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The electronic and magnetic properties of ZnS crystal with impurities of transition 3d elements Cr and Fe, which replace Zn atoms, are calculated. The calculations are performed in 2×2×2 sphalerite supercells containing 64 atoms. The same supercells, which contain both Cr and Fe impurities, are also considered. It is established that energies of 3d electrons of the Cr atom with spin up in the Zn31Cr1S32 supercell are localized in narrow energy intervals around the Fermi level and at the top of the valence band. For spindown states, this material exhibits the properties of a semiconductor with a direct band gap of 2.11 eV. The magnetic moment of the supercell equals to 4 μB. The results of the calculation for the Zn31Fe1S32 supercell show that this material is a direct-band semiconductor with the width of the band gap at the Γ point of about 1.96 eV. 3d electrons with spin down are localized in narrow energy bands near the Fermi level and at the bottom of the conduction band. The pseudogap value for electrons with spin down is 0.53 eV. The magnetic moment of the supercell is also equal to 4 μB. It is found from the results of the calculation for the Zn30Cr1Fe1S32 supercell that 3d electrons of the Cr atom with spin up are localized in the vicinity of the top of the valence band and the Fermi level, which is inside the valence band. The optical gap at the Γ point is equal to 0.60 eV and the indirect fundamental gap (Γ – R) is 0.59 eV. The pseudogap value for electrons with spin down is 0.60 eV. The magnetic moment of the supercell equals 8.04 μB. The calculations are performed using the ABINIT program. The hybrid exchange-correlation functional PBE0 is used to eliminate the self-interaction error of Cr and Fe 3d electrons. [ABSTRACT FROM AUTHOR]

The electronic energy band spectra of CdX crystals (X = O, S, Se, Te) in the wurtzite phase have been calculated. We have revealed that Cd 4d electrons are localized in the narrow energy intervals of the valence band. The results obtained here show that the exchange-correlation functionals developed within the LDA and GGA approaches are not applicable to the strongly correlated 4d electron subsystem. Indeed, the band gap found for the CdO crystal in these approaches is Eg = 0, and its values for CdS, CdSe, and CdTe materials are 1.48, 0.67, and 0.84 eV, respectively. They all are much smaller than the measured Eg for CdX materials, which are 0.91, 2.5, 1.8 and 1.8 eV, respectively. We found that “GGA – Green’s function” calculation scheme, which is very effective for s-p electron systems, did not produce successful results for CdX crystals. That is why we have chosen a combination of “hybrid functional – Green’s function” approach. The Eg values found in CdX crystals without spin-orbit interaction are 0.90, 2.49, 1.92 and 2.10 eV, respectively. The spin-orbit interaction causes a negligible narrowing dESO of the Eg parameter in CdO and CdS crystals. However, in CdSe and CdTe materials, the narrowing parameter dESO values are 0.12 and 0.27 eV, respectively. Corrected by dESO band gaps of CdSe and CdTe crystals have the values of 1.80 and 1.83 eV, respectively. The Eg values found here are well compared with the experiment. It is found that the energy bands of the semicore Cd 4d electrons obtained without spin-orbit interaction overlap in CdX crystals. Spin-orbit interaction leads to a widening of Cd 4d energy bands. Moreover, in CdSe and CdTe crystals, the spin-orbit interaction leads to a clear separation of 4d electron energy bands into two parts, the lower of which contains 8 and the upper 12 branches of the spectrum. [ABSTRACT FROM AUTHOR]

First, we have evaluated the spin-resolved electronic structure of crystals Si < B, V>, Si < B, Cr > and Si < B, Mn > with the exchange-correlation energy in a form of generalized gradient approximation (GGA-PBE). Next, the strong correlation effects of the V, Cr and Mn 3d electrons have been taken into account by means of the hybrid exchange-correlation functional PBE0. All the calculations have been done for supercells Si30B1V1, Si30B1Cr1, and Si30B1Mn1, modeling the substitutional alloys based on silicon crystal. In the pseudogap materials we are considering here, the Fermi level lies in the valence band. Without taking into account the strong correlations of the 3d electrons of the transition elements, we found the following properties of the materials under study. For the spin up the SiBV has an indirect pseudogap, the SiBCr is a direct gap material, and the SiBMn is an indirect pseudogap material. For the spin down these materials are a direct gap, direct pseudogap and indirect gap materials, respectively. Taking into account the strong correlations of the 3d electrons of the transition elements, we found the following properties of these materials. For the spin up the SiBV and SiBCr are the direct gap materials and SiBMn has an indirect pseudogap. For the spin down the SiBV is a direct pseudogap material, and the SiBCr is an indirect pseudogap compound, and SiBMn has a direct band gap. [ABSTRACT FROM AUTHOR]

The influence of strong d-electron correlations on the values of the parameters of the electron energy bands in the solid solution Cd1-xMnxTe (x = 0.25) was studied. At the first stage, the spin-resolved electronic energy bands were obtained in the GGA approach. At the second stage, strong correlations were taken into account for the Mn d-electrons by means of the hybrid exchange-correlation function PBE0, and at the third stage, correlations of the Mn and Cd d-electrons were included simultaneously. It was revealed that the material is a semiconductor for both spin channels. Found that for spin-up electrons, after taking into account the strong correlations of the Mn 3d-electrons, the top of the valence band was shifted by - 0.10 eV, and the bottom of the conduction band has increased by 0.02 eV, which resulted in an increase in the band gap, Eg, by 0.12 eV. Strong correlations of the Cd 4d-electrons add to an increase in Eg a value of 0.03 eV. The total increase in Eg for spin-up electrons is equal to 0.15 eV. For the spin-down electrons, the inclusion of strong 3d-electron correlations of Mn led to an increase in the top of the valence band by 0.12 eV, and the bottom of the conduction band has increased by 0.54 eV, resulting in an increase in the band gap by 0.42 eV. Strong correlations of the Cd 4d-electrons add to an increase in Eg a value of 0.02 eV. The total increase in Eg for spin-down states equals 0.44 eV. It was found that for spin-up electrons, the value of Eg of solid solution, without taking into account strong correlations of d-electrons, is 1.32 eV, and the inclusion of 3d-electron correlations of Mn leads to the Eg value of 1.44 eV. And the additional inclusion of 4d-electron correlations of Cd gives the Eg value of 1.47 eV. It was found that for spin-down electrons, the value of Eg of a solid solution, without taking into account the strong correlations of d-electrons, is 1.69 eV, and the inclusion of 3d-electron correlations of Mn leads to the Eg value of 2.11 eV. And the accounting of 4d-electron correlations of Cd has caused only a slight increase in the Eg to 2.13 eV. [ABSTRACT FROM AUTHOR]

The Green’s function method, implemented in the first order of the perturbation theory (GW), was applied for an accurate description of the electronic structure of cubic BaSnO3 and SrSnO3 perovskites. First, the band structure of these materials was calculated within the generalized gradient approximation (GGA). Then, in order to obtain the accurate band gaps, the quasiparticle corrections to the eigenenergies were evaluated. The calculated electronic structure by means of the GW approximation was compared to the structure obtained within the GGA. The application of quasiparticle corrections to the eigenenergies led to a significant widening of the band gaps and provided a much better agreement with the experimental data. The GW corrections to the band energies, found for both crystals at the points of the first Brillouin zone, are quite different. Consequently, the use of a scissor operator can lead to errors in a calculation of the optical constants. [ABSTRACT FROM AUTHOR]

In the present study, the structural, thermal, and electronic properties of some important orthosilicate dielectrics, such as the ZrSiO, ZrGeO, and HfSiO compounds, have been investigated theoretically with the use of first-principle calculations. We attribute the application of the modified Becke-Johnson exchange potential, which is basically an improvement over the local density approximation and the Perdew-Burke-Ernzerhof exchange-correlation functional, for a better description of the band gaps of the compounds. This resulted in a good agreement with our estimated values in comparison with the reported experimental data, specifically for the ZrSiO, and HfSiO compounds. Conversely, for the ZrGeO compound, the calculated electronic band structure shows a direct band gap at the Γ point with the value of 5.79 eV. Furthermore, our evaluated thermal properties that are calculated by using the quasi-harmonic Debye model indicated that the volume variation with temperature is higher in the ZrGeO compound as compared to both the ZrSiO and HfSiO compounds, which is ascribed to the difference between the electron shells of the Si and Ge atoms. Therefore, these results also indicate that while the entropy ( S) and enthalpy ( U) parameters increase monotonically, the free energy ( G), in contrast, decreases monotonically with increasing temperature, respectively. Moreover, the pressure and temperature dependencies of the Debye temperature Θ, thermal expansion coefficient, and heat capacities C were also predicted in our study. [ABSTRACT FROM AUTHOR]

The electronic energy band spectra, as well as partial and total density of electronic states of the crystal argyrodite Ag8SnSe6 have been evaluated within the projector augmented waves (PAW) approach by means of the ABINIT code. The one-electron energies have been evaluated using two functionals for exchange-correlation energy. The first one is the generalized gradient approximation (GGA) approach. The second one is the hybrid functional PBE0 composed of the semilocal GGA part and Hartree-Fock exact exchange non-local energy for strongly correlated 4d electrons of Ag atom. The second approach eliminates the Coulomb self-interaction of the Ag 4d electrons. This leads to a significant restructuring of the energy bands in the filled valence part and to an improved location of the Ag 4d-states on the energy scale, and the resulting value of the band gap is well compared with experiment. The effects of strong correlation on the electronic structure of the crystal argyrodite are considered here for the first time. [ABSTRACT FROM AUTHOR]