Many-body Perturbation Theory Research Articles

Optical absorption study of iron-substituted zirconia and yttria-stabilized zirconia using experimental measurements and many-body perturbation theory

Optical absorption study of iron-substituted zirconia and yttria-stabilized zirconia using experimental measurements and many-body perturbation theory

Excitonic and optoelectronic investigations of bromide-chloride mixed vacancy-ordered double tellurium perovskite systems

Metal-halide perovskites, including vacancy-ordered double perovskites, are versatile semiconductors with applications in photovoltaics and sensors. Here we explore the structural stability, electronic and optical properties of Cs2Te(Br1-xClx)6 series using first-principles-based on density functional theory and state-of-the-art many-body perturbation theory. Our study reveals consistent energy levels across different crystal structures obtained by varying Cl concentrations, indicating uniform structural stability. By employing hybrid density functional theory with spin-orbit coupling, we analyze the effect of Cl-site substitutions on electronic and optical properties. We observe a quasi-linear relationship between band gap and Cl concentration in Cs2Te(Br1-xClx)6 compositions, with alloying resulting in a redshift in the optical absorption coefficient. Moreover, we explore electron-hole interactions using the GW approximation and Bethe−Salpeter equation, revealing exciton binding energies and dark-bright splitting. Remarkably, exciton binding energies, reaching up to 930 meV, can be finely tuned through substitutional engineering. Finally, we investigate the thermoelectric properties of the Br/Cl mixed halide double perovskites, suggesting promising prospects for their application as thermoelectric materials.

Morphology-Dependent Interaction between Tetraphenylporphyrins and a Metal Oxide Surface: Electronic Signature, Photo Physics, and Chemical Interaction

Porphyrin-substrate hybrid systems are the building blocks in a series of materials, such as organic light-emitting diodes, chemical sensors, dye-sensitized solar cells and catalysts. Functional layers and 2D nanostructures on surfaces enable sensors, and nanoscale optical and magnetic materials. 3D porphyrin structures are for instance used in controlling and manipulating light at subwavelength dimensions. Understanding and correctly describing the way molecules interact with the substrate and in thin-film structures gives a handle on geometrical and electronic device properties. Indeed, adsorption and film-formation have a relevant effect on the molecular electronic structure even in the absence of strong covalent bonding, namely in terms of level-alignment with substrate states in the former and polarization-induced renormalization of molecular band gaps in the latter.Using the case of Co(II)-tetraphenylporphyrin (CoTPP) on the MgO(100), we shine light on the adsorption at morphology-related low-coordinated surfaces sites and the effect of multilayer film formation from a theoretical perspective in a frame work of hybrid density functional theory and many-body perturbation theory. We simulate the photoemission spectra for relevant adsorption sites as a fingerprint with distinct site-related features in both valence and core-level regions of the electronic structure that are also observed in experiments on MgO(100) films on Ag(100) and thus aid the identification of sites. Effects of the formation of porphyrin films on the electronic and optical properties will be outlined for H2TPP and MgTPP.We also highlight the self-metalation of H2TPP at morphology-related low-coordinated sites. Metalation is an important pathway for functionalization of porphyrin-based organic-inorganic structures. We address the self-metalation mechanism using ab initio molecular dynamics simulation at room temperature. While H2TPP is mobile on the pristine surface as the steric hindrance by phenyl rings prevents the physisorption of the macrocycle at a specific site, step edges or kink sites provide anchor points exposing low-coordinated, reactive oxygen-sites to hydrogens of the macrocycle. We report a spontaneous proton transfer at these sites forming an intermediate complex before the metalation occurs. This highlights the role of low-coordinated surface sites for the chemical interaction with the porphyrin.

An Attractive Way to Correct for Missing Singles Excitations in Unitary Coupled Cluster Doubles Theory.

Coupled cluster methods based exclusively on double excitations are comparatively "cheap" and interesting model chemistries, as they are typically able to capture the bulk of the dynamic electron correlation effects. The trade-off in such approximations is that the effect of neglected excitations, particularly single excitations, can be considerable. Using standard and electron-pair-restricted T2 operators to define two flavors of unitary coupled cluster doubles (UCCD) methods, we investigate the extent to which missing single excitations can be recovered from low-order corrections in many-body perturbation theory (MBPT) within the unitary coupled cluster (UCC) formalism. Our analysis includes the derivations of finite-order UCC energy functionals, which are used as a basis to define perturbative estimates of missed single excitations. This leads to the novel UCCD[4S] and UCCD[6S] methods, which consider energy corrections for missing single excitations through fourth- and sixth-order in MBPT, respectively. We also apply the same methodology to the electron-pair-restricted ansatz, but the improvements are only marginal. Our findings show that augmenting UCCD with these post hoc perturbative corrections can lead to UCCSD-quality results.

Web-CONEXS: an inroad to theoretical X-ray absorption spectroscopy.

Accurate analysis of the rich information contained within X-ray spectra usually calls for detailed electronic structure theory simulations. However, density functional theory (DFT), time-dependent DFT and many-body perturbation theory calculations increasingly require the use of advanced codes running on high-performance computing (HPC) facilities. Consequently, many researchers who would like to augment their experimental work with such simulations are hampered by the compounding of nontrivial knowledge requirements, specialist training and significant time investment. To this end, we present Web-CONEXS, an intuitive graphical web application for democratizing electronic structure theory simulations. Web-CONEXS generates and submits simulation workflows for theoretical X-ray absorption and X-ray emission spectroscopy to a remote computing cluster. In the present form, Web-CONEXS interfaces with three software packages: ORCA, FDMNES and Quantum ESPRESSO, and an extensive materials database courtesy of the Materials Project API. These software packages have been selected to model diverse materials and properties. Web-CONEXS has been conceived with the novice user in mind; job submission is limited to a subset of simulation parameters. This ensures that much of the simulation complexity is lifted and preliminary theoretical results are generated faster. Web-CONEXS can be leveraged to support beam time proposals and serve as a platform for preliminary analysis of experimental data.

Excitonic and Environmental Screening Effects in Two-Dimensional Janus MSO (M = Ga, In).

In this work, we investigate Janus monolayer MSO (M = Ga, In) systems using the state-of-the-art GW method within the framework of the many-body perturbation theory. Ground-state density functional theory calculations reveal that both the substitution of S atoms with O atoms and the chemisorption of the O atoms on a single side of the MS layer narrow the band gaps and reduce the carrier mobilities. Notably, one-shot GW calculations demonstrate that the GaSO-2 and InSO-1 systems exhibit optimal band gaps for visible light absorption. Based on the Bethe-Salpeter equation, the exciton binding energies of isolated Janus monolayer GaSO-2 and InSO-1 systems are lower than those of their prototype GaS and InS by 0.37 and 0.17 eV, respectively. Further calculations show that the exciton binding energies of the Janus GaSO-2 and InSO-1 systems can be precisely tuned by adjusting their thicknesses and the thicknesses of their substrates. A deep understanding of the mechanisms for tuning the exciton binding energies in Janus GaSO-2 and InSO-1 systems is crucial for the future design of advanced photovoltaic devices.

Optoelectronic properties of electron-acceptor molecules adsorbed on graphene/silicon carbide interfaces

Silicon carbide has emerged as an optimal semiconducting support for graphene growth. In previous studies, the formation of an interfacial graphene-like buffer layer covalently bonded to silicon carbide has been observed, revealing electronic properties distinct from ideal graphene. Despite extensive experimental efforts dedicated to this interface, theoretical investigations have been confined to its ground state. Here, we use many-body perturbation theory to study the electronic and optical characteristics of this interface and demonstrate its potential for optoelectronics. By adsorbing graphene, we show that the quasiparticle band structure exhibits a reduced bandgap, associated with an optical onset in the visible energy window. Furthermore, we reveal that the absorption of two prototypical electron-accepting molecules on this substrate results in a significant renormalization of the adsorbate gap, giving rise to distinct low-lying optically excited states in the near-infrared region. These states are well-separated from the substrate’s absorption bands, ensuring wavelength selectivity for molecular optoelectronic applications.

Nuclear Quantum Effects on the Electronic Structure of Water and Ice.

The electronic properties and optical response of ice and water are intricately shaped by their molecular structure, including the quantum mechanical nature of the hydrogen atoms. Despite numerous previous studies, a comprehensive understanding of the nuclear quantum effects (NQEs) on the electronic structure of water and ice at finite temperatures remains elusive. Here, we utilize molecular simulations that harness efficient machine-learning potentials and many-body perturbation theory to assess how NQEs impact the electronic bands of water and hexagonal ice. By comparing path-integral and classical simulations, we find that NQEs lead to a larger renormalization of the fundamental gap of ice, compared to that of water, ultimately yielding similar bandgaps in the two systems, consistent with experimental estimates. Our calculations suggest that the increased quantum mechanical delocalization of protons in ice, relative to water, is a key factor leading to the enhancement of NQEs on the electronic structure of ice.

Investigation of the M1 transitions from the ground configuration of W23+

To extend the research on the atomic structure of tungsten ions with an open 4f-shell, we conducted an investigation into the M1 transitions originating from the ground configuration of W23+ using the SHHtscEBIT in the 420–600 nm range. Three different calculational methods, including the relativistic configuration interactions (RCI) and the relativistic many-body perturbation theory (RMBPT) implemented in FAC, as well as the multiconfiguration Dirac-Hartree-Fock (MCDHF) in GRASP2018, are performed to investigate the energy level structure of W23+. The excitation energies obtained through RCI and RMBPT are in good agreement, with an average difference of 0.62 %. When comparing RCI and MCDHF, this difference increases to 1.20 %. The observed 12 lines of W23+are identified using the detailed collisional-radiative model with the atomic data calculated by RCI. The average wavelength deviations from experimental results are 0.77 %, 1.38 %, and 2.02 % for RCI, RMBPT, and MCDHF, respectively.

Converging many-body perturbation theory for ab-initio nuclear structure: Brillouin-Wigner perturbation series for open-shell nuclei

Converging many-body perturbation theory for <i>ab-initio</i> nuclear structure: Brillouin-Wigner perturbation series for open-shell nuclei

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.

Analysis and Assessment of Knowles' Partitioning in Many-Body Perturbation Theory.

A detailed analysis of a new partitioning in many-body perturbation theory recently proposed by Knowles (J. Chem. Phys. 156, 011101, 2022), termed "perturbation adapted partitioning" (PAPT), is presented. Level shift and orbital rotation effects are identified as gears of the zero-order Hamiltonian. These two components are examined separately, revealing that, in themselves, neither of the two is competitive with the combined effect. The success of PAPT can be attributed to determining a set of molecular orbitals and corresponding orbital energies that can systematically outperform the canonical orbitals and Koopmans' energy-based Møller-Plesset partitioning. The self-consistent version of the method is also tested in terms of energy and convergence. Previous numerical studies are further complemented with an application to an inherent multireference example and an investigation of van der Waals interaction energies. In addition, a rigorous mathematical analysis of the consequence of the linear dependence of projection functions on the solution of the Knowles' equations is provided.

A new "gold standard": Perturbative triples corrections in unitary coupled cluster theory and prospects for quantum computing.

A major difficulty in quantum simulation is the adequate treatment of a large collection of entangled particles, synonymous with electron correlation in electronic structure theory, with coupled cluster (CC) theory being the leading framework for dealing with this problem. Augmenting computationally affordable low-rank approximations in CC theory with a perturbative account of higher-rank excitations is a tractable and effective way of accounting for the missing electron correlation in those approximations. This is perhaps best exemplified by the "gold standard" CCSD(T) method, which bolsters the baseline CCSD with the effects of triple excitations using considerations from many-body perturbation theory (MBPT). Despite this established success, such a synergy between MBPT and the unitary analog of CC theory (UCC) has not been explored. In this work, we propose a similar approach wherein converged UCCSD amplitudes are leveraged to evaluate energy corrections associated with triple excitations, leading to the UCCSD[T] method. In terms of quantum computing, this correction represents an entirely classical post-processing step that improves the energy estimate by accounting for triple excitation effects without necessitating new quantum algorithm developments or increasing demand for quantum resources. The rationale behind this choice is shown to be rigorous by studying the properties of finite-order UCC energy functionals, and our efforts do not support the addition of the fifth-order contributions as in the (T) correction. We assess the performance of these approaches on a collection of small molecules and demonstrate the benefits of harnessing the inherent synergy between MBPT and UCC theories.

Physical and unphysical regimes of self-consistent many-body perturbation theory

In the standard framework of self-consistent many-body perturbation theory, the skeleton series for the self-energy is truncated at a finite order N and plugged into the Dyson equation, which is then solved for the propagator G_NGN. We consider two examples of fermionic models, the Hubbard atom at half filling and its zero space-time dimensional simplified version. First, we show that G_NGN converges when N\to∞N→∞ to a limit G_∞\,G∞, which coincides with the exact physical propagator G_{exact}Gexact at small enough coupling, while G_∞ ≠ G_{exact}G∞≠Gexact at strong coupling. This follows from the findings of [Phys. Rev. Lett. 114, 156402 (2015)] and an additional subtle mathematical mechanism elucidated here. Second, we demonstrate that it is possible to discriminate between the G_∞=G_{exact}G∞=Gexact and G_∞≠G_{exact}G∞≠Gexact regimes thanks to a criterion which does not require the knowledge of G_{exact}Gexact, as proposed in [Phys. Rev. B 93, 161102 (2016)].

Finite and high-temperature series expansion via many-body perturbation theory: application to Heisenberg spin-1/2 XXZ chain

We present a new algorithm to evaluate the grand potential at high and finite temperatures using many-body perturbation theory. This algorithm enables us to calculate the contribution of any Hugenholtz or Feynman vacuum diagrams and formulate the results as a sum of divided differences. Additionally, the proposed method is applicable to any interaction in any dimension, allowing us to calculate thermodynamic quantities efficiently at any given temperature, particularly at high temperatures.Furthermore, we apply this algorithm to the Heisenberg spin-1/2 XXZ chain. We obtain all coefficients of the high-temperature expansion of the free energy and susceptibility per site of this model up to the sixth order.

Converging many-body perturbation theory for ab-initio nuclear structure: Brillouin-Wigner perturbation series for closed-shell nuclei

Convergence aspects of nuclear many-body perturbation theory for ground states of closed-shell nuclei are explored using a Brillouin-Wigner formulation with a new vertex function enabling high-order calculations. A general formalism for Hamiltonian partitioning and a convergence criterion for the perturbation series are proposed. Analytical derivations show that with optimal partitioning, the convergence criterion for ground states can always be satisfied. This feature attributes to the variational principle and does not depend on the choice of an internucleon interaction or a many-body basis. Numerical calculations of the ground state energies of He4 and O16 with Daejeon16 and a bare N3LO potential in both harmonic-oscillator and Hartree-Fock bases confirm this finding.

Going Beyond the GW Approximation Using the Time-Dependent Hartree-Fock Vertex.

The time-dependent Hartree-Fock (TDHF) vertex of many-body perturbation theory (MBPT) makes it possible to extend TDHF theory to charged excitations. Here we assess its performance by applying it to spherical atoms in their neutral electronic configuration. On a theoretical level, we recast the TDHF vertex as a reducible vertex, highlighting the emergence of a self-energy expansion purely in orders of the bare Coulomb interaction; then, on a numerical level, we present results for polarizabilities, ionization energies (IEs), and photoemission satellites. We confirm the superiority of THDF over simpler methods such as the random phase approximation for the prediction of atomic polarizabilities. We then find that the TDHF vertex reliably provides better IEs than GW and low-order self-energies do in the light-atom, few-electron regime; its performance degrades in heavier, many-electron atoms instead, where an expansion in orders of an unscreened Coulomb interaction becomes less justified. New relevant features are introduced in the satellite spectrum by the TDHF vertex, but the experimental spectra are not fully reproduced due to a missing account of nonlinear effects connected to hole relaxation. We also explore various truncations of the self-energy given by the TDHF vertex, but do not find them to be more convenient than low-order approximations such as GW and second Born (2B), suggesting that vertex corrections should be carried out consistently both in the self-energy and in the polarizability.

Impact of Magnetism on (e-ph) self-energy and thermalization of hot carriers in 2D half-functionalized GeC-hybrid

The present work delves into the analysis of carrier scattering rates, emphasizing the effective strategy of chemical functionalization for introducing magnetism into two-dimensional, initially non-magnetic materials. Specifically, our investigation focuses on the distinct alterations observed in the electron linewidth within two half-fluorinated configurations, namely GeC-F and F-GeC. Leveraging density functional and many-body perturbation theories, our findings highlight a substantial increase in the linewidth upon inducing ferromagnetism and antiferromagnetism in the GeC hybrid. Additionally, we identify the significant contribution of different phonon modes to the linewidth. The outcomes reveal that the out-of-plane acoustic mode ZA exerts global control over the valence and conduction bands. Moreover, the relaxation time of electrons following sunlight illumination demonstrates that electron thermalization occurs within 1.5fs to 130fs for ferromagnetic GeC-F and within 2.32fs to 42fs for antiferromagnetic F-GeC. This study provides a detailed and accurate description of electron behavior at sub-picosecond timescales, a realm that is experimentally challenging to attain.

Multi-objective global optimization approach predicted quasi-layered ternary TiOS crystals with promising photocatalytic properties

Abstract Titanium dioxide (TiO2) has attracted considerable research attentions for its promising applications in solar cells and photocatalytic devices. However, the intrinsic challenge lies in the relatively low energy conversion efficiency of TiO2, primarily attributed to the substantial band gaps (exceeding 3.0 eV) associated with its rutile and anatase phases. Leveraging multi-objective global optimization, we have identified two quasi-layered ternary Ti–O–S crystals, composed of titanium, oxygen, and sulfur. The calculations of formation energy, phonon dispersions, and thermal stability confirm the chemical, dynamical and thermal stability of these newly discovered phases. Employing the state-of-art hybrid density functional approach and many-body perturbation theory (quasiparticle GW approach and Bethe–Salpeter equation), we calculate the optical properties of both the TiOS phases. Significantly, both phases show favorable photocatalytic characteristics, featuring band gaps suitable for visible optical absorption and appropriate band alignments with water for effective charge carrier separation. Therefore, ternary compound TiOS holds the potential for achieving high-efficiency photochemical conversion, showing our multi-objective global optimization provides a new approach for novel environmental and energy materials design with multicomponent compounds.

Computational Discovery of Intermolecular Singlet Fission Materials Using Many-Body Perturbation Theory.

Intermolecular singlet fission (SF) is the conversion of a photogenerated singlet exciton into two triplet excitons residing on different molecules. SF has the potential to enhance the conversion efficiency of solar cells by harvesting two charge carriers from one high-energy photon, whose surplus energy would otherwise be lost to heat. The development of commercial SF-augmented modules is hindered by the limited selection of molecular crystals that exhibit intermolecular SF in the solid state. Computational exploration may accelerate the discovery of new SF materials. The GW approximation and Bethe-Salpeter equation (GW+BSE) within the framework of many-body perturbation theory is the current state-of-the-art method for calculating the excited-state properties of molecular crystals with periodic boundary conditions. In this Review, we discuss the usage of GW+BSE to assess candidate SF materials as well as its combination with low-cost physical or machine learned models in materials discovery workflows. We demonstrate three successful strategies for the discovery of new SF materials: (i) functionalization of known materials to tune their properties, (ii) finding potential polymorphs with improved crystal packing, and (iii) exploring new classes of materials. In addition, three new candidate SF materials are proposed here, which have not been published previously.