Magnetoplasmons in magic-angle twisted bilayer graphene.

The magic-angle twisted bilayer graphene (MATBLG) has been demonstrated to exhibit exotic physical properties due to the special flat bands. However, exploiting the engineering of such properties by external fields is still in it infancy. Here we show that MATBLG under an external magnetic field presents a distinctive magnetoplasmon dispersion, which can be significantly modified by transferred momentum and charge doping. Along a wide range of transferred momentum, there exist special pronounced single magnetoplasmon and horizontal single-particle excitation modes near charge neutrality. We provide an insightful discussion of such unique features based on the electronic excitation of Landau levels quantized from the flat bands and Landau damping. Additionally, charge doping leads to peculiar multiple strong-weight magnetoplasmons. These characteristics make MATBLG a favorable candidate for plasmonic devices and technology applications. .

Profitability of Ichimoku-Based Trading Rule in Vietnam Stock Market in the Context of the COVID-19 Outbreak.

Ichimoku Kinkohyo or Ichimoku Cloud Chart is one of the most popular technical indicators used by traders all over the world. However, its profitability is heavily influenced by the market environment, to which it is applied. Furthermore, the COVID-19 outbreak may have an impact on the market environment as well as the performance of all technical indicators. This study is the first to look into the profitability of Ichimoku-based trading rules in the Vietnamese stock market in the context of the COVID-19 outbreak. More particularly, the COVID-19 outbreak has a positive influence on the performance of this strategy when considering the entire market as well as a variety of industries including real estate industry, food and beverage industry, resource industry, and automotive and electronic components industry. Compared to the pre-pandemic period, the return on investment obtained per each transaction using the Ichimoku-based strategy increased by roughly 8 - 9 % in the pandemic period. Compared to the Buy-and-hold method, the Ichimoku-based strategy could slightly increase Accumulated return while posing a lower risk. The findings indicate that the Ichimoku-based strategy is applicable to the Vietnam stock market, regardless of the adverse effects of the pandemic on the industries.

Role Played by Edge-Defects in the Optical Properties of Armchair Graphene Nanoribbons.

We explore the implementation of specific optical properties of armchair graphene nanoribbons (AGNRs) through edge-defect manipulation. This technique employs the tight-binding model in conjunction with the calculated absorption spectral function. Modification of the edge states gives rise to the diverse electronic structures with striking changes in the band gap and special flat bands at low energy. The optical-absorption spectra exhibit unique excitation peaks, and they strongly depend on the type and period of the edge extension. Remarkably, there exist the unusual transition channels associated with the flat bands for selected edge-modified systems. We discovered the special rule governing how the edge-defect influences the electronic and optical properties in AGNRs. Our theoretical prediction demonstrates an efficient way to manipulate the optical properties of AGNRs. This might be of importance in the search for suitable materials designed to have possible technology applications in nano-optical, plasmonic and optoelectronic devices.

Engineering plasmon modes and their loss in armchair graphene nanoribbons by selected edge-extended defects.

The effect of edge modification of armchair graphene nanoribbons (AGNRs) on the collective excitations are theoretically investigated. The tight-binding method is employed in conjunction with the dielectric function. Unconventional plasmon modes and their association with the flat bands of the specially designed AGNRs are thoroughly studied. We demonstrate the robust relationship between the novel collective excitations and both the type and period of the edge modification. Additionally, we reveal that the main features displayed in the (momentum, frequency)-phase diagrams for both single-particle and collective excitations of AGNRs can be efficiently tuned by edge-extended defects. Our obtained plasmon modes are found to be analogous to magnetoplasmons associated with collective excitations of Landau-quantized electrons. This work provides a unique way to engineer discrete magnetoplasmon-like modes of AGNRs in the absence of magnetic field.

Atomistic Band-Structure Computation for Investigating Coulomb Dephasing and Impurity Scattering Rates of Electrons in Graphene.

In this paper, by introducing a generalized quantum-kinetic model which is coupled self-consistently with Maxwell and Boltzmann transport equations, we elucidate the significance of using input from first-principles band-structure computations for an accurate description of ultra-fast dephasing and scattering dynamics of electrons in graphene. In particular, we start with the tight-binding model (TBM) for calculating band structures of solid covalent crystals based on localized Wannier orbital functions, where the employed hopping integrals in TBM have been parameterized for various covalent bonds. After that, the general TBM formalism has been applied to graphene to obtain both band structures and wave functions of electrons beyond the regime of effective low-energy theory. As a specific example, these calculated eigenvalues and eigen vectors have been further utilized to compute the Bloch-function form factors and intrinsic Coulomb diagonal-dephasing rates for induced optical coherence of electron-hole pairs in spectral and polarization functions, as well as the energy-relaxation time from extrinsic impurity scattering of electrons for non-equilibrium occupation in band transport.

Multi-orbital tight binding model for the electronic and optical properties of armchair graphene nanoribbons in the presence of a periodic potential.

The influences of an external electric field with uniform or modulated potential on the electronic and optical properties of armchair graphene nanoribbons (GNRs) are explored using the multi-orbital tight-binding Hamiltonian. The interplay between an electric field and interaction between (s,px,py,pz) orbitals remarkably enriches the main features of band structures and absorption spectra. The applied electric field can notably alter the energy dispersions ofπandσbands, leading to the deformation of band-edge states, open and close of a band gap, and modification of the Fermi energy. The vertical optical excitations happen among theπbands, while their available channels depend on the Fermi level which is controlled by theσ-edge bands and a finite potential. With the rich and unique properties, GNRs are suitable candidates for applications in the fields of photodetectors, nanoelectronics, and spintronics. The calculated results are expected to be examined by the angle-resolved photoemission spectroscopies and optical spectroscopies.

Adjusting the electronic properties and contact types of graphene/F-diamane-like C4F2 van der Waals heterostructure: a first principles study.

Motivated by the successful exfoliation of two-dimensional F-diamane-like C4F2 monolayer and the superior properties of graphene-based vdW heterostructures, in this work, we perform a first principles study to investigate the atomic structure, electronic properties and contact types of the graphene/F-diamane-like C4F2 heterostructure. The graphene/C4F2 vdW heterostructure is structurally stable at room temperature. In the ground state, the graphene/C4F2 heterostructure forms n-type Schottky contact with a Schottky barrier height of 0.46/1.03 eV given by PBE/HSE06. The formation of the graphene/C4F2 heterostructure tends to decrease in the band gap of the semiconducting C4F2 layer, suggesting that such a heterostructure may have strong optical absorption. Furthermore, the electronic properties and contact types of the graphene/C4F2 heterostructure can be adjusted by applying an external electric field, which leads to the change in the Schottky barrier height and the transformation from Schottky to ohmic contact. Our findings reveal the potential of the graphene/C4F2 heterostructure as a tunable hybrid material with strong potential in electronic applications.

Computational insights into structural, electronic, and optical properties of Janus GeSO monolayer.

Although O is an element of chalcogen group, the study of two-dimensional (2D) O-based Janus dichalcogenides/monochalcogenides, especially their 1T-phase, has not been given sufficient attention. In this work, we systematically investigate the structural, electronic, and optical properties of 1T Janus GeSO monolayer by using the density functional theory. Via the analysis of phonon spectrum and evaluation of elastic constants, the GeSO monolayer is confirmed to be dynamically and mechanically stable. Calculated results for the elastic constants demonstrate that the Janus GeSO monolayer is much mechanically flexible than other 2D materials due to its small Young's modulus. At the ground state, while both GeS2 and GeO2 monolayers are indirect semiconductors, the Janus GeSO monolayer is found to be a direct band gap semiconductor. Further, effective masses of both electrons and holes are predicted to be directionally isotropic. The Janus GeSO monolayer has a broad absorption spectrum, which is activated from the visible light region and its absorption intensity is very high in the near-ultraviolet region. The calculated results not only systematically provide the fundamental physical properties of GeSO monolayer, but also stimulate scientists to further studying its importance both theoretically and experimentally.

Type-I band alignment of BX-ZnO (X = As, P) van der Waals heterostructures as high-efficiency water splitting photocatalysts: a first-principles study.

In this work, we perform first-principles calculations to examine the electronic, optical and photocatalytic properties of the BX-ZnO (X = As, P) heterostructures. The interlayer distance and binding energy of the most energetically favorable stacking configuration are 3.31 Å and -0.30 eV for the BAs-ZnO heterostructure and 3.30 Å and -0.25 eV for the BP-ZnO heterostructure. All the stacking patterns of the BX-ZnO heterostructures are proved to have thermal stability by performing AIMD simulations. The BAs-ZnO and BP-ZnO heterostructures are semiconductors with direct band gaps of 1.43 eV and 2.35 eV, respectively, and they exhibit type-I band alignment, which make them suitable for light emission applications with the ultra-fast recombination between electrons and holes. Both the BAs-ZnO and BP-ZnO heterostructures can exhibit a wider optical absorption range for visible-light owing to their reduced band gaps compared with the isolated BAs, BP and ZnO monolayers. The band alignment of both the BAs-ZnO and BP-ZnO heterostructures can straddle the water redox potential and they would have better performances owing to the direct band gap and the reduced band gap. All these findings demonstrate that the BX-ZnO heterostructures can be considered as potential photocatalysts for water splitting.

Electronic and photocatalytic properties of two-dimensional boron phosphide/SiC van der Waals heterostructure with direct type-II band alignment: a first principles study.

Designing van der Waals (vdW) heterostructures of two-dimensional materials is an efficient way to realize amazing properties as well as opening opportunities for applications in solar energy conversion and nanoelectronic and optoelectronic devices. In this work, we investigate the electronic, optical, and photocatalytic properties of a boron phosphide-SiC (BP-SiC) vdW heterostructure using first-principles calculations. The relaxed configuration is obtained from the binding energies, inter-layer distance, and thermal stability. We show that the BP-SiC vdW heterostructure has a direct band gap with type-II band alignment, which separates the free electrons and holes at the interface. Furthermore, the calculated absorption spectra demonstrate that the optical properties of the BP-SiC heterostructure are enhanced compared with those of the constituent monolayers. The intensity of optical absorption can reach up to about 105 cm-1. The band edges of the BP-SiC heterostructure are located at energetically favourable positions, indicating that the BP-SiC heterostructure is able to split water under working conditions of pH = 0-3. Our theoretical results provide not only a fascinating insight into the essential properties of the BP-SiC vdW heterostructure, but also helpful information for the experimental design of new vdW heterostructures.

Rich Magnetic Quantization Phenomena in AA Bilayer Silicene.

The rich magneto-electronic properties of AA-bottom-top (bt) bilayer silicene are investigated using a generalized tight-binding model. The electronic structure exhibits two pairs of oscillatory energy bands for which the lowest conduction and highest valence states of the low-lying pair are shifted away from the K point. The quantized Landau levels (LLs) are classified into various separated groups by the localization behaviors of their spatial distributions. The LLs in the vicinity of the Fermi energy do not present simple wave function modes. This behavior is quite different from other two-dimensional systems. The geometry symmetry, intralayer and interlayer atomic interactions, and the effect of a perpendicular magnetic field are responsible for the peculiar LL energy spectra in AA-bt bilayer silicene. This work provides a better understanding of the diverse magnetic quantization phenomena in 2D condensed-matter materials.

Electric-field-diversified optical properties of bilayer silicene.

The rich optical properties of AA-bottom-top (bt) bilayer silicene (BS) in a uniform perpendicular electric field (E) are investigated through the use of the tight-binding model. The distinctive multivalley band structure presents a semimetallic behavior but with a sizeable intraband gap. The main features of the energy dispersion appear in the optical absorption spectrum through transitions between band-edge states obeying specific selection rules. The E field clearly modifies the energy dispersion, especially opening a band gap, leading to a substantial influence on the optical properties. This field creates additional excitation channels and increases the frequency and intensity of the spectral structures. The inclusion of electron-hole interaction gives rise to a slight difference in the spectral intensity and strongly affects the fluctuation in the threshold intensity as the field strength is varied. The interplay between the special lattice structure, atomic interactions, electron-hole interaction, and a E field robustly diversifies the absorption spectra.

Peculiar optical properties of bilayer silicene under the influence of external electric and magnetic fields.

We conduct a comprehensive investigation of the effect of an applied electric field on the optical and magneto-optical absorption spectra for AB-bt (bottom-top) bilayer silicene. The generalized tight-binding model in conjunction with the Kubo formula is efficiently employed in the numerical calculations. The electronic and optical properties are greatly diversified by the buckled lattice structure, stacking configuration, intralayer and interlayer hopping interactions, spin-orbital couplings, as well as the electric and magnetic fields ([Formula: see text] [Formula: see text] [Formula: see text]). An electric field induces spin-split electronic states, a semiconductor-metal phase transitions and the Dirac cone formations in different valleys, leading to the special absorption features. The Ez-dependent low-lying Landau levels possess lower degeneracy, valley-created localization centers, peculiar distributions of quantum numbers, well-behaved and abnormal energy spectra in Bz-dependencies, and the absence of anti-crossing behavior. Consequently, the specific magneto-optical selection rules exist for diverse excitation categories under certain critical electric fields. The optical gaps are reduced as Ez is increased, but enhanced by Bz, in which the threshold channel might dramatically change in the former case. These characteristics are in sharp contrast with those for layered graphene.

The diverse magneto-optical selection rules in bilayer black phosphorus.

The magneto-optical properties of bilayer phosphorene is investigated by the generalized tight-binding model and the gradient approximation. The vertical inter-Landau-level transitions, being sensitive to the polarization directions, are mainly determined by the spatial symmetries of sub-envelope functions on the distinct sublattices. The anisotropic excitations strongly depend on the electric and magnetic fields. A uniform perpendicular electric field could greatly diversify the selection rule, frequency, intensity, number and form of symmetric absorption peaks. Specifically, the unusual magneto-optical properties appear beyond the critical field as a result of two subgroups of Landau levels with the main and side modes. The rich and unique magnetoabsorption spectra arise from the very close relations among the geometric structures, multiple intralayer and interlayer hopping integrals and composite external fields.

Stacking-enriched magneto-transport properties of few-layer graphenes.

The quantum Hall effects in sliding bilayer graphene and a AAB-stacked trilayer system are investigated using the Kubo formula and a generalized tight-binding model. The various stacking configurations can greatly diversify the magnetic quantization and thus create rich and unique transport properties. The quantum conductivities are very sensitive to the Fermi energy and magnetic-field strength. The diverse features cover the specific non-integer conductivities, the integer conductivities with distinct steps, the splitting-created reduction and complexity of quantum conductivity, a vanishing or non-zero conductivity at the neutral point, and the well-like, staircase, composite, and abnormal plateau structures in the field dependencies. Such stacking-dependent characteristics mainly originate from the crossing, anticrossing and splitting Landau-level energy spectra and three kinds of quantized modes.

Rich magneto-absorption spectra of AAB-stacked trilayer graphene.

A generalized tight-binding model is developed to investigate the feature-rich magneto-optical properties of AAB-stacked trilayer graphene. Three intragroup and six intergroup inter-Landau-level (inter-LL) optical excitations largely enrich magneto-absorption peaks. In general, the former are much higher than the latter, depending on the phases and amplitudes of LL wavefunctions. The absorption spectra exhibit single- or twin-peak structures which are determined by quantum modes, LL energy spectra and Fermion distribution. The splitting LLs, with different localization centers (2/6 and 4/6 positions in a unit cell), can generate very distinct absorption spectra. There exist extra single peaks because of LL anti-crossings. AAB, AAA, ABA, and ABC stackings considerably differ from one another in terms of the inter-LL category, frequency, intensity, and structure of absorption peaks. The main characteristics of LL wavefunctions and energy spectra and the Fermi-Dirac function are responsible for the configuration-enriched magneto-optical spectra.

Magneto-optical properties of ABC-stacked trilayer graphene.

The generalized tight-binding model is developed to investigate the magneto-optical absorption spectra of ABC-stacked trilayer graphene. The absorption peaks can be classified into nine categories of inter-Landau-level optical excitations, including three intra-group and six inter-group ones. Most of them belong to the twin-peak structures because of the asymmetric Landau level spectrum. The threshold absorption peak alone comes from a certain excitation channel, and its frequency is associated with a specific interlayer atomic interaction. The Landau-level anticrossings cause extra absorption peaks. Moreover, a simple relationship between the absorption frequency and the field strength is absent. The magneto-optical properties of ABC-stacked trilayer graphene are totally different from those of AAA- and ABA-stacked ones, such as the number, intensity and frequency of absorption peaks.