arXiv subjects

Yeonghun Lee

Publications and source records attributed to Yeonghun Lee.

16 recordsLinked to original sources

arXiv2026

Electron transport in amorphous materials: from localization to predictive transport modeling

Amorphous materials are disordered solids without long-range structural order, making them useful systems for studying electron transport beyond the crystalline picture. This review discusses how structural disorder changes the spatial character of electron wavefunctions and how these changes govern carrier transport in amorphous materials, especially amorphous semiconductors. Basic concepts of Anderson localization, mobility edges, diffusive transport, and hopping transport are first reviewed, followed by representative amorphous semiconductors, including amorphous silicon and amorphous oxide semiconductors. Computational approaches are then discussed, from conventional Boltzmann and Green's-function-based transport theories to real-space Kubo-Greenwood simulations combined with molecular dynamics. The discussion focuses on localization, spectral broadening, finite-temperature lattice fluctuations, and electron-phonon interactions. Recent progress and remaining issues in predictive transport modeling of amorphous materials are then outlined.

cond-mat.mtrl-sci↗

arXiv2026

Radiative electronic bound states in the continuum from defects in semiconductors

Continuum-buried defect states in semiconductors are generally expected to be optically inactive due to their strong coupling to continuum bands. Here, we show that such defects can instead host radiative electronic bound states in the continuum (BICs), using the silicon G-center as a prototypical example. Hybrid-functional first-principles calculations with a Hubbard $U$ correction reveal that a localized defect state, initially buried below the valence band maximum (VBM) in the ground state, undergoes exchange-driven energy-level reordering under optical excitation and shifts above the VBM. This exchange-induced transition suppresses nonradiative decay and enables robust radiative emission. By computing temperature-dependent nonradiative lifetimes and comparing them with experimental photoluminescence (PL) lifetimes, we quantitatively reproduce the observed temperature dependence of the emission. These results uncover a stabilization mechanism for continuum-embedded defect states and establish electronic BICs as a general paradigm for designing defect-based optical systems, including quantum emitters and qubits.

cond-mat.mtrl-sci↗

arXiv2026

Role of surface states and band modulations in ultrathin ruthenium interconnects

Mitigating the RC delay from transistor miniaturization is essential for next-generation devices, driving a focus on interconnect electrical performance. Current copper-based interconnects face a critical challenge, that their resistivity sharply increases at the nanometer-scale due to surface and grain boundary scattering. Therefore, there is a pressing need for techniques that reduce resistance in ultrathin metal films. In this study, we employ the density functional theory to investigate how the intrinsic electronic structure of thin films impacts conductivity as a function of thickness. Notably, our analysis of ruthenium slab structures shows that surface states significantly influence thickness-dependent resistivity. It reveals that vacuum-terminated Ru slab exhibits decreasing resistivity with the decrease in thickness, whereas oxygen-terminated Ru slab shows the opposite trend. This difference is fundamentally attributed to the presence or absence of surface states, highlighting the importance of surface engineering in optimizing interconnect performance.

cond-mat.mtrl-sci↗

arXiv2026

First-principles high-throughput screening of ruthenium compounds for advanced interconnects

As interconnect dimensions continue to shrink, the industry-standard copper faces a critical increase in resistivity, presenting a significant hurdle to overall device performance. To overcome this limitation, this work investigates the potential of ruthenium (Ru)-based compounds, encompassing binary, ternary, and quaternary systems, as viable alternatives to copper (Cu). Ruthenium is regarded as a strong candidate, owing to its inherent advantages in reliability and more favorable resistivity scaling at reduced dimensions. Moreover, forming compounds offers an effective strategy to engineer novel properties, expanding the material design space beyond the constraints of pure metals. Utilizing a high-throughput screening methodology, we systematically investigated a broad spectrum of 2,106 Ru-based compounds to identify candidates with superior electronic transport and reliability characteristics. Consequently, we successfully identified a total of 61 promising candidates that exhibit excellent resistivity scaling behavior and enhanced reliability. These findings demonstrate that Ru-based compounds offer a viable pathway to overcome the scaling limitations of next-generation interconnects.

cond-mat.mtrl-sci↗

arXiv2026

Cobalt Binary Compounds for Advanced Interconnect Materials

The industrial standard copper (Cu) interconnects face a substantial resistivity increase at thinner linewidths, posing a well-known challenge to limit overall device performance. To address this issue, we have evaluated the potential properties of cobalt (Co) based binary compounds as replacements for Cu. Co is considered as a promising alternative due to its potential for enhanced reliability and low resistivity at sub-nanoscale dimensions. Furthermore, the combination of elements provides a possibility to engineer novel properties, transcending the limitations of elemental metals and expanding the search space for next-generation interconnects. In this study, a high-throughput screening method was used to identify several Co-based binary compounds with superior electronic transport and reliability at reduced thickness. The findings demonstrate that specific Co-based binary compounds hold significant potential to overcome the performance limitations of scaled interconnects.

cond-mat.mtrl-sci↗

arXiv2026

First-principles modeling of electrostatics and transport in 2D topological transistors

We develop a simulation framework for electrostatic and transport modeling of 2D Topological insulator field-effect transistor (2D TIFETs), based solely on first-principles calculations using density functional theory (DFT). We find that careful consideration of basis set and symmetry constraints in DFT calculations is crucial for determining critical electric field ($E_c$), defined as the electric field intensity at which the topological phase transition occurs. Using ballistic Landauer-B$\"u$ttiker formula and local potential profile, the drain current-gate bias voltage ($I_D$-$V_G$) characteristics were obtained and switching behavior was studied. A comparison with the $\mathbf{k}\cdot\mathbf{p}$ model reveals the necessity of DFT calculations for investigating realistic edge dispersions. Our approach provides an efficient and rigorous simulation methodology for mesoscopic transport in 2D TIFETs.

cond-mat.mes-hall↗

arXiv2026

Symmetric Trotterization in digital quantum simulation of quantum spin dynamics

A higher-order Suzuki-Trotter decomposition or Trotterization can be exploited to mitigate the Trotter error in digital quantum simulation. This work revisits the second-order symmetric Trotterization in terms of the Trotter error, where quantum many-body spin dynamics of the transverse-field Ising model is simulated. While the work presents a pedagogical way to exploit a real quantum computer, the effectiveness of the symmetric Trotterization is evaluated in a prototype superconducting quantum device on IBM Quantum Experience. It turns out that the symmetric Trotterization does not provide higher accuracy than the first-order Trotterization in the testbed using the transverse-field Ising model. The result indicates that apart from the quantum errors, such as logical gate error and readout error, the use of a higher-order Trotterization should be circumspect, and the Trotter error would play an insignificant role in particular applications in an early stage of realized noisy intermediate-scale quantum (NISQ) devices.

quant-ph↗

arXiv2026

Designing Extremely Low-Power Topological Transistors with 1T'-MoS2 and HZO for Cryogenic Applications

Large-scale quantum computing requires cryogenic electronic controllers such as control/readout circuit and routing circuit. However, current technologies face high power dissipation problems, hindering large-scale qubit integration. Here, we theoretically propose extremely low-power cryogenic topological transistors, i.e., negative-capacitance topological insulator field-effect transistors (NC-TIFETs). By combining a gate-field-induced two-dimensional 1T'-Molybdenum Disulfide (MoS$_2$) topological channel with a hafnium-zirconium oxide (HZO) ferroelectric gate insulator, NC-TIFETs exhibit an extremely steep-slope transfer curve and ultra-high transconductance at low drain voltage ($V_{\mathrm{D}}$). Therefore, NC-TIFETs are the compelling candidate for minimizing power dissipation in the cryogenic electronic interfaces essential for large-scale quantum computing systems.

cond-mat.mes-hall↗

arXiv2026

Trap-Enhanced Steep-Slope Negative-Capacitance FETs Using Amorphous Oxide Semiconductors

Amorphous oxide semiconductors (AOSs) have recently gained attention as a promising channel material of back-end-of-line (BEOL)-compatible transistors for monolithic three-dimensional (3D) integrations. However, the degradation in device performance resulting from the high trap densities in AOS, compared to conventional crystalline channel materials, has remained an intractable issue. We introduce the negative-capacitance (NC) operation into the AOS-based transistors. Negative-capacitance field-effect transistors (NCFETs) have been proposed for low-power devices, enabling sub-60 mV/decade subthreshold swing SS induced by a ferroelectric layer. In this work, we develop an AOS NCFET model to investigate the influence of traps within the channel on the steep-slope operation. It is revealed that as the trap density of the channel increases, SS of the MOSFET increases, while the SS of the NCFET decreases. The physical interpretation for steep SS is attributed to the fact that the trapped charges enhance the negative potential drop of the NC layer, enabling the abrupt device switching. This finding will accelerate the development of BEOL transistors and other applications based on the AOS materials in conjunction with the NC effect.

cond-mat.other↗

arXiv2026

Validation of constant mean free path and relaxation time approximations for metal resistivity: explicit treatment of electron-phonon interactions

The figure of merit $\rho \lambda$ the product of resistivity and mean free path (MFP) evaluated from first-principles calculations, is widely adopted to screen promising interconnect metals with high electrical conductivity at ultranarrow dimensions. However, the $\rho \lambda$ has been calculated without addressing the validity of the assumption that the MFP is independent of the wavevector $\mathbf{k}$. Here, we assess the validity of the constant MFP approximation, by estimating the $\mathbf{k}$-dependent MFPs for (an)isotropic elemental metals, with explicit treatment of electron-phonon interactions. Additionally, we verify the validity of the constant relaxation time approximation (CRTA) for resistivity calculations. We show that both the constant MFP approximation and CRTA are reasonable even for highly anisotropic Fermi surfaces. Our results support the practical use of those approximations in transport studies, where explicit electron-phonon calculations are not feasible.

cond-mat.mtrl-sci↗

arXiv2026

Tight-Binding Device Modeling of 2-D Topological Insulator Field-Effect Transistors With Gate-Induced Phase Transition

Topological insulator field-effect transistors (TIFETs) built on 2-D quantum spin Hall insulators are being considered as advanced logic transistors due to their potentially superior performance originating from the dissipationless edge transport. This paper presents a device modeling based on the tight-binding model and the nonequilibrium Green's function formalism to simulate the current-voltage characteristics of the TIFETs. We then use the device simulator to demonstrate the effect of channel length on device performance. The device modeling will not only enable a direct estimation of TIFET performance but also shed light on the nontraditional switching operation via the topological phase transition.

cond-mat.mes-hall↗

arXiv2025

NeuroPAL: Punctuated Anytime Learning with Neuroevolution for Macromanagement in Starcraft: Brood War

StarCraft: Brood War remains a challenging benchmark for artificial intelligence research, particularly in the domain of macromanagement, where long-term strategic planning is required. Traditional approaches to StarCraft AI rely on rule-based systems or supervised deep learning, both of which face limitations in adaptability and computational efficiency. In this work, we introduce NeuroPAL, a neuroevolutionary framework that integrates Neuroevolution of Augmenting Topologies (NEAT) with Punctuated Anytime Learning (PAL) to improve the efficiency of evolutionary training. By alternating between frequent, low-fidelity training and periodic, high-fidelity evaluations, PAL enhances the sample efficiency of NEAT, enabling agents to discover effective strategies in fewer training iterations. We evaluate NeuroPAL in a fixed-map, single-race scenario in StarCraft: Brood War and compare its performance to standard NEAT-based training. Our results show that PAL significantly accelerates the learning process, allowing the agent to reach competitive levels of play in approximately half the training time required by NEAT alone. Additionally, the evolved agents exhibit emergent behaviors such as proxy barracks placement and defensive building optimization, strategies commonly used by expert human players. These findings suggest that structured evaluation mechanisms like PAL can enhance the scalability and effectiveness of neuroevolution in complex real-time strategy environments.

cs.AI↗

arXiv2022

Two Types of 3D Quantum Hall Effects in Multilayer WTe$_2$

Interplay between the topological surface states and bulk states gives rise to diverse exotic transport phenomena in topological materials. The recently proposed Weyl orbit in topological semimetals in the presence of magnetic field is a remarkable example. This novel closed magnetic orbit consists of Fermi arcs on two spatially separated sample surfaces which are connected by the bulk chiral zero mode, which can contribute to transport. Here we report Shubnikov-de Haas (SdH) oscillation and its evolution into quantum Hall effect (QHE) in multilayered type-II Weyl semimetal WTe2. We observe both the three-dimensional (3D) QHE from bulk states by parallelly stacking of confined two-dimensional layers in the low magnetic field region and the 3D QHE in the quantized surface transport due to Weyl orbits in the high magnetic field region. Our study of the two types of novel QHEs controlled by magnetic field and our demonstration of the crossover between quantized bulk and surface transport provide an essential platform for the future quantized transport studies in topological semimetals.

cond-mat.mes-hall↗

arXiv2022

Effect of Localization on Photoluminescence and Zero-Field Splitting of Silicon Color Centers

The study of defect centers in silicon has been recently reinvigorated by their potential applications in optical quantum information processing. A number of silicon defect centers emit single photons in the telecommunication $O$-band, making them promising building blocks for quantum networks between computing nodes. The two-carbon G-center, self-interstitial W-center, and spin-$1/2$ T-center are the most intensively studied silicon defect centers, yet despite this, there is no consensus on the precise configurations of defect atoms in these centers, and their electronic structures remain ambiguous. Here we employ \textit{ab initio} density functional theory to characterize these defect centers, providing insight into the relaxed structures, bandstructures, and photoluminescence spectra, which are compared to experimental results. Motivation is provided for how these properties are intimately related to the localization of electronic states in the defect centers. In particular, we present the calculation of the zero-field splitting for the excited triplet state of the G-center defect as the structure is transformed from the A-configuration to the B-configuration, showing a sudden increase in the magnitude of the $D_{zz}$ component of the zero-field splitting tensor. By performing projections onto the local orbital states of the defect, we analyze this transition in terms of the symmetry and bonding character of the G-center defect which sheds light on its potential application as a spin-photon interface.

cond-mat.mtrl-sci↗

arXiv2020

Real-time ab initio simulation of inelastic electron scattering using the exact, density functional, and alternative approaches

To investigate inelastic electron scattering, which is ubiquitous in various fields of study, we carry out ab initio study of the real-time dynamics of a one-dimensional electron wave packet scattered by a hydrogen atom using different methods: the exact solution, the solution provided by time-dependent density functional theory (TDDFT), and the solutions given by alternative approaches. This research not only sheds light on inelastic scattering processes but also verifies the capability of TDDFT in describing inelastic electron scattering. We revisit the adiabatic local-density approximation (ALDA) in describing the excitation of the target during the scattering process along with a self-interaction correction and spin-polarized calculations. Our results reveal that the ALDA severely underestimates the energy transferred in the regime of low incident energy particularly for a spin-singlet system. After demonstrating alternative approaches, we propose a hybrid ab initio method to deal with the kinetic correlation alongside TDDFT. This hybrid method would facilitate first-principles studies of systems in which the correlation of a few electrons among many others is of interest.

physics.atom-ph↗

arXiv2020

First-principles study of metal-graphene edge contact for ballistic Josephson junction

Edge-contacted superconductor-graphene-superconductor Josephson junction have been utilized to realize topological superconductivity, which have shown superconducting signatures in the quantum Hall regime. We perform the first-principles calculations to interpret electronic couplings at the superconductor-graphene edge contacts by investigating various aspects in hybridization of molybdenum d orbitals and graphene $π$ orbitals. We also reveal that interfacial oxygen defects play an important role in determining the doping type of graphene near the interface.

cond-mat.supr-con↗