The approximate calculation of electronic band structures. V. Wavefunctions

Generalized Bloch wave functions of bulk structures, which are composed of not only propagating waves but also decaying and growing evanescent waves, are known to be essential for defining the open boundary conditions in the calculations of the electronic surface states and scattering wave functions of surface and junction structures. Electronic complex band structures being derived from the generalized Bloch wave functions are also essential for studying bound states of the surface and junction structures, which do not appear in conventional band structures. We present a novel calculation method to obtain the generalized Bloch wave functions of periodic bulk structures by solving a generalized eigenvalue problem, whose dimension is drastically reduced in comparison with the conventional generalized eigenvalue problem derived by Fujimoto and Hirose [Phys. Rev. B 67, 195315 (2003)]. The generalized eigenvalue problem derived in this work is even mathematically equivalent to the conventional one, and, thus, we reduce computational cost for solving the eigenvalue problem considerably without any approximation and losing the strictness of the formulations. To exhibit the performance of the present method, we demonstrate practical calculations of electronic complex band structures and electron transport properties of Al and Cu nanoscale systems. Moreover, employing atom-structured electrodes and jellium-approximated ones for both of the Al and Si monatomic chains, we investigate how much the electron transport properties are unphysically affected by the jellium parts.

Tight-binding band structure calculations for the room temperature structure of BEDT-TTF4[ Hg2Cl6] .PhCl show the existence of closed electron and hole Fermi surfaces, in agreement with the 2D metallic conductivity of this salt. It is shown that these closed Fermi surfaces result from the hybridization of two hidden 1D Fermi surfaces. However, our study also shows that a transition associated with either a usual or a hidden nesting type mechanism is unlikely. This explains why this salt retains its metallic properties without any resistivity anomaly down to 1.3 K. Our study suggests that BEDT-TTF4[ Hg2Cl6] .PhCl is a somewhat anisotropic 2D semimetal and should exhibit Shubnikov-de Haas oscillations corresponding to a cross-sectional area of approximately 13 % of the first Brillouin zone.

The combination of cryo-electron spectroscopic imaging (ESI)/electron energy-loss spectroscopy (EELS) and cryo-energy-dispersive X-ray (EDX) analysis in the scanning transmission (STEM) mode was applied for the characterization of Ag(Br, I) nano- and microcrystals. Combined qualitative silver and halide distributions were obtained by ESI (a three-window method) and by EDX/STEM including a high-magnification mapping of nanocrystals. A low-loss fine structure in EEL spectra between 4 and 26eV was attributed to excitons and plasmons possibly super-imposed with interband transitions and many-electron effects. Contrast tuning under energy-filtering was used to image electron excitations in the silver halide particles. The real and imaginary parts, e1 and e2, of dielectric permittivity were determined by means of a Kramers-Kronig analysis. A proposed assignment of exciton peaks based on calculations of electronic band structure of silver halides is discussed.

Relativistic electronic structure and Fermi surface of URu 2Si 2 in antiferromagnetic state

High pressure phase transition and band structures of different phases in CeO 2

A feasible strategy to realize the Majorana fermions is searching for a simple compound with both bulk superconductivity and Dirac surface states. In this paper, we performed calculations of electronic band structure, the Fermi surface and surface states, as well as measured the resistivity, magnetization, specific heat for TlSb compound with a CsCl-type structure. The band structure calculations show that TlSb is a Dirac semimetal when spin-orbit coupling is taken into account. Meanwhile, we first found that TlSb is a type-II superconductor with $T_c$ = 4.38 K, $H_{c1}$(0) = 148 Oe, $H_{c2}$(0) = 1.12 T and $\kappa_{GL}$ = 10.6, and confirmed it to be a moderately coupled s-wave superconductor. Although we can not determine which bands near the Fermi level $E_F$ to be responsible for superconductivity, its coexistence with the topological surface states implies that TlSb compound may be a simple material platform to realize the fault-tolerant quantum computations.

Electronic structure of the heavy fermion superconductor Ce2PdIn8: Experiment and calculations

We present the electronic band structure of bulk gallium arsenide (GaAs) using first principle approach. A series of calculations has been performed by applying norm-conserving pseudopotentials and ultrasoft non-norm-conserving pseudopotentials within the density functional theory. These calculations yield too small band gap as compare to experiment. Thus, we use semiemperical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U), which is quite effective in order to describe the band gap of GaAs.

Systematic X-ray photoelectron spectroscopic (XPES) measurements were performed for bismuth compounds: α-Bi2O3, BaBiO3, and NaBiO3. Data on the valence band structure and positions of core electron levels in atoms were gained. Electronic band structure calculations were performed by the fullpotential linear-muffin-tin-orbital (FP-LMTO) method. A comprehensive analysis of the electronic structures was carried out on the basis of comparison of experimental and theoretical data. Our results imply the occurrence of noticeable electronic correlations in the compounds under study, which should be taken into account in interpreting the unusual properties discovered earlier in α-Bi2O3 and in some other oxygen-containing bismuth compounds.

Ti2AlCxNy solid solutions are synthesized using hot isostatic pressing. The X site solid solution effects are investigated, focusing on the microstructure, electronic structure and transport properties of the Ti2AlCxNy MAX phases. Combining X-ray diffraction with scanning electron microscopy and wavelength dispersive X-ray spectroscopy measurements, it is shown that solid solutions can be synthesized with well-controlled chemical compositions in the entire composition range (x=0–1). The measured a and c lattice-parameter values are shown to be in good agreement with those calculated using the cluster-expansion formalism. Combining electron energy-loss spectroscopy (EELS) and band structure calculations, it is demonstrated by Arroyave et al. that solid solution effects induce weak perturbations on the electronic structure. The solid solution effect thus results mainly in a rigid shift of the Fermi energy in a flat part of the electron density of states of Ti2AlC (or Ti2AlN). From these observations, the variations in two key parameters of the conductivity are rationalized. The relative variations in the residual resistivity in the whole composition range are shown to be small, which agrees well with the weak disorder evidenced by EELS. However, the values of the slope of the variation in resistivity vs. temperature are shown to vary quite significantly, evidencing a deviation from Matthiessen’s rule in these systems. By comparing the experimental data with calculations based on the semi-classical Boltzmann equation, it is demonstrated that the observed variations are due to a combination of band structure effects and changes in the scattering mechanisms. Finally, vacancies are also shown to have a prominent effect on the transport properties of Ti2AlCxNy solid solutions. In particular, both residual resistivity and dρ(T)/dT values are shown to be significantly higher than those of the corresponding stoichiometric compounds.

Electronic band structure calculations have been carried out for Y3AlZ (Z=B, C, N, O) system to check the possibility of new superconductor. The band structure calculations were carried out by augmented plane wave and local orbital (APW+LO) method using WIEN2K software, based on density functional theory (DFT). Lattice parameters of Y3AlZ system were found with volume optimization. It is observed that lattice parameter decreases with decrease in atomic radii of Z interstitials. Maximum density of states at Fermi level is found for Y3AlB among Y3AlZ system.

Discussion of band gap behavior based on first principles calculations of electronic band structures for various short period nitride superlattices is presented. Binary superlattices, as InN/GaN and GaN/AlN as well as superlattices containing alloys, as InGaN/GaN, GaN/AlGaN, and GaN/InAlN are considered. Taking into account different crystallographic directions of growth (polar, semipolar and nonpolar) and different strain conditions (free-standing and pseudomorphic) all the factors influencing the band gap engineering are analyzed. Dependence on internal strain and lattice geometry is considered, but the main attention is devoted to the influence of the internal electric field and the hybridization of well and barrier wave functions. The contributions of these two important factors to band gap behavior are illustrated and estimated quantitatively. It appears that there are two interesting ranges of layer thicknesses; in one (few atomic monolayers in barriers and wells) the influence of the wave function hybridization is dominant, whereas in the other (layers thicker than roughly five to six monolayers) dependence of electric field on the band gaps is more important. The band gap behavior in superlattices is compared with the band gap dependence on composition in the corresponding ternary and quaternary alloys. It is shown that for superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary alloys. The calculated values of the band gaps are compared with the photoluminescence emission energies, when the corresponding data are available. Finally, similarities and differences between nitride and oxide polar superlattices are pointed out by comparison of wurtzite GaN/AlN and ZnO/MgO.

Evolution of ferromagnetism and electrical resistivity in Sb-doped Cr4PtGa17

Based upon the density functional theory (DFT) in this paper, the first-principles approach is used to study the electronic band structure of Nb-doped wurtzite ZnO with different concentration. Three doped structures were considered: A Nb atom replaced by a Zn atom and two Nb atoms replaced by two Zn atoms in different positions. For the pure ZnO, the Fermi level is in the valence band maximum, but in the Nb-Doped ZnO supercell, Fermi level shifts to the conduction band and exhibits similar metallic properties.

CALCULATIONS OF THE SUPERCONDUCTING AND TRANSPORT PROPERTIES OF A15 STRUCTURE COMPOUNDS