arXiv subjects

Guan-Hao Peng

Publications and source records attributed to Guan-Hao Peng.

11 recordsLinked to original sources

arXiv2025

Machine-learning-enabled methodology for the ab-initio simulations of sub-$μ$m-wide nanoribbons

Simulation of mesoscopic nanostructures is a central challenge in condensed matter physics and device applications. First-principles methods provide accurate electronic structures but are computationally prohibitive for large systems, while empirical band theories are efficient yet limited by parameter fitting that neglects wavefunction information and often yields non-transferable parameters. We propose a methodology that bridges these approaches, achieving first-principles-level reliability with computational efficiency through a machine-learning-enabled tight-binding framework. Our approach starts with Wannier tight-binding (WTB) parameters from small nanostructures, which serve as training data for machine learning (ML). To remove the gauge freedom of Wannier functions that obscures size- and geometry-dependent parameter trends, we construct gauge-independent (GI) bases and transform the WTB model into a gauge-independent WTB (GI-WTB) model. This enables robust parameter fitting and ML prediction of parameter variations, yielding the machine-learning GI-WTB (ML-GI-WTB) model. Applied to MoS2 armchair-edge nanoribbons, the ML-GI-WTB model shows excellent agreement with first-principles results and enables reliable simulations of sub-$μ$m-wide nanoribbons. This framework provides a scalable tool for predicting electronic properties of realistic nanostructures beyond the reach of conventional first-principles methods.

cond-mat.mtrl-sci↗

arXiv2025

Signatures of valley drift in the diversified band dispersions of bright, gray, and dark excitons in MoS2 monolayers under uni-axial strains

We present a comprehensive theoretical investigation of the strain-modulated excitonic properties of uni-axially strained transition-metal dichalcogenide monolayers (TMD-MLs) by solving the Bethe-Salpeter equation (BSE) established on the basis of first principles. We show that imposing an uni-axial strain onto a MoS_$2$ monolayers leads to the diversified band dispersions of the bright exciton (BX), gray exciton (GX), and dark exciton (DX) states, as a consequence of the competitive interplay between strain-induced valley drift (VD) and momentum-dependent electron-hole exchange interaction (EHEI). While the band dispersions of BX doublet in the light-accessible small reciprocal area remain almost unchanged against strain, the band dispersion of DX is reshaped by an increasing uni-axial strain from a parabola to a Mexican-hat-like profile, featured with unusual sign-reversal of the heavy effective mass and strain-activated brightness. In contrast, the effective mass of GX is drastically lightened by uni-axial strain and remains always positive. We show that the strain-diversified exciton band dispersions leads to the distinct exciton diffusivities and angle-resolved optical patterns of BX, GX, and DX in a strained TMD-ML, suggesting the feasibility of {\it spatially} resolving spinallowed and -forbidden excitons in exciton transport experiments and angle-resolved optical spectroscopies.

physics.app-ph↗

arXiv2025

Efficient light upconversion via resonant exciton-exciton annihilation of dark excitons in few-layer transition metal dichalcogenides

In this work, we report a pronounced light upconversion in few-layer transition metal dichalcogenides. Our joint theory-experiment study attributes the upconversion photoluminescence to a resonant exciton-exciton annihilation involving a pair of dark excitons with opposite momenta, followed by the spontaneous emission of upconverted bright excitons, which can have a high upconversion efficiency. Additionally, the upconversion photoluminescence is generic in MoS2, MoSe2, WS2, and WSe2, showing a high tuneability from green to ultraviolet light.

cond-mat.mtrl-sci↗

arXiv2024

Enhanced photo-excitation and angular-momentum imprint of gray excitons in WSe$_{2}$ monolayers by spin-orbit-coupled vector vortex beams

A light beam can be spatially structured in the complex amplitude to possess orbital angular momentum (OAM), which introduces a new degree of freedom alongside the intrinsic spin angular momentum (SAM) associated with circular polarization. Moreover, super-imposing two twisted lights with distinct SAM and OAM produces a vector vortex beam (VVB) in non-separable states where not only complex amplitude but also polarization are spatially structured and entangled with each other. In addition to the non-separability, the SAM and OAM in a VVB are intrinsically coupled by the optical spin-orbit interaction and constitute the profound spin-orbit physics in photonics. In this work, we present a comprehensive theoretical investigation, implemented on the first-principles base, of the intriguing light-matter interaction between VVBs and WSe$_{2}$ monolayers (WSe$_{2}$-MLs), one of the best-known and promising two-dimensional (2D) materials in optoelectronics dictated by excitons, encompassing bright exciton (BX) as well as various dark excitons (DXs). One of the key findings of our study is the substantial enhancement of the photo-excitation of gray excitons (GXs), a type of spin-forbidden dark exciton, in a WSe$_2$-ML through the utilization of a twisted light that possesses a longitudinal field associated with the optical spin-orbit interaction. Our research demonstrates that a spin-orbit-coupled VVB surprisingly allows for the imprinting of the carried optical information onto gray excitons in 2D materials, which is robust against the decoherence mechanisms in materials. This observation suggests a promising method for deciphering the transferred angular momentum from structured lights to excitons.

cond-mat.mes-hall↗

arXiv2023

The key role of non-local screening in the environment-insensitive exciton fine structures of transition-metal dichalcogenide monolayers

In this work, we present a comprehensive theoretical and computational investigation of exciton fine structures of WSe$_2$-monolayers, one of the best known two-dimensional (2D) transition-metal dichalcogenides (TMD's), in various dielectric-layer environments by solving the first-principles-based Bethe-Salpeter equation. While the physical and electronic properties of atomically thin nano-materials are normally sensitive to the variation of surrounding environment, our studies reveal that the influence of dielectric environment on the exciton fine structures of TMD-ML's is surprisingly limited. We point out that the non-locality of Coulomb screening plays a key role to suppress the factor of dielectric environment and drastically shrink the fine structure splittings between bright exciton (BX) states and various dark exciton (DX) states of TMD-ML's. The intriguing non-locality of screening in 2D materials can be manifested by the measurable {\it non-linear} correlation between the BX-DX splittings and exciton binding energies with varying the surrounding dielectric environments. The revealed environment-insensitive exciton fine structures of TMD-ML's suggest the robustness of prospective dark-exciton-based opto-electronics against the inevitable variation of inhomogeneous dielectric environment.

cond-mat.mes-hall↗

arXiv2022

Twisted-light-induced exciton wave packets in transition-metal dichalcogenide monolayers

We present a comprehensive theoretical investigation of the photo-generated excitons in transition-metal dichalcogenide monolayers (TMD-ML's) by Laguerre-Gaussian beams, a celebrated kind of twisted lights (TL's) carrying quantized orbital angular momenta (OAM). We show that the photo-excitation of TL incident to a TMD-ML leads to the formation of spatially localized exciton wave packets, constituted by the superposition of finite-momentum exciton states determined by the intriguing interplay between the multiple degrees of freedom of the optical and excitonic subsystems. Consequently, the TL-induced exciton wave packets yield profound directional photo-luminescences whose polar-angle-dependences are encoded by the transferred optical OAM and azimuthal angle part, despite OAM-irrelevant, optically resolves the exchange-split longitudinal and transverse exciton bands. Interestingly, the application of linearly polarized TL onto a valley-excitonic system mimics an exciton multiplexer allowing for selectively detecting the individual valley-mixed exciton bands, which are normally hardly measured spectrally.

cond-mat.mes-hall↗

arXiv2021

Inherently high valley polarizations of momentum-forbidden dark excitons in transition-metal dichalcogenide monolayers

High degree of valley polarization of optically active excitons in transition-metal dichalcogenide monolayers (TMD-MLs) is vital in valley-based photonic applications but known to be likely spoiled by the intrinsic electron-hole exchange interactions. In this study, we present a theoretical investigation of the valley and optical properties of finite-momentum dark excitons in WSe$_2$-MLs by solving the density-functional-theory(DFT)-based Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. %We reveal that, in general, finite-momentum excitons are actually well immune from the exchange-induced valley depolarization, except for those with specific exciton momenta coincident with the $3σ_v$ and $3C_2'$ symmetries in the $D_{3h}$ point group of TMD-MLs. We reveal that, unlike the bright exciton inevitably subjected to electron-hole exchange interaction, inter-valley finite-momentum dark excitons in WSe$_2$-MLs are well immune from the exchange-induced valley depolarization and inherently highly valley-polarized under the enforcement of the crystal symmetries. More importantly, the superior valley polarizations of the inter-valley dark excitons in WSe$_2$-MLs are shown almost fully transferable to the optical polarization in the phonon-assisted photo-luminescences because of the native suppression of exchange-induced depolarization in the second-order optical processes. The analysis of phonon-assisted photo-luminescences accounts for the recently observed brightness, high degree of optical polarization and long lifetime of the inter-valley dark exciton states in tungsten-based TMD-MLs.

cond-mat.mes-hall↗

arXiv2020

Symmetry governed valley-pseudospin textures of the full-zone excitonic bands of transition-metal dichalcogenide monolayers

Preserving a high degree of valley polarization of excitons in photo-excited transition-metal dichalcogenide monolayers (TMD-MLs) is desirable for the valley-based photonic applications, but widely recognized as a hard task hindered by the intrinsic electron-hole exchange interaction. In this study, we present a comprehensive investigation of valley-polarized finite-momentum excitons in WSe$_2$-MLs over the entire Brillouin zone by solving the density-functional-theory(DFT)-based Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. We reveal that finite-momentum excitons are actually in general well immune from the exchange-induced valley depolarization, except for those with specific exciton momenta directionally coincide with the axes associated with the $3σ_v$ and $3C_2'$ symmetries in TMD-MLs. Governed by the symmetries, the valley pseudo-spin texture of the full-zone exciton band in the momentum space is locally featured by individual skyrmion-like structures where highly valley-polarized finite-momentum exciton states are centred. Remarkably, we show that the high degrees of valley polarizations of the finite-momentum exciton states are excellently well transferable to the optical polarizations in the resulting phonon-assisted photo-luminescences, suggesting the prospective usefulness of those inter-valley excitons in valley-based photonics.

cond-mat.mes-hall↗

arXiv2020

Twisted-light-revealed Lightlike Exciton Dispersion in Monolayer MoS2

Twisted light carries a well-defined orbital angular momentum (OAM) per photon. The quantum number l of its OAM can be arbitrarily set, making it an excellent light source to realize high-dimensional quantum entanglement and ultra-wide bandwidth optical communication structures. To develop solid-state optoelectronic systems compatible with such promising light sources, a timely challenging task is to efficiently and coherently transfer the optical OAM of light to certain solid-state optoelectronic materials. Among the state-of-the-art emergent materials, atomically thin monolayer transition metal dichalcogenide (ML-TMD), featured by ultra-strong light-matter interaction due to its reduced dimensionality, renders itself a potential material suitable for novel applications. In this study, we carried out photoluminescence (PL) spectroscopy studies of ML-MoS2 under photoexcitation of twisted light with well-defined quantized OAM. We mainly observed pronounced increases in the spectral peak energy for every increment of l of the incident twisted light. The observed non-linear l-dependence of the spectral blue shifts evidences the OAM transfer from the exciting twisted light to the valley excitons in ML-TMDs, which is well accounted for by our analysis and computational simulation. Even more excitingly, the twisted light excitation is shown to make excitonic transitions relative to the transferred OAM, enabling us to infer the exciton band dispersion from the measured spectral shifts. Consequently, the measured non-linear l-dependent spectral shifts revealed an unusual lightlike exciton band dispersion of valley excitons in ML-TMDs that is predicted by previous theoretical studies and evidenced for the first time via our experimental setup that utilizes the unique twisted light source.

cond-mat.mtrl-sci↗

arXiv2019

Distinctive signatures of the spin- and momentum-forbidden dark exciton states in the photo-luminescences of strained WSe$_2$ monolayers under thermalization

With the both spin and valley degrees of freedom, the low-lying excitonic spectra of photo-excited transition-metal dichalcogenide monolayers (TMDC-MLs) are featured by rich fine structures, comprising the intra-valley bright exciton states as well as various intra- and inter-valley dark ones. The latter states can be classified as those of the spin- and momentum-forbidden dark excitons according to the violated optical selection rules. Because of the optical invisibility, the two types of the dark states are in general hardly observed and even distinguished in conventional spectroscopies although their impacts on the optical and dynamical properties of TMDC-MLs have been well noticed. In this Letter, we present a theoretical and computational investigation of the exciton fine structures and the temperature-dependent photo-luminescence spectra of strained tungsten diselenide monolayers (WSe$_2$-MLs) where the intra-valley spin-forbidden dark exciton lies in the lowest exciton states and other momentum-forbidden states are in the higher energies that are tunable by external stress. The numerical computations are carried out by solving the Bethe-Salpeter equation for an exciton in a WSe$_2$-ML under the stress-control in the tight-binding scheme established from the first principle computation in the density functional theory. According to the numerical computation and supportive model analysis, we reveal the distinctive signatures of the spin- and momentum-forbidden exciton states of strained WSe$_2$-MLs in the temperature-dependent photo-luminescences and present the guiding principle to infer the relative energetic locations of the two types of DX's.

cond-mat.mes-hall↗

arXiv2018

Optimal electro-mechanical control of the excitonic fine structures of droplet epitaxial quantum dots

The intrinsic fine structure splittings (FSSs) of the exciton states of semiconductor quantum dots (QDs) are known to be the major obstacle for realizing the QD-based entangled photon pair emitters. In this study, we present a theoretical and computational investigation of the excitonic fine structures of droplet-epitaxial (DE) GaAs/AlGaAs QDs under the electro-mechanical control of micro-machined piezoelectricity actuators. From the group theory analysis with numerical confirmation based on the developed exciton theory, we reveal the general principle for the optimal design of micro-machined actuators whose application on to an elongated QD can certainly suppress its FSS. We show that the use of two independently tuning stresses is sufficient to achieve the FSS-elimination but is not always necessary as widely deemed. The use of a single tuning stress to eliminate the FSS of an elongated QD is possible as long as the crystal structure of the actuator material is in coincidence with that of the QD. As a feasible example, we show that a {\it single} symmetric bi-axial stress naturally generated from the $(001)$ PMN-PT actuator can be used as a single tuning knob to make the full FSS-elimination for elongated DE GaAs QDs.

cond-mat.mes-hall↗