arXiv subjects

Seoung-Hun Kang

Publications and source records attributed to Seoung-Hun Kang.

At least 19 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

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↗

arXiv2026

High Photovoltaic Efficiency in Bulk-Stacked One-Dimensional GeSe$_{2}$ van der Waals Crystal

Germanium diselenide (GeSe$_{2}$) has recently attracted substantial interest as a rare example of one-dimensional (1D) van der Waals material. Here, we investigate the photovoltaic potential of bulk-stacked GeSe$_{2}$ chains using first-principles calculations within the $GW0$ approximation and the Bethe-Salpeter equation (BSE) to capture quasiparticle and excitonic effects. The bulk GeSe$_{2}$ exhibits indirect GW band gaps of 1.92 eV (type-I) and 1.08 eV (type-II). Optical calculations show markedly stronger visible-light absorption in type-II, yielding a spectroscopically limited maximum efficiency (SLME) of ~25.6% at a 0.5 $\mu$m thickness. Phonon and room-temperature ab initio molecular dynamics analyses indicate that type-II is dynamically stable, whereas type-I shows imaginary phonon modes, suggesting a propensity for structural distortion. These results identify type-II GeSe2 as a promising stable absorber for thin-film photovoltaics with enhanced flexibility compared to typical 2D vdW systems.

cond-mat.mtrl-sci↗

arXiv2025

Overcoming Quantum Resistivity Scaling in Nanoscale Interconnects Using Delafossite PdCoO2

Continued scaling into the sub 7 nm regime exacerbates quantum limited resistivity in Cu interconnects. We evaluated layered PdCoO2 and explicitly benchmarked it against Cu to identify mechanisms that maintain conductivity under confinement. Using a momentum resolved relaxation time formalism derived from the conductivity tensor, we link k and energy resolved velocities, life times, and mean free paths (MFPs) to thickness dependent resistivity for films and wires. PdCoO2 exhibits quasi 2D transport with high inplane velocities and strongly anisotropic MFPs (15 nm inplane, 3 nm outofplane near EF), whereas Cu shows an isotropic 22 nm MFP. Under identical boundary conditions including a realistic 2 nm liner/diffusion barrier for Cu, PdCoO2 displays suppressed boundary scattering and a much slower resistivity increase from bulk down to sub 30 nm, preserving near bulk conductivity and remaining viable at 2 nm. Thickness trends reveal dual slope changes in PdCoO2 (35 nm and 7 nm) set by anisotropic MFPs, contrasting with the single characteristic scale of Cu (40 nm). The calculated bulk values and scaling curves track available measurements for both materials. These results establish PdCoO2 as a scalable interconnect that outperforms Cu under quantum confinement and provide a quantitative framework to screen layered conductors for next generation nanoelectronic interconnects.

cond-mat.mtrl-sci↗

arXiv2025

Momentum-Resolved Relaxation-Time Approach for Size-Dependent Conductivity in Anisotropic Metallic Films

Shrinking CMOS interconnect dimensions to the nanometer scale intensifies electron scattering at surfaces, interfaces, and grain boundaries, causing severe conductivity loss and challenging copper-based designs. Here we present a momentum-resolved relaxation time framework that integrates density functional theory with the semiclassical Boltzmann transport equation to predict size-dependent resistivity in metallic thin films. Electron phonon interactions are computed from first principles, and anisotropic surface and grain boundary scattering is captured through a momentum dependent mean free path, allowing relaxation times to vary spatially and directionally without empirical fitting. Applied to isotropic (Cu, Ag, Au) and anisotropic (W, Ti$_2$GeC) metals, the model achieves excellent agreement with experiments and uncovers the critical role of crystallographic anisotropy in transport. We further identify layered MAX phase compounds as promising ultrathin interconnects. This work provides a predictive, physically rigorous, and computationally efficient route to designing high-performance conductors for next generation nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic and Crystal Symmetry Effects on Spin Hall Conductivity in Altermagnets

Altermagnets, which reconcile zero net magnetization with pronounced spin splitting, offer fresh opportunities for spin-based functionalities in next-generation electronic and spintronic devices. In this paper, we explore the unconventional spin Hall conductivity (USHC) in three prototypical altermagnets -- RuO$_2$, CrSb, and MnTe -- and elucidate how distinct magnetic and crystal symmetries modulate their spin Hall responses. RuO$_2$ exhibits only trivial USHC contributions under a tilted geometry, demonstrating that symmetry projections alone can induce apparent unconventional elements. In contrast, CrSb and MnTe manifest robust, symmetry-driven USHC without structural tilts, enabled by easy-axis orientations that reduce magnetic symmetry. Through extensive first-principles calculations, we demonstrate the complementary roles of the time-reversal-even and time-reversal-odd components in determining the overall SHC. Our findings indicate that controlling the interplay between crystal and magnetic symmetry -- for instance, by epitaxial strain or doping -- can provide an experimental avenue to tune USHC magnitudes and directions in altermagnets. These results pave the way for the engineering of multifunctional spintronic devices, where enhanced coherence and robust spin transport are realized in zero-net-moment materials with easily tailored spin configurations.

cond-mat.mtrl-sci↗

arXiv2025

Interfacial Magnetic Anisotropy of Iron-Adsorbed Ferroelectric Perovskites: First-Principles and Machine Learning Study

The advancement of spin-based devices as a replacement for CMOS technology demands lower spin-switching energy in ferromagnetic (FM) materials. Ferroelectric (FE) materials offer a promising avenue for influencing FM properties, yet the mechanisms driving this interplay remain inadequately understood. In this study, we investigate iron-adsorbed FE ABO$_3$ perovskites using a combination of first-principles calculations and machine learning. Our findings reveal a universal correlation between the magnetic anisotropy energy (MAE) of iron and the induced magnetic dipole moments within the BO$_2$ layer and basal oxygen atoms of ABO$_3$ at the FE/FM interface. By identifying key material descriptors and achieving high predictive accuracy, this research provides a robust framework for selecting and optimizing ABO$_3$ substrates for energy-efficient spintronic devices. These insights contribute to the rational design of novel low-power spin-based technologies.

cond-mat.mtrl-sci↗

arXiv2025

Reversible Modification of Rashba States in Topological Insulators at Room Temperature by Edge Functionalization

Quantum materials with novel spin textures from strong spin-orbit coupling (SOC) are essential components for a wide array of proposed spintronic devices. Topological insulators have necessary strong SOC that imposes a unique spin texture on topological states and Rashba states that arise on the boundary, but there is no established methodology to control the spin texture reversibly. Here, we demonstrate that functionalizing Bi2Se3 films by altering the step-edge termination directly changes the strength of SOC and thereby modifies the Rashba strength of 1D edge states. Scanning tunneling microscopy/spectroscopy shows that these Rashba edge states arise and subsequently vanish through the Se functionalization and reduction process of the step edges. The observations are corroborated by density functional theory calculations, which show that a subtle chemical change of edge termination fundamentally alters the underlying electronic structure. Importantly, we experimentally demonstrated fully reversible and repeatable switching of Rashba edge states across multiple cycles at room temperature. The results imply Se functionalization as a practical method to control SOC and spin texture of quantum states in topological insulators.

cond-mat.mes-hall↗

arXiv2025

Coexistence of superconductivity and topological band in a van der Waals Sn1-xInxBi2Te4 crystal

The realization of topological surface states and superconductivity within a single material platform is a crucial step toward achieving topologically nontrivial superconductivity. This can be achieved at an interface between a superconductor and a topological insulator, or within a single material that intrinsically hosts both superconductivity and topological surface states. Here we use scanning tunneling microscopy to study Sn1-xInxBi2Te4 crystals. Spectroscopic evidence reveals the coexistence of topological surface states and superconductivity on the same surface of the crystals. The Te-terminated surface exhibits a single U-shaped superconducting gap with a size of up to 311 μeV, alongside Dirac bands outside the gap. Analysis of the vortex structure and differential conductance suggests weak-coupling s-wave superconductivity. The absence of observed zero modes suggests that shifting the Fermi level closer to the Dirac point of the topological bands is necessary to realize a topological superconducting state.

cond-mat.supr-con↗

arXiv2024

Thickness-Dependent Polaron Crossover in Tellurene

Polarons, quasiparticles arising from electron-phonon coupling, are crucial in understanding material properties such as high-temperature superconductivity and colossal magnetoresistance. However, scarce studies have been performed to investigate the formation of polarons in low-dimensional materials with phonon polarity and electronic structure transitions. In this work, we studied polarons of tellurene that are composed of chiral chains of tellurium atoms. The frequency and linewidth of the A1 phonon, which becomes increasingly polar for thinner tellurene, exhibit an abrupt change when the thickness of tellurene is below 10 nm. Meanwhile, the field effect mobility of tellurene drops rapidly as the thickness is smaller than 10 nm. These phonon and transport signatures, combined with the calculated phonon polarity and band structure, suggest a crossover from large polarons for bulk tellurium to small polarons for few-layer tellurene. Effective field theory considers the phonon renormalization in the strong coupling (small polaron) regime, and semi-quantitatively reproduces the observed phonon hardening and broadening effects in few-layer tellurene. This polaron crossover stems from the quasi-1D nature of tellurene where modulation of the interchain distance reduces the dielectric screening and promotes electron-phonon coupling. Our work provides valuable insights into the influence of polarons on phononic, electronic, and structural properties in low-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2024

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

In this study, we evaluate the predictive power of density functional theory (DFT) for the magnetic properties of MnBi$_2$Te$_4$ (MBT), an intrinsically magnetic topological insulator with potential applications in spintronics and quantum computing. Our theoretical understanding of MBT has been challenged by discrepancies between experimental results and \textit{ab initio} calculations, particularly with respect to its electronic and magnetic properties. Our results show that the magnetic phase diagram of MBT varies significantly depending on the Hubbard $U$ parameter in the DFT framework, highlighting the importance of benchmark calculations. To address these challenges, we establish an optimized Hubbard $U$ approach derived from Diffusion Monte Carlo (DMC) calculations, which directly solves the many-body Schrödinger equation based on the stochastic process, and implement it in the DFT framework. Once the optimized $U$ value is determined as a function of strain, we apply it to achieve DMC-level accuracy within our DFT framework. This approach is instrumental in accurately describing the magnetic states of MBT and understanding the underlying mechanisms governing its magnetic properties and their dependence on external factors.

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↗

arXiv2024

Distance-Dependent Evolution of Electronic States in Kagome- Honeycomb Lateral Heterostructures in FeSn

In this work, we demonstrate the formation and electronic influence of lateral heterointerfaces in FeSn containing Kagome and honeycomb layers. Lateral heterostructures offer spatially resolved property control, enabling the integration of dissimilar materials and promoting phenomena not typically observed in vertical heterostructures. Using the molecular beam epitaxy technique, we achieve a controllable synthesis of lateral heterostructures in the Kagome metal FeSn. With scanning tunneling microscopy/spectroscopy in conjunction with first-principles calculations, we provide a comprehensive understanding of the bonding motif connecting the Fe3Sn-terminated Kagome and Sn2-terminated honeycomb surfaces. More importantly, we reveal a distance-dependent evolution of the electronic states in the vicinity of the heterointerfaces. This evolution is significantly influenced by the orbital character of the flat bands. Our findings suggest an approach to modulate the electronic properties of the Kagome lattice, which should be beneficial for the development of future quantum devices.

cond-mat.str-el↗

arXiv2023

Interplay between Topological States and Rashba States as Manifested on Surface Steps at Room Temperature

The unique spin texture of quantum states in topological materials underpins many proposed spintronic applications. However, realizations of such great potential are stymied by perturbations, such as temperature and local fields imposed by impurities and defects, that can render a promising quantum state uncontrollable. Here, we report room-temperature observation of interaction between Rashba states and topological surface states, which manifests unique spin textures controllable by layer thickness of thin films. Specifically, we combine scanning tunneling microscopy/spectroscopy with the first-principles theoretical calculation to find the robust Rashba states coexisting with topological surface states along the surface steps with characteristic spin textures in momentum space. The Rashba edge states can be switched off by reducing the thickness of a topological insulator Bi2Se3 to bolster their interaction with the hybridized topological surface states. The study unveils a manipulating mechanism of the spin textures at room temperature, reinforcing the necessity of thin film technology in controlling quantum states.

cond-mat.mes-hall↗

arXiv2023

Theoretical investigation of delafossite-Cu2ZnSnO4 as a promising photovoltaic absorber

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

cond-mat.mtrl-sci↗

arXiv2023

Universal sublinear resistivity in vanadium kagome materials hosting charge density waves

The recent discovery of a charge density (CDW) state in ScV$_6$Sn$_6$ at $T_{\textrm{CDW}}$ = 91 K offers new opportunities to understand the origins of electronic instabilities in topological kagome systems. By comparing to the isostructural non-CDW compound LuV$_6$Sn$_6$, we unravel interesting electrical transport properties in ScV$_6$Sn$_6$, above and below the charge ordering temperature. We observed that by applying a magnetic field along the $a$ axis, the temperature behavior of the longitudinal resistivity in ScV$_6$Sn$_6$ changes from metal-like to insulator-like above the CDW transition. We show that in the charge ordered state ScV$_6$Sn$_6$ follows the Fermi liquid behavior while above that, it transforms into a non-Fermi liquid phase in which the resistivity varies sublinearly over a broad temperature range. The sublinear resistivity, which scales by $T^{3/5}$ is a common feature among other vanadium-containing kagome compounds exhibiting CDW states such as KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$. By contrast, the non-Fermi liquid behavior does not occur in LuV$_6$Sn$_6$. We explain the $T^{3/5}$ universal scaling behavior from the Coulomb scattering between Dirac electrons and Van Hove singularities; common features in the electronic structure of kagome materials. Finally, we show anomalous Hall-like behavior in ScV$_6$Sn$_6$ below $T_{\textrm{CDW}}$, which is absent in the Lu compound. Comparing the transport properties of ScV$_6$Sn$_6$ and LuV$_6$Sn$_6$ is valuable to highlight the impacts of the unusual CDW in the Sc compound.

cond-mat.str-el↗