Experimental Momentum Densities Research Articles

Electronic structure, optical properties and Compton profiles of Bi 2S 3 and Bi 2Se 3

In this paper, we present the Compton profiles of Bi 2S 3 and Bi 2Se 3 using our 20 Ci 137Cs Compton spectrometer. To compare our experimental data, we have computed the Compton profiles, energy bands and density of states using linear combination of atomic orbitals with density functional theory (DFT) and Hartree-Fock (HF) scheme. It is seen that hybrid functional involving HF and DFT approximations gives a relatively better agreement with experimental momentum densities than other approximations of DFT. We have also reported the band structure, density of states, valence charge densities, dielectric functions and electron energy loss spectra using full potential linearized augmented plane wave scheme. On the basis of charge densities, Mulliken’s population data and equal-valence-electron-density profiles, Bi 2S 3 is found to be more ionic than Bi 2Se 3. The calculated dielectric functions for the parallel and perpendicular polarizations show a small anisotropic effect. The electron energy loss spectrum for Bi 2Se 3 is found to be in good agreement with the available experimental data.

On the role of Al doping in La0.7Ca0.3MnO3: A magnetic Compton scattering study

Spin and orbital magnetic moments in hole doped manganite La0.7Ca0.3Mn1−xAlxO3 (x = 0, 0.02, and 0.06) have been scrutinized using experimental spin momentum densities. An analysis of magnetic Compton profiles shows that the spin moment of Mn 3d has a major contribution towards the total spin moment and its contribution decreases as we increase the concentration of Al. The present experiment along with magnetization data predict orbital moment of −0.29 ± 0.03 μB/f.u. in the parent compound (x = 0), which almost disappears on substitution of Al. The results are explained on the basis of imbalance in Mn3+ and Mn4+ charge ratio in these compounds.

Compton scattering studies and electronic properties of cerium

Experimental electron momentum density of γ-Ce was measured at an intermediate resolution (0.38 a.u.) using 20 Ci 137Cs Compton spectrometer. The experiment was compared with the first ever theoretical Compton profiles computed using spin polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) method within atomic sphere approximation; and linear combination of atomic orbitals (LCAO) within generalised gradient approximation (GGA) and recently developed second order generalised gradient approximation (SOGGA). It was seen that the Compton profile derived from LCAO with GGA and SOGGA schemes gave a reasonable agreement with the experimental Compton line shape. Theoretical anisotropies in Compton profiles derived along [100], [110] and [111] directions were explained in terms of degenerate states in the energy bands.

Investigation of electronic properties of crystalline arsenic chalcogenides: Theory and experiment

The electronic structure of crystalline As 2S 3 and As 2Se 3 has been calculated in this paper. We present the energy bands, density of states (DOS) and the Compton profiles using the linear combination of atomic orbitals (LCAO) scheme based on the density functional theory (DFT). From the calculated total and partial density of states it is seen that the lone-pair p-states of sulphur/selenium contribute closest to the Fermi energy level. To interpret the theoretical data on the Compton line shape, we have measured the Compton profiles on a 100 mCi 241Am spectrometer. It is seen that the density functional theory within generalised gradient approximation gives a slightly better agreement with the experimental momentum densities. The nature of chemical bonding in arsenic chalcogenides is studied using Mulliken's population analysis and the experimentally measured equal-valence-electron-density profiles; As 2S 3 is found to be more ionic compared to As 2Se 3.

Compton scattering study and electronic properties of vanadium carbide: A validation of hybrid functional

In this paper, we have reported the isotropic Compton profile of VC measured using high energy (661.65 keV) γ-radiations from a 137Cs isotope. To compare the experimental momentum densities, we have also employed the linear combination of atomic orbitals (LCAO). In addition, energy bands, density of states and Fermi surface topology of VC have been computed using FP-LAPW and LCAO methods. It is seen that the LCAO with hybridization of density functional theory and Hartree–Fock (so called B3LYP) gives a better agreement with the present Compton profile experiment. This shows validation of an exact exchange part in hybrid density functional. On the basis of energy bands, we have discussed the microscopic origin for the anomalous behavior of hardness of VC. The relative nature of bonding in VC and NbC is also discussed in terms of valence charge densities and Mulliken′s population analysis. To establish the role of Compton profiles in computation of cohesive properties of refractory materials, we have also calculated for the first time the cohesive energy using the present experimental Compton profile and compared it with the existing data.

Electronic structure and Compton profiles of CdO and HgO

AbstractWe present the first ever measured experimental electron momentum densities of CdO and HgO at an intermediate resolution using 20 Ci 137Cs Compton spectrometer. The measurements are compared with the theoretical Compton profiles using density functional theory (DFT) and hybridization of DFT with the Hartree–Fock as embodied in linear combination of atomic orbitals (LCAO) scheme. In addition, we have also calculated the energy bands and density of states using the full potential linearized augmented plane wave method and the LCAO schemes. It is seen that the hybridization scheme, which overestimates the band gap, gives a better agreement with the experimental profiles. A comparative nature of bonding is also discussed in terms of equal‐valence‐electron‐density scaling of both the experimental profiles and valence charge densities.

Electronic structure, Compton profiles and cohesive properties of Cs-halides

We have reported energy bands, density of states, valence electron charge densities and Compton profiles of CsCl, CsBr and CsI using linear combination of atomic orbitals with Hartree–Fock and density functional theories. We have also computed these properties, except the momentum densities, using full potential linearized augmented plane wave method. The general features of the energy bands and the density of states in these halides are found to be almost similar. To interpret the theoretical data on Compton line shapes, we have also measured the Compton profiles using our 20 Ci 137Cs spectrometer. It is seen that the Hartree–Fock calculations give relatively a better agreement with the experimental momentum densities. On the basis of equal-valence-electron-density profiles, a comparison of relative nature of bonding is made which is in agreement with the valence charge densities and atomic charges by means of Mulliken analysis. Using our experimental and theoretical Compton profiles, we have also computed the cohesive energy of the halides.

Compton profiles and electronic structure of HgBr 2 and HgI 2

In this paper, we present the first-ever experimental Compton line shapes of HgBr 2 and HgI 2 using 137Cs Compton spectrometer. To compare our experimental momentum densities, we have computed the Compton profiles using Hartree–Fock and density functional theory within linear combination of atomic orbitals. We have also computed the energy bands and density of states using the linear combination of atomic orbitals and full potential linearized augmented plane wave method. On the basis of equal-valence-electron-density profiles, it is seen that HgI 2 is more covalent than HgBr 2 which is in agreement with the valence charge densities. The experimental isotropic profiles are found to be relatively in better agreement with the Hartree–Fock data. We have also discussed the photoluminescence and detection properties of both the halides.

Electronic structure of layer type tungsten metal dichalcogenides WX2 (X=S, Se) using Compton spectroscopy: Theory and experiment

Abstract In this paper, we report the first ever experimental Compton profile study of WS 2 and WSe 2 employing 20 Ci 137 Cs Compton spectrometer. To interpret our experimental data, the electronic properties of these compounds have been determined by linear combination of atomic orbitals, full potential linearised augmented plane-wave and spin polarised relativistic Korringa–Kohn–Rostoker (SPR-KKR) schemes. The band structure calculations show that both the WS 2 and WSe 2 are indirect-gap semiconductors. The SPR-KKR calculations are found to be relatively in poor agreement with the experimental electron momentum densities. The relative nature of bonding in both the dichalcogenides is explained in terms of equal-valence-electron-density profiles, Mulliken's population and valence band charge densities.

(e, 2e) Momentum spectroscopic study of the valence orbitals of hydrogen selenide

Electron momentum densities of the four valence molecular orbitals of H 2Se have been measured by (e, 2e) spectroscopy. Although the three outermost orbitals all show momentum densities that are qualitatively similar to the 4p orbital of a free Se atom, a detailed comparison of the orbital momentum densities using the autocorrelation function approach reveals differences consistent with the usual qualitative molecular orbital (MO) theoretical representations of these orbitals. For H 2Se, as for H 2S previously studied by others, the experimental (e,2e) momentum densities are accurately described by near-Hartree-Fock level orbital momentum density calculations. Results from direct calculations of the SCF-CI overlap densities and the SCF orbital momentum densities with a 3-21G * basis are found to be very similar. This suggests that relaxation and correlation effects are small for H 2Se. This is contrast to H 2O where such effects are substantial, particularly for the highest-occupied molecular orbital (HOMO).

Three-dimensional momentum density of magnetic electrons in ferromagnetic iron.

Experimental and theoretical three-dimensional momentum densities of magnetic electrons in ferromagnetic iron are reported for the first time and are found to be in good qualitative agreement. The experimental momentum density is reconstructed from fourteen one-dimensional magnetic Compton profiles, and the theoretical one is calculated by the full-potential linearized augmented-plane-wave method. The experimental indicates that the theory slightly underestimates the negative spin polarization of the s,p-like electrons in the first Brillouin zone and slightly overestimates umklapp processes.

Study on Momentum Density and Fermi Surface in Paramagnetic Cr and W by Positron Annihilation

AbstractThe three‐dimensional electron‐positron momentum densities in paramagnetic Cr and W are reconstructed from measurements of two‐dimensional angular correlation of positron annihilation radiations (2D‐ACAR) by the image reconstruction technique based on a direct Fourier transformation. The experimental momentum density for paramagnetic Cr and W shows the contribution of the Fermi surface sheets; the electron surface at Γ, the hole surface at H, the hole surface at N. The Fermi surface topology obtained from the present results agrees with the theory and other experiments.

Experimental electron momentum densities in two-dimensional solids

Experimental electron momentum densities in two-dimensional solids

Three‐dimensional momentum density reconstructed from positron annihilation measurements of Si single crystals in comparison with compton effect results

AbstractThe three‐dimensional electron‐positron pair momentum density ϱep(p) is reconstructed with the help of the MIJNARENDS'‐analysis using angular correlation curves of 2γ‐positron annihilation‐radiation measured with pz parallel to the [100], [110], and [111] direction of a Si single crystal. ϱep(p) is compared with the electron momentum density ϱe(p) and ϱe(p) which are calculated on the basis of the independent particle model. These momentum densities are taken from the literature. It is shown that the most important differences between the experimental momentum densities ϱep(p) and ϱe(p) can be reproduced by the calculation on the basis of the independent particle model. Some experiences concerning the handling of experimental data and accuracy of reconstructed momentum densities are reported.

Reconstructed Three‐Dimensional Valence Electron Momentum Density of Si from Compton Profile Measurements

AbstractRecently measured anisotropies of Si valence electron Compton profiles with the scattering vector in the [100], [110], and [111] direction are analyzed together with calculated anisotropies of the corresponding Compton profiles from a modified free electron model. From both sets of profiles the momentum density ϱ(p) is reconstructed. In comparison with the modified free electron model the experimental momentum density is strongly reduced in the vicinity of p =0.6 to 1.0 at. units along the [100] direction and enhanced mainly in the vicinity of p =1.1 at. units along the [110] direction. These main features of the reconstructed experimental momentum density are well reproduced by a pseudopotential band calculation of valence electron momentum densities along the [100], [110], and [111] directions. Also the influence of the orthogonalization term on the momentum density is estimated.

250. Preparation of translucent coating for As 2S 3 glass : I F Kopinets et al, Neorg Mater, 8 (9), 1972, 1551–1553 ( in Russian)

The electronic structure of crystalline As2S3 and As2Se3 has been calculated in this paper. We present the energy bands, density of states (DOS) and the Compton profiles using the linear combination of atomic orbitals (LCAO) scheme based on the density functional theory (DFT). From the calculated total and partial density of states it is seen that the lone-pair p-states of sulphur/selenium contribute closest to the Fermi energy level. To interpret the theoretical data on the Compton line shape, we have measured the Compton profiles on a 100 mCi 241Am spectrometer. It is seen that the density functional theory within generalised gradient approximation gives a slightly better agreement with the experimental momentum densities. The nature of chemical bonding in arsenic chalcogenides is studied using Mulliken's population analysis and the experimentally measured equal-valence-electron-density profiles; As2S3 is found to be more ionic compared to As2Se3.

Charge density and momentum density – a comparison between theory and experiment

X-ray structure factors and Compton line shape measurements provide complementary information on charge and momentum density, the latter measurement evidencing very much greater sensitivity to the valence electrons. Theoretical calculations of structure factors and Compton line shapes for solids have been sparse and have probable errors in excess of 3%. When these errors are compounded with experimental errors of several per cent the cases for which comparison between theory and experiment is meaningful are presently limited to diamond, silicon, germanium, MgO, LiH, Li, Be, Mg, Al and perhaps a few others. In diamond the 111 and 222 structure factors are in agreement with theory as are the 111 structure factors of LiH and MgO. The experimental momentum densities in Li, Be, Mg, and Al do not agree with conventional band calculations since these are unable at present to account for a significant electron-electron correlation effect.