arXiv subjects

Jeonghwan Ahn

Publications and source records attributed to Jeonghwan Ahn.

15 recordsLinked to original sources

arXiv2026

Hybridization-controlled re-entrant electronic phase switching and moire-confined states in twisted bilayer PtTe2

Twisting a van der Waals bilayer changes not only the moir\'e periodicity but also the local stacking and interlayer hybridization. Here, we show, using fully relaxed first-principles calculations including spin--orbit coupling, band unfolding, and Brillouin-zone-integrated densities of states, that bilayer PtTe$_2$ exhibits a non-monotonic evolution between gapless and gapped electronic regimes. The $7.34^\circ$ structure remains gapless, whereas finite direct gaps appear at the sampled intermediate angles. The gap closes at the sampled $60^\circ$ configuration and reopens at higher angles. The direct gap shows an overall increase with the minimum local interlayer Pt--Pt separation, although the complete distribution of local stacking environments is required to account for deviations from this trend. At $7.34^\circ$, the low-energy states are concentrated predominantly in the AA-like regions of the otherwise gapless moir\'e cell. Controlled interlayer-separation scans show that increasing the layer spacing removes the near-$E_F$ crossings and opens a gap, consistent with weakened interlayer Te-$p_z$ hybridization. These results identify the redistribution of interlayer hybridization as the microscopic origin of the re-entrant gap evolution in twisted bilayer PtTe$_2$.

cond-mat.mtrl-sci↗

arXiv2026

Many-body benchmarking of DFT local-registry energetics in bilayer InSe

Density functional theory (DFT) is widely used to model twisted bilayers, but the accuracy of the local stacking energetics underlying such models remains uncertain. Here, we benchmark the local-registry landscape of bilayer InSe using diffusion quantum Monte Carlo (DMC). DFT predicts that AB, AAr, and ABr stackings, which share the same interfacial Se registry, are nearly degenerate within 1.5 meV/f.u. and exhibit nearly indistinguishable DFT charge-density responses. DMC instead separates these stackings by 8(5) and 41(4) meV/f.u., while the energy difference between the most stable and least stable registries reaches 60(7) meV/f.u.. These large energy separations show that the stacking energetics are not determined by the interfacial atomic motif alone but depend on the full registry and its associated many-body electronic response. More broadly, these results show that DFT-based moir\'e models can substantially underestimate local stacking-energy corrugation, with direct consequences for predicted structural relaxation, domain formation, and electronic reconstruction in twisted layered materials.

cond-mat.mtrl-sci↗

arXiv2026

Thickness-Dependent Interlayer Coupling and Semiconductor-to-Semimetal Crossover in Arsenene Multilayers

Interlayer interactions in layered materials are often assumed to transfer from the bilayer to the bulk, but this assumption can fail when chemically active out-of-plane orbitals participate in bonding. We combine diffusion quantum Monte Carlo (DMC) and density functional theory (DFT) to determine how interlayer coupling evolves in arsenene multilayers. DMC shows that bulk gray arsenic is compact, whereas the corresponding few-layer structures remain at substantially larger interlayer separations despite sharing the same nominal A$_{1}$B$_{-1}$ adjacent-layer registry. Registry alone therefore does not determine the bonding regime; thickness and coordination reshape the interlayer interaction. Among the tested functionals, SCAN+rVV10 most closely reproduces DMC equilibrium separations and stacking energetics. Using the DMC-benchmarked SCAN+rVV10 calculations, we predict a thickness-driven stacking sequence from A$_{1}$A$_{1}$ to A$_{1}$B$_{1}$ and finally bulk-like A$_{1}$B$_{-1}$. The structural crossover coincides with a stacking-dependent DFT band-gap collapse driven by enhanced interlayer As p$_{z}$ hybridization.

cond-mat.mtrl-sci↗

arXiv2026

Nonmagnetic Ground State of Rutile RuO$_2$ from Diffusion Quantum Monte Carlo

Rutile RuO$_2$ has been proposed as an altermagnet, but its bulk magnetic ground state is still under debate because density-functional calculations give conflicting predictions. Using fixed-node diffusion quantum Monte Carlo, we find that stoichiometric bulk RuO$_2$ is nonmagnetic in the pristine structure, lying 23(9) meV per formula unit below the lowest antiferromagnetic state considered. A 3$\%$ compressive strain instead stabilizes antiferromagnetism, placing RuO$_2$ near a strain-tunable magnetic instability and helping reconcile apparently conflicting experimental reports.

cond-mat.mtrl-sci↗

arXiv2026

Optimizing Density Functional Theory for Strain-Dependent Magnetic Properties of Monolayer MnBi$_2$Te$_4$ with Diffusion Monte Carlo

Monolayer MnBi$_{2}$Te$_{4}$ (MBT) is an intrinsically magnetic topological insulator whose magnetic response is strongly affected by strain and electron correlation. In density functional theory with an on-site Hubbard correction (DFT+$U$), however, predictions vary substantially with the choice of Hubbard $U$, making it difficult to establish a reliable strain-dependent picture of magnetism in this system. Here we use diffusion Monte Carlo (DMC) to benchmark DFT+$U$ for monolayer MBT and to determine an effective $U$ as a function of strain. We find that the predicted magnetic phase diagram depends strongly on $U$, indicating that a single fixed value is not sufficient across the strain range considered. DMC nodal optimization further shows that the optimal $U$ increases with strain magnitude and is well captured by a simple quadratic form. When this DMC-informed strain-dependent $U$ is used in PBE+$U$, the calculated Mn local moments are brought into close agreement with DMC and are improved relative to commonly used fixed-$U$ choices. These results show that, for monolayer MBT, correlation strength itself should be treated as strain dependent, and they provide a practical many-body-guided strategy for improving strain-dependent DFT+$U$ descriptions of magnetic van der Waals materials.

cond-mat.mtrl-sci↗

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↗

arXiv2024

Identifying Band Inversions in Topological Materials Using Diffusion Monte Carlo

Topological insulators are characterized by insulating bulk states and robust metallic surface states. Band inversion is a hallmark of topological insulators: at time-reversal invariant points in the Brillouin zone, spin-orbit coupling (SOC) induces a swapping of orbital character at the bulk band edges. In this work, we develop a novel method to detect band inversion within continuum quantum Monte Carlo (QMC) methods that can accurately treat the electron correlation and spin-orbit coupling crucial to the physics of topological insulators. Our approach applies a momentum-space-resolved atomic population analysis throughout the first Brillouin zone utilizing the Löwdin method and the one-body reduced density matrix produced with Diffusion Monte Carlo (DMC). We integrate this method into QMCPACK, an open source ab initio QMC package, so that these ground state methods can be used to complement experimental studies and validate prior DFT work on predicting the band structures of correlated topological insulators. We demonstrate this new technique on the topological insulator bismuth telluride, which displays band inversion between its Bi-p and Te-p states at the $Γ$-point. We show an increase in charge on the bismuth p orbital and a decrease in charge on the tellurium p orbital when comparing band structures with and without SOC. Additionally, we use our method to compare the degree of band inversion present in monolayer Bi$_2$Te$_3$, which has no interlayer van der Waals interactions, to that seen in the bulk. The method presented here will enable future, many-body studies of band inversion that can shed light on the delicate interplay between correlation and topology in correlated topological materials.

cond-mat.str-el↗

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↗

arXiv2024

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

Exploring interlayer coupling in the twisted bilayer PtTe$_{2}$

We have investigated interlayer interactions in the bilayer PtTe$_{2}$ system, which influence the electronic energy bands near the Fermi levels. Our diffusion Monte Carlo (DMC) calculations for the high-symmetry bilayer stackings (AA, AB, AC) manifest distinct interlayer binding characteristics among the stacking modes by revealing significantly different interlayer separations depending on the stackings, which is critical to understanding the interlayer coupling of the twisted bilayers consisting of various local stacking arrangements. Furthermore, a comparison between the interlayer separations obtained from DMC and density functional theory (DFT) shows that meta-GGA-based vdW-DFT results agree with DMC for different layer stackings, including twisted bilayers, but only the ground-state AA stacking matches well with GGA-based DFT predictions. This underscores the importance of accurate exchange-correlation potentials even for capturing the stacking-dependent interlayer binding properties. We further show that the variability in DFT-predicted interlayer separations is responsible for the large discrepancy of band structures in the 21.79$^{\circ}$-twisted bilayer PtTe$_{2}$, affecting its classification as metallic or semiconducting. These results demonstrate the importance of obtaining a correct description of stacking-dependent interlayer coupling in modeling delicate bilayer systems at finite twists.

cond-mat.mtrl-sci↗

arXiv2023

Metastability and topology in the magnetic topological insulator MnBi$_{2}$Te$_{4}$

We study the effect of stacking faults on the topological properties of the magnetic topological insulator MnBi$_{2}$Te$_{4}$ (MBT) using density functional theory calculations and the Hubbard $U$ being tuned with many-body diffusion Monte Carlo techniques. We show that a modest deviation from the equilibrium interlayer distance leads to a topological phase transition from a non-trivial to a trivial topology, suggesting that tuning the interlayer coupling by adjusting the interlayer distance alone can lead to different topological phases. Interestingly, due to the locally increased interlayer distance of the top layer, a metastable stacking fault in MBT leads to a nearly gapless state at the topmost layer due to charge redistribution as the topmost layer recedes. We further find evidence of spin-momentum locking in the surface state along with a weak preservation of the band inversion in the near gapless state, which is indicative of the non-trivial topological surface states for the metastable stacking fault. Our findings provide a possible explanation for reconciling the long-standing puzzle of gapped and gapless states on MBT surfaces.

cond-mat.mtrl-sci↗

arXiv2023

Procedures for assessing the stability of proposed topological materials

We investigate the stability of MnPb$_{2}$Bi$_{2}$Te$_{6}$ (MPBT), which is predicted to be a magnetic topological insulator (TI), using density functional theory calculations. Our analysis includes various measures such as enthalpies of formation, Helmholtz free energies, defect formation energies, and dynamical stability. Our thermodynamic analysis shows that the phonon contribution to the energy gain from finite temperature is estimated to be less than 10~meV/atom, which may not be sufficient to stabilize MPBT at high temperatures, even with the most favorable reactions starting from binaries. While MPBT is generally robust against the formation of various defects, we find that anti-site defect formation of $\text{Mn}_{\text{Pb}}$ is the most likely to occur, with corresponding energy less than 60~meV. This can be attributed to the significant energy cost from compressive strain at the PbTe layer. Our findings suggest that MPBT is on the brink of stability in terms of thermodynamics and defect formation, underscoring the importance of conducting systematic analyses of the stability of proposed TIs, including MPBT, for their practical utilization. This study offers valuable insights into the design and synthesis of desirable magnetic TI materials with robust stabilities.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic measures of purity for MnBi$_2$Te$_4$

The intrinsically anti-ferromagnetic topological insulator, MnBi$_2$Te$_4$ (MBT), has garnered significant attention recently. The excitement for this layered van der Waals bonded compound stems from its potential to host numerous exotic topological quantum states. For instance, quantum anomalous Hall states are predicted for odd-layer compounds and axion insulator states for the even-layer compounds. Unfortunately, the realization of these phenomena has been hindered by experimental challenges such as the existence of negative charge carriers, i.e., electron doping, which have been linked to anti-site defects among the Mn and Bi sub-lattices. Based on high level diffusion Monte Carlo (DMC) and DMC-tuned DFT+U calculations, we provide benchmark quality results for the bulk Mn magnetization as well as for Mn$_{Bi}$ and Bi$_{Mn}$ defects. We use this information to refine and extend models that estimate the anti-site defect concentration in actual MBT samples when combined with data from magnetic susceptibility and intermediate field magnetization measurements. Our models are validated through favorable comparison with prior experimental studies that obtained both magnetic and site occupancy data. We then extend our estimates to a larger set of prior samples to identify a probable zone of low defect density that has yet to be reached in synthesis. We anticipate our theoretically based magnetic purity measures may be used as minimization targets in the cycle of refinement needed to synthesize MBT samples with low anti-site defect concentrations and more reproducible topological properties.

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↗