arXiv subjects

Young-Woo Son

Publications and source records attributed to Young-Woo Son.

At least 19 recordsLinked to original sources

arXiv2026

First-principles carrier mobility and optical absorption of strained ZnO with self-consistent Hubbard interactions

Carrier mobility and optical absorption are key performance parameters of oxide semiconductors in transparent and flexible displays. We use a newly developed density-functional perturbation theory with a self-consistent Hubbard correction (DFPT+U) to study phonon-limited electron transport and phonon-assisted optical absorption in strained zinc oxide (ZnO). This parameter-free approach accounts for electron-phonon interactions and on-site correlation effects simultaneously. Electronic structures and phonon dispersions are computed under three distinct uniaxial strain directions. Uniaxial tensile strain up to 4.8% along [\bar110] is found to increase the room-temperature electron mobility by 19% while leaving visible-range optical absorption essentially unchanged. These results demonstrate that moderate strain can selectively enhance carrier transport without degrading optical transparency, and establish DFPT+U as an effective framework for predicting strain-dependent transport and optical properties in wide-band-gap oxides with implications for strain-engineered display and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Unconventional and Fragile Magnetic Exciton in a van der Waals Quantum Magnet

The recently discovered magnetic exciton in the van der Waals (vdW) antiferromagnet NiPS3 exemplifies these phenomena, exhibiting several distinctive characteristics. Despite extensive investigation, much of its physics remains unresolved, with key questions about why the NiPS3 magnetic exciton is so sharp and optically bright despite the nominally spin-forbidden transition, posing significant challenges to a proper understanding and practical manipulation of the exciton. An urgent question is to what extent it is due to chemical disorder, magnetic weakening, lattice modification, or intrinsic instability of the bright exciton itself: answers to which will put stringent constraints on possible theoretical models. Here we address these questions using hydrostatic pressure as a clean, continuous, reversible, and in-situ tuning parameter. We find that the sharp photoluminescence peak is drastically suppressed by as little as 0.4 GPa and completely quenched by 1.5 GPa, with demonstrating its reversibility. Crucially, this bright-to-dark conversion occurs without magnetic, crystallographic, or electronic reconstruction despite an increase in the Neel temperature, as established by Raman, X-ray absorption, nuclear magnetic resonance spectroscopy, and first-principles many-body calculations. Our results demonstrate that the optical brightness of the magnetic exciton is independent of chemical disorder, lattice expansion, and weakening of magnetic order, indicating that a higher-order correlated mechanism governs the bright exciton. We further propose experimentally constrained microscopic scenarios involving exciton pairing, crystal-field-controlled spin-orbit mixing, and symmetry breaking, providing a framework for future tests of entangled magnetic exciton in correlated quantum magnets.

cond-mat.mtrl-sci↗

arXiv2026

Super Moiré Domain Tessellations, Sliding Ferroelectricity, and Reconfigurable Quantum Dot Arrays in Twisted Trilayer Hexagonal Boron Nitride

At very small twist angles, bilayer moiré systems exhibit characteristic stacking domain patterns, where the moiré length scale is determined solely by the twist angle. In contrast, the additional stacking and twisting degrees of freedom in twisted trilayer systems give rise to richer and more intricate domain tessellations. In twisted trilayer hexagonal boron nitride (TTBN), the interplay between polar and nonpolar domains and their domain walls is shown to result in unconventional responses to external electric fields, including electric-field tunability of the moiré-of-moiré or super moiré pattern--features absent in bilayer counterparts. We demonstrate that at the vertices of super moiré domains, TTBN can support arrays of quantum dots hosting localized quantum harmonic oscillator (QHO) states with diverse spatial symmetries. Futhermore, we show that the shape of the array and the spacing between the localized QHO states can be dynamically reconfigured by electric fields, enabling facile switching between fully isolated and strongly coupled regimes. The local potentials for the quantum dot state are predicted to be sufficiently deep to support a series of QHO states with nonzero angular momentum. This tunability enables control over the transport of quantum dot states and their interdot coupling, facillitating long-range quantum state transfer. Combined with the feasibility of large-scale fabrication of homogeneous twisted trilayer materials, these properties position TTBN as a promising platform for a wide range of quantum technologies.

cond-mat.mes-hall↗

arXiv2025

Comparing Hubbard parameters from linear-response theory and Hartree-Fock-based approach

Density-functional theory with on-site $U$ and inter-site $V$ Hubbard corrections (DFT+$U$+$V$) is a powerful and accurate method for predicting various properties of transition-metal compounds. However, its accuracy depends critically on the values of these Hubbard parameters. Although they can be determined empirically, first-principles methods provide a more consistent and reliable approach; yet, their results can vary, and a comprehensive comparison between methods is still lacking. Here, we present a systematic comparison of two widely used approaches for computing $U$ and $V$, namely linear-response theory (LRT) and the Hartree-Fock-based pseudohybrid functional formalism, applied to a representative set of oxides (MnO, NiO, CoO, FeO, BaTiO$_3$, ZnO, and ZrO$_2$). We find that for partially occupied transition-metal $d$ states, these two methods yield consistent $U$ values, but they differ for nearly empty or fully filled $d$ shells. For O-$2p$ states, LRT always predicts large $U$ values ($\sim$10 eV), whereas the pseudohybrid formalism produces system-dependent values depending on the level of localization and hybridization for the electronic states. Even larger differences are found for the inter-site $V$: the former predicts consistently small values ($<1$ eV), while the latter produces larger values ($\sim3$ eV), reflecting its explicit dependence on relative charge redistribution. Our results show that while parallels between these two methods exist, they rely on distinct assumptions for determining $U$ and $V$, leading to variations in predictions of material properties.

cond-mat.mtrl-sci↗

arXiv2025

Probing Intrinsic Elastic Properties of Multilayer Graphene -- a New Mechanical Constant

We present measurements on in-plane Young's modulus and the Grüneisen parameter of multilayer graphene with varying number of layers, obtained through {\it in situ} bulge tests. Accurate determination of their elastic parameters poses a significant experimental challenge due to the substantial differences in mechanical behavior between intra- and inter-layers. To address this, we develop a novel theoretical model with first-principles calculations to investigate thickness-dependent incomplete strain transfer between the layers. Our findings show that the experimentally measured elastic constants, which deviate from computed intrinsic values, fail to fully capture ideal mechanical couplings between layers. As a solution, we propose a new mechanical modulus that integrates the Grüneisen parameter and in-plane Young's modulus, providing a more reliable representation of their mechanical properties, independent of unavoidable interlayer effects.

cond-mat.mtrl-sci↗

arXiv2025

Self-consistent tight-binding calculations with extended Hubbard interactions in rhombohedral multilayer graphene

We study the mean-field broken symmetry phases of charge neutral multilayer rhombohedral graphene within tight-binding approximations including self-consistent extended Hubbard interactions. We used on-site and inter-site Hubbard interactions obtained from a newly developed first-principles calculation method. Our calculations for systems up to eight layers give rise to electron-hole asymmetries, band flatness, band gaps, and layer anti-ferromagnetic ground states in keeping with available experiments. By including the intersite Hubbard interactions up to the next-nearest neighboring sites, the band gaps are shown to open when the number of layers is larger than three, while the trilayer system maintains its metallic nature with two low energy density of state peaks near the Fermi energy whose separation increases with the range of inter-site Hubbard parameters. Within our framework, the calculated band gaps reflect mean-field ground states with extended Hubbard interactions, in closer agreement with experimental estimates. The tight-binding formulation further enables efficient treatment of large rhombohedral chiral systems, including twisted multilayer graphene.

cond-mat.mes-hall↗

arXiv2025

First-principles electron-phonon interactions with self-consistent Hubbard interaction: an application to transparent conductive oxides

The ab initio computational method known as Hubbard-corrected density functional theory (DFT+$U$) captures well ground electronic structures of a set of solids that are poorly described by standard DFT alone. Since lattice dynamical properties are closely linked to electronic structures, the Hubbard-corrected density functional perturbation theory (DFPT+$U$) can calculate them at the same level of accuracy. To investigate the effects of $U$ on electron-phonon (el-ph) interactions, we implemented DFPT+$U$ with a Hartree-Fock-based pseudohybrid functional formalism to determine $U$ self-consistently and applied our method to compute optical and transport properties of transparent conductive oxides of CdO and ZnO. For CdO, we find that opening a band gap due to $U$ restores the long-range Fröhlich interaction and that its calculated mobility and absorption spectrum are in excellent agreement with experiments. For ZnO where a band gap already appears at the DFT level, DFPT+$U$ brings the results into much closer alignment with experiment, thus demonstrating improved accuracy of our method in dealing with el-ph interactions in these technologically important materials.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional domain tessellations in moiré-of-moiré lattices

Imposing incommensurable periodicity on the periodic atomic lattice can lead to complex structural phases consisting of locally periodic structure bounded by topological defects. Twisted trilayer graphene (TTG) is an ideal material platform to study the interplay between different atomic periodicities, which can be tuned by twist angles between the layers, leading to moiré-of-moiré lattices. Interlayer and intralayer interactions between two interfaces in TTG transform this moiré-of-moiré lattice into an intricate network of domain structures at small twist angles, which can harbor exotic electronic behaviors. Here we report a complete structural phase diagram of TTG with atomic scale lattice reconstruction. Using transmission electron microscopy combined with a new interatomic potential simulation, we show several large-scale moiré lattices, including triangular, kagome, and a corner-shared hexagram-shaped domain pattern. Each domain is bounded by a two-dimensional network of domain wall lattices. In the limit of small twist angles, two competing structural orders-rhombohedral and Bernal stackings-with a slight energy difference, cause unconventional lattice reconstruction with spontaneous symmetry breaking and nematic instability, highlighting the importance of long-range interlayer interactions across entire van der Waals layers. The diverse tessellation of distinct domains, whose topological network can be tuned by the adjustment of the twist angles, establishes TTG as a platform for exploring the interplay between emerging quantum properties and controllable nontrivial lattices.

cond-mat.mtrl-sci↗

arXiv2025

Electronic structures of crystalline and amorphous GeSe and GeSbTe compounds using machine learning empirical pseudopotentials

The newly developed machine learning (ML) empirical pseudopotential (EP) method overcomes the poor transferability of the traditional EP method with the help of ML techniques while preserving its formal simplicity and computational efficiency. We apply the new method to binary and ternary systems such as GeSe and Ge-Sb-Te (GST) compounds, well-known materials for non-volatile phase-change memory and related technologies. Using a training set of {\it ab initio} electronic energy bands and rotation-covariant descriptors for various GeSe and GST compounds, we generate transferable EPs for Ge, Se, Sb, and Te. We demonstrate that the new ML model accurately reproduces the energy bands and wavefunctions of structures outside the training set, closely matching first-principles calculations. This accuracy is achieved with significantly lower computational costs due to the elimination of self-consistency iterations and the reduced size of the plane-wave basis set. Notably, the method maintains accuracy even for diverse local atomic environments, such as amorphous phases or larger systems not explicitly included in the training set.

cond-mat.mtrl-sci↗

arXiv2025

A DFT+$U$+$V$ study of pristine and oxygen-deficient HfO$_2$ with self-consistent Hubbard parameters

HfO$_2$-based ferroelectrics have emerged as promising materials for advanced nanoelectronics, with their robust polarization and silicon compatibility making them ideal for high-density, non-volatile memory applications. Oxygen vacancies, particularly in positively charged states, are suggested to profoundly impact the polymorphism kinetics and phase stability of hafnia, thereby affecting its ferroelectric behavior. The electronic structures of pristine and oxygen-deficient hafnia polymorph have been extensively studied using density functional theory, primarily employing (semi-)local exchange-correlation functionals. However, these methods often underestimate band gaps and may not accurately capture the localized nature of $d$-electrons. In this work, we investigate hafnia in various phases using DFT + $U$ + $V$, with onsite $U$ and intersite $V$ Hubbard parameters computed self-consistently via the pseudohybrid Hubbard density functional, ACBN0, and its extended version eACBN0. We find that the self-consistent DFT + $U$ method provides comparable accuracy to the computationally more expensive Heyd-Scuseria-Ernzerhof (HSE) hybrid density functional in predicting relative thermodynamic stability, band gaps, and density of states. Furthermore, it is a cost-effective approach for estimating the formation energies of oxygen vacancies. Additionally, we demonstrate that environmentally dependent Hubbard parameters serve as useful indicators for analyzing bond strengths and electronic structures in real space.

cond-mat.mtrl-sci↗

arXiv2025

First-principles study of dielectric properties of ferroelectric perovskite oxides with on-site and inter-site Hubbard interactions

We study the atomic and electronic structures of ferroelectric perovskite oxides, BaTiO$_3$, LiNbO$_3$, and PbTiO$_3$ using ab initio extended Hubbard functionals in which the on-site and inter-site Hubbard interactions are determined self-consistently, adapted from the pseudohybrid density functional proposed by Agapito-Curtarolo-Buongiorno Nardelli. Band structures, ferroelectric distortions, polarization, Born effective charges, and switching barriers are calculated with extended Hubbard functionals, that are compared with those using local density approximation (LDA), generalized gradient approximation (GGA), and Hybrid (HSE06) functionals. The properties of all three compounds calculated by extended Hubbard functionals are in good agreement with experimental data. We find a substantial increase in band gaps due to the inter-site Coulomb interactions, which show better agreement with $GW$ results compared to those from LDA and GGA functionals. The crucial role of the inter-site Coulomb interactions in restoring the suppressed polar instability, which is computed when only the on-site Hubbard interactions are considered, is also highlighted. Overall, we find that the properties calculated using our extended Hubbard functionals exhibit trends similar to those obtained with the HSE06 functional, while reducing computational costs by over an order of magnitude. Thus, we propose that the current method is well-suited for high-throughput calculations for perovskite oxides, offering significantly improved accuracy in computing band gap and other related physical properties such as the shift current photovoltaic effect and band alignments in ferroelectric heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

Effects of self-consistent extended Hubbard interactions and spin-orbit couplings on energy bands of semiconductors and topological insulators

A first-principles computational method with self-consistent on-site and inter-site Hubbard functionals is able to treat local and non-local Coulomb interactions on an equal footing. To apply the method to understand solids with strong spin-orbit coupling (SOC), we have extended a psuedohybrid functional approach developed by Agapito-Curtarolo-Buongiorno Nardelli to implement self-consistent extended Hubbard energy functionals for noncollinear spin states. With this, energy bands of semiconductors with various SOC strengths such as Si, Ge, GaAs, GaSb, CdSe and PdO are obtained, agreeing with results from fully relativistic $GW$ approximation (FR-GWA) as well as experiments. We also compute energy gaps of HgTe, CuTlS$_2$, and CuTlSe$_2$ and assign them to be topological insulators correctly, unlike characteristic failures for judging topological properties from typical hybrid functionals. We demonstrate feasibility of our method to handle large systems by computing surface bands of topological insulators, Bi$_{\text{2}}$Se$_{\text{3}}$ and Bi$_2$Te$_3$ with varying thickness up to eight quintuple layers. Considering its low computational cost comparable to conventional {\it ab intio} methods and improved accuracy to FR-GWA, we expect that our method provides an opportunity to study large scale correlated systems with the strong SOC efficiently and reliably.

cond-mat.mtrl-sci↗

arXiv2024

Transferable empirical pseudopotenials from machine learning

Machine learning is used to generate empirical pseudopotentials that characterize the local screened interactions in the Kohn-Sham Hamiltonian. Our approach incorporates momentum-range-separated rotation-covariant descriptors to capture crystal symmetries as well as crucial directional information of bonds, thus realizing accurate descriptions of anisotropic solids. Trained empirical potentials are shown to be versatile and transferable such that the calculated energy bands and wave functions without cumbersome self-consistency reproduce conventional ab initio results even for semiconductors with defects, thus fostering faster and faithful data-driven materials researches.

cond-mat.mtrl-sci↗

arXiv2023

Condensation of preformed charge density waves in kagome metals

Charge density wave (CDW) is a spontaneous spatial modulation of electric charges in solids whose general microscopic descriptions are yet to be completed. Layered kagome metals of $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) provide a unique chance to realize its emergence intertwined with dimensional effects as well as their special lattice. Here, based on a state-of-the-art molecular dynamics simulation, we uncover that the phase transition to charge ordered states in kagome metals is a condensation process of incoherently preformed CDWs. We demonstrate that charge modulation first preforms on each kagome layer at a well defined temperature but its phase fluctuation proliferates across the entire layers with a $10^5$ times slower frequency than typical phonon vibrations until reaching its freezing temperature. We find that the fluctuation is not random but confined to a limited number of states as a consequence of unavoidable degeneracy in stacking layered charge orders. As the size of interfacial alkali atom increases, the fluctuating phases are shown to counterbalance the condensation of orderings, resulting in a maximized transition temperature for RbV$_3$Sb$_5$. Our results resolve several controversial observations on their CDW formations and highlight a crucial role of interlayer interactions for the charge ordering in kagome metals.

cond-mat.mtrl-sci↗

arXiv2023

Rapid suppression of quantum many-body magnetic exciton in doped van der Waals antiferromagnet (Ni,Cd)PS3

The unique discovery of magnetic exciton in van der Waals antiferromagnet NiPS3 arises between two quantum many-body states of a Zhang-Rice singlet excited state and a Zhang-Rice triplet ground state. Simultaneously, the spectral width of photoluminescence originating from this exciton is exceedingly narrow as 0.4 meV. These extraordinary properties, including the extreme coherence of the magnetic exciton in NiPS3, beg many questions. We studied doping effects using Ni1-xCdxPS3 using two experimental techniques and theoretical studies. Our experimental results show that the magnetic exciton is drastically suppressed upon a few % Cd doping. All these happen while the width of the exciton only gradually increases, and the antiferromagnetic ground state is robust. These results highlight the lattice uniformity's hidden importance as a prerequisite for coherent magnetic exciton. Finally, an exciting scenario emerges: the broken charge transfer forbids the otherwise uniform formation of the coherent magnetic exciton in (Ni,Cd)PS3.

cond-mat.str-el↗

arXiv2023

Intersite Coulomb Interactions in Charge Ordered Systems

Using {\it ab initio} approaches for extended Hubbard interactions coupled to phonons, we reveal that the intersite Coulomb interaction plays important roles in determining various distinctive phases of the paradigmatic charge ordered materials of Ba$_{1-x}$K$_x A$O$_3$ ($A=$ Bi and Sb). We demonstrated that all their salient doping dependent experiment features such as breathing instabilities, anomalous phonon dispersions, and transition between charge-density wave and superconducting states can be accounted very well if self-consistently obtained nearest neighbor Hubbard interaction are included, thus establishing a minimal criterion for reliable descriptions of spontaneous charge orders in solids.

cond-mat.str-el↗

arXiv2022

Metal to Mott Insulator Transition in Two-dimensional 1T-TaSe$_2$

When electron-electron interaction dominates over other electronic energy scales, exotic, collective phenomena often emerge out of seemingly ordinary matter. The strongly correlated phenomena, such as quantum spin liquid and unconventional superconductivity, represent a major research frontier and a constant source of inspiration. Central to strongly correlated physics is the concept of Mott insulator, from which various other correlated phases derive. The advent of two-dimensional (2D) materials brings unprecedented opportunities to the study of strongly correlated physics in the 2D limit. In particular, the enhanced correlation and extreme tunability of 2D materials enables exploring strongly correlated systems across uncharted parameter space. Here, we discover an intriguing metal to Mott insulator transition in 1T-TaSe$_2$ as the material is thinned down to atomic thicknesses. Specifically, we discover, for the first time, that the bulk metallicity of 1T-TaSe$_2$ arises from a band crossing Fermi level. Reducing the dimensionality effectively quenches the kinetic energy of the initially itinerant electrons and drives the material into a Mott insulating state. The dimensionality-driven Metal to Mott insulator transition resolves the long-standing dichotomy between metallic bulk and insulating surface of 1T-TaSe$_2$. Our results additionally establish 1T-TaSe$_2$ as an ideal variable system for exploring various strongly correlated phenomena.

cond-mat.str-el↗

arXiv2022

Lattice dynamical properties of antiferromagnetic oxides calculated using self-consistent extended Hubbard functional method

We study the lattice dynamics of antiferromagnetic transition-metal oxides by using self-consistent Hubbard functionals. We calculate the ground states of the oxides with the on-site and intersite Hubbard interactions determined self-consistently within the framework of density functional theory. The on-site and intersite Hubbard terms fix the errors associated with the electron self-interaction in the local and semilocal functionals. Inclusion of the intersite Hubbard terms in addition to the on-site Hubbard terms produces accurate phonon dispersion of the transition-metal oxides. Calculated Born effective charges and high-frequency dielectric constants are in good agreement with experiment. Our study provides a computationally inexpensive and accurate set of first-principles calculations for strongly-correlated materials and related phenomena.

cond-mat.str-el↗