environment. The agent is trained with an episode-level objective that rewards verified O2 dissociation while preferring low effective activation barriers. We demonstrate that the learned policy discovers kinetically favorable O2 diffusion and dissociation pathways in a disordered Si/a-SiO2 environment, progressively improving success rate while reducing effective activation barriers over training. We also discuss how the approach can be generalized to other processing and synthesis problems. △ Less

Submitted 15 May, 2026; originally announced May 2026.

Comments: 35 pages, 9 figures

Monolithic axial InGaAs quantum dot emitters in GaAs-based nanowires via Sb-mediated facet engineering

Authors: Hyowon W. Jeong, Aris Koulas-Simos, Imad Limame, Markus Döblinger, Sang Kyu Kim, Chirag C. Palekar, Jonathan J. Finley, Stephan Reitzenstein, Gregor Koblmüller

Abstract: GaAs-based nanowires hosting active quantum heterostructures provide a promising route toward monolithic integration of single-photon sources on silicon, a key requirement for scalable quantum photonics. However, ultrathin axial quantum-emitter formation is often hindered by facet-dependent growth dynamics and rotational twins, which induce lateral overgrowth and compromise interface abruptness. H… ▽ More GaAs-based nanowires hosting active quantum heterostructures provide a promising route toward monolithic integration of single-photon sources on silicon, a key requirement for scalable quantum photonics. However, ultrathin axial quantum-emitter formation is often hindered by facet-dependent growth dynamics and rotational twins, which induce lateral overgrowth and compromise interface abruptness. Here, we develop InGaAs-based quantum emitters by tailoring facet evolution via dilute Sb incorporation, which efficiently suppresses twins and promotes confined axial insertion at the growth-front facet. This approach significantly enhances the probability of obtaining abrupt, few-nanometer-thin quantum dots at the nanowire tip. Single-nanowire optical spectroscopy reveals intense, spatially localized emission from the active region with lifetimes as short as (0.51 $\pm$ 0.02) ns, and second-order photon-correlation measurements consistently exhibit pronounced antibunching with $g^{(2)}(0)<0.4$, confirming single-photon emission. These results establish a strong correlation between twin density and axial heterostructure formation, identifying defect control as a key factor in realizing monolithically integrated nanowire single-photon sources. △ Less

Submitted 16 June, 2026; v1 submitted 13 May, 2026; originally announced May 2026.

Ligand Engineering for Precise Control of Ultrathin CsPbI3 Nanoplatelet Superlattices for Efficient Light-Emitting Diodes

Authors: Jongbeom Kim, Woo Hyeon Jeong, Junzhi Ye, Allison Nicole Arber, Vikram, Donghan Kim, Yi-Teng Huang, Yixin Wang, Dongeun Kim, Dongryeol Lee, Chia-Yu Chang, Xinyu Shen, Sung Yong Bae, Ashish Gaurav, Akshay Rao, Henry J. Snaith, M. Saiful Islam, Bo Ram Lee, Myoung Hoon Song, Robert L. Z. Hoye

Abstract: Strongly-confined perovskite nanoplatelets (PeNPLs) offer opportunities not found in conventional isotropic nanocubes, especially in producing linearly polarized light, as well as enhancing outcoupling through control over the transition dipole moment. But this requires ultrathin nanoplatelets with three or fewer monolayers of PbI6 octahedra across the thickness, which are challenging to synthesis… ▽ More Strongly-confined perovskite nanoplatelets (PeNPLs) offer opportunities not found in conventional isotropic nanocubes, especially in producing linearly polarized light, as well as enhancing outcoupling through control over the transition dipole moment. But this requires ultrathin nanoplatelets with three or fewer monolayers of PbI6 octahedra across the thickness, which are challenging to synthesise uniformly, and their luminescence is strongly affected by surface defects. Together, these limit the performance of ultrathin PeNPLs in light-emitting diodes (LEDs). Here, we address these challenges with an ancillary ligand engineering strategy. We demonstrate that ligands with phosphoryl functional groups strongly bind to the perovskite surface, while having an organic backbone that is not sterically bulky ensures high ligand density. By modulating nucleation and growth, these ancillary ligands lead to monodisperse PeNPLs that stack more uniformly when self-assembled into superlattices, with suppressed agglomeration. As a result, from edge-up PeNPL superlattices, we achieve enhanced degree of polarization, while from face-down PeNPL superlattices, we achieve enhanced outcoupling that results in LEDs with 13.1% external quantum efficiency, the highest reported for ultrathin PeNPL LEDs. This work establishes ancillary ligand-induced synthesis as a decisive route to achieve uniform nanoplatelets with robust orientation control, enabling full utilization of the multifunctionality of anisotropic PeNPLs. △ Less

Submitted 23 January, 2026; v1 submitted 14 November, 2025; originally announced November 2025.

Comments: 25 pages, 4 figures, 1 table

Quantum simulation approach to ultra-weak magnetic anisotropy in a frustrated spin-1/2 antiferromagnet

Authors: Ki Won Jeong, Jae Yeon Seo, Sunghyun Lim, Jae Min Hong, Hyeon Jun Ryu, Jongseok Byeon, Kyungsun Moon, Nara Lee, Young Jai Choi

Abstract: The intrinsic equivalence between electron spin and qubit offers a natural foundation for quantum simulations of magnetic materials. However, incorporating magnetocrystalline anisotropy (MCA), a key feature of real magnets, remains a major challenge. Here, we develop a quantum simulation framework for MCA in CuSb2O6, a spin-1/2 antiferromagnet with alternating ferromagnetic chains arising from fru… ▽ More The intrinsic equivalence between electron spin and qubit offers a natural foundation for quantum simulations of magnetic materials. However, incorporating magnetocrystalline anisotropy (MCA), a key feature of real magnets, remains a major challenge. Here, we develop a quantum simulation framework for MCA in CuSb2O6, a spin-1/2 antiferromagnet with alternating ferromagnetic chains arising from frustrated, anisotropic exchange interactions in a nearly square lattice. The $\mathrm{Cu}^{2+}$ spin network is modeled as a four-qubit square lattice, with four paired ancilla qubits introduced to encode angle-dependent MCA. This two-qubit representation per spin site resolves the limitation that squared Pauli operators yield only the identity, enabling MCA terms to be faithfully embedded into quantum circuits. Using the variational quantum eigensolver, we determine an exceptionally small easy-axis MCA constant, just 0.00022% of the nearest-neighbor exchange interaction, yet sufficient to drive a spin-flop transition with $90^{\circ}$ spin reorientation and strong angular variation in magnetic torque. Beyond this regime, the simulations uncover a half-saturated magnetic phase at ultra-high fields, stabilized by anisotropic next-nearest-neighbor interactions. Our findings demonstrate the feasibility of resource-efficient quantum simulations of complex magnetic phenomena in real materials. △ Less

Submitted 1 October, 2025; v1 submitted 26 September, 2025; originally announced September 2025.

Enhanced Stability and Linearly Polarized Emission from CsPbI$_3$ Perovskite Nanoplatelets through A-site Cation Engineering

Authors: Woo Hyeon Jeong, Junzhi Ye, Jongbeom Kim, Rui Xu, Xinyu Shen, Chia-Yu Chang, Eilidh L. Quinn, Myoung Hoon Song, Peter Nellist, Henry J. Snaith, Yunwei Zhang, Bo Ram Lee, Robert L. Z. Hoye

Abstract: The anisotropy of perovskite nanoplatelets (PeNPLs) opens up many opportunities in optoelectronics, including enabling the emission of linearly polarized light. But the limited stability of PeNPLs is a pressing challenge, especially for red-emitting CsPbI$_3$. Herein, we address this limitation by alloying FA into the perovskite cuboctahedral site. Unlike Cs/FA alloying in bulk thin films or nonco… ▽ More The anisotropy of perovskite nanoplatelets (PeNPLs) opens up many opportunities in optoelectronics, including enabling the emission of linearly polarized light. But the limited stability of PeNPLs is a pressing challenge, especially for red-emitting CsPbI$_3$. Herein, we address this limitation by alloying FA into the perovskite cuboctahedral site. Unlike Cs/FA alloying in bulk thin films or nonconfined nanocubes, FA incorporation in nanoplatelets requires meticulous control over the reaction conditions, given that nanoplatelets are obtained in kinetically-driven growth regimes instead of thermodynamically-driven conditions. Through in-situ photoluminescence (PL) measurements, we find that excess FA leads to uncontrolled growth, where phase-impurities and nanoplatelets of multiple thicknesses co-exist. Restricting the FA content to up to 25% Cs substitution enables monodisperse PeNPLs, and increases the PL quantum yield (from 53% to 61%), exciton lifetime (from 18 ns to 27 ns), and stability in ambient air (from ~2 days to >7 days) compared to CsPbI$_3$. This arises due to hydrogen bonding between FA and the oleate and oleylammonium ligands, anchoring them to the surface to improve optoelectronic properties and stability. The reduction in non-radiative recombination, improvement in the nanoplatelet aspect ratio, and higher ligand density lead to FA-containing PeNPLs more effectively forming edge-up superlattices, enhancing the PL degree of linear polarization from 5.1% (CsPbI$_3$) to 9.4% (Cs$_{0.75}$FA$_{0.25}$PbI$_3$). These fundamental insights show how the stability limitations of PeNPLs could be addressed, and these materials grown more precisely to improve their performance as polarized light emitters, critical for utilizing them in next-generation display, bioimaging and communications applications. △ Less

Submitted 28 May, 2025; originally announced May 2025.

Comments: 19 pages, 5 figures

Kaleidoscopic reorganization of network communities across different scales

Authors: Wonhee Jeong, Daekyung Lee, Heetae Kim, Sang Hoon Lee

Abstract: The notion of structural heterogeneity is pervasive in real networks, and their community organization is no exception. Still, a vast majority of community detection methods assume neatly hierarchically organized communities of a characteristic scale for a given hierarchical level. In this work, we demonstrate that the reality of scale-dependent community reorganization is convoluted with simultan… ▽ More The notion of structural heterogeneity is pervasive in real networks, and their community organization is no exception. Still, a vast majority of community detection methods assume neatly hierarchically organized communities of a characteristic scale for a given hierarchical level. In this work, we demonstrate that the reality of scale-dependent community reorganization is convoluted with simultaneous processes of community splitting and merging, challenging the conventional understanding of community-scale adjustment. We provide a mathematical argument concerning the modularity function, the results from real-network analysis, and a simple network model for a comprehensive understanding of the nontrivial community reorganization process. The reorganization is characterized by a local drop in the number of communities as the resolution parameter varies. This study suggests a need for a paradigm shift in the study of network communities, which emphasizes the importance of considering scale-dependent reorganization to better understand the genuine structural organization of networks. △ Less

Submitted 21 January, 2025; v1 submitted 27 September, 2024; originally announced September 2024.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. E 111, 014312 (2025)

Global decomposition of networks into multiple cores formed by local hubs

Authors: Wonhee Jeong, Unjong Yu, Sang Hoon Lee

Abstract: Networks are ubiquitous in various fields, representing systems where nodes and their interconnections constitute their intricate structures. We introduce a network decomposition scheme to reveal multiscale core-periphery structures lurking inside, using the concept of locally defined nodal hub centrality and edge-pruning techniques built upon it. We demonstrate that the hub-centrality-based edge… ▽ More Networks are ubiquitous in various fields, representing systems where nodes and their interconnections constitute their intricate structures. We introduce a network decomposition scheme to reveal multiscale core-periphery structures lurking inside, using the concept of locally defined nodal hub centrality and edge-pruning techniques built upon it. We demonstrate that the hub-centrality-based edge pruning reveals a series of breaking points in network decomposition, which effectively separates a network into its backbone and shell structures. Our local-edge decomposition method iteratively identifies and removes locally least connected nodes, and uncovers an onion-like hierarchical structure as a result. Compared with the conventional $k$-core decomposition method, our method based on relative information residing in local structures exhibits a clear advantage in terms of discovering locally crucial substructures. As an application of the method, we present a scheme to detect multiple core-periphery structures and the decomposition of coarse-grained supernode networks, by combining the method with the network community detection. △ Less

Submitted 12 May, 2025; v1 submitted 29 June, 2024; originally announced July 2024.

Comments: 12 pages, 11 figures, 1 table

Journal ref: Phys. Rev. E 111, 054302 (2025)

Spectroscopy-Guided Discovery of Three-Dimensional Structures of Disordered Materials with Diffusion Models

Authors: Hyuna Kwon, Tim Hsu, Wenyu Sun, Wonseok Jeong, Fikret Aydin, James Chapman, Xiao Chen, Matthew R. Carbone, Deyu Lu, Fei Zhou, Tuan Anh Pham

Abstract: The ability to rapidly develop materials with desired properties has a transformative impact on a broad range of emerging technologies. In this work, we introduce a new framework based on the diffusion model, a recent generative machine learning method to predict 3D structures of disordered materials from a target property. For demonstration, we apply the model to identify the atomic structures of… ▽ More The ability to rapidly develop materials with desired properties has a transformative impact on a broad range of emerging technologies. In this work, we introduce a new framework based on the diffusion model, a recent generative machine learning method to predict 3D structures of disordered materials from a target property. For demonstration, we apply the model to identify the atomic structures of amorphous carbons ($a$-C) as a representative material system from the target X-ray absorption near edge structure (XANES) spectra--a common experimental technique to probe atomic structures of materials. We show that conditional generation guided by XANES spectra reproduces key features of the target structures. Furthermore, we show that our model can steer the generative process to tailor atomic arrangements for a specific XANES spectrum. Finally, our generative model exhibits a remarkable scale-agnostic property, thereby enabling generation of realistic, large-scale structures through learning from a small-scale dataset (i.e., with small unit cells). Our work represents a significant stride in bridging the gap between materials characterization and atomic structure determination; in addition, it can be leveraged for materials discovery in exploring various material properties as targeted. △ Less

Submitted 9 December, 2023; originally announced December 2023.

E-beam-enhanced solid-state mechanical amorphization of alpha-quartz: Reducing deformation barrier via localized excess electrons as mobile anions

Authors: Sung-Gyu Kang, Wonseok Jeong, Hwangsun Kim, Jeongin Paeng, Seungwu Han, Heung Nam Han, In-Suk Choi

Abstract: Under hydrostatic pressure, alpha-quartz undergoes solid-state mechanical amorphization wherein the interpenetration of SiO4 tetrahedra occurs and the material loses crystallinity. This phase transformation requires a high hydrostatic pressure of 14 GPa because the repulsive forces resulting from the ionic nature of the Si-O bonds prevent the severe distortion of the atomic configuration. Herein,… ▽ More Under hydrostatic pressure, alpha-quartz undergoes solid-state mechanical amorphization wherein the interpenetration of SiO4 tetrahedra occurs and the material loses crystallinity. This phase transformation requires a high hydrostatic pressure of 14 GPa because the repulsive forces resulting from the ionic nature of the Si-O bonds prevent the severe distortion of the atomic configuration. Herein, we experimentally and computationally demonstrate that e-beam irradiation changes the nature of the interatomic bonds in alpha-quartz and enhances the solid-state mechanical amorphization at nanoscale. Specifically, during in situ uniaxial compression, a larger permanent deformation occurs in alpha-quartz micropillars compressed during e-beam irradiation than in those without e-beam irradiation. Microstructural analysis reveals that the large permanent deformation under e-beam irradiation originates from the enhanced mechanical amorphization of alpha-quartz and the subsequent viscoplastic deformation of the amorphized region. Further, atomic-scale simulations suggest that the delocalized excess electrons introduced by e-beam irradiation move to highly distorted atomic configurations and alleviate the repulsive force, thus reducing the barrier to the solid-state mechanical amorphization. These findings deepen our understanding of electron-matter interactions and can be extended to new glass forming and processing technologies at nano- and microscale. △ Less

Submitted 26 December, 2022; originally announced December 2022.

Comments: 24 pages, 6 figures

Sign-tunable anisotropic magnetoresistance and electrically detectable dual magnetic phases in a helical antiferromagnet

Authors: Jong Hyuk Kim, Hyun Jun Shin, Mi Kyung Kim, Jae Min Hong, Ki Won Jeong, Jin Seok Kim, Kyungsun Moon, Nara Lee, Young Jai Choi

Abstract: The helimagnetic order describes a non-collinear spin texture of antiferromagnets, arising from competing exchange interactions. Although collinear antiferromagnets are elemental building blocks of antiferromagnetic (AFM) spintronics, the potential of implementing spintronic functionality in non-collinear antiferromagnets has not been clarified thus far. Here, we propose an AFM helimagnet of EuCo2… ▽ More The helimagnetic order describes a non-collinear spin texture of antiferromagnets, arising from competing exchange interactions. Although collinear antiferromagnets are elemental building blocks of antiferromagnetic (AFM) spintronics, the potential of implementing spintronic functionality in non-collinear antiferromagnets has not been clarified thus far. Here, we propose an AFM helimagnet of EuCo2As2 as a novel single-phase spintronic material that exhibits a remarkable sign reversal of anisotropic magnetoresistance (AMR). The contrast in the AMR arises from two electrically distinctive magnetic phases with spin reorientation driven by magnetic field lying on the easy-plane, which switches the sign of the AMR from positive to negative. Further, various AFM memory states associated with the evolution of the spin structure under magnetic fields were identified theoretically, based on an easy-plane anisotropic spin model. These results reveal that non-collinear antiferromagnets hold potential for developing spintronic devices. △ Less

Submitted 21 March, 2022; originally announced March 2022.

Critical Phenomena and Strategy Ordering with Hub Centrality Approach in the Aspiration-based Coordination Game

Authors: Wonhee Jeong, Unjong Yu

Abstract: We study the coordination game with an aspiration-driven update rule in regular graphs and scale-free networks. We prove that the model coincides exactly with the Ising model and shows a phase transition at the critical selection noise when the aspiration level is zero. It is found that the critical selection noise decreases with clustering in random regular graphs. With a non-zero aspiration leve… ▽ More We study the coordination game with an aspiration-driven update rule in regular graphs and scale-free networks. We prove that the model coincides exactly with the Ising model and shows a phase transition at the critical selection noise when the aspiration level is zero. It is found that the critical selection noise decreases with clustering in random regular graphs. With a non-zero aspiration level, the model also exhibits a phase transition as long as the aspiration level is smaller than the degree of graphs. We also show that the critical exponents are independent of clustering and aspiration level to confirm that the coordination game belongs to the Ising universality class. As for scale-free networks, the effect of aspiration level on the order parameter at a low selection noise is examined. In model networks (Barabási-Albert network and Holme-Kim network), the order parameter abruptly decreases when the aspiration level is the same as the average degree of the network. In real-world networks, in contrast, the order parameter decreases gradually. We explain this difference by proposing the concepts of hub centrality and local hub. The histogram of hub centrality of real-world networks separates into two parts unlike model networks, and local hubs exist only in real-world networks. We conclude that the difference of network structures in model and real-world networks induces qualitatively different behavior in the coordination game. △ Less

Submitted 24 August, 2021; originally announced August 2021.

Comments: 9 pages, 7 figures

Accelerated identification of equilibrium structures of multicomponent inorganic crystals using machine learning potentials

Authors: Sungwoo Kang, Wonseok Jeong, Changho Hong, Seungwoo Hwang, Youngchae Yoon, Seungwu Han

Abstract: The discovery of new multicomponent inorganic compounds can provide direct solutions to many scientific and engineering challenges, yet the vast size of the uncharted material space dwarfs current synthesis throughput. While the computational crystal structure prediction is expected to mitigate this frustration, the NP-hardness and steep costs of density functional theory (DFT) calculations prohib… ▽ More The discovery of new multicomponent inorganic compounds can provide direct solutions to many scientific and engineering challenges, yet the vast size of the uncharted material space dwarfs current synthesis throughput. While the computational crystal structure prediction is expected to mitigate this frustration, the NP-hardness and steep costs of density functional theory (DFT) calculations prohibit material exploration at scale. Herein, we introduce SPINNER, a highly efficient and reliable structure-prediction framework based on exhaustive random searches and evolutionary algorithms, which is completely free from empiricism. Empowered by accurate neural network potentials, the program can navigate the configuration space faster than DFT by more than 10$^{2}$-fold. In blind tests on 60 ternary compositions diversely selected from the experimental database, SPINNER successfully identifies experimental (or theoretically more stable) phases for ~80% of materials within 5000 generations, entailing up to half a million structure evaluations for each composition. When benchmarked against previous data mining or DFT-based evolutionary predictions, SPINNER identifies more stable phases in the majority of cases. By developing a reliable and fast structure-prediction framework, this work opens the door to large-scale, unbounded computational exploration of undiscovered inorganic crystals. △ Less

Submitted 7 July, 2021; v1 submitted 6 July, 2021; originally announced July 2021.

Comments: 3 figures

Journal ref: npj Comput. Mater. 8, 108 (2022)

Evolution of cooperation in multi-agent systems with time-varying tags, multiple strategies, and heterogeneous invasion dynamics

Authors: Wonhee Jeong, Tarik Hadzibeganovic, Unjong Yu

Abstract: Cooperation in an open dynamic system fundamentally depends upon information distributed across its components. Yet in an environment with rapidly enlarging complexity, this information may need to change adaptively to enable not only cooperation but also the mere survival of an organism. Combining the methods of evolutionary game theory, agent-based simulation, and statistical physics, we develop… ▽ More Cooperation in an open dynamic system fundamentally depends upon information distributed across its components. Yet in an environment with rapidly enlarging complexity, this information may need to change adaptively to enable not only cooperation but also the mere survival of an organism. Combining the methods of evolutionary game theory, agent-based simulation, and statistical physics, we develop a model of the evolution of cooperation in an ageing population of artificial decision makers playing spatial tag-mediated prisoner's dilemma games with their ingroup neighbors and with genetically unrelated immigrant agents. In our model with six strategies we introduce the concept of time-varying tags such that the phenotypic features of 'new' agents can change into 'approved' following variable approval times. Our Monte Carlo simulations show that ingroup-biased ethnocentric cooperation can dominate only at low costs and short approval times. In the standard 4-strategy model with fixed tags, we identified a critical cost $c_{\mathrm{crit}}$ above which cooperation transitioned abruptly into the phase of pure defection, revealing remarkable fragility of ingroup-biased generosity. In our generalized 6-strategy model with time-varying tags, elevated cooperation was observed for a wider region of the parameter space, peaking at intermediate approval times and cost values above $c_{\mathrm{crit}}$. Our findings show that in an open system subject to immigration dynamics, high levels of social cooperation are possible if a fraction of the population adopts the strategy with an egalitarian generosity directed towards both native and approved naturalized citizens, regardless of their actual origin. These findings also suggest that instead of relying upon arbitrarily fixed approval times, there is an optimal duration of the naturalization procedure from which the society as a whole can profit most. △ Less

Submitted 3 April, 2021; originally announced April 2021.

Comments: 13 pages, 10 figures, 1 table, 1 Appendix

Training machine-learning potentials for crystal structure prediction using disordered structures

Authors: Changho Hong, Jeong Min Choi, Wonseok Jeong, Sungwoo Kang, Suyeon Ju, Kyeongpung Lee, Jisu Jung, Yong Youn, Seungwu Han

Abstract: Prediction of the stable crystal structure for multinary (ternary or higher) compounds with unexplored compositions demands fast and accurate evaluation of free energies in exploring the vast configurational space. The machine-learning potential such as the neural network potential (NNP) is poised to meet this requirement but a dearth of information on the crystal structure poses a challenge in ch… ▽ More Prediction of the stable crystal structure for multinary (ternary or higher) compounds with unexplored compositions demands fast and accurate evaluation of free energies in exploring the vast configurational space. The machine-learning potential such as the neural network potential (NNP) is poised to meet this requirement but a dearth of information on the crystal structure poses a challenge in choosing training sets. Herein we propose constructing the training set from densityfunctional-theory (DFT) based dynamical trajectories of liquid and quenched amorphous phases, which does not require any preceding information on material structures except for the chemical composition. To demonstrate suitability of the trained NNP in the crystal structure prediction, we compare NNP and DFT energies for Ba2AgSi3, Mg2SiO4, LiAlCl4, and InTe2O5F over experimental phases as well as low-energy crystal structures that are generated theoretically. For every material, we find strong correlations between DFT and NNP energies, ensuring that the NNPs can properly rank energies among low-energy crystalline structures. We also find that the evolutionary search using the NNPs can identify low-energy metastable phases more efficiently than the DFTbased approach. By proposing a way to developing reliable machine-learning potentials for the crystal structure prediction, this work will pave the way to identifying unexplored multinary phases efficiently. △ Less

Submitted 2 December, 2020; v1 submitted 18 August, 2020; originally announced August 2020.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. B 102, 224104 (2020)

Highly Clustered Complex Networks in the Configuration Model: Random Regular Small-World Network

Authors: Wonhee Jeong, Hoseung Jang, Unjong Yu

Abstract: We propose a method to make a highly clustered complex network within the configuration model. Using this method, we generated highly clustered random regular networks and analyzed the properties of them. We show that highly clustered random regular networks with appropriate parameters satisfy all the conditions of the small-world network: connectedness, high clustering coefficient, and small-worl… ▽ More We propose a method to make a highly clustered complex network within the configuration model. Using this method, we generated highly clustered random regular networks and analyzed the properties of them. We show that highly clustered random regular networks with appropriate parameters satisfy all the conditions of the small-world network: connectedness, high clustering coefficient, and small-world effect. We also study how clustering affects the percolation threshold in random regular networks. In addition, the prisoner's dilemma game is studied and the effects of clustering and degree heterogeneity on the cooperation level are discussed. △ Less

Submitted 26 November, 2019; originally announced November 2019.

Comments: 6 pages, 5 figures

Journal ref: EPL, 128 (2019) 16001

Atomic energy mapping of neural network potential

Authors: Dongsun Yoo, Kyuhyun Lee, Wonseok Jeong, Satoshi Watanabe, Seungwu Han

Abstract: We show that the intelligence of the machine-learning potential arises from its ability to infer the reference atomic-energy function from a given set of total energies. By utilizing invariant points in the feature space at which the atomic energy has a fixed reference value, we examine the atomic energy mapping of neural network potentials. Through a series of examples on Si, we demonstrate that… ▽ More We show that the intelligence of the machine-learning potential arises from its ability to infer the reference atomic-energy function from a given set of total energies. By utilizing invariant points in the feature space at which the atomic energy has a fixed reference value, we examine the atomic energy mapping of neural network potentials. Through a series of examples on Si, we demonstrate that the neural network potential is vulnerable to 'ad hoc' mapping in which the total energy appears to be trained accurately while the atomic energy mapping is incorrect in spite of its capability. We show that the energy mapping can be improved by choosing the training set carefully and monitoring the atomic energy at the invariant points during the training procedure. △ Less

Submitted 11 March, 2019; originally announced March 2019.

Journal ref: Phys. Rev. Materials 3, 093802 (2019)

Influences of interfacial oxidization on surface magnetic energy, magnetic damping and spin-orbit-torques in Pt / ferromagnet / capping structures

Authors: DongJoon Lee, WonMin Jeong, DeokHyun Yun, Seung-Young Park, Byeong-Kwon Ju, Kyung-Jin Lee, Hyun Cheol Koo, Byoung-Chul Min, OukJae Lee

Abstract: We investigate the effect of capping layer (CAP) on the interfacial magnetic anisotropy energy density (K_S), magnetic damping (α), and spin-orbit torques (SOTs) in heavy-metal (Pt) / ferromagnet (Co or Py) / CAP (MgO/Ta, HfOx, or TaN). At room temperature (RT) the CAP materials influence the effective magnitude of K_S, which is associated with a formation of interfacial magnetic oxides. The dynam… ▽ More We investigate the effect of capping layer (CAP) on the interfacial magnetic anisotropy energy density (K_S), magnetic damping (α), and spin-orbit torques (SOTs) in heavy-metal (Pt) / ferromagnet (Co or Py) / CAP (MgO/Ta, HfOx, or TaN). At room temperature (RT) the CAP materials influence the effective magnitude of K_S, which is associated with a formation of interfacial magnetic oxides. The dynamical dissipation parameters of Co are considerably influenced by the CAP (especially MgO) while those of Py are not. This is possibly due to an extra magnetic damping via spin-pumping process across the Co/CoO interface and incoherent magnon generation (spin fluctuation) in the interfacial CoO. It is also observed that both anti-damping and field-like SOT efficiencies vary marginally with the CAP in the thickness ranges we examined. Our results reveal the crucial role of interfacial oxides on the perpendicular magnetic anisotropy, magnetic damping, and SOTs. △ Less

Submitted 17 January, 2019; originally announced January 2019.

Prisoner's dilemma game on complex networks with a death process: Effects of minimum requirements and immigration

Authors: Wonhee Jeong, Unjong Yu

Abstract: We present results of the prisoner's dilemma game on complex networks that have population change. We introduce a death process with minimum requirements and show that it induces a highly cooperative society. We also study the effects of immigration on the society. We show that the acceptable number of immigrants of the society is determined by the population of the society, the ratio of cooperato… ▽ More We present results of the prisoner's dilemma game on complex networks that have population change. We introduce a death process with minimum requirements and show that it induces a highly cooperative society. We also study the effects of immigration on the society. We show that the acceptable number of immigrants of the society is determined by the population of the society, the ratio of cooperator among immigrants, and the immigration interval. In addition, if immigrants have a preferential attachment link, the acceptable number of immigrants increases. △ Less

Submitted 6 November, 2018; v1 submitted 22 October, 2018; originally announced October 2018.

Comments: 7 pages, 5 figures

Measurement of collective excitations in VO$_2$ by resonant inelastic X-ray scattering

Authors: Haowei He, A. X. Gray, P. Granitzka, J. W. Jeong, N. P. Aetukuri, R. Kukreja, Lin Miao, Y. B. Huang, P. Olalde-Velasco, J. Pelliciari, W. F. Schlotter, E. Arenholz, T. Schmitt, M. G. Samant, S. S. P. Parkin, H. A. Dürr, L. Andrew Wray

Abstract: Vanadium dioxide is of broad interest as a spin-1/2 electron system that realizes a metal-insulator transition near room temperature, due to a combination of strongly correlated and itinerant electron physics. Here, resonant inelastic X-ray scattering is used to measure the excitation spectrum of charge, spin, and lattice degrees of freedom at the vanadium L-edge under different polarization and t… ▽ More Vanadium dioxide is of broad interest as a spin-1/2 electron system that realizes a metal-insulator transition near room temperature, due to a combination of strongly correlated and itinerant electron physics. Here, resonant inelastic X-ray scattering is used to measure the excitation spectrum of charge, spin, and lattice degrees of freedom at the vanadium L-edge under different polarization and temperature conditions. These spectra reveal the evolution of energetics across the metal-insulator transition, including the low temperature appearance of a strong candidate for the singlet-triplet excitation of a vanadium dimer. △ Less

Submitted 3 March, 2016; originally announced March 2016.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 94, 161119(R) (2016)

Dimensionality Control of d-orbital Occupation in Oxide Superlattices

Authors: Da Woon Jeong, Woo Seok Choi, Satoshi Okamoto, Jae-Young Kim, Kyung Wan Kim, Soon Jae Moon, Deok-Yong Cho, Ho Nyung Lee, Tae Won Noh

Abstract: Manipulating the orbital state in a strongly correlated electron system is of fundamental and technological importance for exploring and developing novel electronic phases. Here, we report an unambiguous demonstration of orbital occupancy control between t2g and eg multiplets in quasi-twodimensional transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO3 and a band insulator… ▽ More Manipulating the orbital state in a strongly correlated electron system is of fundamental and technological importance for exploring and developing novel electronic phases. Here, we report an unambiguous demonstration of orbital occupancy control between t2g and eg multiplets in quasi-twodimensional transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO3 and a band insulator LaAlO3. As the LaCoO3 sublayer thickness approaches its fundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs changed from a Mott insulator, in which both t2g and eg orbitals are partially filled, to a band insulator by completely filling (emptying) the t2g (eg) orbitals. We found the reduction of dimensionality has a profound effect on the electronic structure evolution, which is, whereas, insensitive to the epitaxial strain. The remarkable orbital controllability shown here offers a promising pathway for novel applications such as catalysis and photovoltaics, where the energy of d level is an essential parameter. △ Less

Submitted 22 December, 2014; originally announced December 2014.

Journal ref: Sci. Rep. 4, 6124 (2014)

Latent Instabilities in Metallic LaNiO3 Films by Strain Control of Fermi-Surface Topology

Authors: Hyang Keun Yoo, Seung Ill Hyun, Luca Moreschini, Hyeong-Do Kim, Young Jun Chang, Chang Hee Sohn, Da Woon Jeong, Soobin Sinn, Yong Su Kim, Aaron Bostwick, Eli Rotenberg, Ji Hoon Shim, Tae Won Noh

Abstract: Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by m… ▽ More Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by misfit strain to change its fermiology. As tensile strain increases, the hole pocket centered at the A point elongates along the kz-axis and seems to become open, thus changing Fermi-surface (FS) topology from three- to quasi-two-dimensional. Concomitantly, the FS shape becomes flattened to enhance FS nesting. A FS superstructure with Q1 = (1/2,1/2,1/2) appears in all LNO films, while a tensile-strained LNO film has an additional Q2 = (1/4,1/4,1/4) modulation, indicating that some instabilities are present in metallic LNO films. Charge disproportionation and spin-density-wave fluctuations observed in other nickelates might be their most probable origins. △ Less

Submitted 4 March, 2015; v1 submitted 10 June, 2014; originally announced June 2014.

Comments: 5 pages, 4 figures, including Supplemental Material

Journal ref: Scientific Reports 5, 8746 (2015)

Phonon-assisted optical excitation in the narrow bandgap Mott insulator Sr3Ir2O7

Authors: H. J. Park, C. H. Sohn, D. W. Jeong, G. Cao, K. W. Kim, S. J. Moon, Hosub Jin, Deok-Yong Cho, T. W. Noh

Abstract: We examined the temperature (T) evolution of the optical conductivity spectra of Sr$_3$Ir$_2$O$_7$ over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\sim$0.1 eV. The low-energy features of the optical d-d excitation (${\hbar}ω$ $<$ 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge X-ray absorption spectra. T… ▽ More We examined the temperature (T) evolution of the optical conductivity spectra of Sr$_3$Ir$_2$O$_7$ over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\sim$0.1 eV. The low-energy features of the optical d-d excitation (${\hbar}ω$ $<$ 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge X-ray absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead, presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involve, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr$_3$Ir$_2$O$_7$. △ Less

Submitted 24 January, 2014; v1 submitted 7 January, 2014; originally announced January 2014.

Comments: submitted to Phys. Rev. B

Temperature Evolution of Itinerant Ferromagnetism in SrRuO3 Probed by Optical Spectroscopy

Authors: D. W. Jeong, Hong Chul Choi, Choong H. Kim, Seo Hyoung Chang, C. H. Sohn, H. J. Park, T. D. Kang, Deok-Yong Cho, S. H. Baek, C. B. Eom, J. H. Shim, J. Yu, K. W. Kim, S. J. Moon, T. W. Noh

Abstract: The temperature ($T$) dependence of the optical conductivity spectra $σ(ω)$ of a single crystal SrRuO$_3$ thin film is studied over a $T$ range from 5 to 450 K. We observed significant $T$ dependence of the spectral weights of the charge transfer and interband $d$-$d$ transitions across the ferromagnetic Curie temperature ($T_c$ ~ 150 K). Such $T$ dependence was attributed to the increase in the R… ▽ More The temperature ($T$) dependence of the optical conductivity spectra $σ(ω)$ of a single crystal SrRuO$_3$ thin film is studied over a $T$ range from 5 to 450 K. We observed significant $T$ dependence of the spectral weights of the charge transfer and interband $d$-$d$ transitions across the ferromagnetic Curie temperature ($T_c$ ~ 150 K). Such $T$ dependence was attributed to the increase in the Ru spin moment, which is consistent with the results of density functional theory calculations. $T$ scans of $σ(Ω, T)$ at fixed frequencies $Ω$ reveal a clear $T^2$ dependence below $T_c$, demonstrating that the Stoner mechanism is involved in the evolution of the electronic structure. In addition, $σ(Ω, T)$ continues to evolve at temperatures above $T_c$, indicating that the local spin moment persists in the paramagnetic state. This suggests that SrRuO$_3$ is an intriguing oxide system with itinerant ferromagnetism. △ Less

Submitted 4 September, 2013; originally announced September 2013.

Journal ref: Physical Review Letters, 110, 247202 (2013)

Dimensional crossover of the electronic structure in LaNiO3 ultrathin films: Orbital reconstruction, Fermi surface nesting, and the origin of the metal-insulator transition

Authors: Hyang Keun Yoo, Seung Ill Hyun, Luca Moreschini, Young Jun Chang, Da Woon Jeong, Chang Hee Sohn, Yong Su Kim, Hyeong-Do Kim, Aaron Bostwick, Eli Rotenberg, Ji Hoon Shim, Tae Won Noh

Abstract: Dimensionality control in the LaNiO3 (LNO) heterostructure has attracted attention due to its two-dimensional (2D) electronic structure was predicted to have an orbital ordered insulating ground state, analogous to that of the parent compound of high-Tc cuprate superconductors [P. Hansmann et al., Phys. Rev. Lett. 103, 016401 (2009)]. Here, we directly measured the electronic structure of LNO ultr… ▽ More Dimensionality control in the LaNiO3 (LNO) heterostructure has attracted attention due to its two-dimensional (2D) electronic structure was predicted to have an orbital ordered insulating ground state, analogous to that of the parent compound of high-Tc cuprate superconductors [P. Hansmann et al., Phys. Rev. Lett. 103, 016401 (2009)]. Here, we directly measured the electronic structure of LNO ultrathin films using in situ angle-resolved photoemission spectroscopy (ARPES). We recognized the dimensional crossover of the electronic structure around 3-unit cells (UC)-thick LNO film and observed the orbital reconstruction. However, complete orbital ordering was not achieved. Instead, we observed that the Fermi surface nesting effect became strong in the 2D LNO ultrathin film. These results indicated that the orbital reconstruction should be described by taking into account the strong nesting effect to search for the novel phenomena, such as superconductivity in 2D LNO heterostructure. In addition, the APRES spectra showed that the Fermi surface existed down to a 1-UC-thick film, which showed insulating behavior in transport measurements. We suggested that the metal-insulator transition in the transport properties may originate from Anderson localization. △ Less

Submitted 3 September, 2013; v1 submitted 3 September, 2013; originally announced September 2013.

Comments: 17 pages, 5 figures

Heat dissipation and its relation to thermopower in single-molecule junctions

Authors: Linda Angela Zotti, Marius Bürkle, Fabian Pauly, Woochul Lee, Kyeongtae Kim, Wonho Jeong, Yoshihiro Asai, Pramod Reddy, Juan Carlos Cuevas

Abstract: Motivated by recent experiments [Lee et al. Nature 498, 209 (2013)], we present here a detailed theoretical analysis of the Joule heating in current-carrying single-molecule junctions. By combining the Landauer approach for quantum transport with ab initio calculations, we show how the heating in the electrodes of a molecular junction is determined by its electronic structure. In particular, we sh… ▽ More Motivated by recent experiments [Lee et al. Nature 498, 209 (2013)], we present here a detailed theoretical analysis of the Joule heating in current-carrying single-molecule junctions. By combining the Landauer approach for quantum transport with ab initio calculations, we show how the heating in the electrodes of a molecular junction is determined by its electronic structure. In particular, we show that in general the heat is not equally dissipated in both electrodes of the junction and it depends on the bias polarity (or equivalently on the current direction). These heating asymmetries are intimately related to the thermopower of the junction as both these quantities are governed by very similar principles. We illustrate these ideas by analyzing single-molecule junctions based on benzene derivatives with different anchoring groups. The close relation between heat dissipation and thermopower provides general strategies for exploring fundamental phenomena such as the Peltier effect or the impact of quantum interference effects on the Joule heating of molecular transport junctions. △ Less

Submitted 31 July, 2013; originally announced July 2013.

Comments: 26 pages, 9 figures, submitted to New Journal of Physics

Journal ref: New J. Phys. 16, 015004 (2014)

Heat dissipation in atomic-scale junctions

Authors: Woochul Lee, Kyeongtae Kim, Wonho Jeong, Linda Angela Zotti, Fabian Pauly, Juan Carlos Cuevas, Pramod Reddy

Abstract: Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits. They are also ideal platforms to test quantum transport theories that are required to describe charge and energy transfer in novel functional nanodevices. Recent work has successfully probed electric and thermoelectric phenomena in atomic-scale junctions. However, heat dissipation and t… ▽ More Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits. They are also ideal platforms to test quantum transport theories that are required to describe charge and energy transfer in novel functional nanodevices. Recent work has successfully probed electric and thermoelectric phenomena in atomic-scale junctions. However, heat dissipation and transport in atomic-scale devices remain poorly characterized due to experimental challenges. Here, using custom-fabricated scanning probes with integrated nanoscale thermocouples, we show that heat dissipation in the electrodes of molecular junctions, whose transmission characteristics are strongly dependent on energy, is asymmetric, i.e. unequal and dependent on both the bias polarity and the identity of majority charge carriers (electrons vs. holes). In contrast, atomic junctions whose transmission characteristics show weak energy dependence do not exhibit appreciable asymmetry. Our results unambiguously relate the electronic transmission characteristics of atomic-scale junctions to their heat dissipation properties establishing a framework for understanding heat dissipation in a range of mesoscopic systems where transport is elastic. We anticipate that the techniques established here will enable the study of Peltier effects at the atomic scale, a field that has been barely explored experimentally despite interesting theoretical predictions. Furthermore, the experimental advances described here are also expected to enable the study of heat transport in atomic and molecular junctions, which is an important and challenging scientific and technological goal that has remained elusive. △ Less

Submitted 31 July, 2013; originally announced July 2013.

Comments: supporting information available in the journal web site or upon request

Journal ref: Nature 498, 209 (2013)

Charge states and magnetic ordering in LaMnO3/SrTiO3 superlattices

Authors: Woo Seok Choi, D. W. Jeong, S. S. A. Seo, Y. S. Lee, T. H. Kim, S. Y. Jang, H. N. Lee, K. Myung-Whun

Abstract: We investigated the magnetic and optical properties of [(LaMnO3)n/(SrTiO3)8]20 (n = 1, 2, and 8) superlattices grown by pulsed laser deposition. We found a weak ferromagnetic and semiconducting state developed in all superlattices. An analysis of the optical conductivity showed that the LaMnO3 layers in the superlattices were slightly doped. The amount of doping was almost identical regardless of… ▽ More We investigated the magnetic and optical properties of [(LaMnO3)n/(SrTiO3)8]20 (n = 1, 2, and 8) superlattices grown by pulsed laser deposition. We found a weak ferromagnetic and semiconducting state developed in all superlattices. An analysis of the optical conductivity showed that the LaMnO3 layers in the superlattices were slightly doped. The amount of doping was almost identical regardless of the LaMnO3 layer thickness up to eight unit cells, suggesting that the effect is not limited to the interface. On the other hand, the magnetic ordering became less stable as the LaMnO3 layer thickness decreased, probably due to a dimensional effect. △ Less

Submitted 2 March, 2011; originally announced March 2011.

Comments: 17 pages including 4 figures, accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 83, 195113 (2011)

Monovacancy-induced magnetism in graphene bilayers

Authors: Sangkook Choi, Byoung Wook Jeong, Seungchul Kim, Gunn Kim

Abstract: Vacancy-induced magnetism in graphene bilayers is investigated using spin-polarized density functional theory calculations. One of two graphene layers has a monovacancy. Two atomic configurations for bilayers are considered with respect to the position of the monovacancy. We find that spin magnetic moments localized at the vacancy site decreases by ~10 % for our two configurations, compared with… ▽ More Vacancy-induced magnetism in graphene bilayers is investigated using spin-polarized density functional theory calculations. One of two graphene layers has a monovacancy. Two atomic configurations for bilayers are considered with respect to the position of the monovacancy. We find that spin magnetic moments localized at the vacancy site decreases by ~10 % for our two configurations, compared with the graphene monolayer with a monovacancy. The reduction of the spin magnetic moment in the graphene bilayers is attributed to the interlayer charge transfer from the adjacent layer to the layer with the monovacancy, compensating for spin magnetic moments originating from quasilocalized defect state. △ Less

Submitted 15 May, 2008; originally announced May 2008.

Comments: 14 pages, 1 table and 4 figures

Journal ref: Physics: Condensed Matter v.20, p.235220 (2008)