[en] The band structures of CrN are calculated using the quasiparticle self-consistent GW approach in both the low-temperature antiferromagnetic (AFM) [110]₂ Pmna orthorhombic phase and in a hypothetical ferromagnetic phase representing the paramagnetic cubic state in a saturating magnetic field. A gap of about 1 eV is found in the AFM state, while the ferromagnetic band structure is found to be half metallic. Another hypothetical AFM-1 structure [001]₁ is further considered and gives a smaller band gap. The orbital nature of the bands is studied and reveals a gap between Cr-d states, indicating a strongly correlated behavior. Here, optical dielectric functions are calculated from the interband transitions and compared to experimental data.

[en] The free-exciton photoluminescence (PL) spectrum of4H-SiC exhibits a detailed fine structure due to the different phonons involved in the indirect gap transition. In this paper we present a first-principles calculation of these phonons, their symmetry labeling and their contribution to the photoluminescence spectrum. The calculation uses phonons and electron-phonon coupling matrix elements computed via density functional perturbation theory and energy bands and optical phonon matrix elements calculated in density functional theory. The results are in excellent agreement with the experiment for the phonon energies and the polarization dependence of the spectrum. The relative intensities are also in fair agreement if we allow for some phonons within a few meV to be interchanged. There is however a remarkable discrepancy that the experimental spectrum shows a distinct behavior for phonons with energy below ~55 meV and above that energy, which is absent in the theory. The experimental PL lines corresponding to phonon energies below 55 meV are about a factor 5–10 smaller in intensity. This is not found in our calculations. The calculations show a similar peak distribution as the experiment in this range but with intensities comparable to those above 55 meV. This indicates that another mechanism outside the scope of the electron-phonon mediated transitions is operative for photon energies of the PL lines closer to the indirect exciton gap than this 55 meV cutoff, which reduces the overall intensity of these lines. We propose that this may result from competition between the phonon-assisted PL and trapping of the electron in the available unoccupied hexagonal site N-donor shallow level at 53-meV binding energy. As part of this study, we also present the phonon dispersions and density of states in 4H-SiC and the electronic band structure including quasiparticle corrections.

[en] Because of its narrow split-off conduction band, doping of ${\mathrm{V}}_2{\mathrm{O}}_5$ leads to interesting strongly correlated electrons. We study the effects of doping on ${\mathrm{V}}_2{\mathrm{O}}_5$'s electronic and magnetic properties, either by adding electrons compensated by an artificial homogeneous background, or a virtual crystal approximation (VCA), by changing the atomic number $Z_{\mathrm{V}}$, so as to keep charge neutrality, or by explicitly introducing Na as a dopant. The former two are considered as a way to simulate injected charge by gating, the latter occurs in the vanadium bronze ${\mathrm{NaV}}_2{\mathrm{O}}_5$. We also simulate ${\mathrm{Na}}_{1-x}{\mathrm{V}}_2{\mathrm{O}}_5$ using a virtual crystal approximation by changing the atomic number $10\le Z_{\mathrm{Na}}\le 11$. The differences in the band structure, which result from how the electrons added to the band are compensated by positive charge in the three models, are compared. The electronic band structures are calculated using the quasiparticle self-consistent $QSGW$ method including a lattice polarization correction and the local spin density functional method with Hubbard-$U$ corrections (LSDA+$U$). For ${\mathrm{NaV}}_2{\mathrm{O}}_5$, the half-filling leads to a splitting of the up- and down-spin lowest $d_{xy}$ band. The spins are found to prefer an antiferromagnetic ordering along the chain direction. Other spin configurations are shown to have higher energy and the exchange interactions are extracted and compared with literature. The optical conductivities are calculated and compared with experiment. Similar results are found for simply doping the band compensated by a background or virtual crystal approximation. Yet, the position of the occupied bands depends on the method chosen for compensating the charge. The most realistic way to simulate gating in which the compensating charge is kept away from the ${\mathrm{V}}_2{\mathrm{O}}_5$ layer is the VCA with varying $Z_{\mathrm{Na}}$. The splitting between the up- and down-spin bands depends on the filling. Furthermore, we find that below a certain concentration of about 0.88 electrons per V, the FM arrangement becomes preferable over the antiferromagnetic one. The magnetic moments then gradually decrease as we lower the filling of the split-off band.

[en] Electronic structure calculations were carried out for the europium chalcogenides (EuO, EuS, EuSe, EuTe) using the 'LSDA+U' approach, in which orbital-dependent Coulomb and exchange effects are added to the local spin density approximation (LSDA) for the f electrons. The usual LSDA gap underestimate is also corrected by adding U_d terms, which shift up the empty d states. While both GdN and EuO are found to be magnetic semiconductors, there are significant differences in the electronic structure, in particular in the location of the filled f bands. The origin of these differences and their effect on various other aspects of the band structure are discussed. The exchange coupling between magnetic moments is studied by mapping the energy differences of different magnetic configurations to a Heisenberg Hamiltonian with first- and second-nearest-neighbour interactions. The exchange interactions are in fair agreement with experimental values and explain the trends in magnetic properties

[en] The trends in electronic band structure are investigated in the cubic ABX₃ halide perovskites fo rA=Cs;B=Pb,Sn, Ge, Si; andX=I, Br, Cl. The gaps are found to decrease from Pb to Sn and from Ge to Si, but increase from Sn to Ge. The trend is explained in terms of the atomic levels of the group-IV element and the atomic sizes which changes the amount of hybridization with X-p and hence the valence bandwidth. Along the same series spin-orbit coupling also decreases and this tends to increase the gap because of the smaller splitting of the conduction band minimum. Both effects compensate each other to a certain degree. The trend with halogens is to reduce the gap from Cl to I, i.e., with decreasing electronegativity. The part of the tolerance factor in avoiding octahedron rotations and octahedron edge sharing is discussed. The Ge containing compounds have tolerance factor t>1 and hence do not show the series of octahedral rotation distortions and the existence of edge-sharing octahedral phases known for Pb and Sn-based compounds, but rather a rhombohedral distortion. CsGeI₃ is found to have a suitable gap for photovoltaics both in its cubic (high-temperature) and rhombohedral (low-temperature) phases. The structural stability of the materials in the different phases is also discussed. We find the rhombohedral phase to have lower total energy and slightly larger gaps but to present a less significant distortion of the bandstructure than the edge-sharing octahedral phases, such as the yellow phase in CsSnI3. The corresponding silicon based compounds have not yet been synthesized and therefore our estimates are less certain but indicate a small gap for cubic CsSiI₃ and CsSiBr₃of about 0.2±0.2 eV and 0.8±0.6 eV for CsSiCl₃. The intrinsic stability of the Si compounds is discussed.

[en] Density functional perturbation theory is used here to calculate the phonons at the zone center for monoclinic CsSnCl₃. Furthermore, we report the calculated normal mode frequencies classified according to irreducible representations for both infrared-active and Raman-active modes. We also report the dielectric constants and Born-effective charges and investigate the anisotropy of the nonanalyticities near the zone center. The first-principles calculated Raman-active phonon frequencies and simulated Raman spectra are compared with experimental results from literature. We find that aside from the first-order allowed Raman modes of A_g and B_g symmetries, forbidden LO Raman modes appear prominently in the measured spectra. Several of these modes are close to or coincide with allowed modes and alter their intensity, while others appear as separate otherwise unexplained peaks. With this interpretation, good agreement is obtained with experimental mode frequencies to within a few cm^-1.

[en] The first-principles linear response approach is used within the local-density approximation to calculate the full phonon band structures and phonon density of states (DOS) of CsSnX₃ (X=Cl, Br, or I) in different phases. The relations between soft phonon modes and phase transitions are investigated. We find soft phonon modes only in the cubic and tetragonal phases, not in the orthorhombic and monoclinic phases. A dispersionless soft phonon branch spreads from the k point M to R in the Brillouin zone of the cubic phase. The lower symmetry tetragonal phase results from the condensation of the soft phonon mode at the k point M. Additionally, the condensation of the soft phonon mode at the k point Z in the Brillouin zone of tetragonal phase results in the orthorhombic γ phase. To facilitate comparison with experimental data, we calculate infrared spectra for the cubic phase. At this point only a limited comparison with experimental data is possible. We find that the calculated modes agree with the available experimental data when we assign the second and third calculated modes to the experimental first and second modes. The lowest calculated mode is at a frequency where the phonon DOS has a maximum value. So the strong phonon-phonon interaction results in short phonon lifetime or strong broadening, which could explain why this mode has not been observed. Our first-principles calculated IR spectra show that the third observed mode in IR absorption is actually the highest longitudinal optical (LO) rather than transverse optical mode. We show, furthermore, that a strong LO-plasmon coupling may be expected in these materials and can explain observed Raman data for CsSnI₃.

[en] Here, the zone-center phonons are calculated for yellow phase CsSnI₃ using density functional perturbation theory in a plane-wave pseudopotential method. The infrared absorption and reflection spectra are simulated and show that the absorption has a strong contribution from the LO as well as TO modes. The polarization-dependent Raman spectra for various configurations are simulated. The results for the Raman and the infrared reflectionspectra are found to be in good agreement with the available experimental data when averaging over directions.The Born effective charges and high- and low-frequency dielectric tensors are calculated. The ratio of the static to the high-frequency dielectric constants is high in the y direction.

[en] The electronic band structures of BeSiN₂ and BeGeN₂ compounds are calculated using the quasiparticle self-consistent GW method. The lattice parameters are calculated for the wurtzite based crystal structure commonly found in other II–IV-N₂ compounds with the Pbn2₁ space group. They are determined both in the local density approximation and generalized gradient approximation, which provide lower and upper limits. At the GGA lattice constants, which gives lattice constants closer to the experimental ones, BeSiN₂ is found to have an indirect band gap of 6.88 eV and its direct gap at is 7.77 eV, while in BeGeN₂ the gap is direct at and equals 5.03 eV. To explain the indirect gap in BeSiN₂ comparisons are made with the parent III-N compound w-BN band structure. The effective mass parameters are also evaluated and found to decrease from BeSiN₂ to BeGeN₂. (paper)

[en] Using quasiparticle self-consistent GW calculations, we demonstrate that DyN has an unusual nearly zero indirect gap semimetallic band structure in which the states near the valence band maximum are fully minority spin polarized at Γ while the states near the conduction band minimum (at X) have fully majority spin character. This arises due to a strong hybridization of one of the minority spin f states of dysprosium with the N-2p bands. The reason why only one of the f bands hybridizes is explained using symmetry arguments. Also, we show that in HoN, this hybridization is already strongly reduced because of the deeper Ho-4f minority spin states.