arXiv subjects

Yia-Chung Chang

Publications and source records attributed to Yia-Chung Chang.

At least 19 recordsLinked to original sources

arXiv2026

Dielectric function in WSe2

We develop a Hartree-Fock numerical method for computing the band structure of a two-dimensional Wigner crystal in an electron gas at zero temperature. The ground state is assumed to be fully spin-polarized. Single-particle excitation spectra are evaluated in spin-conserving channel. As an application, we use the developed code to compute the static dielectric function epsilon(q,0) of a Wigner-crystal state formed in a two-dimensional transition-metal dichalcogenide, specifically monolayer WSe2. The dielectric response is obtained from the Hartree-Fock band structure and eigenfunctions through a static Lindhard-type polarizability. The method provides a theoretical tool for investigating screening, band-structure reconstruction, and interaction effects in low-density two-dimensional systems, with possible relevance for future experimental studies.

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arXiv2026

Semi-empirical Pseudopotential Method for Monolayer Transition Metal Dichalcogenides

We present a semi-empirical pseudopotential method for accurately computing the band structures and bloch states of monolayer transition metal dichalcogenides(TMDCs), including MoS2, MoSe2, WS2, and WSe2. Our approach combines local and nonlocal pseudopotentials, carefully fitted to reproduce fully self-consistent density-functional theory results while using only a minimal set of empirical parameters. By expressing the total potential as a sum of a few separable components, we achieve both accuracy and computational efficiency. The transferability of the monolayer-fitted pseudopotentials is assessed through a direct application to bilayer TMDCs without additional refitting, where good agreement with self-consistent DFT band structures is obtained near the band edges. The resulting framework provides an efficient and flexible platform for band-structure and Bloch-state calculations in TMDC-based low-dimensional materials. Keywords: transition metal dichalcogenides, semi-empirical pseudopotentials, density functional theory, electronic structure, computational physics

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arXiv2026

Exciton-polaron Umklapp scattering in Wigner crystals

Strong Coulomb interactions in two-dimensional (2D) semiconductors give rise to tightly bound excitons, exciton polarons, and correlated electronic phases such as Wigner crystals (WCs), yet their mutual interplay remains poorly understood. Here we report the observation of multi-branch excitonic Umklapp scattering in both electron and hole WCs realized in ultraclean monolayer WSe$_2$, exhibiting exceptionally high melting temperatures (T$_c$ $\approx$ 20-30 K). Robust Wigner crystallization activates multiple finite-momentum optical resonances, including quasilinearly dispersing, light-like excitons and exciton polarons, extending far beyond the single excitonic Umklapp feature reported previously. Helicity-resolved magneto-optical measurements reveal a pronounced valley dependence of the scattering processes. Combined experiment and theory identify a polaron-induced brightening mechanism in which exciton polarons transfer oscillator strength from bright zero-momentum states to otherwise dark finite-momentum states, explaining the emergence of multiple Umklapp branches where conventional exciton-WC scattering is ineffective. These results establish WC polarons as a new quasiparticle paradigm and introduce polaron-induced Umklapp scattering as a general route to accessing finite-momentum many-body excitations in 2D quantum materials.

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arXiv2025

Brightening interlayer excitons by electric-field-driven hole transfer in bilayer WSe2

We observe the interlayer A1s^I, A2s^I, and B1s^I excitons in bilayer WSe2 under applied electric fields using reflectance contrast spectroscopy. Remarkably, these interlayer excitons remain optically bright despite being well separated from symmetry-matched intralayer excitons-a regime where conventional two-level coupling models fail unless unphysically large coupling strengths are assumed. To uncover the origin of this brightening, we perform density functional theory (DFT) calculations and find that the applied electric field distorts the valence-band Bloch states, driving the hole wavefunction from one layer to the other. This field-driven interlayer hole transfer imparts intralayer character to the interlayer excitons, thereby enhancing their oscillator strength without requiring hybridization with bright intralayer states. Simulations confirm that this mechanism accounts for the major contribution to the observed brightness, with excitonic hybridization playing only a minor role. Our results identify interlayer hole transfer as a robust and general mechanism for brightening interlayer excitons in bilayer transition metal dichalcogenides (TMDs), especially when inter- and intralayer excitons are energetically well separated.

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arXiv2024

Efficient Band Structure Calculation for Transitional-Metal Dichalcogenides Using the Semiempirical Pseudopotential Method

The Semiempirical Pseudopotential Method (SEPM) has emerged as a valuable tool for accurately determining band structures, especially in the realm of low-dimensional materials. SEPM operates by utilizing atomic pseudopotentials, which are derived from DFT calculations. SEPM calculations offer a unique advantage compared to DFT as they eliminate the requirement for iterative self-consistent solutions in solving the Schrödinger equation, leading to a substantial reduction in computational complexity. The incorporation of both non-local and local Semiempirical Pseudopotentials in our current approach yields band structures and wavefunctions with enhanced precision compared to traditional empirical methods. When applied to monolayer TMDCs, adjusting the parameters to align with pertinent values obtained from DFT computations enables us to faithfully replicate the band structure, opening avenues for investigating the optoelectronic properties of TMDCs and exploring their potential applications in nanodevices.

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arXiv2022

Quantum anomalous Hall effect and electric-field-induced topological phase transition in AB-stacked MoTe${}_2$/WSe${}_2$ moiré heterobilayers

We propose a new mechanism to explain the quantum anomalous Hall (QAH) effect and the electric-field-induced topological phase transition in AB-stacked MoTe${}_2$/WSe${}_2$ moiré heterobilayers at $ν=1$ hole filling. We suggest that the Chern band of the QAH state is generated from an intrinsic band inversion composed of the highest two moiré hole bands with opposite valley numbers and a gap opening induced by two Coulomb-interaction-driven magnetic orders. These magnetic orders, including an in-plane $120^{\circ}$-Néel order and an in-plane ferromagnetic order, interact with moiré bands via corresponding in-plane exchange fields. The Néel order ensures the insulating gap, the ferromagnetic order induces the non-zero Chern number, and both orders contribute to time-reversal symmetry breaking. The Néel order is acquired from the Hartree-Fock exchange interaction, and the formation of ferromagnetic order is attributed to interlayer-exciton condensation and exciton ferromagnetism. The exciton ferromagnetism can be demonstrated by excitonic Bose-Hubbard physics and Berezinskii-Kosterlitz-Thouless transition. In low electric fields, the equilibrium state is a Mott-insulator state. At a certain electric field, a correlated insulating state composed of the hole-occupied band and the exciton condensate becomes a new thermodynamically stable phase, and the topological phase transition occurs as the ferromagnetic order emerges. The consistency between the present theory and experimental observations is discussed. Experimental observations, including the spin-polarized/valley-coherent nature of the QAH state, the absence of charge gap closure at the topological phase transition, the canted spin texture, and the insulator-to-metal transition, are interpreted by the mechanism.

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arXiv2022

Contact effects on thermoelectric properties of textured graphene nanoribbons

Transport and thermoelectric properties of finite textured graphene nanoribbons (t-GNRs) connected to electrodes with various coupling strengths are theoretically studied in the framework of the tight-binding model and Green's function approach. Due to quantum constriction induced by the indented edges, such t-GNRs behave like serially-coupled graphene quantum dots (SGQDs). These types of SGQDs can be formed by tailoring zigzag GNRs (ZGNRs) or armchair GNRs (AGNRs). Their bandwidths and gaps can be engineered by varying the size of the quantum dot and the neck width at indented edges. Effects of defects and contact junction on electrical conductance, Seebeck coefficient and electron thermal conductance of t-GNRs are calculated. When a defect occurs in the interior site of textured ZGNRs (t-ZGNRs), the maximum power factor within the central gap or near the band edges is found to be insensitive to the defect scattering. Furthermore, we found that SGQDs formed by t-ZGNRs have significantly better electrical power outputs than those of textured ANGRs due to the improved functional shape of the transmission coefficient in t-ZGNRs. With a proper design of contact the maximum power factor ( figure of merit) of t-ZGNRs could reach $90\%$ ($95\%$) of the theoretical limit.

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arXiv2021

Density functional calculations of atomic structure, charging effect, and static dielectric constant of two-dimensional systems based on B-splines

We implement a total-energy minimization scheme to allow for relaxation of atomic positions in density functional calculations for two-dimensional (2D) systems using a mixed basis set. The basis functions consist of products of 2D plane waves in the plane of the material and localized B-splines along the perpendicular direction. By using this mixed basis approach (MBA), we studied the atomic relaxation and charge polarization of 2D systems under an applied electric field. Compared to the conventional supercell approach (SCA) which adopts repeated slabs sandwiched between vacuum regions, MBA makes no requirement of compensating background charge for treating electrically charged 2D systems due to carrier injection. Furthermore, with MBA, the sawtooth potentials for systems under the applied field to maintain periodicity as needed in SCA is automatically avoided. From the linear response of charge polarization to the applied field, we introduced a simple method to determine the out-of-plane dielectric constants of 2D materials without the ambiguity of defining their effective thickness. Selected 2D systems including graphene and transition-metal dichalcogenides are tested. Our MBA results are consistent with previous SCA calculations when both approaches are equally applicable. However, for the charged system with high carrier density, we found significant deviation from SCA results obtained by imposing artificial charge neutrality condition. PACS: 71.15.

cond-mat.mtrl-sci↗

arXiv2021

Foldy-Wouthuysen transformation for gapped Dirac fermions in two-dimensional semiconducting materials and valley excitons under external fields

In this work, we provide a detailed derivation of Foldy-Wouthuysen (FW) transformation for two-dimensional (2D) gapped Dirac fermions under external fields and apply the formalism to study valley excitons in 2D semiconducting materials. Similar to relativistic quantum few-body problem, the gapped Dirac equation can be transformed into a Schrödinger equation with "relativistic" correction terms. In this 2D materials system, the correction terms can be interpreted as the Berry-curvature effect. The Hamiltonian for a valley exciton in external fields can be written based on the FW transformed Dirac Hamiltonian. Various valley-dependent effects on excitons, such as fine-structure splittings of exciton energy levels, valley-selected exciton transitions, and exciton valley Zeeman effect are discussed within this framework.

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arXiv2021

Modeling Photocurrent Spectra of In$_{0.91}$Ga$_{0.09}$N/In$_{0.4}$Ga$_{0.6}$N Disk-in-Wire Photodiode on Silicon for $1.3$ $μ$m $-$ $1.55$ $μ$m Operation

This work reports comprehensive theoretical modeling of photocurrent spectra generated by an In$_{0.91}$Ga$_{0.09}$N/In$_{0.4}$Ga$_{0.6}$N disk-in-wire photodiode. The strain distribution is calculated by valence-force-field (VFF) model, while a realistic band structure of the InN/InGaN heterostructure is incorporated using an eight-band effective bond-orbital model (EBOM) with spin-orbit coupling neglected. The electrostatic potential is obtained from self-consistent calculation employing the non-equilibrium Green's function (NEGF) method. With the strain distribution and band profile determined, a multi-band transfer-matrix method (TMM) is used to calculate the tunneling coefficients of optically-pumped carriers in the absorbing region. The photocurrent spectra contributed by both single-photon absorption (SPA) and two-photon absorption (TPA) are calculated. The absorption coefficient is weighted by the carrier tunneling rate and the photon density-of-state (DOS) in the optical cavity formed in the nanowire region to produce the photocurrent. The calculated photocurrent spectra is in good agreement with experimental data, while physical mechanisms for the observed prominent peaks are identified and investigated.

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arXiv2021

Variationally optimized orbital approach to trions in two-dimensional materials

In this work, trions in two-dimensional (2D) space are studied by variational method with trial wavefunctions being constructed by linear combinations of 2D slater-type orbitals (STOs). Via this method, trion energy levels and wavefunctions can be calculated efficiently with fairly good accuracy. We first apply this method to study trion energy levels in a 2D hydrogen-like system with respect to a wide range of mass ratios and screening lengths. We find that the ground-state trion is bound for the whole parameter range, and an excited-state trion with antisymmetric permutation of electrons with finite angular momentum is bound for large electron-hole mass ratios or long screening lengths. The binding energies of ground-state trions calculated by the present method agree well with those calculated by more sophisticated but computationally-demanding methods. We then calculate trion states in various monolayer transition metal dichalcogenides (TMDCs) by using this method with the inclusion of electron-hole exchange (EHX) interaction. For TMDCs, we found that the effect of EHX can be significant in determining the trion binding energy and the possible existence of stable excited-state trions.

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arXiv2021

Electric-field-tunable intervalley excitons and phonon replicas in bilayer WSe$_2$

We report the direct observation of intervalley exciton between the Q conduction valley and $Γ$ valence valley in bilayer WSe$_2$ by photoluminescence. The Q$Γ$ exciton lies at ~18 meV below the QK exciton and dominates the luminescence of bilayer WSe$_2$. By measuring the exciton spectra at gate-tunable electric field, we reveal different interlayer electric dipole moments and Stark shifts between Q$Γ$ and QK excitons. Notably, we can use the electric field to switch the energy order and dominant luminescence between Q$Γ$ and QK excitons. Both Q$Γ$ and QK excitons exhibit pronounced phonon replicas, in which two-phonon replicas outshine the one-phonon replicas due to the existence of (nearly) resonant exciton-phonon scatterings and numerous two-phonon scattering paths. We can simulate the replica spectra by comprehensive theoretical modeling and calculations. The good agreement between theory and experiment for the Stark shifts and phonon replicas strongly supports our assignment of Q$Γ$ and QK excitons.

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arXiv2020

Bloch-Grüneisen temperature and universal scaling of normalized resistivity in doped graphene revisited

In this work, we resolved some controversial issues on the Bloch-Grüneisen (BG) temperature in doped graphene via analytical and numerical calculations based on full inelastic electron-acoustic-phonon (EAP) scattering rate and various approximation schemes. Analytic results for BG temperature obtained by semi-inelastic (SI) approximation (which gives scattering rates in excellent agreement with the full inelastic scattering rates) are compared with those obtained by quasi-elastic (QE) approximation and the commonly adopted value of $Θ^{LA}_{F} = 2\hbar v_{LA} k_F/k_B$. It is found that the commonly adopted BG temperature in graphene ($Θ^{LA}_{F}$) is about 5 times larger than the value obtained by the QE approximation and about 2.5 times larger than that by the SI approximation, when using the crossing-point temperature where low-temperature and high-temperature limits of the resistivity meet. The corrected analytic relation based on SI approximation agrees extremely well with the transition temperatures determined by fitting the the low- and high-$T$ behavior of available experimental data of graphene's resistivity. We also introduce a way to determine the BG temperature including the full inelastic EAP scattering rate and the deviation of electron energy from the chemical potential ($μ$) numerically by finding the maximum of $\partial ρ(μ,T)/\partial T$. Using the analytic expression of $Θ_{BG,1}$ we can prove that the normalized resistivity defined as $R_{1}=ρ(μ,T)/ρ(μ,Θ_{BG,1})$ plotted as a function of $(T/Θ_{BG,1})$ is independent of the carrier density. Applying our results to previous experimental data extracted shows a universal scaling behavior, which is different from previous studies.

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arXiv2020

Gate-tunable exciton-polaron Rydberg series with strong roton effect

The electronic exciton polaron is a hypothetical many-body quasiparticle formed by an exciton dressed with a polarized electron-hole cloud in the Fermi sea (FS). It is predicted to display rich many-body physics and unusual roton-like dispersion. Exciton polarons were recently evoked to explain the excitonic spectra of doped monolayer transition metal dichalcogenides (TMDs), but these studies are limited to the ground state. Excited-state exciton polarons can exhibit richer many-body physics due to their larger spatial extent, but detection is challenging due to their inherently weak signals. Here we observe gate-tunable exciton polarons for the 1s - 3s excitonic Rydberg series in ultraclean monolayer MoSe$_2$ devices by optical spectroscopy. When the FS expands, we observe increasingly severe suppression and steep energy shift from low to high Rydberg states. Their gate-dependent energy shifts go beyond the trion description but match our exciton-polaron theory. Notably, the exciton-polaron absorption and emission bands are separated with an energy gap, which increases from ground to excited state. Such peculiar characteristics are attributed to the roton-like exciton-polaron dispersion, where energy minima occur at finite momenta. The roton effect increases from ground to excited state. Such exciton-polaron Rydberg series with progressively significant many-body and roton effect shall provide a new platform to explore complex many-body phenomena.

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arXiv2020

Multipath optical recombination of intervalley dark excitons and trions in monolayer WSe$_2$

Excitons and trions (or exciton-polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violate the conservation of momentum. Here we reveal the intervalley recombination of dark excitons and trions through more than one path in monolayer WSe$_2$. We observe the intervalley dark excitons, which can recombine by the assistance of defect scattering or chiral-phonon emission. We also reveal that a trion can decay in two distinct paths - through intravalley or intervalley electron-hole recombination - into two different final valley states. Although these two paths are energy degenerate, we can distinguish them by lifting the valley degeneracy under a magnetic field. In addition, the intra- and inter-valley trion transitions are coupled to zone-center and zone-corner chiral phonons, respectively, to produce distinct phonon replicas. The observed multipath optical decays of dark excitons and trions provide much insight into the internal quantum structure of trions and the complex excitonic interactions with defects and chiral phonons in monolayer valley semiconductors.

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arXiv2020

Landau-quantized excitonic absorption and luminescence in a monolayer valley semiconductor

We investigate Landau-quantized excitonic absorption and luminescence of monolayer WSe$_2$ under magnetic field. We observe gate-dependent quantum oscillations in the bright exciton and trions (or exciton-polarons) as well as the dark trions and their phonon replicas. Our results reveal spin- and valley-polarized Landau levels (LLs) with filling factors $n = +0, +1$ in the bottom conduction band and $n = -0$ to $-6$ in the top valence band, including the Berry-curvature-induced $n = \pm0$ LLs of massive Dirac fermions. The LL filling produces periodic plateaus in the exciton energy shift accompanied by sharp oscillations in the exciton absorption width and magnitude. This peculiar exciton behavior can be simulated by semi-empirical calculations. The experimentally deduced g-factors of the conduction band (g ~ 2.5) and valence band (g ~ 15) exceed those predicted in a single-particle model (g = 1.5, 5.5, respectively). Such g-factor enhancement implies strong many-body interactions in gated monolayer WSe$_2$. The complex interplay between Landau quantization, excitonic effects, and many-body interactions makes monolayer WSe$_2$ a promising platform to explore novel correlated quantum phenomena.

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arXiv2019

Transfer current in p-type graphene/MoS2 heterostructures

Transfer characteristics of p-doped graphene/monolayer-MoS2 heterostructure at 300 K are measured experimentally and analyzed based on a model calculation. In the model, we first discretize the Poisson equation (PE) into multiple zones. In each zone the charge density is assumed constant and the PE can be transformed into a linear equation to determine the chemical potential self-consistently. To calculate the electrical conductivity, we solve the Boltzmann transport equation in the relaxation-time approximation using the inelastic acoustic phonon scattering by solving the Dirac equation. The relationship between the Dirac voltage (the voltage at which the Fermi level is located at the Dirac point) and the Fermi energy (i.e. the initial chemical potential at 0K) is derived. These calculations are performed without and with optical pumping and the results obtained agree well with the experimental data.

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arXiv2019

Effective bond-orbital model of III-nitride wurtzite structures based on modified interaction parameters of zinc-blende structures

A simple theoretical method for deducing the effective bond-orbital model (EBOM) of III-nitride wurtzite (WZ) semiconductors is presented. In this model, the interaction parameters for zinc-blende (ZB) structures are used as an initial guess for WZ structure based on the two-center approximation. The electronic band structure of III-nitride WZ semiconductors can hence be produced by utilizing this set of parameters modified to include effects due to three-center integrals and fitting with first-principles calculations. Details of the semi-empirical fitting procedure for constructing the EBOM Hamiltonian for bulk III-nitride WZ semiconductors are presented. The electronic band structure of bulk AlN, GaN, and InN with WZ structure calculated by EBOM with modified interaction parameters are shown and compared to the results obtained from density functional (DFT) theory with meta-generalized gradient approximation (mGGA). The set of parameters are further optimized by using a genetic algorithm. In the end, electronic band structures and electron (hole) effective masses near the zone center calculated by the proposed model with best fitting parameters are analyzed and compared with the $\mathbf{k}\cdot \mathbf{p}$ model.

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