arXiv subjects

Yongkyung Kwon

Publications and source records attributed to Yongkyung Kwon.

18 recordsLinked to original sources

arXiv2025

Quantum Monte Carlo Benchmarking of Molecular Adsorption on Graphene-Supported Single Pt Atom

The precise understanding of adsorption energetics and molecular geometry at catalytic sites is fundamental for advancing catalysis, particularly under the constraints of resource efficiency and environmental sustainability. This study benchmarks the performance of density functional theory (DFT) calculations against diffusion Monte Carlo (DMC) calculations for adsorption properties of small gas molecules relevant to CO oxidation -- namely O$_2$, CO, CO$_2$, and atomic oxygen -- on a single Pt atom supported by pristine graphene. Our findings reveal that DMC calculations provide a significantly different landscape of adsorption energetics compared to DFT results. Notably, DFT predicts different lowest-energy configurations and spin states, particularly for O$_2$, which suggests potential discrepancies in predicting the catalytic behavior. Furthermore, this study identifies the critical issue of CO poisoning, highlighted by the large disparity between the DMC adsorption energies of O$_2$ ($-1.23(2)$ eV) and CO ($-3.37(1)$ eV), which can inhibit the catalytic process. These results emphasize the necessity for more sophisticated computational approaches in catalysis research, aiming to refine the prediction accuracy of reaction mechanisms and to enhance the design of more effective catalysts.

cond-mat.mtrl-sci↗

arXiv2025

Toward improved property prediction of 2D materials using many-body quantum Monte Carlo methods

The field of two-dimensional (2D) materials has grown dramatically in the last two decades. 2D materials can be utilized for a variety of next-generation optoelectronic, spintronic, clean energy, and quantum computation applications. These 2D structures, which are often exfoliated from layered van der Waals (vdW) materials, possess highly inhomogeneous electron densities and can possess short- and long-range electron correlations. The complexities of 2D materials make them challenging to study with standard mean-field electronic structure methods such as density functional theory (DFT), which relies on approximations for the unknown exchange-correlation functional. In order to overcome the limitations of DFT, highly accurate many-body electronic structure approaches such as Diffusion Monte Carlo (DMC) can be utilized. In the past decade, DMC has been used to calculate accurate magnetic, electronic, excitonic, and topological properties in addition to accurately capturing interlayer interactions and cohesion and adsorption energetics of 2D materials. This approach has been applied to 2D systems of wide interest including graphene, phosphorene, MoS$_2$, CrI$_3$, VSe$_2$, GaSe, GeSe, borophene, and several others. In this review article, we highlight some successful recent applications of DMC to 2D systems for improved property predictions beyond standard DFT.

cond-mat.mtrl-sci↗

arXiv2024

Stacking Polymorphism of PtSe$_{2}$: Its Implication to Layer-dependent Metal-insulator Transitions

Using diffusion Monte Carlo (DMC) and density functional theory (DFT) calculations, we examined the structural stability and interlayer binding properties of PtSe$_2$, a representative transition metal dichalcogenide (TMD) with strong interlayer interaction. Our DMC results for the bilayer revealed that AA and AB-r stacking modes are nearly degenerate, highlighting the significant role of interlayer hybridization in offsetting the energy cost due to larger interlayer separations in the AB-r mode. Additionally, our DMC-benchmarked DFT studies with the r$^2$SCAN+rVV10 functional demonstrated pronounced stacking polymorphism in few-layer PtSe$_2$, suggesting the potential for stacking faults and the formation of grain boundaries between different stacking domains which could develop metallic electronic structures. Thus this polymorphism, along with selenium vacancies, influences a layer-dependent metal-insulator transition observed in few-layer PtSe$_2$. Our findings emphasize the importance of both van der Waals interactions and interlayer hybridization in determining the phase stability and electronic properties of TMDs, advancing our understanding of their fundamental properties and refining theoretical models for practical applications in nanoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Diffusion Monte Carlo Study of the O$_2$ Adsorption on a Single Layer Graphene

Diffusion Monte Carlo (DMC) calculations were performed for an accurate description of the nature of the O$_2$ adsorption on a single layer graphene. We investigated the stable orientation of O$_2$ at a specific adsorption site as well as its equilibrium adsorption energy. At equilibrium adsorption distances, an O$_2$ molecule was found to prefer a horizontal orientation, where the O-O bond is parallel to the graphene surface, to the vertical orientation. However, the vertical orientation is favored at the O$_2$-graphene distances shorter than the equilibrium distance, which could be understood by the steric repulsion between O and C atoms. Contrary to previous DFT calculations, our DMC calculations show that the midpoint of a C-C bond (a bridge site) is energetically preferred for the O$_2$ adsorption to a center of a hexagonal ring (a hollow site). The lowest DMC adsorption energy was found at an intermediate point between a hollow and a bridge site, where the O$_2$ adsorption energy was estimated to be -0.142(4) eV that was in very good agreement with the recently-reported experimental value. Finally, we have found that O$_2$ is very diffusive on the surface of graphene with the diffusion barrier along a bridge-hollow-bridge path being as small as ~ 11 meV.

cond-mat.mtrl-sci↗

arXiv2018

Spatial distribution of superfluidity and superfluid distillation of Bose liquids

Under the assumption of two fluid kinematics of a nonrelativistic Bose liquid in the presence of a local velocity field $v(x)$, local Galilei transformations are used to derive formulas for the spatial distribution of superfluidity. The local formulation is shown to subsume several descriptions of superfluidity, from Landau's free quasiparticle picture of the normal fluid to the fully microscopic winding number formula for superfluid density. We derive the superfluid distribution of generic pure states of 1-d bosonic systems by using the continuum analog of matrix product states. With a view toward spatially structured superfluid-based quantum devices, we consider the limits to local distillation of superfluidity within the framework of localized resource theories of quantum coherence.

quant-ph↗

arXiv2017

The Nature of Interlayer Binding and Stacking of $sp$-$sp^{2}$ Hybridized Carbon Layers: A Quantum Monte Carlo Study

$α$-graphyne is a two-dimensional sheet of $sp$-$sp^2$ hybridized carbon atoms in a honeycomb lattice. While the geometrical structure is similar to that of graphene, the hybridized triple bonds give rise to electronic structure that is different from that of graphene. Similar to graphene, $α$-graphyne can be stacked in bilayers with two stable configurations, but the different stackings have very different electronic structures: one is predicted to have gapless parabolic bands and the other a tunable band gap which is attractive for applications. In order to realize applications, it is crucial to understand which stacking is more stable. This is difficult to model, as the stability is a result of weak interlayer van der Waals interactions which are not well captured by density functional theory (DFT). We have used quantum Monte Carlo simulations that accurately include van der Waals interactions to calculate the interlayer binding energy of bilayer graphyne and to determine its most stable stacking mode. Our results show that interlayer bindings of $sp$- and $sp^{2}$-bonded carbon networks are significantly underestimated in a Kohn-Sham DFT approach, even with an exchange-correlation potential corrected to include, in some approximation, van der Waals interactions. Finally, our quantum Monte Carlo calculations reveal that the interlayer binding energy difference between the two stacking modes is only 0.9(4) meV/atom. From this we conclude that the two stable stacking modes of bilayer $α$-graphyne are almost degenerate with each other, and both will occur with about the same probability at room temperature unless there is a synthesis path that prefers one stacking over the other.

cond-mat.mtrl-sci↗

arXiv2016

Path integral Monte Carlo simulation of global and local superfluidity in liquid $^{4}$He reservoirs separated by nanoscale apertures

We present a path integral Monte Carlo study of the global superfluid fraction and local superfluid density in cylindrically-symmetric reservoirs of liquid $^{4}$He separated by nanoaperture arrays. The superfluid response to both translations along the axis of symmetry (longitudinal response) and rotations about the cylinder axis (transverse response) are computed, together with radial and axial density distributions that reveal the microscopic inhomogeneity arising from the combined effects of the confining external potential and the $^4$He-$^4$He interatomic potentials. We make a microscopic determination of the length-scale of decay of superfluidity at the radial boundaries of the system by analyzing the local superfluid density distribution to extract a displacement length that quantifies the superfluid mass displacement away from the boundary. We find that the longitudinal superfluid response is reduced in reservoirs separated by a septum containing sufficiently small apertures compared to a cylinder with no intervening aperture array, for all temperatures below $T_λ$. For a single aperture in the septum, a significant drop in the longitudinal superfluid response is seen when the aperture diameter is made smaller than twice the empirical temperature-dependent $^4$He healing length, consistent with the formation of a weak link between the reservoirs. Increasing the diameter of a single aperture or the number of apertures in the array results in an increase of the superfluid density toward the expected bulk value.

cond-mat.quant-gas↗

arXiv2016

High-throughput screening of metal-porphyrin-like graphenes for selective capture of carbon dioxide

Nano-materials, such as metal-organic frameworks, have been considered to capture CO$_2$. However, their application has been limited largely because they exhibit poor selectivity for flue gases and low capture capacity under low pressures. We perform a high-throughput screening for selective CO$_2$ capture from flue gases by using first principles thermodynamics. We find that elements with empty d orbitals selectively attract CO$_2$ from gaseous mixtures under low CO$_2$ pressures at 300 K and release it at ~450 K. CO$_2$ binding to elements involves hybridization of the metal d orbitals with the CO$_2$ $π$ orbitals and CO$_2$-transition metal complexes were observed in experiments. This result allows us to perform high-throughput screening to discover novel promising CO$_2$ capture materials with empty d orbitals and predict their capture performance under various conditions. Moreover, these findings provide physical insights into selective CO$_2$ capture and open a new path to explore CO$_2$ capture materials.

cond-mat.mtrl-sci↗

arXiv2015

Interlayer correlation between two $^4$He monolayers adsorbed on both sides of $α$-graphyne

Path-integral Monte Carlo calculations have been performed to study the $^4$He adsorption on both sides of a single $α$-graphyne sheet. For investigation of the interlayer correlation between the upper and the lower monolayer of $^4$He adatoms, the $^4$He-substrate interaction is described by the sum of the $^4$He-C interatomic pair potentials, for which we use both Lennard-Jones and Yukawa-6 anisotropic potentials. When the lower $^4$He layer is a C$_{4/3}$ commensurate solid, the upper-layer $^4$He atoms are found to form a Kagomé lattice structure at a Mott insulating density of 0.0706 Å$^{-2}$, and a commensurate solid at an areal density of 0.0941 Å$^{-2}$ for both substrate potentials. The correlation between upper- and lower-layer pseudospins, which were introduced in Ref. [1] for two degenerate configurations of three $^4$He atoms in a hexagonal cell, depends on the substrate potential used; With the substrate potential based on the anisotropic Yukawa-6 pair potentials, the Ising pseudo-spins of both $^4$He layers are found to be anti-parallel to each other while the parallel and anti-parallel pseudo-spin alignments between the two $^4$He layers are nearly degenerate with the Lennard-Jones potentials. This is attributed to the difference in the interlayer distance, which is $\sim 4$ Å~ with the Yukawa-6 substrate potential but as large as $\sim 4.8$ Å~with the Lennard-Jones potential. [1] Y. Kwon, H. Shin, and H. Lee, Phys. Rev. B 88, 201403(R) (2013)

cond-mat.mtrl-sci↗

arXiv2014

Commensurate-incommensurate solid transition in the $^4$He monolayer on $γ$-graphyne

Path-integral Monte Carlo calculations have been performed to study the $^4$He adsorption on $γ$-graphyne, a planar network of benzene rings connected by acetylene bonds. Assuming the $^4$He-substrate interaction described by a pairwise sum of empirical $^4$He-carbon interatomic potentials, we find that unlike $α$-graphyne, a single sheet of $γ$-graphyne is not permeable to $^4$He atoms in spite of its large surface area. One-dimensional density distributions computed as a function of the distance from the graphyne surface reveal a layer-by-layer growth of $^4$He atoms. A partially-filled $^4$He monolayer is found to exhibit different commensurate solid structures depending on the helium coverage; it shows a C$_{2/3}$ commensurate structure at an areal density of 0.0491Å$^{-2}$, a C$_{3/3}$ structure at 0.0736Å$^{-2}$, and a C$_{4/3}$ structure at 0.0982Å$^{-2}$. While the promotion to the second layer starts beyond the C$_{4/3}$ helium coverage, the first $^4$He layer is found to form an incommensurate triangular solid when compressed with the development of the second layer.

cond-mat.mtrl-sci↗

arXiv2014

Density-Functional Theory and Tight-Binding Studies of the Geometry of Hydrogen Adsorbed on Graphynes

Using density-functional theory and a tight-binding approach we investigate the physical origin of distinct favourable geometries of adsorbed hydrogen atoms in various graphyne structures, and the relation with electronic properties. In particular, H atoms are adsorbed in-plane for $α$-graphyne, and they assume an oblique configuration in all other graphynes, including 6,6,12-graphyne. The origin of different configurations is identified by means of a simple tight-binding model and it is controlled by the tuning of the hopping between sp$^2$-bonded C atoms and sp-bonded C atoms hybridized with the H atoms. We discuss in details how the geometry change of the attached H atom tunes the electronic properties like energy gap.

cond-mat.mtrl-sci↗

arXiv2014

Semiclassical approximation solved by Monte Carlo as an efficient impurity solver for dynamical mean field theory and its cluster extensions

We propose that a combination of the semiclassical approximation with Monte Carlo simulations can be an efficient and reliable impurity solver for dynamical mean field theory equations and their cluster extensions with large cluster sizes. In order to show the reliability of the method, we consider two test cases: (i) the single-band Hubbard model within the dynamical cluster approximation with 4- and 8-site clusters and (ii) the anisotropic two-orbital Hubbard model with orbitals of different band width within the single-site dynamical mean field theory. We compare our results with those obtained from solving the dynamical mean field equations with continuous time and determinant quantum Monte Carlo. In both test cases we observe reasonable values of the metal-insulator critical interaction strength $U_c/t$ and, while some details of the spectral functions cannot be captured by the semiclassical approximation due to the freezing of dynamical fluctuations, the main features are reproduced by the approach.

cond-mat.str-el↗

arXiv2014

Cohesion Energetics of Carbon Allotropes : Quantum Monte Carlo Study

We have performed quantum Monte Carlo calculations to study the cohesion energetics of carbon allotropes, including $sp^3$-bonded diamond, $sp^2$-bonded graphene, $sp$-$sp^2$ hybridized graphynes, and $sp$-bonded carbyne. The computed cohesive energies of diamond and graphene are found to be in excellent agreement with the corresponding values determined experimentally for diamond and graphite, respectively, when the zero-point energies, along with the interlayer binding in the case of graphite, are included. We have also found that the cohesive energy of graphyne decreases systematically as the ratio of $sp$-bonded carbon atoms increases. The cohesive energy of $γ$-graphyne, the most energetically-stable graphyne, turns out to be 6.766(6) eV/atom, which is smaller than that of graphene by 0.698(12) eV/atom. Experimental difficulty in synthesizing graphynes could be explained by their significantly smaller cohesive energies. Finally we conclude that the cohesive energy of a newly-proposed graphyne can be accurately estimated with the carbon-carbon bond energies determined from the cohesive energies of graphene and three different graphynes considered here.

cond-mat.mtrl-sci↗

arXiv2013

Mott-insulator to commensurate-solid transition in a 4He layer on α-graphyne: Pseudo-spin symmetry breaking under a particle-induced pseudo-magnetic field

Path-integral Monte Carlo calculations were performed to study the adsorption of $^4$He atoms on $α$-graphyne. We find that one $^4$He atom can be embedded onto the in-plane center of each hexagon of the graphyne. In the first $^4$He layer above the $^4$He-embedded graphyne surface, a Mott insulating state was observed at the areal density of 0.0706 Å$^{-2}$ with three $^4$He atoms occupying each hexagonal cell while the helium atoms form a commensurate triangular solid at a density of 0.0941 Å$^{-2}$. Here we show that the Ising pseudo-spin symmetry introduced for two degenerate configurations of three $^4$He atoms in a hexagonal cell can be broken by additional $^4$He atoms placed at the hexagon vertices and the Mott-insulator to commensurate-solid transition is a transition from a nonmagnetic spin liquid of frustrated antiferromagnets to a spin-aligned ferromagnet under a particle-induced pseudo-magnetic field.

cond-mat.mtrl-sci↗

arXiv2003

Nanoscale Quantum Solvation of para-H$_2$ around the Linear OCS Molecule inside $^4$He Droplets

We present a microscopic analysis of the quantum solvation structures of para-H$_2$ around the OCS molecule when embedded in low temperature $^4$He droplets. The structures of clusters containing M=5 and 6 para-H$_2$ molecules are compared with corresponding structures for M=1 (OCS-H$_2$ complex) and M=17 (a full solvation shell), as well as with the clusters in the absence of helium. We find that the helium has negligible effect on the structures for the small and large OCS(H$_2$)$_M$ clusters, but that it modifies the cluster structure for M=6. We discuss implications of these results for the onset of superfluidity in the solvating hydrogen shell and for spectroscopic measurements.

cond-mat↗

arXiv2003

Microscopic two-fluid theory of rotational constants of the OCS-H$_2$ complex in $^4$He droplets

We present a microscopic quantum analysis for rotational constants of the OCS-H$_2$ complex in helium droplets using the local two-fluid theory in conjunction with path integral Monte Carlo simulations. Rotational constants are derived from effective moments of inertia calculated assuming that motion of the H$_2$ molecule and the local non-superfluid helium density is rigidly coupled to the molecular rotation of OCS and employing path integral methods to sample the corresponding H$_2$ and helium densities. The rigid coupling assumption for H$_2$-OCS is calibrated by comparison with exact calculations of the free OCS-H$_2$ complex. The presence of the H$_2$ molecule is found to induce a small local non-superfluid helium density in the second solvation shell which makes a non-negligible contribution to the moment of inertia of the complex in helium. The resulting moments of inertia for the OCS-H$_2$ complex embedded in a cluster of 63 helium atoms are found to be in good agreement with experimentally measured values in large helium droplets. Implications for analysis of rotational constants of larger complexes of OCS with multiple H$_2$ molecules in helium are discussed.

cond-mat↗

arXiv2002

The finite-temperature Monte Carlo method and its application to superfluid helium clusters

We review the use of the path integral Monte Carlo (PIMC) methodology to the study of finite-size quantum clusters, with particular emphasis on recent applications to pure and impurity-doped He clusters. We describe the principles of PIMC, the use of the multilevel Metropolis method for sampling particle permutations, and the methods used to accurately incorporate anisotropic molecule-helium interactions into the path integral scheme. Applications to spectroscopic studies of embedded atoms and molecules are summarized, with discussion of the new concepts of local and nanoscale superfluidity that have been generated by recent PIMC studies of the impurity-doped He clusters.

physics.chem-ph↗

arXiv1998

Effects of Backflow Correlation in the Three-Dimensional Electron Gas: Quantum Monte Carlo Study

The correlation energy of the homogeneous three-dimensional interacting electron gas is calculated using the variational and fixed-node diffusion Monte Carlo methods, with trial functions that include backflow and three-body correlations. In the high density regime the effects of backflow dominate over those due to three-body correlations, but the relative importance of the latter increases as the density decreases. Since the backflow correlations vary the nodes of the trial function, this leads to improved energies in the fixed-node diffusion Monte Carlo calculations. The effects are comparable to those found for the two-dimensional electron gas, leading to much improved variational energies and fixed-node diffusion energies equal to the release-node energies of Ceperley and Alder within statistical and systematic errors.

cond-mat.str-el↗