Ab initio calculations of electronic band structure of CdMnS semimagnetic semiconductors

This work is devoted to theoretical investigations of Cd1-xMnxS semimagnetic semiconductors (SMSC). The purpose of this work was to calculate the electronic band structure of ideal and defective Cd1- xMnxS SMSC in both antiferromagnetic (AFM) and ferromagnetic (FM) phases. Ab initio, calculations are performed in the Atomistix Toolkit (ATK) program within the Density Functional Theory (DFT) and Local Spin Density Approximation (LSDA) on Double Zeta Double Polarized (DZDP) basis. We have used Hubbard U potential UMn = 3.59 eV for 3d states for Mn atoms. Supercells of 8 and 64 atoms were constructed. After the construction of Cd1-xMnxS (x = 6.25 %; 25 %) supercells and atom relaxation and optimization of the crystal structure were carried out. Electronic band structure and density of states were calculated, the total energy has been defined in antiferromagnetic (AFM) and ferromagnetic (FM) phases. Our calculations show that the band gap increases with the increase in Mn ion concentration. It has been established that Cd or S vacancy in the crystal structure leads to the change of band gap, Fermi level shifts towards the valence or conduction band.

Electron–plasmon and electron–magnon scattering in ferromagnets from first principles by combining GW and GT self-energies

This work combines two powerful self-energy techniques: the well-known GW method and a self-energy recently developed by us that describes renormalization effects caused by the scattering of electrons with magnons and Stoner excitations. This GT self-energy, which is fully k-dependent and contains infinitely many spin-flip ladder diagrams, was shown to have a profound impact on the electronic band structure of Fe, Co, and Ni. In the present work, we refine the method by combining GT with the GW self-energy. The resulting GWT spectral functions exhibit strong lifetime effects and emergent dispersion anomalies. They are in an overall better agreement with experimental spectra than those obtained with GW or GT alone, even showing partial improvements over local-spin-density approximation dynamical mean-field theory. The performed analysis provides a basis for applying the GWT technique to a wider class of magnetic materials.

Effect of Edge Fluorination on Electronic and Transport Properties of Aarmchair Boron Nitride Nanoribbons: A First-Principles Study

We have systematically investigated the effect of edge fluorination on structural stability, electronic and transport properties of armchair boron nitride nanoribbons (ABNNRs), using first-principles calculations within the frame work of local spin-density approximation (LSDA). ABNNRs with F-passivation on only edge B atoms, regardless of their width, are found to be half-metallic and energetically most stable. For these ribbons, completely spin polarized charge transport is predicted across the Fermi level as a result of giant spin splitting. Transmission spectrum analysis also confirms the separation of spin-up and spin-down electronic channels. It is revealed that F-passivation of only edge N atoms transforms nonmagnetic bare ribbons into magnetic semiconductors whereas fluorination of both the edges does not affect the electronic and magnetic state of bare ribbons significantly. Predicted Half-metallicity projects these ABNNRs as potential candidates for inorganic spintronic devices.

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.

Benchmarking of Density Functionals for Z-Azoarene Half-Lives via Automated Transition State Search

Molecular photoswitches use light to interconvert from a thermodynamically stable isomer into a meta-stable isomer. Chemists and materials scientists have applied photoswitches in photopharmacology, catalysis, and molecular solar thermal (MOST) materials. Visible-light-absorbing photoswitches are attractive because the relatively low-energy light minimizes undesired photochemical reactions and enables biological applications. Designing ideal photoswitches requires long-lived metastable states; predicting their half-lives with theory is difficult because it requires locating transition structures. We now report the EZ-TS code, which automates the prediction of rate constants for the thermal Z → E isomerization. We leverage EZ-TS to automate the location of the favored transition structure and to comprehensively benchmark the performance of 140 model chemistries against the experimental rate constants of 11 azoarenes. We used 28 density functionals [local spin density approximation, generalized gradient approximation, meta-GGA, hybrid GGA, hybrid meta-GGA], and five basis sets [6-31G(d), 6-31+G(d,p), 6-311+G(d,p), cc-pvdz, and aug-cc-pvdz]. The hybrid GGA functionals performed the best of all tested functional classes. We demonstrate that the mean absolute errors of 14 model chemistries approach chemical accuracy, and mPWPW91/6-31+G(d,p) achieves chemical accuracy and should be used with EZ-TS.<br>

Benchmarking of Density Functionals for Z-Azoarene Half-Lives via Automated Transition State Search

Molecular photoswitches use light to interconvert from a thermodynamically stable isomer into a meta-stable isomer. Chemists and materials scientists have applied photoswitches in photopharmacology, catalysis, and molecular solar thermal (MOST) materials. Visible-light-absorbing photoswitches are attractive because the relatively low-energy light minimizes undesired photochemical reactions and enables biological applications. Designing ideal photoswitches requires long-lived metastable states; predicting their half-lives with theory is difficult because it requires locating transition structures. We now report the EZ-TS code, which automates the prediction of rate constants for the thermal Z → E isomerization. We leverage EZ-TS to automate the location of the favored transition structure and to comprehensively benchmark the performance of 140 model chemistries against the experimental rate constants of 11 azoarenes. We used 28 density functionals [local spin density approximation, generalized gradient approximation, meta-GGA, hybrid GGA, hybrid meta-GGA], and five basis sets [6-31G(d), 6-31+G(d,p), 6-311+G(d,p), cc-pvdz, and aug-cc-pvdz]. The hybrid GGA functionals performed the best of all tested functional classes. We demonstrate that the mean absolute errors of 14 model chemistries approach chemical accuracy, and mPWPW91/6-31+G(d,p) achieves chemical accuracy and should be used with EZ-TS.<br>

Structural Stability and Magnetic Ordering in BiFeO3 Perovskite Oxide: A Comparative Study GGA+U vs L(S)DA+U

Ab initio calculations of BiFeO3 magnetic perovskite are carried. Accurate density functional theory calculations were performed considering a U-Hubbard correction (DFT+U) to account for on-site Coulomb interactions of the 3d-Fe states. We have applied the Full-potential linearized augmented plane waves (FP-LAPW) method. Exchange-correlation effects are treated using the Local Spin Density approximation (L(S)DA+U) vs generalized gradient approximations (GGA+U). Equilibrium lattices agree very well with other theoretical and experimental data. The magnetization energy differences between Spin Up and Spin Dn states are small. Spin effect and magnetic moment obtained from subsequent (L(S)DA+U) and (GGA+U) calculations are also discussed in different magnetic configurations: The Ferromagnetic cubic phase (Pm-3m), The A-type Antiferromagnetic (P4/mmc) and The G-type Antiferromagnetic (Fm-3m). The nature of magnetism arises mainly from the Fe-site exhibiting a G-type antiferromagnetic ordering. The electronic structure shows that BiFeO3 has a metallic band gap. This multiferroic exhibit strong hybridization of the 3d-Fe and 2p-O orbitals. Therefore, the Multiferroic BiFeO3 perovskite has driven significant research interest due to their promising technological potential. It’s a good candidate for potential applications in spintronic, and to aid the development of the next generation of data storage and multi-functional technological devices.

Effect of Electronic Correlations on the Electronic Structure, Magnetic and Optical Properties of the Ternary RCuGe Compounds with R = Tb, Dy, Ho, Er

In this study, the ab initio and experimental results for RCuGe ternary intermetallics were reported for R = Tb, Dy, Ho, Er. Our theoretical calculations of the electronic structure, employing local spin density approximation accounting for electron–electron correlations in the 4f shell of Tb, Dy, Ho, Er ions were carried in DFT+U method. The optical properties of the RCuGe ternary compounds were studied at a broad range of wavelengths. The spectral and electronic characteristics were obtained. The theoretical electron densities of states were taken to interpret the experimental energy dependencies of the experimental optical conductivity in the interband light–absorption region. From the band calculations, the 4f shell of the rare-earth ions was shown to provide the major contribution to the electronic structure, magnetic and optical properties of the RCuGe intermetallics. The accounting for electron–electron correlations in Tb, Dy, Ho, Er resulted in a good agreement between the calculated and experimental magnetic and optical characteristics.

Электронные и магнитные свойства сплавов DyFe-=SUB=-4-=/SUB=-Ge-=SUB=-2-=/SUB=- в области фазового перехода

The first-principles studies have been performed for the electronic and magnetic properties of DyFe_4Ge_2 alloys near the P 4_2/ mmm – Cmmm phase transition. The calculations are carried out in a local spin density approximation taking into account the Coulomb interaction within the limit of strong localization in a mean field approximation. The electronic and magnetic properties of the tetragonal structure are shown to be weakly changed in the dependence on the Coulomb and exchange interactions and also on the choice of the approximations. In the case of the orthorhombic structure, a change in the parameters of the Coulomb and exchange interactions leads to a change in the magnetic ordering: from the ferromagnetic to ferrimagnetic in the strong localization limit and from the ferromagnetic to paramagnetic in the mean field approximation.

DFT-focused estimation of mechanical, thermoelectric and thermodynamic properties of ACdF3 (A=K, Rb, Cs) fluroperovskites

Structural, electronic, mechanical, thermodynamic and thermoelectric properties of Alkaline fluroperovskites ACdF3 (A[Formula: see text]=[Formula: see text]K, Rb, Cs) have been explored using the FP-LAPW method from the perspective of density functional theory (DFT) as employed in the Wien2k package within the generalized gradient approximation (GGA) as well as the local spin density approximation (LSDA). The volume optimization calculations of these materials agree well with experimental as well as available theoretical values. From the energy dispersion curves as well as DOS plots, all the investigated alkali fluroperovskites are found to have an indirect bandgap in [Formula: see text]-M direction using both approximations. The second-order elastic constants as well as mechanical properties such as Young’s modulus, shear modulus, density, anisotropic factor, B/G[Formula: see text] ratio are also computed for these materials. In order to study the thermal and vibrational effects on the studied fluroperovskites, we have employed the quasi-harmonic Debye model that uses a set of energy versus volume calculations. The effects of temperature as well as pressure on the structural parameters are also studied using the non-equilibrium Gibbs function. The transport properties of these perovskites have been investigated using BoltzTrap Code. Comparatively, a good figure of merit of these compounds reveals their possible use in thermoelectric applications.