Showing papers on "Random phase approximation published in 2016"
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23 Aug 2016
TL;DR: In this article, reduced density operators were used to estimate the mean field and the mean matrix of an EM field, and the spin statistics of the spin statistic was used to determine the distance to the ground truth.
Abstract: Introduction.- Reduced Density Operators.- Correlations due to the Spin Statistics.- Mean-Field Approximation.- Correlations and their Dynamics.- Non-Markovian Effects.- Kinetic Equations with Selfenergy.- Properties of the Kinetic Equation.- T-Matrix Approximation.- Random Phase Approximation.- Screened Ladder Approximation.- Charged Carriers in EM Fields.- Non-Equilibrium Green's Functions.- Kinetics vs. Molecular Dynamics.- Conclusion.
362 citations
TL;DR: This paper demonstrates that end-to-end multiscale modeling, starting at detailed many-body quantum mechanics and ending with macroscopic properties, with the approximations controlled along the way, is feasible for these systems.
Abstract: The study of hexagonal boron nitride (hBN) in microfluidic and nanofluidic applications at the atomic level requires accurate force field parameters to describe the water-hBN interaction. In this work, we begin with benchmark quality first principles quantum Monte Carlo calculations on the interaction energy between water and hBN, which are used to validate random phase approximation (RPA) calculations. We then proceed with RPA to derive force field parameters, which are used to simulate water contact angle on bulk hBN, attaining a value within the experimental uncertainties. This paper demonstrates that end-to-end multiscale modeling, starting at detailed many-body quantum mechanics and ending with macroscopic properties, with the approximations controlled along the way, is feasible for these systems.
91 citations
TL;DR: An atomic-orbital reformulation of the random-phase approximation (RPA) correlation energy is presented allowing to reduce the steep computational scaling to linear, so that large systems can be studied on simple desktop computers with fully numerically controlled accuracy.
Abstract: An atomic-orbital (AO) reformulation of the random-phase approximation (RPA) correlation energy is presented allowing to reduce the steep computational scaling to linear, so that large systems can be studied on simple desktop computers with fully numerically controlled accuracy. Our AO-RPA formulation introduces a contracted double-Laplace transform and employs the overlap-metric resolution-of-the-identity. First timings of our pilot code illustrate the reduced scaling with systems comprising up to 1262 atoms and 10 090 basis functions.
75 citations
TL;DR: In this article, the Hartree-Fock-Bogoliubov plus QRPA method is applied to even-even nuclei, the strength function for odd nuclei being derived by interpolation, and the convergence with respect to the adopted number of harmonic oscillator shells and the cutoff energy introduced in the 2-quasiparticle QP excitation space is analyzed.
Abstract: Valuable theoretical predictions of nuclear dipole excitations in the whole chart are of great interest for different nuclear applications, including in particular nuclear astrophysics. Here we present large-scale calculations of the $E1\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$-ray strength function obtained in the framework of the axially symmetric deformed quasiparticle random-phase approximation based on the finite-range Gogny force. This approach is applied to even-even nuclei, the strength function for odd nuclei being derived by interpolation. The convergence with respect to the adopted number of harmonic oscillator shells and the cutoff energy introduced in the 2-quasiparticle $(2\ensuremath{-}qp)$ excitation space is analyzed. The calculations performed with two different Gogny interactions, namely D1S and D1M, are compared. A systematic energy shift of the $E1$ strength is found for D1M relative to D1S, leading to a lower energy centroid and a smaller energy-weighted sum rule for D1M. When comparing with experimental photoabsorption data, the Gogny-QRPA predictions are found to overestimate the giant dipole energy by typically $\ensuremath{\sim}2$ MeV. Despite the microscopic nature of our self-consistent Hartree-Fock-Bogoliubov plus QRPA calculation, some phenomenological corrections need to be included to take into account the effects beyond the standard $2\ensuremath{-}qp$ QRPA excitations and the coupling between the single-particle and low-lying collective phonon degrees of freedom. For this purpose, three prescriptions of folding procedure are considered and adjusted to reproduce experimental photoabsorption data at best. All of them are shown to lead to somewhat similar predictions of the $E1$ strength, both at low energies and for exotic neutron-rich nuclei. Predictions of $\ensuremath{\gamma}$-ray strength functions and Maxwellian-averaged neutron capture rates for the whole Sn isotopic chain are also discussed and compared with previous theoretical calculations.
69 citations
TL;DR: In this paper, the dynamic polarizability of the dice lattice with pseudospin-1/2 Dirac fermions with flat-band dispersion was investigated.
Abstract: We calculate the dynamic polarizability under the random phase approximation for the dice lattice. This two-dimensional system gives rise to massless Dirac fermions with pseudospin-1 in the low-energy quantum excitation spectrum, providing a Dirac-cone plus flat-band dispersion. Due to the presence of the flat band, the polarizability shows key differences to that of graphene (the pseudospin-1/2 Dirac material). We find that the plasmon branch is pinched in to a single point, ${\ensuremath{\omega}}_{p}=q=\ensuremath{\mu}$, independent of the background dielectric constant. Finally, screening effects are discussed with regard to impurities.
69 citations
TL;DR: A massively parallel implementation of RI-RPA in a Gaussian basis that is the key for the application to large systems is reported and cubic-scaling RPA is applied to a thousand water molecules using a correlation-consistent triple-ζ quality basis.
Abstract: We present an algorithm for computing the correlation energy in the random phase approximation (RPA) in a Gaussian basis requiring O(N3) operations and O(N2) memory. The method is based on the resolution of the identity (RI) with the overlap metric, a reformulation of RI-RPA in the Gaussian basis, imaginary time, and imaginary frequency integration techniques, and the use of sparse linear algebra. Additional memory reduction without extra computations can be achieved by an iterative scheme that overcomes the memory bottleneck of canonical RPA implementations. We report a massively parallel implementation that is the key for the application to large systems. Finally, cubic-scaling RPA is applied to a thousand water molecules using a correlation-consistent triple-ζ quality basis.
66 citations
TL;DR: In this paper, the exchange parameter of the Perdew-burke-Ernzerhof (PBE) hybrid functionals was derived for the tetrathiafulvalene-tetracyanoquinodimethane complex for which standard density functional theory functionals predict artificial electron transfer.
Abstract: We obtain the exchange parameter of hybrid functionals by imposing the fundamental condition of a piecewise linear total energy with respect to electron number. For the Perdew-Burke-Ernzerhof (PBE) hybrid family of exchange-correlation functionals (i.e., for an approximate generalized Kohn-Sham theory) this implies that (i) the highest occupied molecular orbital corresponds to the ionization potential $(I)$, (ii) the energy of the lowest unoccupied molecular orbital corresponds to the electron affinity $(A)$, and (iii) the energies of the frontier orbitals are constant as a function of their occupation. In agreement with a previous study [N. Sai et al., Phys. Rev. Lett. 106, 226403 (2011)], we find that these conditions are met for high values of the exact exchange admixture $\ensuremath{\alpha}$ and illustrate their importance for the tetrathiafulvalene-tetracyanoquinodimethane complex for which standard density functional theory functionals predict artificial electron transfer. We further assess the performance for atomization energies and weak interaction energies. We find that atomization energies are significantly underestimated compared to PBE or PBE0, whereas the description of weak interaction energies improves significantly if a $1/{R}^{6}$ van der Waals correction scheme is employed.
59 citations
TL;DR: The study indicates the importance of considering high-order dynamical correlation effects to correctly predict the relative phase stability of polymorphic materials, especially for those systems in which the less stable phase as predicted by conventional local, semilocal or even hybrid density functional approximations has a smaller band gap than the more stable one.
Abstract: The relative stability of TiO2 in the rutile and anatase structure is wrongly described by density functional theory in various local, semilocal, or even hybrid functional approximations. In this work, we have found that by considering high-order correlations in the adiabatic connection fluctuation–dissipation theory with the random phase approximation (ACFDT-RPA), rutile is correctly predicted to be more stable than anatase, which can be physically attributed to different characters in the electronic band structure of rutile and anatase, including, in particular, that rutile has a smaller band gap than anatase. We further consider the zero-point energy and finite-temperature effects based on the harmonic approximation, and we found that the inclusion of the zero-point energy correction can further increase the relative stability of rutile, and leads to a better quantitative agreement with available experimental measurements. Our study indicates the importance of considering high-order dynamical correlation effects to correctly predict the relative phase stability of polymorphic materials, especially for those systems in which the less stable phase as predicted by conventional local, semilocal or even hybrid density functional approximations has a smaller band gap than the more stable one.
54 citations
TL;DR: In this paper, the authors consider the external momenta which enter the integrals representing the magnetized polarization tensor and employ a field-independent regularization scheme to obtain more accurate results.
Abstract: We consider the two-flavor Nambu--Jona-Lasinio model within the random phase approximation (RPA) framework to evaluate the masses of the $\ensuremath{\sigma}$ and ${\ensuremath{\pi}}^{0}$ mesons and the ${\ensuremath{\pi}}^{0}$ decay constant in the presence of a magnetic field at vanishing temperatures and baryonic densities. The present work extends other RPA applications by fully considering the external momenta which enter the integrals representing the magnetized polarization tensor. We employ a a field-independent regularization scheme so that more accurate results can be obtained in the evaluation of physical quantities containing pionic contributions. As we show, this technical improvement generates results which agree well with those produced by lattice simulations and chiral perturbation theory. Our method may also prove to be useful in future evaluations of quantities, such as the shear viscosity and the equation of state of magnetized quark matter with mesonic contributions.
53 citations
TL;DR: In this article, the authors compared the axially symmetric-deformed quasiparticle random phase approximation (QRPA) based on the finite-range D1M Gogny force with available experimental data and showed that a relatively good agreement is obtained provided the strength is shifted globally by about 2 MeV and increased by an empirical factor of 2.
Abstract: Valuable theoretical predictions of nuclear dipole excitations in the whole chart are of great interest for different nuclear applications, including in particular nuclear astrophysics. Here we extend our large-scale calculations of the $E1 \ensuremath{\gamma}$-ray strength function, obtained in the framework of the axially- symmetric-deformed quasiparticle random phase approximation (QRPA) based on the finite-range D1M Gogny force, to the calculation of the $M1$ strength function. We compare our QRPA prediction of the $M1$ strength with available experimental data and show that a relatively good agreement is obtained provided the strength is shifted globally by about 2 MeV and increased by an empirical factor of 2. Predictions of the $M1$ strength function for spherical and deformed nuclei within the valley of $\ensuremath{\beta}$ stability as well as in the neutron-rich region are discussed. Its impact on the radiative neutron capture cross section is also analyzed.
46 citations
22 Feb 2016
TL;DR: In this paper, the optical and plasmonic properties of a graphene sheet carrying a dc current were analyzed and it was shown that an applied dc current modifies the Pauli blocking mechanism and absorption acquired a birefringent character with respect to the angle between the in-plane light polarization and current flow.
Abstract: We present extensive calculations of the optical and plasmonic properties of a graphene sheet carrying a dc current. By calculating analytically the density–density response function of current-carrying states at finite temperature, we demonstrate that an applied dc current modifies the Pauli blocking mechanism and that absorption acquires a birefringent character with respect to the angle between the in-plane light polarization and current flow. Employing random phase approximation at finite temperature, we show that graphene plasmons display a degree of non-reciprocity and collimation that can be tuned with the applied current. We discuss the possibility to measure these effects.
TL;DR: In this article, a relativistic meson-exchange nuclear Lagrangian of Quantum Hadrodynamics was developed for spin-isospin excitations in open-shell nuclei.
Abstract: A new theoretical approach to spin-isospin excitations in open-shell nuclei is presented. The developed method is based on the relativistic meson-exchange nuclear Lagrangian of Quantum Hadrodynamics and extends the response theory for superfluid nuclear systems beyond relativistic quasiparticle random phase approximation in the proton-neutron channel (pn-RQRPA). The coupling between quasiparticle degrees of freedom and collective vibrations (phonons) introduces a time-dependent effective interaction, in addition to the exchange of pion and \( \rho\) -meson taken into account without retardation. The time-dependent contributions are treated in the resonant time-blocking approximation, in analogy to the previously developed relativistic quasiparticle time-blocking approximation (RQTBA) in the neutral (non-isospin-flip) channel. The new method is called proton-neutron RQTBA (pn-RQTBA) and is applied to the Gamow-Teller resonance in a chain of neutron-rich nickel isotopes 68-78Ni . A strong fragmentation of the resonance along with quenching of the strength, as compared to pn-RQRPA, is obtained. Based on the calculated strength distribution, beta-decay half-lives of the considered isotopes are computed and compared to pn-RQRPA half-lives and to experimental data. It is shown that a considerable improvement of the half-life description is obtained in pn-RQTBA because of the spreading effects, which bring the lifetimes to a very good quantitative agreement with data.
TL;DR: For La-substituted barium hexaferrites, the electronic structure has been determined by the density functional theory (DFT) and generalized gradient approximation (GGA) and the molecular field approximation (MFA) and random phase approximation (RPA) have been adopted to deduce the Curie temperature Tc.
Abstract: As the macro behavior of the strength of exchange interaction, state of the art of Curie temperature Tc, which is directly proportional to the exchange integrals, makes sense to the high-frequency and high-reliability microwave devices Challenge remains as finding a quantitative way to reveal the relationship between the Curie temperature and the exchange integrals for doped barium hexaferrites Here in this report, for La-substituted barium hexaferrites, the electronic structure has been determined by the density functional theory (DFT) and generalized gradient approximation (GGA) By means of the comparison between the ground and relative state, thirteen exchange integrals have been calculated as a function of the effective value Ueff Furthermore, based on the Heisenberg model, the molecular field approximation (MFA) and random phase approximation (RPA), which provide an upper and lower bound of the Curie temperature Tc, have been adopted to deduce the Curie temperature Tc In addition, the Curie temperature Tc derived from the MFA are coincided well with the experimental data Finally, the strength of superexchange interaction mainly depends on 2b-4f1, 4f2-12k, 2a-4f1, and 4f1-12k interactions
TL;DR: In this article, a self-consistent quasiparticle random-phase approximation (QRPA) plus QPVC model with Skyrme interactions was proposed to describe the width and the line shape of giant resonances in open-shell nuclei, in which the effect of superfluidity should be taken into account in both the ground state and excited states.
Abstract: We propose a self-consistent quasiparticle random-phase approximation (QRPA) plus quasiparticle-vibration coupling (QPVC) model with Skyrme interactions to describe the width and the line shape of giant resonances in open-shell nuclei, in which the effect of superfluidity should be taken into account in both the ground state and the excited states. We apply the new model to the Gamow-Teller resonance in the superfluid nucleus $^{120}\mathrm{Sn}$, including both the isoscalar spin-triplet and the isovector spin-singlet pairing interactions. The strength distribution in $^{120}\mathrm{Sn}$ is well reproduced and the underlying microscopic mechanisms, related to QPVC and also to isoscalar pairing, are analyzed in detail.
TL;DR: It is shown that one can take advantage of density fitting or Cholesky decomposition techniques to improve the computational efficiency and exchange effects are instrumental for improvement over direct RPA results.
Abstract: Starting from the general expression for the ground state correlation energy in the adiabatic-connection fluctuation–dissipation theorem (ACFDT) framework, it is shown that the dielectric matrix formulation, which is usually applied to calculate the direct random phase approximation (dRPA) correlation energy, can be used for alternative RPA expressions including exchange effects. Within this famework, the ACFDT analog of the second order screened exchange (SOSEX) approximation leads to a logarithmic formula for the correlation energy similar to the direct RPA expression. Alternatively, the contribution of the exchange can be included in the kernel used to evaluate the response functions. In this case, the use of an approximate kernel is crucial to simplify the formalism and to obtain a correlation energy in logarithmic form. Technical details of the implementation of these methods are discussed, and it is shown that one can take advantage of density fitting or Cholesky decomposition techniques to improve ...
TL;DR: In this article, the selfconsistent quasiparticle random-phase approximation (QRPA) approach is applied to study the isobaric analog states (IAS) and Gamov-Teller resonances (GTR) by taking Sn isotopes as examples.
Abstract: The self-consistent quasiparticle random-phase approximation (QRPA) approach is formulated in the canonical single-nucleon basis of the relativistic Hatree-Fock-Bogoliubov (RHFB) theory. This approach is applied to study the isobaric analog states (IAS) and Gamov-Teller resonances (GTR) by taking Sn isotopes as examples. It is found that self-consistent treatment of the particle-particle residual interaction is essential to concentrate the IAS in a single peak for open-shell nuclei and the Coulomb exchange term is very important to predict the IAS energies. For the GTR, the isovector pairing can increase the calculated GTR energy, while the isoscalar pairing has an important influence on the low-lying tail of the GT transition. Furthermore, the QRPA approach is employed to predict nuclear $\beta$-decay half-lives. With an isospin-dependent pairing interaction in the isoscalar channel, the RHFB+QRPA approach almost completely reproduces the experimental $\beta$-decay half-lives for nuclei up to the Sn isotopes with half-lives smaller than one second. Large discrepancies are found for the Ni, Zn, and Ge isotopes with neutron number smaller than $50$, as well as the Sn isotopes with neutron number smaller than $82$. The potential reasons for these discrepancies are discussed in detail.
TL;DR: Using density-functional-theory-based molecular dynamics and thermodynamic integration, the free energy levels of six redox couples in water are calculated at the level of the random phase approximation and a double hybrid density functional.
Abstract: Understanding charge transfer at electrochemical interfaces requires consistent treatment of electronic energy levels in solids and in water at the same level of the electronic structure theory. Using density-functional-theory-based molecular dynamics and thermodynamic integration, the free energy levels of six redox couples in water are calculated at the level of the random phase approximation and a double hybrid density functional. The redox levels, together with the water band positions, are aligned against a computational standard hydrogen electrode, allowing for critical analysis of errors compared to the experiment. It is encouraging that both methods offer a good description of the electronic structures of the solutes and water, showing promise for a full treatment of electrochemical interfaces.
TL;DR: In this article, the authors present results of the time blocking approximation (TBA) for giant resonances in light-, medium-, and heavy-mass nuclei, which is an extension of the widely used random-phase approximation (RPA) adding complex configurations by coupling to phonon excitations.
Abstract: We present results of the time blocking approximation (TBA) for giant resonances in light-, medium-, and heavy-mass nuclei. The TBA is an extension of the widely used random-phase approximation (RPA) adding complex configurations by coupling to phonon excitations. A new method for handling the single-particle continuum is developed and applied in the present calculations. We investigate in detail the dependence of the numerical results on the size of the single-particle space and the number of phonons as well as on nuclear matter properties. Our approach is self-consistent, based on an energy-density functional of Skyrme type where we used seven different parameter sets. The numerical results are compared with experimental data.
TL;DR: In this paper, a short-ranged, exchange-like kernel that is one-electron self-correlation free and exact for twoelectron systems in the high-density limit was proposed.
Abstract: In order to remedy some of the shortcomings of the random phase approximation (RPA) within adiabatic connection fluctuation-dissipation (ACFD) density functional theory, we introduce a short-ranged, exchange-like kernel that is one-electron self-correlation free and exact for two-electron systems in the high-density limit. By tuning a free parameter in our model to recover an exact limit of the homogeneous electron gas correlation energy, we obtain a nonlocal, energy-optimized kernel that reduces the errors of RPA for both homogeneous and inhomogeneous solids. Using wave-vector symmetrization for the kernel, we also implement RPA renormalized perturbation theory for extended systems, and demonstrate its capability to describe the dominant correlation effects with a low-order expansion in both metallic and nonmetallic systems. The comparison of ACFD structural properties with experiment is also shown to be limited by the choice of norm-conserving pseudopotential.
TL;DR: In this paper, the authors studied the dynamical polarization and collective excitations in an anisotropic two-dimensional system undergoing a quantum phase transition with merging of two Dirac points.
Abstract: We have studied the dynamical polarization and collective excitations in an anisotropic two-dimensional system undergoing a quantum phase transition with merging of two Dirac points. Analytical results for the one-loop polarization function are obtained at the finite momentum, frequency, and chemical potential. The evolution of the plasmon dispersion across the phase transition is then analyzed within the random phase approximation. We derive analytically the long-wavelength dispersion of the undamped anisotropic collective mode and find that it evolves smoothly at the critical merging point. The effects of the van Hove singularity on the plasmon excitations are explored in detail.
TL;DR: In this paper, the authors investigate metal nano-particle size influence on plasmon resonance within theoretical and numerical approaches and compare results with available experimental data in order to improve resolution of optical identification of metallic nanoparticle sizes and shape.
Abstract: We investigate metal nano-particle size influence on plasmon resonance within theoretical and numerical approaches and compare results with available experimental data in order to improve resolution of optical identification of metallic nano-particle size and shape. The developed microscopic approach is the quantum random phase approximation model of plasmons in metallic nano-particles including plasmon damping by electron scattering and by radiative losses (i.e., by the so-called Lorentz friction). The numerical approach is by the finite element method solution of Maxwell equations for incident planar wave in spherical (also nano-rod, spheroid) geometry upon the system COMSOL and Mie treatment, supplemented with phenomenologically modeled dielectric function of metallic nano-particle. Comparison with experimental data for light extinction in Au and Ag nano-particle colloidal solutions with different particle sizes is presented. The crucial role of the Lorentz friction in the size effect of plasmon resonance in large (e.g., 20–60 nm for Au in vacuum) metallic nanoparticles is evidenced.
TL;DR: The RPA is identical with the variational Gaussian approximation derived within the field-theoretical framework, originally derived and used for charged particles and applies to the Gaussian core model and Coulomb charges.
Abstract: Using the adiabatic connection, we formulate the free energy in terms of the correlation function of a fictitious system, h_{λ}(r,r^{'}), in which interactions λu(r,r^{'}) are gradually switched on as λ changes from 0 to 1. The function h_{λ}(r,r^{'}) is then obtained from the inhomogeneous Ornstein-Zernike equation and the two equations constitute a general liquid-state framework for treating inhomogeneous fluids. The two equations do not yet constitute a closed set. In the present work we use the closure c_{λ}(r,r^{'})≈-λβu(r,r^{'}), known as the random-phase approximation (RPA). We demonstrate that the RPA is identical with the variational Gaussian approximation derived within the field-theoretical framework, originally derived and used for charged particles. We apply our generalized RPA approximation to the Gaussian core model and Coulomb charges.
TL;DR: In this article, the multiorbital Hubbard model for superconductors has been investigated and it has been shown that the leading instability is the spin-driven nematic phase.
Abstract: Nematic order resulting from the partial melting of density waves has been proposed as the mechanism to explain nematicity in iron-based superconductors. An outstanding question, however, is whether the microscopic electronic model for these systems\char22{}the multiorbital Hubbard model\char22{}displays such an ordered state as its leading instability. In contrast to usual electronic instabilities, such as magnetic and charge order, this fluctuation-driven phenomenon cannot be captured by the standard random phase approximation (RPA) method. Here, by including fluctuations beyond RPA in the multiorbital Hubbard model, we derive its nematic susceptibility and contrast it with its ferro-orbital order susceptibility, showing that its leading instability is the spin-driven nematic phase. Our results also demonstrate the primary role played by the ${d}_{xy}$ orbital in driving the nematic transition and reveal that high-energy magnetic fluctuations are essential to stabilize nematic order in the absence of magnetic order.
TL;DR: In this paper, the effect of including a Hubbard-U term in a single-particle Hamiltonian was investigated to better describe the on-site correlation of 3D electrons in the transition metal compounds ZnS, TiO2, and NiO.
Abstract: In non-self-consistent calculations of the total energy within the random-phase approximation (RPA) for electronic correlation, it is necessary to choose a single-particle Hamiltonian whose solutions are used to construct the electronic density and noninteracting response function. Here we investigate the effect of including a Hubbard-U term in this single-particle Hamiltonian, to better describe the on-site correlation of 3d electrons in the transition metal compounds ZnS, TiO2, and NiO. We find that the RPA lattice constants are essentially independent of U, despite large changes in the underlying electronic structure. We further demonstrate that the non-self-consistent RPA total energies of these materials have minima at nonzero U. Our RPA calculations find the rutile phase of TiO2 to be more stable than anatase independent of U, a result which is consistent with experiments and qualitatively different from that found from calculations employing U-corrected (semi)local functionals. However we also find that the +U term cannot be used to correct the RPA's poor description of the heat of formation of NiO.
TL;DR: In this article, the real and imaginary part of the dielectric function in addition to the energy loss spectra of Tantalum arsenide and Na3Bi have been calculated within random phase approximation (RPA); then, the electron−hole interaction is included by solving the Bethe-Salpeter equation for the electron-hole Green's function.
Abstract: We present ab initio a study on linear and nonlinear optical properties of topological semimetal Tantalum arsenide and Sodium bismuthate. The real and imaginary part of the dielectric function in addition to the energy loss spectra of TaAs and Na3Bi have been calculated within random phase approximation (RPA); then, the electron–hole interaction is included by solving the Bethe–Salpeter equation for the electron–hole Green’s function. In spite of being in the single category of topological materials, we have found obvious distinction between linear optical responses of TaAs and Na3Bi at a high energy region where, in contrast to Na3Bi, Tantalum arsenide has excitonic peaks at 9 eV and 9.5 eV. It is remarkable that the excitonic effects in the high energy range of the spectrum are stronger than in the lower one. The dielectric function is overall red shifted compared with that of RPA approximation. The resulting static dielectric constants for Na3Bi are smaller than corresponding ones in TaAs. At a low energy region, the absorption intensity of TaAs is more than Na3Bi. The calculated second-order nonlinear optical susceptibilities χ
(2)
(ω) show that Tantalum arsenide acts as a Weyl semimetal, and has high values of nonlinear responses in the low energy region which makes it promising candidate for the second harmonic generation in the terahertz frequency region. In the low energy regime, optical spectra are dominated by the 2ω intra-band contributions.
TL;DR: In this paper, the electronic structure and linear optical properties of oxygen-functionalized graphene are investigated by means of density functional theory (DFT), and the optical spectra are anisotropic along the two polarizations of the electric field of the incident light.
TL;DR: In this article, the authors exploit this feature to add the missing correlation to these methods via meta-GGA (generalised gradient approximation) density functionals (Tao-Perdew-Staroverov-Scuseria and strongly constrained and appropriately normed).
Abstract: Contrary to standard coupled cluster doubles (CCD) and Brueckner doubles (BD), singlet-paired analogues of CCD and BD (denoted here as CCD0 and BD0) do not break down when static correlation is present, but neglect substantial amounts of dynamic correlation. In fact, CCD0 and BD0 do not account for any contributions from multielectron excitations involving only same-spin electrons at all. We exploit this feature to add – without introducing double counting, self-interaction, or increase in cost – the missing correlation to these methods via meta-GGA (generalised gradient approximation) density functionals (Tao–Perdew–Staroverov–Scuseria and strongly constrained and appropriately normed). Furthermore, we improve upon these CCD0+DFT blends by invoking range separation: the short- and long-range correlations absent in CCD0/BD0 are evaluated with density functional theory and the direct random phase approximation, respectively. This corrects the description of long-range van der Waals forces. Comprehe...
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TL;DR: In this paper, a simple analytical theory of flexible polymer chain dissolved in a good solvent, carrying permanent freely oriented dipoles on the monomers is presented, where the dipole correlations of monomers can be taken into account as pairwise ones only when the polymer chain is in a coil conformation.
Abstract: We present a simple analytical theory of flexible polymer chain dissolved in a good solvent, carrying permanent freely oriented dipoles on the monomers. We take into account the dipole correlations within the random phase approximation (RPA), as well as a dielectric heterogeneity in the internal polymer volume relative to the bulk solution. We demonstrate that the dipole correlations of monomers can be taken into account as pairwise ones only when the polymer chain is in a coil conformation. In this case the dipole correlations manifest themselves through the Keesom interactions of the permanent dipoles. On the other hand, the dielectric heterogeneity effect (dielectric mismatch effect) leads to effective interaction between the monomers of the polymeric coil. Both of these effects can be taken into account by the renormalizing the second virial coefficient of the volume interactions monomer-monomer. We establish that in the case when the solvent dielectric permittivity exceeds the dielectric permittivity of the polymeric material, the dielectric mismatch effect competes with the dipole attractive interactions, leading to polymer coil expansion. In the opposite case, both the dielectric mismatch effect and the dipole attractive interaction lead to the polymer coil collapse. We analyse the coil-globule transition caused by the dipole correlations of monomers within the many-body theory. We demonstrate that accounting for the dipole correlations higher than pairwise ones smooths this pure electrostatics driven coil-globule transition of the polymer chain.
TL;DR: In this paper, optical properties of various polymorphs of hafnia were predicted within the framework of density functional theory, using the Tran-Blaha modified Becke-Johnson potential (TB-mBJ) for exchange and local density approximation for correlation.
Abstract: We report on optical properties of various polymorphs of hafnia
predicted within the framework of density functional theory.
The full potential linearised augmented plane wave method was
employed together with the Tran-Blaha modified Becke-Johnson
potential (TB-mBJ) for exchange and local density approximation
for correlation. Unit cells of monoclinic, cubic and tetragonal
crystalline, and a simulated annealing-based model of amorphous
hafnia were fully relaxed with respect to internal positions
and lattice parameters. Electronic structures and band gaps for
monoclinic, cubic, tetragonal and amorphous hafnia were
calculated using three different TB-mBJ parametrisations and
the results were critically compared with the available
experimental and theoretical reports. Conceptual differences
between a straightforward comparison of experimental
measurements to a calculated band gap on the one hand and to a
whole electronic structure (density of electronic states) on
the other hand, were pointed out, suggesting the latter should
be used whenever possible. Finally, dielectric functions were
calculated at two levels, using the random phase approximation
without local field effects and with a more accurate
Bethe-Salpether equation (BSE) to account for excitonic
effects. We conclude that a satisfactory agreement with
experimental data for HfO2 was obtained only in the latter
case.
TL;DR: In this article, the electronic and optical properties of silicon carbide nanosheet (2D monolayer) have been studied using full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) within WIEN2k package.