Many-body Perturbation Theory Approach Research Articles

Exciton-Phonon Coupling Induces a New Pathway for Ultrafast Intralayer-to-Interlayer Exciton Transition and Interlayer Charge Transfer in WS2-MoS2 Heterostructure: A First-Principles Study.

Despite the weak, van der Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50 fs time scales. An early theoretical understanding of charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate optical properties of such materials. We employ an ab initio many-body perturbation theory approach, which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton-phonon coupling in the charge dynamics of the WS2/MoS2 heterobilayer. We find that the exciton-phonon interaction induced relaxation time of photoexcited excitons at the K valley of MoS2 and WS2 is 67 and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways that are otherwise inaccessible to noninteracting electrons and holes.

Transitions in Xenes between excitonic, topological and trivial insulator phases: Influence of screening, band dispersion and external electric field

Using a variational approach, the binding energies E_bEb of the lowest bound excitons in Xenes under varying electric field are investigated. The internal exciton motion is described both by Dirac electron dispersion and in effective-mass approximation, while the screened electron-hole attraction is modeled by a Rytova-Keldysh potential with a 2D electronic polarizability \alpha_{2D}α2D. The most important parameters as spin-orbit-induced gap E_gEg, Fermi velocity v_FvF and \alpha_{2D}α2D are taken from ab initio density functional theory calculations. In addition, \alpha_{2D}α2D is approximated in two different ways. The relation of E_bEb and E_gEg is ruled by the screening. The existence of an excitonic insulator phase with E_b>E_gEb>Eg sensitively depends on the chosen \alpha_{2D}α2D. The values of E_gEg and \alpha_{2D}α2D are strongly modified by a vertical external electric bias UU, which defines a transition from the topological into a trivial insulator at U=E_g/2U=Eg/2, with the exception of plumbene. Within the Dirac approximation, but also within the effective mass description of the kinetic energy, the treatment of screening dominates the appearance or non-appearance of an excitonic insulator phase. Gating does not change the results: the prediction done at zero electric field is confirmed when a vertical electric field is applied. Finally, Many-Body perturbation theory approaches based on the Green’s function method, applied to stanene, confirm the absence of an excitonic insulator phase, thus validating our results obtained by ab initio modeling of \alpha_{2D}α2D.

Exploring the structural properties and the optoelectronic features of RbPbX3 (X [formula omitted] Cl, F) perovskite crystals for solar cells solicitations: Showcasing the DFT predictions.

A comprehensive study of the structural, optical, and electronic properties of RbPbX3 (where X is Cl or F) perovskite crystals is reported herein. The calculations of the different physical properties of these two distinguishable RbPbCl3 and RbPbF3 perovskite crystals have been carried within the framework of the Density Functional Theory (DFT) approach for the cubic temperature-dependent phase. The study is motivated by the exploration of innovative perovskites materials for solar cells application due to the marvelous augmentation in power conversion efficiency (PCE) obtained by perovskite crystals (≈ 25% up to date). The investigation were undertaken for determining the perceivable role of the X element in their band gap energy features as well as the band structure proprieties. The simulations were commenced by determining the optimal lattice constants and the lowest total energy of both perovskite crystals. The exploration of the electronic properties of RbPbCl3 and RbPbF3 perovskite crystals have been studied by estimating their variation upon employing spin–orbit-coupling (SOC) and without taking into consideration the SOC interaction by means of LDA-pz and GGA-PBE exchange potentials within the framework of projector augmented wave method (PAW) and the Perdew–Burke–Ernzerh of LDA and (PBE) variant of the generalized gradient approximation (GGA). The integrated suite quantum-ESPRESSO is used for materials modeling, structure calculations, and electronic properties of the studied RbPbCl3 and RbPbF3 perovskite crystals. The thermo_pw driver is employed for the determination of the elastic properties whereas the optical properties were elucidated by aid of Yambo code through solving the many-body perturbation theory (MBPT) approach as introduced in the open-source code. The optical band gap of cubic RbPbCl3 and RbPbCF3 within LDA and PBE, were observed at (2.18, 2.2) eV and (2.4, 2.5) eV respectively. Compared and contrasted with those revealed by the electronic band gap energy of the systems, these obtained values are very consistent with the experimental. The two maximum peaks for the absorption coefficients are found to be within the visible range rending the two perovskite crystals in promising candidate for solar cell applications. Despite some quantitative differences, the material’s enhancement potential is well demonstrated by the absorption coefficient spectrum, refractive index, and conductivity. The study holds promise for solar cell applications and may pave the way for consideration of the RbPbCl3 and RbPbF3 as well as their doped analogs for perovskites-based solar cells as the next generation in the photovoltaic industry.

XPS Core-Level Chemical Shift by Ab Initio Many-Body Theory.

X-ray photoemission spectroscopy (XPS) provides direct information on atomic composition and stoichiometry by measuring core-electron binding energies. Moreover, from the shift of the binding energy, the so-called chemical shift, the precise chemical type of bonds can be inferred, which brings additional information on the local structure. In this work, we present a theoretical study of the chemical shift first by comparing different theories, from Hartree-Fock and density functional theory to many-body perturbation theory approaches like the GW approximation and its static version (COHSEX). The accuracy of each theory is assessed with respect to a carbon 1s chemical shift experimental benchmark measured on a set of gas-phase molecules. More importantly, by decomposing the chemical shift into different contributions according to terms in the total Hamiltonian, classical electrostatics is identified as the major contributor to the chemical shift, one order of magnitude larger than the correlation.

A Many‐Body Perturbation Theory Approach to Energy Band Alignment at the Crystalline Tetracene–Silicon Interface (Adv. Theory Simul. 11/2022)

A Many-Body Perturbation Theory Approach to Energy Band Alignment at the Crystalline Tetracene–Silicon Interface The interactions between electrons at the interface between crystalline silicon and tetracene can be described using the many-body perturbation theory that extensively uses the language of Feynman diagrams. In article 2200413, Klymenko and coworkers analyse the interface, revealing the changes in the charge density and electrostatic potential caused by the contact of two slabs.

Exciton-phonon interaction calls for a revision of the “exciton” concept

The concept of \textit{optical} exciton - a photo-excited bound electron-hole pair within a crystal - is routinely used to interpret and model a wealth of excited-state phenomena in semiconductors. Beside originating sub-band gap signatures in optical spectra, optical excitons have also been predicted to condensate, diffuse, recombine, relax. However, all these phenomena are rooted on a theoretical definition of the excitonic state based on the following simple picture: "excitons" are actual particles that both appear as peaks in the linear absorption spectrum and also behave as well-defined quasiparticles. In this paper we show, instead, that the electron-phonon interaction decomposes the initial optical excitons into \textit{elemental} excitons, the latter being a different kind of bound electron-hole pairs lacking the effect caused by the induced, classical, electric field. This is demonstrated within a many-body perturbation theory approach starting from the interacting electronic Hamiltonian including both electron-phonon and electron-hole interactions. We then apply the results on two realistic systems, monolayer MoS$_2$ (where the lowest-bound exciton is optically inactive) and monolayer MoSe$_2$ (where it is optically active), using first-principles methods to compute the exciton-phonon coupling matrix elements. Among the consequences of optical-elemental decomposition, we point to a homogeneous broadening of absorption peaks occurring even for the lowest-bound optical exciton, we demonstrate this by computing exciton-phonon transition rates. More generally, our findings suggest that the optical excitons gradually lose their initial structure and evolve as elemental excitons. These states can be regarded as the real intrinsic excitations of the interacting system, the ones that survive when the external perturbation and the induced electric fields have vanished.

Optical and Electronic Properties of Organic NIR-II Fluorophores by Time-Dependent Density Functional Theory and Many-Body Perturbation Theory: GW-BSE Approaches.

Organic-molecule fluorophores with emission wavelengths in the second near-infrared window (NIR-II, 1000–1700 nm) have attracted substantial attention in the life sciences and in biomedical applications because of their excellent resolution and sensitivity. However, adequate theoretical levels to provide efficient and accurate estimations of the optical and electronic properties of organic NIR-II fluorophores are lacking. The standard approach for these calculations has been time-dependent density functional theory (TDDFT). However, the size and large excitonic energies of these compounds pose challenges with respect to computational cost and time. In this study, we used the GW approximation combined with the Bethe-Salpeter equation (GW-BSE) implemented in many-body perturbation theory approaches based on density functional theory. This method was used to perform calculations of the excited states of two NIR molecular fluorophores (BTC980 and BTC1070), going beyond TDDFT. In this study, the optical absorption spectra and frontier molecular orbitals of these compounds were compared using TDDFT and GW-BSE calculations. The GW-BSE estimates showed excellent agreement with previously reported experimental results.

Strong out-of-plane excitons in 2D hybrid halide double perovskites

By means of an ab initio investigation, exploiting many-body perturbation theory approaches on top of ground-state density functional theory calculations, we disclose the impact of the electronic correlation on the band structure and optical properties of a class of recently synthesized Pb-free 2D-layered halide perovskites, namely, (BA)4AgBiBr8 and (BA)2CsAgBiBr7. We observe strong out-of-plane excitonic dipoles characterizing the optical spectra, a result that suggests the potential applicability of these layered hybrid materials in photonics and plasmonics, where the perpendicular field direction plays a relevant role.

Ab Initio Many-Body Perturbation Theory Calculations of the Electronic and Optical Properties of Cyclometalated Ir(III) Complexes

Cyclometalated Ir(III) compounds are the preferred choice as organic emitters in organic light-emitting diodes. In practice, the presence of the transition metal surrounded by carefully designed ligands allows fine-tuning of the emission frequency as well as good efficiency of the device. To support the development of new compounds, experimental measurements are generally compared with absorption and emission spectra obtained from ab initio calculations. The standard approach for these calculations is time-dependent density functional theory (TDDFT) with a hybrid exchange-correlation functional like B3LYP. Because of the size of these compounds, the application of more complex quantum chemistry approaches can be challenging. In this work, we used many-body perturbation theory approaches, in particular the GW approximation with the Bethe-Salpeter equation (BSE) implemented in Gaussian basis sets, to calculate the quasiparticle properties and the absorption spectra of six cyclometalated Ir(III) complexes, going beyond TDDFT. In the presented results, we compared standard TDDFT simulations with BSE calculations performed on top of perturbative G0W0 and accounting for eigenvalue self-consistency. Moreover, in order to investigate in detail the effect of the DFT starting point, we concentrated on Ir(ppy)3 and performed GW-BSE simulations starting from different DFT exchange-correlation potentials.

Bogoliubov many-body perturbation theory for open-shell nuclei

A Rayleigh–Schrödinger many-body perturbation theory (MBPT) approach is introduced by making use of a particle-number-breaking Bogoliubov reference state to tackle (near-)degenerate open-shell fermionic systems. By choosing a reference state that solves the Hartree–Fock–Bogoliubov variational problem, the approach reduces to the well-tested Møller–Plesset, i.e., Hartree–Fock based, MBPT when applied to closed-shell systems. Due to its algorithmic simplicity, the newly developed framework provides a computationally simple yet accurate alternative to state-of-the-art non-perturbative many-body approaches. At the price of working in the quasi-particle basis associated with a single-particle basis of sufficient size, the computational scaling of the method is independent of the particle number. This paper presents the first realistic applications of the method ranging from the oxygen to the nickel isotopic chains on the basis of a modern nuclear Hamiltonian derived from chiral effective field theory.

Correlation beyond the random phase approximation: A consistent many-body perturbation theory approach

Correlation beyond the random phase approximation: A consistent many-body perturbation theory approach

Many-body perturbation theory and non-perturbative approaches: screened interaction as the key ingredient

Many-body perturbation theory is often formulated in terms of an expansion in the dressed instead of the bare Green’s function, and in the screened instead of the bare Coulomb interaction. However, screening can be calculated on different levels of approximation, and it is important to define what is the most appropriate choice. We explore this question by studying a zero-dimensional model (so called ‘one-point model’) that retains the structure of the full equations. We study both linear and non-linear response approximations to the screening. We find that an expansion in terms of the screening in the random phase approximation is the most promising way for an application in real systems. Moreover, by making use of the nonperturbative features of the Kadanoff–Baym equation for the one-body Green’s function, we obtain an approximate solution in our model that is very promising, although its applicability to real systems has still to be explored.

Screening in Orbital-Density-Dependent Functionals.

Electronic-structure functionals that include screening effects, such as Hubbard or Koopmans' functionals, are required to describe the response of a system to the fractional addition or removal of an electron from an orbital or a manifold. Here, we present a general method to incorporate screening based on linear-response theory, and we apply it to the case of orbital-by-orbital screening of Koopmans' functionals. We illustrate the importance of such generalization when dealing with challenging systems containing orbitals with very different chemical character, also highlighting the simple dependence of the screening on the localization of the orbitals. We choose a set of 46 transition-metal complexes for which experimental data and accurate many-body perturbation theory calculations are available. When compared to experiment, results for ionization potentials show a very good performance, with a mean absolute error of 0.2 eV, comparable to the most accurate many-body perturbation theory approaches. These results reiterate the role of Koopmans-compliant functionals as simple and accurate quasiparticle approximations to the exact spectral functional, bypassing diagrammatic expansions and relying only on the physics of the local density or generalized-gradient approximation.

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.

Responsivity calibration of the extreme ultraviolet spectrometer in the range of 175-435 Å

We reported the relative responsivity calibration of the grazing-incidence flat-field EUV spectrometer between 175 and 435 Å by means of two methods. The first method is implemented by measuring the diffraction efficiency of the grating with synchrotron radiation light source. Considering the transmission efficiency and quantum efficiency of the other optical components in the spectrometer, the total responsivity was then obtained. The second one was carried out by measuring line emissions from C3+, N4+ and O3+ ions at Shanghai high temperature super conductor electron beam ion trap (SH-HtscEBIT). The EUV spectra were also simulated theoretically via a collisional radiative model. In the calculation, the second-order relativistic many-body perturbation theory approach based on the flexible atomic code was used to calculate the energy levels and transition rates; the close-coupling R-matrix approach and relativistic distorted wave method were utilized to calculate the collision strength of electron impact excitation. In comparison with the spectroscopic measurements at EBIT device, the differences between the measured and simulated relative line intensities were obtained. The responsivity calibration for the spectrometer was then achieved by a 3rd degree polynomial function fitting. Our measurement shows that the responsivity between 175 and 435 Å varies by factor of ∼ 46. The two results of calibration demonstrated a consistency within an average deviation of 24%. In addition, an evaluation of our calculations on C iv, N v and O iv line emissions in this wavelength region was given.

Excitonic effects in third-harmonic generation: The case of carbon nanotubes and nanoribbons

Linear and nonlinear optical properties of low-dimensional nanostructures have attracted great interest from the scientific community as tools to probe the strong confinement of electrons and for possible applications in optoelectronic devices. In particular it has been shown that the linear optical response of carbon nanotubes [F. Wang et al., Science 308, 838 (2005)] and graphene nanoribbons [Nat. Commun. 5 4253 (2014)] is dominated by bounded electron-hole pairs, excitons. The role of excitons in linear response has been widely studied, but still, little is known about their effect on nonlinear susceptibilities. Using a recently developed methodology [Phys. Rev. B 88, 235113 (2013)] based on well-established ab initio many-body perturbation theory approaches, we find that quasiparticle shifts and excitonic effects significantly modify the third-harmonic generation in carbon nanotubes and graphene nanoribbons. For both systems the net effect of many-body effects is to reduce the intensity of the main peak in the independent-particle spectrum and redistribute the spectral weight among several excitonic resonances.

Electron-vibration coupling induced renormalization in the photoemission spectrum of diamondoids.

The development of theories and methods devoted to the accurate calculation of the electronic quasi-particle states and levels of molecules, clusters and solids is of prime importance to interpret the experimental data. These quantum systems are often modelled by using the Born–Oppenheimer approximation where the coupling between the electrons and vibrational modes is not fully taken into account, and the electrons are treated as pure quasi-particles. Here, we show that in small diamond cages, called diamondoids, the electron–vibration coupling leads to the breakdown of the electron quasi-particle picture. More importantly, we demonstrate that the strong electron–vibration coupling is essential to properly describe the overall lineshape of the experimental photoemission spectrum. This cannot be obtained by methods within Born–Oppenheimer approximation. Moreover, we deduce a link between the vibronic states found by our many-body perturbation theory approach and the well-known Jahn–Teller effect.

Many-body perturbation theory approach to the electron-phonon interaction with density-functional theory as a starting point

The electron-phonon interaction plays a crucial role in many fields of physics and chemistry. Nevertheless, its actual calculation by means of modern many-body perturbation theory is weakened by the use of model Hamiltonians that are based on parameters difficult to extract from the experiments. Such shortcoming can be bypassed by using density-functional theory to evaluate the electron-phonon scattering amplitudes, phonon frequencies, and electronic bare energies. In this work, we discuss how a consistent many-body diagrammatic expansion can be constructed on top of density-functional theory. In that context, the role played by screening and self-consistency when all the components of the electron-nucleus and nucleus-nucleus interactions are taken into account, is paramount. A way to avoid overscreening is notably presented. Finally, we derive cancellation rules as well as internal consistency constraints in order to draw a clear, sound, and practical scheme to merge the many-body perturbation and density-functional theory.

Exchange-correlation potentials with proper discontinuities for physically meaningful Kohn-Sham eigenvalues and band structures

It is shown how to properly construct exchange-correlation (xc) potentials in the Kohn-Sham (KS) formalism of density-functional theory such that physically meaningful KS eigenvalues result. A potential adjustor ${\mathrm{\ensuremath{\Delta}}}_{Hxc}^{N\ensuremath{-}}$ is derived which enables for any approximate functional for the xc energy the construction of a consistent xc potential leading to an eigenvalue of the energetically highest occupied KS orbital equal to the negative of the ionization potential. Together with a second potential adjustor ${\mathrm{\ensuremath{\Delta}}}_{Hxc}^{N+}$ KS band structures can be converted in approximate quasiparticle band structures exhibiting the exact physical instead of the KS band gap. This represents an alternative route to the fundamental quasiparticle band gap, completely within the KS formalism without the need to resort to many-body perturbation theory approaches like the $GW$ method. It is shown that, for any finite system, approximate xc potentials including those in the local density and generalized gradient approximations, in contrast to common belief, always exhibit consistent nonzero discontinuities at integer electron numbers, if constructed properly. Thus the discontinuity of xc potentials is identified as a derived quantity which emerges automatically by properly constructing xc potentials without the need to be specifically incorporated in approximate xc functionals. It is demonstrated that all relevant objects of the ensemble KS formalism, in particular functional derivatives and their discontinuities, can be expressed in terms of quantities readily available in the KS formalism of integer electron numbers within an approach named integer electron ensemble approach (IEEA). Attempts to specifically construct ensemble density functionals are shown to be needless. Taking the known asymptotic behavior of the electron density into account an internal consistency condition for xc potentials is presented which justifies tuning procedures of xc functionals and, furthermore, indicates how asymptotic corrections for xc potentials have to be properly employed.

SYSTEMATIC CALCULATIONS OF ENERGY LEVELS AND TRANSITION RATES OF C-LIKE IONS WITH Z = 13-36

Based on systematic calculations using a combined relativistic configuration interaction and a many-body perturbation theory (MBPT) approach, we provide a complete and consistent data set for 46 levels belonging to the 2s 22p 2, 2s2p 3, 2p 4, 2s 22p3s, 2s 22p3p, and 2s 22p3d configurations in C-like ions with 13 ≤ Z ≤ 36. The data set includes energy levels as well as electric dipole, magnetic dipole, electric quadrupole, and magnetic quadrupole transition properties. Extensive comparisons with available observed and calculated results are made and indicate that the present MBPT calculations are highly accurate. The present data set can be used reliably for many purposes, such as the line identification of observed spectra, and modeling and diagnostics of astrophysical and fusion plasmas.