Compton Profiles Research Articles

Electronic, magnetic, optical and elastic properties of Fe2YAl (Y=Ti, V and Cr) using first principles methods

Using first principles computations, we have investigated the structural, electronic, magnetic, optical and elastic properties of Fe2YAl (Y=Ti, V and Cr) Heusler alloys. The present study reveals that for the majority-spin state, Fe2CrAl compound has high density of states and shows a 100% spin polarization in the vicinity of the Fermi level. Our results also suggest that both the electronic and magnetic properties and also the Fermi surface of the Fe2CrAl are intrinsically related to the appearance of the minority-spin gap. The origin of energy gap in the minority-spin states is discussed in terms of the d–d hybridization between the Fe and Cr atoms. The Fermi surface structure and optical properties are explored within the framework of the full potential linearized augmented plane wave (FP-LAPW) method. The origin of Fermi surface structure is discussed in terms of energy bands in the majority-spin states near the Fermi level. The calculated dielectric functions and other optical properties are found to be in good agreement with earlier experimental results. In addition, the independent elastic constants of these Heusler alloys are also derived from the derivative of total energy as a function of lattice strain in full detail. The analysis of the ratio between the bulk and shear moduli confirms that all three compounds are found to be brittle in nature. Furthermore, as a representative case, we have also carried out the spin-polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) method based on calculations to obtain the magnetic Compton profiles (MCPs) for momentum transfer along [100], [110], and [111] principal directions and also investigated the anisotropy in the spin-dependent Compton profiles.

Electronic properties and electron momentum density of monoclinic WO3

We present Compton profiles and Mulliken population (MP) data on charge re-organization of tungsten trioxide using pseudopotential approach within linear combination of atomic orbitals (LCAOs). Different exchange and correlation potentials to density functional theory (DFT) as embodied in the LCAO calculations have been validated using the first ever Compton profile measurement employing 20 Ci 137Cs source. The MP data have shown transfer of electrons from W sites which are found to be unevenly distributed among the non-equivalent six O atoms. The computed Compton profiles are found to be in reasonable agreement with the experimental data. Going beyond the LCAO calculations, energy bands, density of states (DOSs) and band gap are also reported using full potential augmented plane wave (FP-LAPW) method with DFT scheme. The FP-LAPW based partial and total DOS have also been discussed in terms of the MP data.

Compton profile study and electronic properties of tantalum diboride

We have reported the first-ever experimental Compton profile (CP) of TaB2 using 20 Ci137Cs Compton spectrometer. To compare the experimental data, we have also computed the theoretical CPs using density functional theory (DFT) and hybridization of DFT and Hartree–Fock (HF) within linear combination of the atomic orbitals (LCAO) method. In addition, we have reported energy bands and density of states of TaB2 using LCAO and full potential-linearized augmented plane wave (FP-LAPW) methods. A real space analysis of CP of TaB2 confirms its metallic character which is in tune with the cross-overs of Fermi level by energy bands and Fermi surface topology. A comparison of equal-valence-electron-density (EVED) experimental profiles of isoelectronic TaB2 and NbB2 show more covalent (or less ionic) character of TaB2 than that of NbB2 which is in agreement with available ionicity data.

Magnetic Compton Scattering Study of Ni<sub>2</sub>Mn<sub>1.4</sub>Sn<sub>0.6</sub> Shape Memory Alloy

Spin momentum density in Ni2Mn1.4Sn0.6 shape memory alloy has been measured using magnetic Compton scattering technique. The experiment has been performed using 182 keV circularly polarized synchrotron radiation at temperature 10 K at SPring-8, Japan. Experimental magnetic Compton profile has been decomposed to determine the site specific magnetic moments, where the magnetic moment at Mn site was found to be dominating. It is seen that the total spin moment in present non-stoichiometric alloy is smaller than that of stoichiometric Ni2MnSn.

Electronic structure of ceramic CrSi2 and WSi2: Compton spectroscopy and ab-initio calculations

We report electron momentum densities of stable phases of transition metal disilicides CrSi2 (C40 hexagonal) and WSi2 (C11b body centered tetragonal) using a high energy 137Cs Compton spectrometer. To support the experimental investigations, we have computed theoretical Compton profiles, energy bands and density of states of both the isoelectronic compounds using density functional theory within linear combination of atomic orbitals. Theoretical calculations for the energy bands and density of states are performed for both the hexagonal and tetragonal phases of these silicides. In the tetragonal phase, both the silicides show metallic behavior. The hexagonal CrSi2 shows an energy gap (0.28eV) between the valence band maximum at the L point and the conduction band minimum at the M point of the Brillouin zone, whereas the hexagonal WSi2 shows a semimetallic character. We have also scaled the experimental and theoretical profiles of the stable phases (hexagonal for CrSi2 and tetragonal for WSi2) on equal-valence-electron-density, which show more covalent character of WSi2 than that of CrSi2. The nature of bonding is in accordance with a present Mulliken's population analysis.

An X-ray Raman spectrometer for EXAFS studies on minerals: bent Laue spectrometer with 20 keV X-rays

An X-ray Raman spectrometer for studies of local structures in minerals is discussed. Contrary to widely adopted back-scattering spectrometers using ≤10 keV X-rays, a spectrometer utilizing ~20 keV X-rays and a bent Laue analyzer is proposed. The 20 keV photons penetrate mineral samples much more deeply than 10 keV photons, so that high intensity is obtained owing to an enhancement of the scattering volume. Furthermore, a bent Laue analyzer provides a wide band-pass and a high reflectivity, leading to a much enhanced integrated intensity. A prototype spectrometer has been constructed and performance tests carried out. The oxygen K-edge in SiO(2) glass and crystal (α-quartz) has been measured with energy resolutions of 4 eV (EXAFS mode) and 1.3 eV (XANES mode). Unlike methods previously adopted, it is proposed to determine the pre-edge curve based on a theoretical Compton profile and a Monte Carlo multiple-scattering simulation before extracting EXAFS features. It is shown that the obtained EXAFS features are reproduced fairly well by a cluster model with a minimal set of fitting parameters. The spectrometer and the data processing proposed here are readily applicable to high-pressure studies.

Electronic structure of nano-sized ZnO: A Compton profile study

The electronic structure of nano-sized ZnO using Compton scattering technique is reported. The nano-crystalline ZnO of size 10.5 nm is synthesized using the chemical route and characterized by XRD, SEM, TEM probes. Using 59.54 keV gamma-rays, the Compton profile measurements are performed on nano-sized as well as bulk ZnO. The present study reveals the narrower electron momentum density in nano-sized ZnO as compared to bulk sample. The ionic model based free atom Compton profiles for a number of configurations of Zn+x O−x (0.0 ≤ x ≤ 2.0) are also performed to estimate the charge transfer on compound formation. The present study suggests different amount of charge transfer from Zn to O atom in the nano-sized and bulk ZnO.

Electronic Structure and Momentum Density of BaO and BaS

The electronic structure and electron momentum density distribution in BaO and BaS are presented using Compton spectroscopy. The first-ever Compton profile measurements on polycrystalline BaO and BaS were performed using 59.54 keV gamma-rays. To interpret the experimental data, we have computed the theoretical Compton profiles of BaO and BaS using the linear combination of atomic orbitals method. In the present computation, the correlation scheme proposed by Perdew-Burke-Ernzerhof and the exchange scheme of Becke were considered. The hybrid B3PW and Hartree-Fock based profiles were also computed for both compounds. The ionic configurations are performed to estimate the charge transfer on compound formation, and the present study suggests charge transfer from Ba to O and S atoms. On the basis of equal-valence-electron-density profiles, it is found that BaO is more ionic as compared to BaS.

Electronic structure and electron momentum density in TiSi

We report the electron momentum density in titanium monosilicide using 241Am Compton spectrometer. Experimental Compton profile has been compared with the theoretical profiles computed using linear combination of atomic orbitals (LCAO). The energy bands, density of states and Fermi surface structures of TiSi are reported using the LCAO and the full potential linearized augmented plane wave methods. Theoretical anisotropies in directional Compton profiles are interpreted in terms of energy bands. To confirm the conducting behavior, we also report the real space analysis of experimental Compton profile of TiSi.

From neutron Compton profiles to momentum distribution: Assessment of direct numerical determination

Inelastic neutron scattering at high momentum transfers, in the neutron Compton scattering regime, provides an access to the neutron Compton profiles, the analogous of Compton profiles in X-ray scattering. The line shape analysis of the neutron Compton profiles is usually carried out making use of multiparametric nonlinear fitting, garnering detailed information about the momentum distribution of the target atoms. This paper presents the proposal to directly determine numerically the momentum distribution from the profiles, thus eliminating the possible instabilities present in multiparametric fitting. A comparison with Monte Carlo simulations and with previous measurements on polycrystalline ice provides quantitative assessments of the proposed method.

Momentum distribution and Compton profile by theab initioGW approximation

We present two possible approaches to calculate the momentum distribution $n(p)$ and the Compton profile within the framework of the ab initio GW approximation on the self-energy. The approaches are based on integration of the Green's function along either the real or the imaginary axes. Examples will be presented on the jellium model and on real bulk sodium. Advantages and drawbacks of both methods are discussed in comparison with accurate quantum Monte Carlo calculations and x-ray Compton scattering experiments. We illustrate the effect of many-body correlations and disentangle them from band-structure and anisotropy effects by a comparison with density functional theory in the local density approximation. Our results suggest the use of ${\text{G}}_{0}{\text{W}}_{0}$ momentum distributions as reference for future experiments and theory developments.

Ionization of H2 and He in collision of 3 keV electrons and the Bethe binary-encounter peak

We have studied the energy distributions of double differential cross sections (DDCS) of electrons emitted in single-ionization of H2 and He induced by 3keVelectron impact in the Bethe binary-encounter (BE) region for wide range of ejection angles.We compare the absolute cross sections for these two isoelectronic systems at high ejection energies. We demonstrate that while Compton profile causes undulation in the BE peak, the constructive interference effect enhances the overall cross section of H2 compared to that of atomic targets.

Electronic structure of CaCO3: A Compton scattering study

In the present work, we have studied electron momentum density of CaCO3 using a Compton scattering technique. The experiment has been performed using a 100mCi 241Am (59.54keV) Compton spectrometer. The experimental data have been interpreted in terms of theoretical Compton profiles. To compute the theoretical momentum densities, energy bands and density of states, we have used linear combination of atomic orbitals method as embodied in CRYSTAL09 code. We have used local density approximation, generalized gradient approximation (GGA) and second order GGA (SOGGA) within the frame work of density functional theory. It is seen that the GGA gives a better agreement with the experimental data than other approximations. We have also discussed the energy bands and density of states of CaCO3.

Dielectronic recombination and resonant transfer excitation processes for helium-like krypton

The relativistic configuration interaction method is employed to calculate the dielectronic recombination (DR) cross sections of helium-like krypton via the 1s2lnl′ (n = 2, 3, …, 15) resonances. Then, the resonant transfer excitation (RTE) processes of Kr34+ colliding with H, He, H2, and CHx (x = 0−4) targets are investigated under the impulse approximation. The needed Compton profiles of targets are obtained from the Hartree—Fock wave functions. The RTE cross sections are strongly dependent on DR resonant energies and strengths, and the electron momentum distributions of the target. For H2 and H targets, the ratio of their RTE cross sections changes from 1.85 for the 1s2l2l′ to 1.88 for other resonances, which demonstrates the weak molecular effects on the Compton profiles of H2. For CHx (x = 0−4) targets, the main contribution to the RTE cross section comes from the carbon atom since carbon carries 6 electrons; as the number of hydrogen increases in CHx, the RTE cross section almost increases by the same value, displaying the strong separate atom character for the hydrogen. However, further comparison of the individual orbital contributions of C(2p, 2s, 1s) and CH4(1t2, 2a1, 1a1) to the RTE cross sections shows that the molecular effects induce differences of about 25.1%, 19.9%, and 0.2% between 2p−1t2, 2s−2a1, and 1s−1a1 orbitals, respectively.

Electronic properties of WC nano-compound: Compton spectroscopy and band structure calculations

We present the electronic properties of tungsten carbide (WC) nano-compound using the Compton scattering technique. We have measured the Compton profile (CP) of nano-powder using our 137Cs Compton spectrometer. To determine the theoretical CPs of WC nano-compound, we have employed linear combination of atomic orbitals (LCAO) with Hartree-Fock and density functional theory. Although all the LCAO-based calculations show similar agreement with the experiment, the second-order generalised gradient approximation gives a marginally better agreement with the measured CP data. Layered structures and slight over-lapping in energy bands show a small role of exchange and correlation potentials in case of WC nano-compound, which is in contrast to bulk WC.

Ab-initio study of structural and electronic properties of AlAs

The structural properties, i.e. equilibrium lattice constant, transition pressure, bulk modulus and its pressure derivatives, together with electronic properties, i.e. energy bands, Compton profile and autocorrelation function, of AlAs are presented in this work. The linear combination of atomic orbitals (LCAO) method of the CRYSTAL code was applied considering the Perdew–Burke–Ernzerhof correlation energy functional and Becke's ansatz for the exchange. The total energy of AlAs as a function of primitive cell volume has also been calculated for the zincblende (B3), nickel arsenide (B8), sodium chloride (B1) and cesium chloride (B2) phases. Structural parameters of the B3, B8, B1 and B2 phases are determined. The calculated structural parameters are found to be in good agreement with the results of previous investigations. The spherically averaged theoretical values of Compton profile are in good agreement with an earlier measurement. The LCAO calculation shows an indirect band gap of 1.85 eV, in reasonable agreement with earlier data. On the basis of the equal-valence-electron-density Compton profile, it is found that AlAs is more ionic compared to AlSb.

Nuclear motion effects on the density matrix of crystals: An ab initio Monte Carlo harmonic approach

In the frame of the Born-Oppenheimer approximation, nuclear motions in crystals can be simulated rather accurately using a harmonic model. In turn, the electronic first-order density matrix (DM) can be expressed as the statistically weighted average over all its determinations each resulting from an instantaneous nuclear configuration. This model has been implemented in a computational scheme which adopts an ab initio one-electron (Hartree-Fock or Kohn-Sham) Hamiltonian in the CRYSTAL program. After selecting a supercell of reasonable size and solving the corresponding vibrational problem in the harmonic approximation, a Metropolis algorithm is adopted for generating a sample of nuclear configurations which reflects their probability distribution at a given temperature. For each configuration in the sample the "instantaneous" DM is calculated, and its contribution to the observables of interest is extracted. Translational and point symmetry of the crystal as reflected in its average DM are fully exploited. The influence of zero-point and thermal motion of nuclei on such important first-order observables as x-ray structure factors and Compton profiles can thus be estimated.

Electronic structure of Ni2TiAl: Theoretical aspects and Compton scattering measurement

Abstract In this paper, we report electron momentum density of Ni2TiAl alloy using an in-house 20 Ci 137Cs (661.65 keV) Compton spectrometer. The experimental data have been analyzed in terms of energy bands and density of states computed using linear combination of atomic orbitals (LCAO) method. In the LCAO computations, we have considered local density approximation, generalized gradient approximation and recently developed second order generalized gradient approximation within the frame work of density functional theory. Anisotropies in theoretical Compton profiles along [1 0 0], [1 1 0] and [1 1 1] directions are also explained in terms of energy bands.

Describing Compton scattering and two-quanta positron annihilation based on Compton profiles: two models suited for the Monte Carlo method

An accurate description of the basic physics processes of Compton scattering and positron annihilation in matter requires the consideration of atomic shell structure effects and, in specific, the momentum distributions of the atomic electrons.Two algorithms which model Compton scattering and two-quanta positron annihilation at rest accounting for shell structure effects are proposed. Two-quanta positron annihilation is a physics process which is of particular importance for applications such as positron emission tomography (PET). Both models use a detailed description of the processes which incorporate consistently Doppler broadening and binding effects. This together with the relatively low level of complexity of the models makes them particularly suited to be employed by fast sampling methods for Monte Carlo particle transport. Momentum distributions of shell electrons are obtained from parametrized one-electron Compton profiles. For conduction electrons, momentum distributions are derived in the framework of a Fermi gas.The Compton scattering model uses an approach which does not employ any free parameter. In contrast, a few semi-empirical approximations are included for the description of the complex physics of electron-positron annihilation resulting in acollinear photons.Comparisons of the Compton scattering model with simpler approaches illustrate the detailed accounting for shell structure effects. A satisfactory agreement is found for comparisons of both newly-developed models with experimental data.

The role of 3d electrons in the appearance of ferromagnetism in the antiferromagnetic Ru2MnGe Heusler compound: a magnetic Compton scattering study

The antiferromagnetism in Ru2MnGe can be suppressed by the substitution of V by Mn and ferromagnetism appears. Synchrotron-based magnetic Compton scattering experiments are used in order to investigates the role of 3d electrons in the indirect/direct exchange interactions for the appearance of ferromagnetism. A small spin moment for the itinerant electron part on the magnetic Compton profile indicates that the metallic ferromagnet Ru2Mn0.5V0.5Ge has a weak indirect exchange interaction between the d-like and sp-like (itinerant) electrons. This suggests that the appearance of ferromagnetism is caused by the enhancement of the direct exchange interactions between d–d electrons in the Ru2MnGe Heusler compound. These findings indicate that the indirect exchange interaction between itinerant electrons and localized electrons is a significant key point for the appearance of ferromagnetism in this system.