Quasiparticle Band Research Articles

Many-body effects of Cu-Phthalocyanine crystal for Solar Cell application

Performance of organic semiconductor material is very sensitive to the reactivity of electrical and optical properties. Structural properties of Cu-Phthalocyanine molecular crystal (β-CuPc) are studied via first-principles approach within density functional theory (DFT) framework. The calculated structural parameters are close to experimental result. Many-body perturbation theory (MBPT) based on convolution of non-interacting Green’s function (G0) and a screened Coulomb interaction (W0), G0W0 approximation were used for quasiparticle (QP) band structure and optical properties calculations. The bandgap value of 1.71 eV calculated with G0W0+RPA is in good agreement with experimental value. Optical properties calculations show that the results obtained within G0W0 plus random phase approximation (RPA) are close to available experimental results. Interestingly, optical gap of 1.71 eV and strong absorption of β-CuPc in the visible light and ultraviolet regions shows that the investigated material is suitable for optoelectronic and solar cells applications.

Study of the nucleon-rich effect in 158Er and 185Os rare-earth nuclei using the projected shell model

In this paper, the structure evolution in 158Er and185Os isotopes is calculated using the projected shell model (PSM) framework. The yrast spectrum, nucleon-rich effects, back-bending phenomenon, and the ratio of reduced electromagnetic transition probabilities, B(M1)/B(E2), are calculated in the spins ranges 8+–22+ and 17/2+–45/2+ for the mentioned isotopes, respectively. In the yrast spectrum, band crossing of one, two, and three quasi-particle bands at spins around 12+ and 41/2+, which is a sign of nucleon alignments, is investigated and 27 additional nucleons are found to be the reasons for back bending at lower spins in 158Er than 185Os, which is coincident with two great drops in the B(M1)/B(E2) ratio plots.

Topological Mott transition in a Weyl-Hubbard model: Dynamical mean-field theory study

Weyl semimetals are three-dimensional, topologically protected, gapless phases which show exotic phenomena such as Fermi arc surface states or negative magnetoresistance. It is an open question whether interparticle interactions can turn the topological semimetal into a topologically nontrivial Mott insulating phase. We investigate an experimentally motivated model for Weyl physics of cold atoms in optical lattices, with the main focus on interaction effects and topological properties by means of dynamical mean-field theory (DMFT). We characterize topological phases by numerically evaluating the Chern number via the Ishsikawa-Matsuyama formula for interacting phases. Within our studies, we find that the Chern numbers become trivial when interactions lead to insulating behavior. For a deeper understanding of the Weyl-semimetal-to-Mott-insulator topological phase transition, we evaluate the topological properties of quasiparticle bands as well as so-called blind bands. Our study is complementary to recent studies of Weyl semimetals with DMFT.

Angle-resolved photoemission spectroscopy view on the nature of Ce 4f electrons in the antiferromagnetic Kondo lattice CePd5Al2

We report an angle-resolved photoemission spectroscopy study of the antiferromagnetic Kondo lattice $\mathrm{Ce}{\mathrm{Pd}}_{5}{\mathrm{Al}}_{2}$, focusing on the quasi-two-dimensional $k$-space nature of its Fermi surface and, tuning photon energy to the Ce $4d--4f$ on-resonance transition, the hybridization of the Ce $4f$ state. A strong shoulder feature on the ${f}^{0}$ peak was detected, suggesting hybridization between conduction and $f$ bands. On-resonance spectra revealed narrow, yet hybridized quasiparticle bands with sharp peaks and $\ensuremath{\sim}$ 9 meV energy dispersion near the Fermi energy ${E}_{F}$. The observed dispersive hybridized $f$ band can be well described by a hybridization-band picture based on the periodic Anderson model (PAM). Hence, the $4f$ electrons in $\mathrm{Ce}{\mathrm{Pd}}_{5}{\mathrm{Al}}_{2}$ display a dual nature, with both localized and itinerant features, but with dominantly localized character.

Advanced First-Principle Modeling of Relativistic Ruddlesden—Popper Strontium Iridates

In this review, we provide a survey of the application of advanced first-principle methods on the theoretical modeling and understanding of novel electronic, optical, and magnetic properties of the spin-orbit coupled Ruddlesden–Popper series of iridates Srn+1IrnO3n+1 (n = 1, 2, and ∞). After a brief description of the basic aspects of the adopted methods (noncollinear local spin density approximation plus an on-site Coulomb interaction (LSDA+U), constrained random phase approximation (cRPA), GW, and Bethe–Salpeter equation (BSE)), we present and discuss select results. We show that a detailed phase diagrams of the metal–insulator transition and magnetic phase transition can be constructed by inspecting the evolution of electronic and magnetic properties as a function of Hubbard U, spin–orbit coupling (SOC) strength, and dimensionality n, which provide clear evidence for the crucial role played by SOC and U in establishing a relativistic (Dirac) Mott–Hubbard insulating state in Sr2IrO4 and Sr3Ir2O7. To characterize the ground-state phases, we quantify the most relevant energy scales fully ab initio—crystal field energy, Hubbard U, and SOC constant of three compounds—and discuss the quasiparticle band structures in detail by comparing GW and LSDA+U data. We examine the different magnetic ground states of structurally similar n = 1 and n = 2 compounds and clarify that the origin of the in-plane canted antiferromagnetic (AFM) state of Sr2IrO4 arises from competition between isotropic exchange and Dzyaloshinskii–Moriya (DM) interactions whereas the collinear AFM state of Sr3Ir2O7 is due to strong interlayer magnetic coupling. Finally, we report the dimensionality controlled metal–insulator transition across the series by computing their optical transitions and conductivity spectra at the GW+BSE level from the the quasi two-dimensional insulating n = 1 and 2 phases to the three-dimensional metallic n=∞ phase.

Fingerprints of optical absorption in the perovskite LaInO3 : Insight from many-body theory and experiment

We provide a combined theoretical and experimental study of the electronic structure and the optical absorption edge of the orthorhombic perovskite LaInO$_{3}$. Employing density-functional theory and many-body perturbation theory, we predict a direct electronic quasiparticle band gap of about 5 eV and an effective electron (hole) mass of 0.31 (0.48) m$_{0}$. We find the lowest-energy excitation at 0.2 eV below the fundamental gap, reflecting a sizeable electron-hole attraction. Since the transition from the valence band maximum (VBM, $\Gamma$ point) is, however, dipole forbidden the onset is characterized by weak excitations from transitions around it. The first intense excitation appears about 0.32 eV above. Interestingly, this value coincides with an experimental value obtained by ellipsometry (4.80 eV) which is higher than the onset from optical absorption spectroscopy (4.35 eV). The latter discrepancy is attributed to the fact that the weak transitions that define the optical gap are not resolved by the ellipsometry measurement. The absorption edge shows a strong dependency on the light polarization, reflecting the character of the involved valence states. Temperature-dependent measurements show a redshift of the optical gap by about 120 meV by increasing the temperature from 5 to 300 K. Renormalization due to zero-point vibrations is extrapolated from the latter measurement to amount to 150 meV. By adding the excitonic binding energy of 0.2 eV obtained theoretically to the experimental optical absorption onset, we determine the fundamental band gap at room temperature to be 4.55 eV.

GW band structure of monolayer MoS2 using the SternheimerGW method and effect of dielectric environment

Monolayers of transition-metal dichalcogenides (TMDs) hold great promise as future nanoelectronic and optoelectronic devices. An essential feature for achieving high device performance is the use of suitable supporting substrates, which can affect the electronic and optical properties of these two-dimensional (2D) materials. Here, we perform many-body GW calculations using the SternheimerGW method to investigate the quasiparticle band structure of monolayer ${\mathrm{MoS}}_{2}$ subject to an effective dielectric screening model, which is meant to approximately describe substrate polarization in real device applications. We show that, within this model, the dielectric screening has a sizable effect on the quasiparticle band gap; for example, the gap renormalization is as large as 250 meV for ${\mathrm{MoS}}_{2}$ with model screening corresponding to ${\mathrm{SiO}}_{2}$. Within the ${\mathrm{G}}_{0}{\mathrm{W}}_{0}$ approximation, we also find that the inclusion of the effective screening induces a direct band gap, in contrast to the unscreened monolayer. We also find that the dielectric screening induces an enhancement of the carrier effective masses by as much as 27% for holes, shifts plasmon satellites, and redistributes quasiparticle weight. Our results highlight the importance of the dielectric environment in the design of 2D TMD-based devices.

Superfluid states in α – T 3 lattice* *Project supported by the National Natural Science Foundation of China (Grant No. 11874127) and the startup grant from Guangzhou University.

The superfluid states of attractive Hubbard model in α–T 3 lattice are investigated. It is found that one usual needs three non-zero superfluid order parameters to describe the superfluid states due to three sublattices. When two hopping amplitudes are equal, the system has particle–hole symmetry. The flat band plays an important role in superfluid pairing near half filling. For example, when the filling factor falls into the flat band, the large density of states in the flat band favors superfluid pairing and the superfluid order parameters reach relatively large values. When the filling factor is in the gap between the flat band and upper band, the superfluid order parameters take small values due to the vanishing of density of states. The superfluid order parameters show nonmonotonic behaviors with the increase of filling factor. At last, we also investigate the edge states with open boundary conditions. It is shown that there exist some interesting edge states in the middle of quasi-particle bands.

Structural and electronic properties of solid molecular hydrogen from many-electron theories

We study the structural and electronic properties of phase III of solid hydrogen using accurate many-electron theories and compare to state-of-the-art experimental findings. The atomic structures of phase III modelled by C2/c-24 crystals are fully optimized on the level of second-order perturbation theory, demonstrating that previously employed structures optimized on the level of approximate density functionals exhibit errors in the H$_2$ bond lengths that cause significant discrepancies in the computed quasi particle band gaps and vibrational frequencies compared to experiment. Using the newly optimized atomic structures, we study the band gap closure and change in vibrational frequencies as a function of pressure. Our findings are in good agreement with recent experimental observations and may prove useful in resolving long-standing discrepancies between experimental estimates of metallization pressures possibly caused by disagreeing pressure calibrations.

Exchange interactions and sensitivity of the Ni two-hole spin state to Hund's coupling in doped NdNiO2

Using density functional based LDA+U method and linear-response theory, we study the magnetic exchange interactions of the superconductor Nd{1-x}Sr{x}NiO2. Our calculated nearest-neighbor exchange constant J1=82 meV is large, weakly affected by doping and is only slightly smaller than that found in the sister compound CaCuO2. We however find that the hole doping significantly enhances the inter--layer exchange coupling as it affects the magnetic moment of the Ni-3d{3z2-r2} orbital. This can be understood in terms of small hybridization of Ni-3d{3z2-r2} within the NiO2 plane which results in a flat band near the Fermi level, and its large overlap along z direction. We also demonstrate that the Nd-5d states appearing at the Fermi level, do not affect the magnetic exchange interactions, and thus may not participate in the superconductivity of this compound. Whereas many previous works emphasize the importance of the Ni-3d{x2-y2} and Nd-5d orbitals, we instead analyze the solution of Ni-3d{x2-y2}/Ni-3d{3z2-r2} minimal model using Dynamical Mean Field Theory. It reveals an underlying Mott insulating state which, depending on precise values of the intra--atomic Hund's coupling less or larger than 0.83 eV, selects upon doping either S=0 or S=1 two--hole states at low energies leading to very different quasiparticle band structures. We propose that trends upon doping in spin excitational spectrum and quasiparticle density of state can be a way to probe Ni 3d8 configuration.

Analytical Calculation of Exciton Binding Energy, Quasi-Particle Band Gap and Optical Gap in Strained Mono-layer MoS2

Strain in two-dimensional (2D) materials induces shifts in the visible spectrum due to strained binding energies, an accurate study of which is critical for materials engineering in the design of optoelectronic applications. Different from previous studies that employed computationally demanding theoretical approaches, we present an analytical approach, based on the difference method to calculate the impact of strained binding energy on the optical gap in mono-layer $${\hbox {MoS}}_{2}$$ by exploiting an existing tight binding (TB) and a fractional Coulomb potential model, for strained band gap and binding energy calculation, respectively. The inclusion of strained binding energy, changing at a rate of $$-8.1 \, \hbox {meV}/\bigtriangleup 1\%$$ of biaxial in-plane tensile strain accompanied with a variation of $$-110 \, \hbox {meV}/\bigtriangleup 1\%$$ of strain in the TB band gap, causes the optical gap to alter at a rate of $$-105.3 \, \hbox {meV}/\bigtriangleup 1\%$$ of strain, supported by first-principles calculations and is benchmarked with reported experimental and theoretical values. The effect of strained binding energies causes a blueshift in the optical gap by a correction factor, increasing from $$\approx 0.5\%$$ to $$\approx 12\%$$ , with an increase in equi-biaxial in-plane tensile strain from 1% to 11%, respectively. Furthermore, binding energy sensitivity to strain decreases linearly with a structural change from a mono-layer to a few layers in the 2D regime and saturates in the bulk regime. The presented framework can be used for the calculation of strained binding energies and optical gaps of other 2D materials and thus allows us to tune optical properties of 2D nanomaterials that are sensitive to lattice deformations.

Tunable interlayer excitons in two-dimensional SiC/MoSSe van der Waals heterostructures

In two-dimensional (2D) van der Waals heterostructures, the excitonic effects are significant enhanced due to the reduction of screening. Accurate description of light-matter interaction of 2D heterostructures is essential to understand their excitonic optical response and radiative lifetime. In this paper, the quasiparticle band structures and excitonic optical properties of SiC/MoSSe heterostructures are investigated by the GW + Bethe-Salpeter equation (GW + BSE) approach. Here the four stable structures 1-AA, 1-A'B, 2-AA and 2-A'B of SiC/MoSSe heterostructures are all type-II band alignment semiconductors, which are favorable for producing interlayer excitons. We find that low energy interlayer excitons of all the four structures posess high binding energies, and the optical dipole oscillator strength and radiative lifetimes can be modulated by several orders of magnitude as a result of the different intrinsic dipole moments of MoSSe. More interestingly, we also find that in 1-A'B and 2-A'B, the lowest-energy exciton is bright interlayer exciton with a strong absorption peak. The detailed analysis of the binding energies, transition contributions and exciton lifetimes of these interlayer excitons will be helpful for SiC/MoSSe heterostructures applications in the optoelectronics.

Transfer learning for materials informatics using crystal graph convolutional neural network

For successful applications of machine learning in materials informatics, it is necessary to overcome the inaccuracy of predictions ascribed to insufficient amount of data. In this study, we propose a transfer learning using a crystal graph convolutional neural network (TL-CGCNN). Herein, TL-CGCNN is pretrained with big data such as formation energies for crystal structures, and then used for predicting target properties with relatively small data. We confirm that TL-CGCNN can improve predictions of various properties such as bulk moduli, dielectric constants, and quasiparticle band gaps, which are computationally demanding, to construct big data for materials. Moreover, we quantitatively observe that the prediction of properties in target models via TL-CGCNN becomes more accurate with an increase in size of training dataset in pretrained models. Finally, we confirm that TL-CGCNN is superior to other regression methods in the predictions of target properties, which suffer from small amount of data. Therefore, we conclude that TL-CGCNN is promising along with compiling big data for materials that are easy to accumulate and relevant to the target properties.

Long-lived isomeric states and quasiparticle band structures in neutron-rich Gd162,164 nuclei from β decay

Long-lived isomeric states and quasiparticle band structures in neutron-rich <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Gd</mml:mi><mml:mprescripts /><mml:none /><mml:mrow><mml:mn>162</mml:mn><mml:mo>,</mml:mo><mml:mn>164</mml:mn></mml:mrow></mml:mmultiscripts></mml:math> nuclei from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>β</mml:mi></mml:math> decay

Quasiparticle band structure of SrTiO3 and BaTiO3 : A combined LDA+U and G0W0 approach

We present the quasiparticle band structures of prototypical oxide perovskites ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$, two seemingly simple oxides for which accurate calculations of the electronic structure have been met with significant (and somewhat unexpected) challenges. Previous ${G}^{0}{W}^{0}$ calculations predicted a band gap ranging from 3.36 to 3.82 eV for ${\mathrm{SrTiO}}_{3}$, with a majority of the studies giving a band gap around 3.7 $\ensuremath{\sim}$ 3.8 eV, to be compared with the experimental value of 3.25 eV. A similar discrepancy between theory and experiment is also observed for ${\mathrm{BaTiO}}_{3}$. We show that the ${G}^{0}{W}^{0}$ approach can predict reasonably accurate band gaps of ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$, provided that the calculations are carried out on top of the local density approximation (LDA) plus $U$ ($\mathrm{LDA}+U$) solutions and are fully converged. The deficiency of the LDA in describing the localized $3d$ states, although not particularly critical in this case, still results in a poor mean-field starting point for subsequent many-body perturbation calculations. ${G}^{0}{W}^{0}$ calculations starting from the $\mathrm{LDA}+U$ solutions, on the other hand, give significantly improved results for both systems. Our work demonstrates the accuracy and applicability of the combined ${G}^{0}{W}^{0}$ and $\mathrm{LDA}+U$ approach in calculating the quasiparticle band structures for materials involving localized $3d$ states, not only for systems with fully occupied $3d$ semicore states as has been shown before, but also for systems in which the $3d$ states are nominally unoccupied.

All-Electron Gaussian-Based G0W0 for Valence and Core Excitation Energies of Periodic Systems

We describe an all-electron G0W0 implementation for periodic systems with k-point sampling implemented in a crystalline Gaussian basis. Our full-frequency G0W0 method relies on efficient Gaussian density fitting integrals and includes both analytic continuation and contour deformation schemes. Due to the compactness of Gaussian bases, no virtual state truncation is required as is seen in many plane-wave formulations. Finite size corrections are included by taking the q → 0 limit of the Coulomb divergence. Using our implementation, we study quasiparticle energies and band structures across a range of systems including molecules, semiconductors, rare gas solids, and metals. We find that the G0W0 band gaps of traditional semiconductors converge rapidly with respect to the basis size, even for the conventionally challenging case of ZnO. Using correlation-consistent bases of polarized triple-ζ quality, we find the mean absolute relative error of the extrapolated G0W0@PBE band gaps to be only 5.2% when compared to experimental values. For core excitation binding energies (CEBEs), we find that G0W0 predictions improve significantly over those from DFT if the G0W0 calculations are started from hybrid functionals with a high percentage of exact exchange.

Pros and cons of the time-dependent hybrid density functional approach for calculating the optical spectra of solids: a case study of CeO2.

The prediction of optical spectra of complex solids remains a great challenge for first-principles calculations due to the huge computational cost of the state-of-the-art many-body perturbation theory based GW-Bethe Salpeter equation (BSE) approach. An alternative method is the time-dependent density-functional theory (TDDFT) based on hybrid exchange-correlation functionals, which involves the essential ingredients of electron-hole interactions in its formalism in contrast to its local/semi-local functional counterparts. In this work, we investigate the optical absorption spectra of ceria (CeO2), a prototypical lanthanide oxide with a 4f0 configuration, utilizing TDDFT based on four well-established hybrid functionals for ground state DFT calculations. All four functionals reproduce well the excitonic features of the experimental optical spectra, in spite of the significant differences in their band structures arising from different hybridization parameters (i.e. the fraction of the Hartree-Fock exchange and the screening parameter). It is demonstrated that the apparently weak dependence of the resulting optical spectra on the employed functionals is quite universal and applies to simple semiconductors such as Si and GaAs and insulator LiF as well. This study highlights the feasibility of TDDFT based on existing hybrids to describe optical spectra of solids, and also, points out the difficulty of obtaining accurate exciton binding energies using these hybrid functionals due to the strong functional dependence of quasi-particle band structures.

Quasiparticle band structures, spontaneous polarization, and spin-splitting in noncentrosymmetric few-layer and bulk γ-GeSe

Noncentrosymmetric γ-GeSe is a ferroelectric semiconductor with the spin-split valence band.

Self-consistent T-matrix approach to gap renormalization in quantum magnets with bond disorder

The analytical theory of density of states (DOS) in three-dimensional quantum magnets with the bond disorder is proposed based on the self-consistent T-matrix approximation (SCTMA). It successfully describes the DOS both for resonant and non-resonant scattering, whose emergence is governed by the ratio of scattering length and the average distance between impurities, which concentration is denoted as c. Corrections to the quasiparticle band gap in these cases are shown to scale as c2/3 and c, respectively. Moreover, the theory yields a semi-circle form of the DOS for the bound states inside the gap, which results in highly nontrivial DOS in the intermediate parameter region between the two limiting cases when the band DOS and the semi-circle overlap. Long-wavelength excitations are discussed. In the resonant regime their damping scales as c2/3, which, according to Ioffe-Regel criterion, corresponds to their localization. Applicability of the theory is illustrated by its quantitative agreement with the recent experimental data on spin-dimer system Ba3−xSrxCr2O8.

G0W0 plus BSE calculations of quasiparticle band structure and optical properties of nitrogen-doped antimony trisulfide for near infrared optoelectronic and solar cells application

Theoretical calculations of structural, electronic, excitonic and optical properties of N-doped Sb2S3 are studied using highly accurate first-principles approach within many-body perturbation theory (MBPT) formalism. The calculated structural parameters of undoped Sb2S3 within Wu-Cohen’s generalized gradient approximation (WC-GGA) are reasonably close to those obtained in experimental measurement. Many-body perturbation theory (MBPT) based on the G0W0 approximation is used for the quasiparticle (QP) band structure. The bandgap value of 1.70 eV for the undoped Sb2S3 crystal within G0W0 approximation is consistent with the experimental value of 1.70–1.80 eV. When one atom of N is introduced into Sb2S3 at Sb site, the doping effects modified the band gap from 1.70 to 1.17 eV. Also, by introducing one atom of N to S site, the band gap value reduced to 0.96 eV. Our findings confirmed that non-metal doping narrow the energy gap of semiconductor materials. The optical properties of pure and N-doped Sb2S3 are computed using G0W0 plus Bethe-Salpeter Equation (BSE) which include both electron-electron (e-e) and electron-hole (e-h) interactions. The optical gap for Sb16S24, Sb15N1S24 and Sb16S23N1 were found to be 1.54, 0.97 and 0.82 eV, respectively. The narrowing effects and strong optical absorption of N-doped Sb2S3 suggest that the investigated material is suitable for solar cells and near infrared optoelectronic applications.