Bethe-Salpeter Equation Approach Research Articles

Combined analysis on nature of X(3960), chi _{c0}(3930), and X_0(4140)

In this work, a study of the D_s{bar{D}}_s interaction and its couplings to the channels D{bar{D}} and J/psi phi is performed in a quasipotential Bethe–Salpeter equation approach. The D_s^+{D}_s^- and D^+{D}^- invariant mass spectra in three-body B decays are investigated in order to understand the origin of X(3960), chi _{c0}(3930), and X_0(4140) structures reported at LHCb. With the help of effective Lagrangians, the potential kernel can be constructed with meson exchanges, from which the scattering amplitudes can be obtained. By inserting it into the three-body decay processes, the invariant mass spectra can be calculated with an additional Breit–Wigner resonance introduced. The D_s^+{D}_s^- invariant mass spectrum in the decay process B^+rightarrow D^+_sD^-_sK^+ is well reproduced, and the X(3960) structure can be explained as a molecular state from the D_s{bar{D}}_s interaction. The state also exhibits as a peak in the D^+{D}^- invariant mass spectrum in the decay B^+rightarrow D^+D^-K^+, however, it is too narrow to reproduce the experimental structure around 3930 MeV. The dip around 4140 MeV in the D_s^+{D}_s^- invariant mass spectrum can be reproduced by the additional resonance with interference effect. However, a small bump instead of a dip is produced from the D_s{bar{D}}_s-J/psi phi coupled-channel effect, which suggests that the X_0(4140) cannot be interpreted as the D_s{bar{D}}_s-J/psi phi coupling in the current model.

Chemical Mapping of Excitons in Halide Double Perovskites.

Halide double perovskites comprise an emerging class of semiconductors with tremendous chemical and electronic diversity. While their band structure features can be understood from frontier-orbital models, chemical intuition for optical excitations remains incomplete. Here, we use ab initio many-body perturbation theory within the GW and the Bethe-Salpeter equation approach to calculate excited-state properties of a representative range of Cs2BB'Cl6 double perovskites. Our calculations reveal that double perovskites with different combinations of B and B' cations display a broad variety of electronic band structures and dielectric properties and form excitons with binding energies ranging over several orders of magnitude. We correlate these properties with the orbital-induced anisotropy of charge-carrier effective masses and the long-range behavior of the dielectric function by comparing them with the canonical conditions of the Wannier-Mott model. Furthermore, we derive chemically intuitive rules for predicting the nature of excitons in halide double perovskites using computationally inexpensive density functional theory calculations.

Stacking up Electron-Rich and Electron-Deficient Monolayers to Achieve Extraordinary Mid- to Far-Infrared Excitonic Absorption: Interlayer Excitons in the C3B/C3N Bilayer

Our ability to efficiently detect and generate far-infrared (i.e., terahertz) radiation is vital in areas spanning from biomedical imaging to interstellar spectroscopy. Despite decades of intense research, bridging the terahertz gap between electronics and optics remains a major challenge due to the lack of robust materials that can efficiently operate in this frequency range, and two-dimensional (2D) type-II heterostructures may be ideal candidates to fill this gap. Herein, using highly accurate many-body perturbation theory within the GW plus Bethe-Salpeter equation approach, we predict that a type-II heterostructure consisting of an electron-rich ${\mathrm{C}}_{3}\mathrm{N}$ and an electron deficient ${\mathrm{C}}_{3}\mathrm{B}$ monolayers can give rise to extraordinary optical activities in the mid- to far-infrared range. ${\mathrm{C}}_{3}\mathrm{N}$ and ${\mathrm{C}}_{3}\mathrm{B}$ are two graphene-derived 2D materials that have attracted increasing research attention. Although both ${\mathrm{C}}_{3}\mathrm{N}$ and ${\mathrm{C}}_{3}\mathrm{B}$ monolayers are moderate-gap 2D materials, and they couple only through the rather weak van der Waals interactions, the bilayer heterostructure surprisingly supports extremely bright, low-energy interlayer excitons with large binding energies of 0.2--0.4 eV, offering an ideal material with interlayer excitonic states for mid- to far-infrared applications at room temperature. We also investigate in detail the properties and formation mechanism of the inter- and intralayer excitons.

Rydberg Excitons and Trions in Monolayer MoTe2.

Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances, which serve as a microscopic, noninvasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS2, MoSe2, WS2, and WSe2), but detailed exploration of excitonic phenomena has been lacking in the important TMDC material molybdenum ditelluride (MoTe2). Here, we report an experimental investigation of excitonic luminescence properties of monolayer MoTe2 to understand the excitonic Rydberg series, up to 3s. We report a significant modification of emission energies with temperature (4 to 300 K), thereby quantifying the exciton-phonon coupling. Furthermore, we observe a strongly gate-tunable exciton-trion interplay for all the Rydberg states governed mainly by free-carrier screening, Pauli blocking, and band gap renormalization in agreement with the results of first-principles GW plus Bethe-Salpeter equation approach calculations. Our results help bring monolayer MoTe2 closer to its potential applications in near-infrared optoelectronics and photonic devices.

Disentangling photoexcitation and photoluminescence processes in defective MgO

Oxygen vacancies are ubiquitous in oxides and strongly influence the material's electronic structure and catalytic and transport properties. Here we focus on a seemingly simple defective oxide, MgO, and on the electronic properties of oxygen vacancies, which remain controversial in spite of numerous studies. We present an ab initio investigation of the photoexcitation and photoionization process of these defects, using a newly developed embedding Bethe-Salpeter equation approach, implemented in the WEST code. We find absorption and emission energies in good agreement with experiments. Our results provide a detailed, microscopic understanding of the absorption and emission processes of the neutral and positively charged oxygen vacancy, reconciling different views present in the chemistry and condensed-matter physics communities.

Nanoscale Surface Structure of Nanometer-Thick Ferroelectric BaTiO3 Films Revealed by Synchrotron X-ray Scanning Tunneling Microscopy: Implications for Catalytic Adsorption Reactions

Ferroelectric nanomaterials are of interest in catalysis, nonvolatile memory, and neuromorphic computing among other applications because of their switchable structure that can alter the electronic and interface properties of a single material. The investigation of the role of polarization on the surface structure and chemistry of ferroelectric nanomaterials is a longstanding challenge, as it ideally requires a combination of both nanoscale imaging and chemical spectroscopy. In this work, we study a model ferroelectric BaTiO3 thin film by synchrotron X-ray scanning tunneling microscopy (SX-STM), a unique method that integrates nanoscale surface imaging and chemically sensitive spectroscopy. We find that polarization switching from downward to upward in (001) single-crystalline BaTiO3 thin films increases the intensity of X-ray absorption across Ba M, Ti L, and O K edges. Chemical mapping of nanometer-sized domains further demonstrates the modulation of surface structures upon polarization switching, as well as confirming the trends observed in single-point experiments across the surface. We complement these measurements with ab initio computational absorption spectroscopy to elucidate the effect of polarization switching on the core–hole excitations using the Bethe–Salpeter equation approach. Our experimental and theoretical results thus confirm a stronger binding strength for the upward-polarized surface with molecular O2 as a model reactant, offering mechanistic evidence that supports previous reports. This work advances the understanding of the surface chemistry and electronic structure of ferroelectrics, which can ultimately aid strategies to design interfaces with tailored properties.

An investigation of dynamical stability, electronic, and optical properties of sodium metaborate (NaBO2): A many-body G0W0+BSE approach

This study investigated the electronic, optical, and phonon properties of bulk sodium metaborate (NaBO2) using density functional theory (DFT), and the many-body G0W0 plus Bethe-Salpeter equation (BSE) approach. The investigation of the phonon dispersion curve indicates the dynamic stability of the NaBO2 compound in the rhombohedral phase, which is in good agreement with experimental results. Also, cohesive energy calculation shows the thermodynamic stability of this material. The band structure results indicated that the direct band gap of NaBO2 at the Γ point is 3.81 eV/4.16 eV within LDA/PBE approximation, which increased to 6.78 eV/6.57 eV by considering the quasi-particle effects in G0W0@LDA/PBE calculations. Considering the electron-hole effects in the G0W0+BSE approach, the optical gap showed more accurate results, and the value of 5.83 eV/5.71 eV was obtained for the LDA/PBE starting point. Therefore, excitonic effects within the G0W0+BSE approximation, in comparison to G0W0+RPA, leads to the redshift of the optical absorption spectrum. The optical results estimate the exciton binding energy of 0.95 eV/0.86 eV within LDA/PBE approximation. The investigation of the optical absorption spectrum introduces sodium metaborate as a transparent material with the prospect of use in optical devices.

WanTiBEXOS: A Wannier based Tight Binding code for electronic band structure, excitonic and optoelectronic properties of solids

The Bethe–Salpeter equation (BSE) approach becomes a methodology commonly used for simulating excitonic and optical properties in computer materials sciences. However, BSE approach based directly on first principles demands a high computational cost, being prohibitive for larger systems. In order to overcome this challenge, we have developed WanTiBEXOS, a parallel computational FORTRAN code, constituted of a maximally localized Wannier functions based tight-binding (MLWF-TB) model in conjunction with BSE framework. The MLWF-TB Hamiltonian used in WanTiBEXOS package can be obtained via any density functional theory package interfaced with Wannier90 code. It's expected, due MLWF-TB formalism, a computational time reduction around one or more orders of magnitude in comparison with BSE ab initio implementations. In order to demonstrate its reliability, flexibility, efficiency and versatility of WanTiBEXOS, we simulate electronic and optical property calculations for the representative materials, including conventional bulk semiconductors, perovskites, nano-monolayer materials and van der Waals heterostructures. Program summaryProgram Title: WanTiBEXOSCPC Library link to program files:https://doi.org/10.17632/4vh76mnkv5.1Developer's repository link:https://github.com/ac-dias/wantibexosLicensing provisions: GNU General Public Licence 3.0Programming language: Fortran90 (Intel compiler)External routines: OpenMP and Intel Math Kernel Library (MKL)Nature of problem: Calculate electronic, optical, and excitonic properties of solids.Solution method: Tight-binding approach under maximally localized Wannier functions (MLWF) parametrization to describe standard electronic properties and the Bethe–Salpeter formalism for excitonic properties.

Possible [formula omitted] molecular states and their productions in nucleon-antinucleon collision

In this work, a study of possible molecular states from the ΛcΛ¯c interaction and their productions in nucleon-antinucleon collision is performed in a quasipotential Bethe-Salpeter equation approach. Two bound states with quantum numbers JPC=0−+ and 1−− are produced with almost the same binding energy from the ΛcΛ¯c interaction which is described by the light meson exchanges. However, the result does not support the assignment of experimentally observed Y(4630) as a ΛcΛ¯c molecular state because it is hard to obtain a peak near experimental mass of the Y(4630) which is far above the ΛcΛ¯c threshold. The possibility to search these states in nucleon-antinucleon collision is studied by including couplings to NN¯ and D(⁎)D¯(⁎) channels. The peaks can be found obviously near the ΛcΛ¯c threshold in the D⁎D¯⁎ channel at an order of amplitude of 10 μb. Too small width of state with 0−+ may lead to the difficulty to be observed in experiment. Based on the results in the current work, search for the ΛcΛ¯c molecular state with 1−− is suggested in process NN¯→D⁎D¯⁎, which is accessible at P¯ANDA.

Modeling of excited state potential energy surfaces with the Bethe-Salpeter equation formalism: The 4-(dimethylamino)benzonitrile twist.

We present a benchmark study of excited state potential energy surfaces (PES) using the many-body Green's function GW and Bethe-Salpeter equation (BSE) formalisms, coupled cluster methods, as well as Time-Dependent Density Functional Theory (TD-DFT). More specifically, we investigate the evolution of the two lowest excited states of 4-(dimethylamino)benzonitrile (DMABN) upon the twisting of the amino group, a paradigmatic system for dual fluorescence and excited-state benchmarks. Our results demonstrate that the BSE/GW approach is able to reproduce the correct topology of excited state PES upon geometry changes in both gas and condensed phases. The vertical transition energies predicted by BSE/GW are indeed in good agreement with coupled cluster values, including triples. The BSE approach ability to include both linear response and state-specific solvent corrections further enables it to accurately describe the solvatochromism of both excited states during the twisting of DMABN. This contribution stands as one of the first proof-of-concept that BSE/GW PES should be accurate in cases for which TD-DFT struggles, including the central case of systems embedded in a dielectric environment.

Near-edge x-ray absorption and emission of cyanates and thiocyanates

The comparability and reliability of the analysis of the electronic structure of selected cyanates and thiocyanates at the nitrogen $K$ edge based on BSE calculations have been investigated in this work. Using high-resolution x-ray spectroscopy with calibrated instrumentation reliable and reproducible experimental results for x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) were achieved. These results were used to validate theoretical modeling with first-principles calculations based on the Bethe-Salpeter equation approach for the excited-state interactions. Furthermore, radiation-induced damages occurring in the samples are correlated with absolute doses.

Forward-backward asymmetries in in the Bethe-Salpeter equation approach * *Supported by National Natural Science Foundation of China (11905117, 11775024)

Using the Bethe-Salpeter equation (BSE), we investigate the forward-backward asymmetries in in the quark-diquark model. This approach provides precise form factors that are different from those of quantum chromodynamics (QCD) sum rules. We calculate the rare decay form factors for b and investigate the (integrated) forward-backward asymmetries in these decay channels. We observe the integrated , , , and ; the hadron side asymmetries ; the lepton-hadron side asymmetries ; and the longitudinal polarization fractions .

Ab Initio Prediction of Excited-State and Polaron Effects in Transient XUV Measurements of α-Fe2O3.

Transient X-ray and extreme ultraviolet (XUV) spectroscopies have become invaluable tools for studying photoexcited dynamics due to their sensitivity to carrier occupations and local chemical or structural changes. One of the most studied materials using transient XUV spectroscopy is α-Fe2O3 because of its rich photoexcited dynamics, including small polaron formation. The interpretation of carrier and polaron effects in α-Fe2O3 is currently carried out using a semi-empirical method that is not transferrable to most materials. Here, an ab initio, Bethe-Salpeter equation (BSE) approach is developed that can incorporate photoexcited-state effects into arbitrary material systems. The accuracy of this approach is proven by calculating the XUV absorption spectra for the ground, photoexcited, and polaron states of α-Fe2O3. Furthermore, the theoretical approach allows for the projection of the core-valence excitons and different components of the X-ray transition Hamiltonian onto the band structure, providing new insights into old measurements. From this information, a physical intuition about the origins and nature of the transient XUV spectra can be built. A route to extracting electron and hole energies is even shown possible for highly angular momentum split XUV peaks. This method is easily generalized to K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Boron-pnictogens: Highly anisotropic two-dimensional semiconductors for nanoelectronics and optoelectronics

Two-dimensional materials open up tremendous opportunities for nanoelectronics and optoelectronics. Using first-principles density functional methods, we predict a family of two-dimensional boron-pnictogen materials. Our results show that these materials have excellent energetic, dynamical, thermal, mechanical, and chemical stabilities. The intrinsic structural anisotropy found in these materials leads to highly direction-dependent mechanical, electronic, and optical properties. They possess highly anisotropic Young's modulus and Poisson's ratio. The tensile strength under uniaxial and biaxial deformations is found to be very high for these materials. Electronically, they are all semiconductors with narrow band gaps. The band gap energies can be tuned by alloying, strain engineering, and chemical functionalization. They exhibit anisotropic and high carrier mobility. All these electronic properties make them promising candidates for nanoelectronic device applications. Using state-of-the-art $GW$- Bethe-Salpeter equation approach, taking the electron-hole effect into account, the prominent optical absorption structure with strong anisotropy in the visible light region endow the boron-pnictogen materials with great potential in optoelectronics.

Study of possible molecular states of Ds(*)Ds(*) and Bs(*)Bs(*)

Recently the LHCb collaboration reported a new exotic state $T^+_{cc}$ which is conjectured to be a molecular state of $D^0D^{*+}$ (or $D^{*0}D^{+}$) theoretically. Belle Collaboration also searched for tetraquark state $X_{ccss}$ in $D_sD_s (D^*_sD^*_s )$ final states but no significant signals were observed, which did not rule out the possibility of $X_{ccss}$ to be a molecular state of $D_sD_s (D^*_sD^*_s )$. Inspired by these experimental results on double charmed exotic state, in this paper we study whether the molecular bound states of $D_s^{(*)}D_s^{(*)}$ and $B_s^{(*)}B_s^{(*)}$ can exist with the Bethe-Salpeter (B-S) equation approach. We employ heavy meson chiral perturbation theory with one boson-exchange approximation to calculate the interaction kernels in the B-S equations. Our numerical results suggest that two $B_s^{*}$ mesons perhaps form a $0^+$ molecular state. Future experimental search for $X_{ccss}$ states in other decay channels may shed light on the structure of double charmed exotic state.

Screening of Excitons by Organic Cations in Quasi-Two-Dimensional Organic-Inorganic Lead-Halide Perovskites.

Interlayer organic cations in quasi-two-dimensional halide perovskites are a versatile tuning vehicle for the optoelectronic properties of these complex systems, but chemical intuition for this design route is yet to be established. Here, we use density functional theory, the GW approximation, and the Bethe-Salpeter equation approach to understand the contribution of the organic cation to the quasiparticle band gap and exciton binding energy of layered perovskites. We show that organic cations in quasi-two-dimensional perovskites contribute significantly to the dielectric screening in these systems, countering quantum confinement effects on the quasiparticle band gap and the exciton binding energy. Using a simple electrostatics model inspired by parallel-plate capacitors, we decouple the organic cation and inorganic layer contributions to the effective dielectric constants and show that dielectric properties of layered perovskites are broadly tunable via the interlayer cation, providing a direct means of tuning photophysical properties for a variety of applications.

Systematical study of Ωc -like molecular states from interactions Ξc(′,*)K¯(*) and Ξ(*)D(*)

In this work, the $\Omega_c$-like molecular states are systematically investigated in a quasipotential Bethe-Salpeter equation approach. The relevant interactions $\Xi_c^{(*,')}\bar{K}^{(*)}$, $\Xi^{(*)}D^{(*)}$, and $\Omega^{(*)}_c(\pi/\eta/\rho/\omega)$ are described by light meson exchanges with the help of the effective Lagrangians with SU(3), chiral, and heavy quark symmetries. The obtained potential kernels of considered interactions are inserted into the quasipotential Bethe-Salpeter equation, and coupled-channel calculations are performed to find possible molecular states and its couplings to the channels considered. The results suggest that an isoscalar state can be produced from the $\Xi^*_c\bar{K}$ interaction with spin parity $3/2^-$, which can be related to state $\Omega_c(3120)$. And its isoscalar partner is predicted with a dominant decay in the $\Omega_c^*\pi$ channel. The isoscalar and isovector states with $1/2^-$ can be produced from the $\Xi'_c\bar{K}$ interaction with a threshold close to the mass of the $\Omega_c(3050)$ and $\Omega_c(3065)$. Their couplings to the $\Xi_c \bar{K}$ channel are very weak, and the isovector one has strong coupling to $\Omega_c\pi$. High precision measurement is helpful to confirm or search such molecular states. Experimental search of states with higher masses generated from interactions $\Xi^{(*,')}_c \bar{K}^*$ and $\Xi^* D^{(*)}$ are also suggested by the current results.

Revealing the quasiparticle electronic and excitonic nature in cubic, tetragonal, and hexagonal phases of FAPbI3

The development of three-dimensional (3D) hybrid organic–inorganic perovskites has sparked much interest because of their rich light-harvesting capabilities in solar cells. However, the understanding of the electronic and optical properties, particularly the excitonic shifts upon structural phase transition with temperature in these materials, is not fully clear. Here, we report the accurate description of electronic and optical properties of mostly studied FAPbI3 across the cubic–tetragonal–hexagonal phases, using the relativistic GW method and Bethe–Salpeter Equation (BSE), including the spin–orbit coupling effects. Our GW calculations reveal that the bandgap values vary from 1.47 to 3.54 eV from the room temperature cubic phase to the low temperature hexagonal phase. Our optical analysis shows that excitonic peaks are blue-shifted, and exciton binding energies estimated by the model BSE approach increase from 74 to 567 meV going from the cubic to hexagonal phases. Our results may have important impacts on the practical uptake of hybrid perovskite based solar cells under different climatic conditions.

Advances in the OCEAN-3 spectroscopy package.

The OCEAN code for calculating valence- and core-level spectra using the Bethe-Salpeter equation is briefly reviewed. OCEAN is capable of calculating optical absorption, near-edge X-ray absorption or non-resonant scattering, and resonant inelastic X-ray scattering, requiring only the structure of the material as input. Improved default behavior and reduced input requirements are detailed as well as new capabilities, such as incorporation of final-state-dependent broadening, finite-temperature dependence, and flexibility in the density-functional theory exchange-correlation potentials. OCEAN is built on top of a plane-wave, pseudopotential, density-functional theory foundation, and calculations are shown for systems ranging in size up to 7 nm3.

JASPER: a software for quantum chromodynamics Dyson-Schwinger equation numerical calculation

The JASPER program is the first part of the high-performance computing information system for estimate some elementary particle properties, developing at Petersburg Nuclear Physics Institute. The JASPER is an implementation of the Dyson-Schwinger equation numerical solution for simple dressed quark propagator calculation in rainbow approximation. The Dyson-Schwinger equation solution with the Marice-Tandy Ansatz is one of several phenomenological approaches to obtain quantitative results in quantum chromodynamics (QCD) within strong coupling regime. The JASPER program is programmed in the C++ language and uses the numerical algorithms from the GNU Scientific Library (GSL). The numerical results for dynamical quark mass in complex Euclidean space were obtained. This result will be employed to study the hadron spectrum with the Bethe-Salpeter equation approach.