arXiv subjects

Xueheng Kuang

Publications and source records attributed to Xueheng Kuang.

10 recordsLinked to original sources

arXiv2025

Review of the tight-binding method applicable to the properties of moiré superlattices

Moiré superlattices have emerged as a versatile platform for exploring a wide range of ex- otic quantum phenomena. Unlike angstrom-scale materials, the moiré length-scale system contains a large number of atoms, and its electronic structure is significantly modulated by the lattice relaxation. These features pose a huge theoretical challenge. Among the available theoretical approaches, tight-binding (TB) methods are widely employed to predict the electronic, transport, and optical properties of systems such as twisted graphene, twisted transition-metal dichalcogenides (TMDs), and related moiré materials. In this review, we pro- vide a comprehensive overview of atomistic TB Hamiltonians and the numerical techniques commonly used to model graphene-based, TMD-based and hBN-based moiré superlattices. We also discuss the connection between atomistic TB descriptions and effective low-energy continuum models. Two examples of different moiré materials and geometries are provided to emphasize the advantages of the TB methods. This review is intended to serve as a theoretical and practical guide for those seeking to apply TB methods to the study of various properties of moiré superlattices.

cond-mat.mtrl-sci↗

arXiv2025

Random State Approach to Quantum Computation of Electronic-Structure Properties

Classical computation of electronic properties in large-scale materials remains challenging. Quantum computation has the potential to offer advantages in memory footprint and computational scaling. However, general and practical quantum algorithms for simulating large-scale materials are still lacking. We propose and implement random-state quantum algorithms to calculate electronic-structure properties of real materials. Using a random state circuit with only a few qubits, we employ real-time evolution with first-order Trotter decomposition and Hadamard test to obtain electronic density of states, and we develop a modified quantum phase estimation algorithm to calculate real-space local density of states via direct quantum measurements. Furthermore, we validate these algorithms by numerically computing the density of states and spatial distributions of electronic states in graphene, twisted bilayer graphene quasicrystals, and fractal lattices, covering system sizes from hundreds to thousands of atoms. Our results manifest that the random-state quantum algorithms provide a general and qubit-efficient route to simulating electronic properties of large-scale periodic and aperiodic materials on quantum computers.

physics.comp-ph↗

arXiv2024

Charge fluctuations, phonons and superconductivity in multilayer graphene

Motivated by the recent experimental detection of superconductivity in Bernal bilayer (AB) and rhombohedral trilayer (ABC) graphene, we study the emergence of superconductivity in multilayer graphene based on a Kohn-Luttinger (KL)-like mechanism in which the pairing glue is the screened Coulomb interaction. We find that electronic interactions alone can drive superconductivity in AB bilayer graphene and ABC trilayer graphene with the critical temperatures in good agreement with the experimentally observed ones, allowing us to further predict superconductivity from electronic interactions in Bernal ABA trilayer and ABAB tetralayer and rhombohedral ABCA tetralayer graphene. By comparing the critical temperatures ($T_c$) of these five non-twisted graphene stacks, we find that the ABC trilayer graphene possesses the highest $T_c\sim100$ mK. After considering the enhancement of superconductivity due to Ising spin-orbit coupling, we observe that the AB bilayer graphene has the largest enhancement in the critical temperature, increasing from 23 mK to 143 mK. The superconducting behaviors in these non-twisted graphene stacks could be explained by the order parameters (OPs). The OPs of Bernal stacks preserve intravalley $C_3$ symmetry, whereas rhombohedral stacks break it. In all stacks, the OPs have zeroes and change signs between valleys, which means that these multilayers of graphene are nodal spin-triplet superconductors. Moreover, dressing the purely electronic interaction with acoustic phonons, we observe minor changes of the critical temperatures in these five stacks. We adopt the KL-like mechanism to investigate the tendency of superconductivity in multilayer graphene without fitting parameters, which could provide guidance to future experiments exploring superconductivity in non-twisted graphene.

cond-mat.mes-hall↗

arXiv2024

Optical properties and plasmons in moiré structures

The discoveries of numerous exciting phenomena in twisted bilayer graphene (TBG) are stimulating significant investigations on moiré structures that possess a tunable moiré potential. Optical response can provide insights into the electronic structures and transport phenomena of non-twisted and twisted moiré structures. In this article, we review both experimental and theoretical studies of optical properties such as optical conductivity, dielectric function, non-linear optical response, and plasmons in moiré structures composed of graphene, hexagonal boron nitride (hBN), and/or transition metal dichalcogenides (TMDCs). Firstly, a comprehensive introduction to the widely employed methodology on optical properties is presented. After, moiré potential induced optical conductivity and plasmons in non-twisted structures are reviewed, such as single layer graphene-hBN, bilayer graphene-hBN and graphene-metal moiré heterostructures. Next, recent investigations of twist-angle dependent optical response and plasmons are addressed in twisted moiré structures. Additionally, we discuss how optical properties and plasmons could contribute to the understanding of the many-body effects and superconductivity observed in moiré structures.

cond-mat.mtrl-sci↗

arXiv2022

TBPLaS: a Tight-Binding Package for Large-scale Simulation

TBPLaS is an open-source software package for the accurate simulation of physical systems with arbitrary geometry and dimensionality utilizing the tight-binding (TB) theory. It has an intuitive object-oriented Python application interface (API) and Cython/Fortran extensions for the performance critical parts, ensuring both flexibility and efficiency. Under the hood, numerical calculations are mainly performed by both exact diagonalizatin and the tight-binding propagation method (TBPM) without diagonalization. Especially, the TBPM is based on the numerical solution of time-dependent Schrödinger equation, achieving linear scaling with system size in both memory and CPU costs. Consequently, TBPLaS provides a numerically cheap approach to calculate the electronic, transport and optical properties of large tight-binding models with billions of atomic orbitals. Current capabilities of TBPLaS include the calculation of band structure, density of states, local density of states, quasi-eigenstates, optical conductivity, electrical conductivity, Hall conductivity, polarization function, dielectric function, plasmon dispersion, carrier mobility and velocity, localization length and free path, Z2 topological invariant, wave-packet propagation, etc. All the properties can be obtained with only a few lines of code. Other algorithms involving tight-binding Hamiltonians can be implemented easily thanks to its extensible and modular nature. In this paper, we discuss the theoretical framework, implementation details and common workflow of TBPLaS, and give a few demonstrations of its applications.

cond-mat.mtrl-sci↗

arXiv2022

Flat-band plasmons in twisted bilayer transition metal dichalcogenides

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in the hole-doped twisted bilayer MoS$_2$ (tb-MoS$_2$) by employing a full tight-binding model and the random phase approximation. When considering lattice relaxations in tb-MoS$_2$, the flat band is not separated from remote valence bands, which makes the contribution of interband transitions in transforming the plasmon dispersion and energy significantly different. In particular, low-damped and quasi-flat plasmons emerge if we only consider intraband transitions in the doped flat band, whereas a $\sqrt q$ plasmon dispersion emerges if we also take into account interband transitions between the flat band and remote bands. Furthermore, the plasmon energies are tunable with twist angles and doping levels. However, in a rigid sample that suffers no lattice relaxations, lower-energy quasi-flat plasmons and higher-energy interband plasmons can coexist. For rigid tb-MoS$_2$ with a high doping level, strongly enhanced interband transitions quench the quasi-flat plasmons. Based on the lattice relaxation and doping effects, we conclude that two conditions, the isolated flat band and a properly hole-doping level, are essential for observing the low-damped and quasi-flat plasmon mode in twisted bilayer transition metal dichalcogenides. We hope that our study on flat-band plasmons can be instructive for studying the possibility of plasmon-mediated superconductivity in twisted bilayer transition metal dichalcogenides in the future.

cond-mat.mes-hall↗

arXiv2021

Electronic properties and quasi-particle model of monolayer MoSi$_2$N$_4$

By a combined study with first-principles calculations and symmetry analysis, we theoretically investigate the electronic properties of monolayer MoSi$_2$N$_4$. While the spin-orbital coupling results in bands splitting, the horizontal mirror symmetry locks the spin polarization along z-direction. In addition, a three-band tight-binding model is constructed to describe the low-energy quasi-particle states of monolayer MoSi$_2$N$_4$, which can be generalized to strained MoSi$_2$N$_4$ and its derivatives. The calculations using the tight-binding model show an undamped $\sqrt{q}$-dependent plasmon mode that agrees well with the results of first-principles calculations. Our model can be extended to be suitable for future theoretical and numerical studies of low-energy properties in MoSi$_2$N$_4$ family materials. Furthermore, the study of electronic properties of monolayer MoSi$_2$N$_4$ paves a way for its applications in spintronics and plasmonics.

cond-mat.mtrl-sci↗

arXiv2021

Magic angle and plasmon mode engineering in twisted trilayer graphene with pressure

Recent experimental and theoretical investigations demonstrate that twisted trilayer graphene (tTLG) is a highly tunable platform to study the correlated insulating states, ferromagnetism, and superconducting properties. Here we explore the possibility of tuning electronic correlations of the tTLG via a vertical pressure. A full tight-binding model is used to accurately describe the pressure-dependent interlayer interactions. Our results show that pressure can push a relatively larger twist angle (for instance, $1.89^{\circ}$) tTLG to reach the flat-band regime. Next, we obtain the relationship between the pressure-induced magic angle value and the critical pressure. These critical pressure values are almost half of that needed in the case of twisted bilayer graphene. Then, plasmonic properties are further investigated in the flat band tTLG with both zero-pressure magic angle and pressure-induced magic angle. Two plasmonic modes are detected in these two kinds of flat band samples. By comparison, one is a high energy damping-free plasmon mode that shows similar behavior, and the other is a low energy plasmon mode (flat-band plasmon) that shows obvious differences. The flat-band plasmon is contributed by both interband and intraband transitions of flat bands, and its divergence is due to the various shape of the flat bands in tTLG with zero-pressure and pressure-induced magic angles. This may provide an efficient way of tuning between regimes with strong and weak electronic interactions in one sample and overcoming the technical requirement of precise control of the twist angle in the study of correlated physics.

cond-mat.mes-hall↗

arXiv2020

Collective excitations and plasmon spectrum in twisted bilayer graphene near the magic angle

Twisted bilayer graphene with tiny rotation angles have drawn significant attention due to the observation of the unconventional superconducting and correlated insulating behaviors. In this paper, we employ a full tight-binding model to investigate collective excitations in twisted bilayer graphene near magic angle. The polarization function is obtained from the tight-binding propagation method without diagonalization of the Hamiltonian matrix. With the atomic relaxation considered in the simulation, damped and undamped interband plasmon modes are discovered near magic angle under both room temperature and superconductivity transition temperature. In particular, an undamped plasmon mode in narrow bands can be directly probed in magic angle twisted bilayer graphene at superconductivity transition temperature. The undamped plasmon mode is tunable with angles and gradually fades away with both temperature and chemical potential. In practice, the flat bands in twisted bilayer graphene can be detected by exploring the collective plasmons from the measured energy loss function.

cond-mat.mes-hall↗

arXiv2019

Quantum time-dependent Monte Carlo simulation of electron devices with 2D linear-band materials: a genuine TeraHertz signature for graphene

An intrinsic electron injection model for linear band two-dimensional (2D) materials, like graphene, is presented and its coupling to a recently developed quantum time-dependent Monte Carlo simulator for electron devices, based on the use of stochastic Bohmian conditional wave functions, is explained. The simulator is able to capture the full (DC, AC, transient and noise) performance of 2D electron devices. In particular, we demonstrate that the injection of electrons with positive and negative kinetic energies is mandatory when investigating high frequency performance of linear band materials with Klein tunneling, while traditional models dealing with holes (defined as the lack of electrons) can lead to unphysical results. We show that the number of injected electrons is bias-dependent, implying that an extra charge is required to get self-consistent results. Interestingly, we provide a successful comparison with experimental DC data. Finally, we predict that a genuine high-frequency signature due to a roughly constant electron injection rate in 2D linear band electron devices (which is missing in 2D parabolic band ones) can be used as a band structure tester.

cond-mat.mes-hall↗