Simple approximate physical orbitals for GW quasiparticle calculations

Generating unoccupied orbitals within density functional theory (DFT) for use in GW calculations of quasiparticle energies becomes prohibitive for large systems. We show that, without any loss of accuracy, the unoccupied orbitals may be replaced by a set of simple approximate physical orbitals made from appropriately prepared plane waves and localized basis DFT orbitals that represent the continuum and resonant states of the system, respectively. This approach allows for accurate quasiparticle calculations using only a very small number of unoccupied DFT orbitals, resulting in an order of magnitude gain in speed.     

Electronic structure and the local electroneutrality level of SiC polytypes from quasiparticle calculations within the GW approximation

The most important interband transitions and the local charge neutrality level (CNL) in silicon carbide polytypes 3C-SiC and nH-SiC (n = 2?C8) are calculated using the GW approximation for the self energy of quasiparticles. The calculated values of band gap E _g for various polytypes fall in the range 2.38 eV (3C-SiC)-3.33 eV (2H-SiC) and are very close to the experimental data (2.42?C3.33 eV). The quasiparticle corrections to E _g determined by DFT-LDA calculations (about 1.1 eV) are almost independent of the crystal structure of a polytype. The positions of CNL in various polytypes are found to be almost the same, and the change in CNL correlates weakly with the change in E _g, which increases with the hexagonality of SiC. The calculated value of CNL varies from 1.74 eV in polytype 3C-SiC to 1.81 eV in 4H-SiC.     

Local vertex corrections from exchange-correlation kernels with a discontinuity

The fundamental gap of an interacting many-electron system is given by the sum of the single-particle Kohn-Sham gap and the derivative discontinuity. The latter can be generated by advanced approximations to the exchange-correlation (XC) energy and is the key quantity to capture strong correlation with density functional theory (DFT). In this work we derive an expression for the derivative discontinuity in terms of the XC kernel of time-dependent density functional theory and demonstrate the crucial role of a discontinuity in the XC kernel itself. By relating approximate XC kernels to approximate local vertex corrections we then generate beyond-GW self-energies that include a discontinuity in the local vertex function. The quantitative importance of this result is illustrated with a numerical study of the local exchange vertex on model systems.     

Antiproton-nucleus scattering with self-consistent model for medium corrections

The antiproton-nucleon t-matrix with propagation in the nuclear medium is calculated selfconsistently and applied to the construction of optical potentials for the elastic scattering of antiprotons from nuclei. We find that this treatment gives results that are sensitive to medium corrections even though the strong absorption acts to mask these corrections partially. The agreement with scattering experiments at 46.8 MeV on ¹²C is very good. We compare potentials containing medium corrections to those based on free $\text{p}\text{N}$ amplitudes for ¹²C and ⁴⁰Ca. The local approximation to the optical potential is found attractive at low energies, becoming shallower with increasing bombardment energy in the range considered here (up to about E_L = 120 MeV).     

GW quasiparticle bandgaps of anatase TiO2 starting from DFT + U

We investigate the quasiparticle band structure of anatase TiO(2), a wide gap semiconductor widely employed in photovoltaics and photocatalysis. We obtain GW quasiparticle energies starting from density-functional theory (DFT) calculations including Hubbard U corrections. Using a simple iterative procedure we determine the value of the Hubbard parameter yielding a vanishing quasiparticle correction to the fundamental bandgap of anatase TiO(2). The bandgap (3.3 eV) calculated using this optimal Hubbard parameter is smaller than the value obtained by applying many-body perturbation theory to standard DFT eigenstates and eigenvalues (3.7 eV). We extend our analysis to the rutile polymorph of TiO(2) and reach similar conclusions. Our work highlights the role of the starting non-interacting Hamiltonian in the calculation of GW quasiparticle energies in TiO(2) and suggests an optimal Hubbard parameter for future calculations.     

Low-temperature thermal conductivity in a d-wave superconductor with coexisting charge order: Effect of self-consistent disorder and vertex corrections

Given the experimental evidence of charge order in the underdoped cuprate superconductors, we consider the effect of coexisting charge order on low-temperature thermal transport in a d  -wave superconductor. Using a phenomenological Hamiltonian that describes a two-dimensional system in the presence of a Q=(π,0)$\mathbf{Q}=(\pi \text{,}0)$ charge density wave and d-wave superconducting order, and including the effects of weak impurity scattering, we compute the self-energy of the quasiparticles within the self-consistent Born approximation, and calculate the zero-temperature thermal conductivity using linear response formalism. We find that vertex corrections within the ladder approximation do not significantly modify the bare-bubble result that was previously calculated. However, self-consistent treatment of the disorder does modify the charge-order-dependence of the thermal conductivity tensor, in that the magnitude of charge order required for the system to become effectively gapped is renormalized, generally to a smaller value.     

Three-nucleon calculations for local potentials with the quasiparticle method

The three-nucleon system for energies below the breakup threshold is investigated with the help of the quasiparticle method. Two types of local potentials are used, namely purely attractive Yukawa potentials and the soft-core potentials of Malfliet and Tjon. The results obtained are compared with those of other calculations employing different methods.Presented at the symposium Theory of lightest nuclei, Liblice, Czechoslovakia, May 1974.     

Systematics of collective correlation energies from self-consistent mean-field calculations

We study nucleon-nucleon scattering on the lattice at next-to-leading order in chiral effective field theory. We determine phase shifts and mixing angles from the properties of two-nucleon standing waves induced by a hard spherical wall in the center-of-mass frame. At fixed lattice spacing we test model independence of the low-energy effective theory by computing next-to-leading-order corrections for two different leading-order lattice actions. The first leading-order action includes instantaneous one-pion exchange and same-site contact interactions. The second leading-order action includes instantaneous one-pion exchange and Gaussian-smeared interactions. We find that in each case the results at next-to-leading order are accurate up to corrections expected at higher order.     

Strong vertex corrections from weak disorder in 2D d-wave superconductors

We show that weak static random potentials have pronounced effects on the quasiparticle states of a 2Dd-wave superconductor close to a node. We prove that the vertex correction coming from the simplest crossed diagram is important even for a nonmagnetic potential. The leading frequency and momentum dependent logarithmic singularities in the self-energy are calculated exactly to second order in perturbation theory. The self-energy corrections lead to a modified low energy density of states which depends strongly on the type of random potential and which can be measured in experiments. There is an exceptional case for a potential with extremely local scatterers and opposite nodes separated by (, ) where an exact cancelation takes place eliminating the leading frequency dependent singularity in the simplest crossed diagram. A comparison of the perturbative results with a self-consistent CPA (coherent potential approximation) for the nonmagnetic disorder reveals qualitative differences in the self-energy at the smallest energies which are due to the neglectance of vertex corrections in CPA.     

Accurate interband-energy measurements from Ellipsometric spectra

The optical constants of AlGaInP, AlGaInP (Si-doping) and AlGaInP (Mg-doping) grown by MOCVD were measured in the region of visible light at room temperature using null spectroscopic ellipsometry, then the dielectric function spectra were obtained. By numerical differentiation, the third-derivative spectra of the dielectric function for three samples were evaluated. Based on the third-derivative theory, it was proved that the third-derivative spectra were proportional to the electroreflectance (ER) spectra under the low-field modulation condition. Thus, the third-derivative spectra given by the ellipsometric spectra were directly related to the band structure of material. Analyzing the third-derivative spectra of the imaginary part of the dielectric function with the three-point method used in ER spectra, the E_g and E_g+Δ₀ values of three samples were gained accurately. The experimental results were in good agreement with those published.     

Spectroscopic properties of 237,239Pu fission isomers from self-consistent calculations

We have studied ^237,239Pu isomeric states: energy levels, M1 and E2 spectroscopic moments and reduced transition probabilities have been calculated within the rotor+quasi-particle model from self-consistently determined single-particle states. The electromagnetic properties of these states have been especially investigated. Without any ad hoc parameter adjustment, a very good reproduction of most of the known spectroscopic data is yieldedwhich assesses the predictive power of the whole approach.     

A self-consistent method for the determination of neutral density from X-ray impurity spectra

A new method for the determination of neutral densities and the electron lifetime from X-ray line spectra through charge exchange processes is proposed. The non-equilibrium population of neutrals and the charge exchange from their excited states in plasma regimes of high density and temperature have been calculated in a self-consistent manner through the introduction of an “effective diffusion rate”.     

Accurate core binding energies of ions from Dirac-Fock calculations combined with experimental atomic binding energies

Core binding energies are reported for the singly-charged alkali ions obtained by removing the outer s electron and for the singly-charged halide ions obtained by adding a P_{$\text{3}\text{2}$} electron. The levels studied are Li 1s, Na 1s, K 2p, Rb 3p, Cs 3d, F 1s, Cl 2p, Br 3p and I 3d. The binding energies were obtained by combining DiracF?ock ΔSCF binding-energy shifts between the ions and the corresponding isoelectronic rare gases with experimental rare-gas binding energies, and also by combining the calculated atomīon shifts with experimental atomic binding energies where available. It is shown that the former approach corrects accurately for the correlation energy, which is not included in single-configuration calculations.     

Self-consistent calculations within the quasiparticle time blocking approximation with exact account taken of the single-particle continuum

A self-consistent version of the model based on the method of Green’s functions which takes into account conventional phonons of the random phase approximation, complex configurations like 2 quasiparticles?phonon ones, and exact single-particle continuum is used to describe many discrete natural-parity states and giant resonances in the ¹²³Sn and ²⁰⁸Pb nuclei. The quasiparticle-phonon interaction is shown to be important not only for low-lying and high-lying collective states but also for low-lying noncollective states.