Solving Bethe-Salpeter Equation Research Articles

Significant improvement in structural, electronic, optical and thermoelectric properties of PdTe2 in bulk and monolayer phase: A G0W0+BSE approach

Using first principles calculations structural, electronic, thermoelectric and optical properties of PdTe2 in bulk and monolayer were investigated. The calculated lattice parameters with different exchange correlation functionals show that the results obtained with addition of van der Waals corrections (vdW-DFC09x) are very well matched with the results obtained experimental. For correct prediction of electronic band gap, G0W0 approximation is used and the calculated band structure reveal that PdTe2 in bulk form has metallic nature as attained in experimental studies. In addition, the electronic band gap results with G0W0 approximation depicted that PdTe2 in monolayer phase is a semiconductor material with a direct band gap of 1.12 eV. The optical spectra have been calculated via many-body perturbation theory (MBPT) by solving Bethe-Salpeter equation (BSE) (G0W0+BSE). Ultimately, the results of optical spectra demonstrated that PdTe2 in monolayer phase has strong optical absorption within visible light wavelengths than the bulk phase which is good for solar cell applications. Thermoelectric (TE) properties of PdTe2 in bulk and monolayer structures have been studied using BoltzTraP code. The TE properties calculations indicated that better performance can be obtained by reducing the dimensionality of materials. The figure of merit was found to be above unity for monolayer phase. This paper successfully exploited the enhancement of the figure of merit values with the monolayer structure of PdTe2 material.

An ab initio study of vertical heterostructures formed by CdO and SnC monolayers

Assembling two dimensional (2D) materials in vertical heterostructures is one of the main techniques for tuning electronic and optical properties. In most cases, known as van der Waals heterostructures (vdWHs), the interlayer distances are larger than typical covalent bond lengths resulting in weak interlayer interactions. It has been shown that reducing the distance between the layers can significantly alter the properties of separated layers, which is not so noticeable in vdWHs and thus creates a new platform for controlling the physical properties of 2D materials. Motivated by enhanced properties of 2D vertical heterostructures, employing ab-initio calculations based on density functional theory we examined CdO/SnC systems in four different configurations. Our results reveal that in spite of thermodynamic and mechanical stabilities of all considered structures, according to the calculated phonon frequencies, only the structure formed by placing the Sn atom on the O atom and the C atom on the Cd atom is dynamically stable at zero temperature. This structure has an interlayer distance of 2.52 Å which is smaller than the interlayer distance in typical vdWHs. We investigated the electronic and optical properties of this dynamically stable structure utilizing GW approximation and solving Bethe–Salpeter equation. Unlike the monolayer CdO which possesses a single optical absorption peak close to the red light energy, the considered CdO/SnC structure has an optical band gap of 1.14 eV, and it can absorb 13% of incident light in the blue light region.

Excitonic effects on the optical spectra of TiB2 nanosheets

Two-dimensional (2D) transition metal borides, have recently attracted tremendous interest and become an emerging class of 2D materials due to their intriguing properties. We report systematic investigation of stability, electronic properties and optical activity of 2D-TiB2 nanosheets using first-principles calculations. By combining elastic and phonon-dispersion calculations, we substantiated the mechanical and dynamic stability of 2D-TiB2 nanosheets. Our results of elastic calculations reveal that addition of an extra layer destroys the auxetic behaviour of monolayer (ML) 2D-TiB2 with significant reduction in the brittleness. Further, based on the spin-polarised electronic structure calculations, we find that a low-dimensional metallic state of ML can be achieved by tuning the distance between the titanium and boron layers. The calculations clearly reveal that metallic nature does not destroys the auxetic behaviour of ML. Subsequently, we investigated the optical response of 2D-TiB2 nanosheets (ML and bilayer (BL)) at the level of density functional theory and many-body perturbation theory. The results obtained by solving Bethe–Salpeter equation (GW+BSE formalism) shows that excitonic effects causes a slight blue-shift in the absorption spectra of the nanosheets with ML being optically active with sharp peaks in infrared (IR) and ultraviolet (UV) regions while BL shows the reduced optical activity across wide range of photon energies spanning the near-IR to UV region. In addition, electronic polarisability of the nanosheet decreases with addition of another layer in ML 2D-TiB2. Further investigation of transition probabilities clearly indicates that the response of ML at Dirac point is tunable by spin–orbital coupling effects which makes these nanosheets promising for spintronic applications. The BL exhibits distinct electronic, and optical properties compared to ML. Our study unravels the structural, elastic, electronic, and optical properties of 2D-TiB2 nanosheets and suggests them as promising candidate for variety of optoelectronic and spintronic applications.

Two-dimensional Al2O3 with ultrawide bandgap and large exciton binding energy for solar-blind ultraviolet photodetectors

New Al2O3 monolayers were predicted using density functional theory, and their stability, geometric structures, and mechanical, electronic, and optical properties were investigated. Among them, Te-Al2O3 with Pmmb space group was the most stable structure with markedly mechanical anisotropy. At the HSE06 level, Te-Al2O3 exhibited an ultra-wide indirect bandgap of 6.338 eV, which was verified to have a relatively reliable value by contrastive analysis on experiments. An exciton with a large binding energy of 1.356 eV was revealed in Te-Al2O3 by solving Bethe–Salpeter equation, and such large binding energy intensely affected the optical properties. As a result, the anisotropic absorption spectra were located at ~4.0–16 eV, nearly covering the spectrum ranging from the near- ultraviolet (UV) to far-UV regions. In particular, the first optical absorption peak appearing at ~5.0 eV indicated that Te-Al2O3 is suitable for solar-blind photodetectors. This work revealed that the ultra-wide bandgap semiconductor Te-Al2O3 has a great application in UV photodetection.

Collective Excitations of Quantum Anomalous Hall Ferromagnets in Twisted Bilayer Graphene

We present a microscopic theory for collective excitations of quantum anomalous Hall ferromagnets (QAHF) in twisted bilayer graphene. We calculate the spin magnon and valley magnon spectra by solving Bethe-Salpeter equations and verify the stability of QAHF. We extract the spin stiffness from the gapless spin wave dispersion and estimate the energy cost of a skyrmion-antiskyrmion pair, which is found to be comparable in energy with the Hartree-Fock gap. The valley wave mode is gapped, implying that the valley polarized state is more favorable compared to the valley coherent state. Using a nonlinear sigma model, we estimate the valley ordering temperature, which is considerably reduced from the mean-field transition temperature due to thermal excitations of valley waves.

Electronic structure and optical properties of F centers in α -alumina

We use state-of-the art GW-BSE formalism to study electronic structure and optical properties of oxygen vacancies in $\alpha$-alumina. Many body perturbation theory within GW approximation in recent years have been used extensively to study excited state properties of a wide range of systems. Moreover, solving Bethe-Salpeter equation (BSE) enable us to capture excitonic effects in a material. We compute the charge transition levels (CTLs) for oxygen vacancies using DFT+GW formalism. We propose an alternative approach to calculate these CTLs, which provides a more efficient way to perform electrostatic correction required because of finite supercell sizes and periodic boundary condition used in first principles calculations. We find that oxygen vacancy in this material has deep donor levels, (+2/+1) at 2.5 eV and a (+1/0) level at 3.8 eV above the VBM. We also study F-center absorption and emission processes using constrained--DFT and BSE. Our calculated absorption and emission energies are in excellent agreement with experimental results.

First-principles investigations of electron-hole inclusion effects on optoelectronic properties of Bi2Te3, a topological insulator for broadband photodetector

Bismuth telluride (Bi2Te3), a layered compound with narrow band gap has been potentially reported for thermoelectric. However, strong light interaction of Bi2Te3 is an exciting feature to emerge it as a promising candidate for optoelectronic applications within broadband wavelengths. In this study, we investigate structural, electronic and optical properties of Bi2Te3 topological insulator using combination of density functional theory (DFT) and many-body perturbation theory (MBPT) approach. With the inclusion of van der Waals (vdW) correction in addition to PBE, the lattice parameters and interlayer distance are in good agreement with experimental results. Furthermore, for the precise prediction of fundamental band gap, we go beyond DFT and calculated band structure using one-shot GW approach. Interestingly, our calculated quasiparticle (QP) band gap, Eg of 0.169eV, is in good agreement with experimental measurements. Taken into account the effects of electron-hole interaction by solving Bethe-Salpeter equation, the calculated optical properties, namely, imaginary and real parts of complex dielectric function, absorption coefficient, refractive index, reflectivity, extinction coefficient, electron energy loss function and optical conductivity all are in better agreement with available experimental results. Consistencies of our findings with experimental data validate the effectiveness of electron-hole interaction for theoretical investigation of optical properties.

Looking for bound states and resonances in the η′KK¯ system

Motivated by the continuous experimental investigations of $X(1835)$ in three-body decay channels like $\eta^\prime \pi^+ \pi^-$, we investigate the $\eta^\prime K \bar K$ system with the aim of searching for bound states and/or resonances when the dynamics involved in the $K\bar K$ subsystem can form the resonances: $f_0(980)$ in isospin 0 or $a_0(980)$ in isospin 1. For this, we solve the Faddeev equations for the three-body system. The input two-body $t$-matrices are obtained by solving Bethe-Salpeter equations in a coupled channel formalism. As a result, no signal of a three-body bound state or resonance is found.

Linear and nonlinear properties of ultraviolet fluorine crystal: A first-principles study

The electronic structure and optical properties of Deep Ultraviolet (DUV) nonlinear optical crystal BaMgF4 and KBe2BO3F2 have been studied in the framework of many-body perturbation theory as well as hybrid functionals. For the electronic properties, the hybrid functionals method makes significant improvement on description of electronic band gap for both crystals. However, the results still underestimate the band gap comparing with the GW results. By considering the self-energy of electrons, electronic band gap and the optical band gap are both described well comparing with the experimental results, which is crucial for prediction the performance of DUV crystals in the DUV region. In addition to the remarkable self-energy effect, the macroscopic dielectric function and related optical properties, such as refractive index n(ω), excitation coefficient k(ω), absorption coefficient α(ω), energy-loss function L(ω), and reflectivity R(ω) have been calculated by solving Bethe–Salpeter equation (BSE). By comparing the RPA results and BSE results, we found that the excitonic effects play an important role in description of optical properties, which is absence in the previous work. Furthermore, taking advantage of 2n+1 theorem, the nonlinear optical susceptibility have been calculated as well. As all key factors that determine the performance of DUV nonlinear crystals have been addressed theoretically, our present work pave the way using first-principle theory to assist the crystal engineering for other candidates of DUV nonlinear optical crystal.

Electronic structure calculations of delafossite Cu-based transparent conducting oxidesCuMO2(M=B,Al,Ga,In)by quasiparticle self-consistentGWapproximation and Tran-Blaha's modified Becke-Johnson exchange potential

In this work, band gaps of the delafossite Cu-based transparent conducting oxides $\mathrm{Cu}M{\mathrm{O}}_{2}$ $(M=\mathrm{B},\mathrm{Al},\mathrm{Ga},\mathrm{In})$ are calculated by density functional theory (DFT) implemented with many-body perturbation theory (MBPT) based on quasiparticle self-consistent GW approximation (QPscGW) and with Tran-Blaha's modified Becke-Johnson functional (DFT-TB09). Their band gaps are explicitly improved from DFT within local density approximation (LDA). Their optical absorption spectra are also calculated by solving Bethe-Salpeter equation (BSE) that includes the electron-hole correlation effect; they show strong excitonic peaks.

Predicting the existence of a 2.9 GeVDf0(980)molecular state

A $D$-like meson resonance with mass around 2.9 GeV has been found in the $DK\overline{K}$ system using two independent and different model calculations based on (1) QCD sum rules and (2) solution of Fadeev equations with input interactions obtained from effective field theories built by considering both chiral and heavy quark symmetries. The QCD sum rules have been used to study the ${D}_{{s}^{*}0}(2317)\overline{K}$ and $D{f}_{0}(980)$ molecular currents. A resonance of mass 2.926 GeV is found with the $D{f}_{0}(980)$ current. Although a state in the ${D}_{{s}^{*}0}(2317)\overline{K}$ current is also obtained, with mass around 2.9 GeV, the coupling of this state is found to be two times weaker than the one formed in $D{f}_{0}(980)$. On the other hand, few-body equations are solved for the $DK\overline{K}$ system and its coupled channels with the input $t$ matrices obtained by solving Bethe-Salpeter equations for the $DK$, $D\overline{K}$, and $K\overline{K}$ subsystems. In this study a $D$-like meson with mass 2.890 GeV and full width $\ensuremath{\sim}55\text{ }\text{ }\mathrm{MeV}$ is found to get dynamically generated when $DK\overline{K}$ gets reorganized as $D{f}_{0}(980)$. However, no clear signal appears for the ${D}_{{s}^{*}0}(2317)\overline{K}$ configuration. The striking similarity between the results obtained in the two different models strongly indicates the existence of a $D{f}_{0}(980)$ molecule with mass nearly 2.9 GeV.

Ab initio many-body study of the electronic and optical properties of MgAl2O4 spinel

The electronic structure and optical properties of MgAl2O4 spinel with and without oxygen vacancies have been studied in the framework of many-body perturbation theory. By considering the self-energy of electrons, we reasonably describe the bandgap of perfect MgAl2O4 and the defect energy levels of MgAl2O4 containing oxygen vacancies. With the inclusion of electron-hole interaction by solving Bethe-Salpeter equation, the calculated dielectric functions and reflectivity spectrum all are in agreement well with the experimental results for perfect MgAl2O4. Our results show that the sharp peak near 7.8 eV in the experimental absorption spectrum is attributed to the excitonic states. The oxygen vacancies produce some new defect energy levels in the forbidden gap. The optical absorption peaks at 5.3 eV, 4.75 eV and 3.2 eV are induced by the VO0 and VO1+ vacancies.

Solving Bethe-Salpeter equation in Minkowski space

We develop a new method of solving Bethe-Salpeter (BS) equation in Minkowski space. It is based on projecting the BS equation on the light-front (LF) plane and on the Nakanishi integral representation of the BS amplitude. This method is valid for any kernel given by the irreducible Feynman graphs. For massless ladder exchange, our approach reproduces analytically the Wick-Cutkosky equation. For massive ladder exchange, the numerical results coincide with the ones obtained by Wick rotation.

QED test of a Bethe-Salpeter solution method.

A method is described of solving Bethe-Salpeter equations for spin-\textonehalf{} particles using a standard quasipotential approximation with a correction series. A three-dimensional, 16-component bound-state equation is obtained whose form is different from the Breit equation. The equation has two equivalent first-order forms which show each particle obeying a Dirac equation in the presence of the other. The formalism also reconstitutes the four-dimensional bound-state vertex function from the three-dimensional wave function. The method is tested on positronium and the hydrogen atom, using a single-photon-exchange kernel in the Coulomb gauge. In the bound-state equation, in addition to the Coulomb potential and Breit interaction, the formalism gives a subtracted box potential $(\frac{1}{2E})\frac{{\ensuremath{\alpha}}^{2}}{{r}^{2}}$ where $E$ is the mass of the bound state. With this term included, the energy levels (fine structure and hyperfine structure) are correct to order ${\ensuremath{\alpha}}^{4}$. The reconstituted bound-state vertex function, when substituted in Feynman triangle diagrams, gives the lowest-order atomic dipole transition amplitudes correctly.