Compton Profiles Research Articles

Compton profiles and nesting of Fermi surfaces for B2-TiNi and B2-TiPd

The band structures of the shape memory alloys B2-TiNi and B2-TiPd are calculated by the full potential linearized augmented plane wave method with the local density approximation. The theoretical Compton profiles for B2-TiNi and B2-TiPd are calculated. In addition, the three-dimensional (3D) occupation number densities obtained by Lock–Crisp–West (LCW) analysis are presented for the first time. These 3D occupation number densities are in good agreement with the Compton scattering experiment for TiNi. Both shape memory alloys are based on martensitic transformation, which is caused by soft phonons. The charge-density wave is created by nesting of Fermi surfaces, which leads to phonon softening. To examine the nesting vectors quantitatively, we calculate the generalized susceptibility χ(q). χ(q) shows peaks at 0.315[110]2π/a and 0.4[111]2π/a for TiNi and at 0.275[110]2π/a and 0.395[111]2π/a for TiPd. Although the nesting vector in the [110] direction agrees with that from the phonon experiment, the nesting vector in the [111] direction differs from that in the experimental results.

Anisotropy of the electron momentum distribution inα-quartz investigated by Compton scattering andab initiosimulations

The Compton profiles of $\ensuremath{\alpha}$-quartz have been measured along 30 crystallographically independent directions with an experimental resolution of 0.11 atomic units in full width at half maximum. They are critically compared to those obtained from periodic quantum-chemical simulations based on the use of density-functional, Hartree-Fock, and post-Hartree-Fock approximations. The anisotropy of the electron momentum distribution, which clearly emerges from both the experiment and the theory, is analyzed and explained in terms of contributions from the localized Wannier functions associated with the Si--O bonds and with the lone pairs on the oxygen atoms.

Calculation of Compton Profiles for Rare Gases Using the DV-Xα Method

Compton profiles related to the electron momentum distribution are calculated using the DV-Xα method for a series of rare gases from He to Rn. To verify the criteria of the DV-Xα-derived Compton profiles, our results are compared with those obtained from the Hartree-Fock method. The trend of Compton profiles obtained from the DV-Xα method is discussed.

Calculation of Compton Profiles Using the DV-Xα Method for 14 Electron Diatomic Molecules

Isotropic and directional Compton profiles are calculated for 14 electron diatomic molecules, N2, CO, and BF, using the DV-Xα method. In order to investigate the effect of chemical bonding for Compton profiles, parallel and perpendicular directional Compton profiles to the molecules are calculated and compared with the results from Hartree-Fock and configuration interaction methods. The DV-Xα method could describe the more detailed character of covalent bonding than that of ionic bonding.

Observation of Magnetic Compton Profile of Interface Controlled Co/Pd Multilayer

We compare two Co/Pd multilayers with correspondingly smooth and rough interfaces. The first is a Co (1.5 nm)/Pd (2.6 nm) multilayer with a smooth interface deposited by the MBE technique, and the second is a Co (1.6 nm)/Pd (4.0 nm) multilayer with a rough interface deposited by the sputter technique. Both multilayers have almost the same perpendicular magnetic anisotropy energy, 1.15 Merg/cc for the Co (1.5 nm)/Pd (2.6 nm) multilayer and 1.20 Merg/cc for the Co (1.6 nm)/Pd (4.0 nm) multilayer, respectively. The symmetry of the wave function, which is measured using the magnetic Compton profile, is almost the same for both multilayers. This suggests that the smooth interface controls the wave function and enhances the PMA energy even if the Co/Pd multilayer has a thinner Pd layer.

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.

Compton profiles and electronic properties of Nd

We report the Compton profile (CP) measurement of Nd using a 20 Ci 137Cs Compton spectrometer. To interpret our experimental Compton data, we have also computed the CPs using the spin-polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) method and the linear combination of atomic orbitals method. To check the role of spin–orbit coupling in KKR calculations, we have also computed the CPs within the scalar relativistic approximation. It is seen that the CPs calculated using relativistic KKR calculations are in better agreement with the present experiment than scalar relativistic KKR calculations and the generalized gradient approximation (GGA) within the pseudopotential density functional theory and the recently developed second-order GGA. Using the SPR-KKR method, we have also computed the energy bands along with total and partial density of states of Nd.

Compton profiles of MoP and WP: Validation of second order generalized gradient approximation

Recently, Zhao and Truhlar (J. Chem. Phys. 128, 184109, 2008) have constructed second order generalized gradient approximation (SOGGA) within the density functional theory. The authors have successfully tested the performance of SOGGA by computing lattice constants, cohesive energies, bond distances and few energetic quantities of different solids and molecules. In this paper, to establish the usefulness of SOGGA in deducing the momentum densities, we have compared our experimental Compton profiles of MoP and WP with those computed using GGA and SOGGA within density functional theory. It is seen that SOGGAPBE based Compton profiles of both the samples are in better agreement with the corresponding experimental data than those derived from BPBE-GGA. In addition, energy bands, density of states and relative nature of bonding in both the phosphides is explained in terms of equal-valence-electron-density profiles and Mulliken’s population analysis.

Magnetic and Transport Properties of Heusler Compound ${\rm Co}_{2}{\rm TiAl}$

The magnetic properties of the polycrystalline Heusler compound Co2TiAl were investigated by the classical experimental methods and the magnetic Compton scattering experiment. The X-ray diffraction profile at room temperature can be identified as the phase of the L21-type ordered structure. The magnetic field and temperature dependence of magnetization show that Co2TiAl has the typical soft ferromagnetic properties with TC of 120 K and the saturation magnetization of 0.70 μB per formula unit. The resistivity shows a typical metallic behavior at temperatures up to 300 K. The magnetoresistance (MR) effect with the MR ratio of - 8% appears in the vicinity of TC. The characteristics of the magnetic Compton profile are similar to that of the well-known ferromagnetic metals such as Ni and Fe. The experimental results indicate that the origin of the MR effect observed for Co2TiAl is similar to that for conventional ferromagnetic materials.

Investigation of orbital magnetization in inverse spinel cobalt ferrite using magnetic Compton scattering

We present the spin momentum densities of CoFe2O4 measured at 8 and 300 K using magnetic Compton scattering. The magnetic Compton profiles were decomposed into component profiles of constituents namely Fe and Co, to determine their role in the formation of total spin moment. It is seen that the major contribution (about 80%) in the spin moment is from Co, whereas the itinerant electrons show a small reverse polarization. Moreover, it is clearly visualized that the spin moment at Co reduces from 2.55 → 2.35 μB/f.u. while going from 8 → 300 K. The magnetic Compton profiles, when compared with the magnetization data, show about 17% contribution of orbital moment to the total magnetic moment at both temperatures. The origin of the orbital moment is explained on the basis of rotation of hole on t2g orbital of Co+2 ion.

Direct kinetic energy extraction from neutron Compton profiles

Abstract Deep inelastic neutron scattering experiments provide access to atomic momentum distributions and mean kinetic energies. These quantities are intimately connected to nuclear quantum effects associated to the equilibrium ground state of condensed systems. The method to derive the single particle mean kinetic energy, directly employing the sum rules associated to the scattering functions, from a set of deep inelastic neutron scattering spectra is discussed. This method does not make use of nonlinear fitting of the scattering spectra.

Sensitivity of peak positions to flight-path parameters in a deep-inelastic scattering neutron TOF spectrometer

The effects of small changes in flight-path parameters (primary and secondary flight paths, detector angles), and of displacement of the sample along the beam axis away from its ideal position, are examined for an inelastic time-of-flight (TOF) neutron spectrometer, emphasising the deep-inelastic regime. The aim was to develop a rational basis for deciding what measured shifts in the positions of spectral peaks could be regarded as reliable in the light of the uncertainties in the calibrated flight-path parameters. Uncertainty in the length of the primary or secondary flight path has the least effect on the positions of the peaks of H, D and He, which are dominated by the accuracy of the calibration of the detector angles. This aspect of the calibration of a TOF spectrometer therefore demands close attention to achieve reliable outcomes where the position of the peaks is of significant scientific interest and is discussed in detail. The corresponding sensitivities of the position of peak of the Compton profile, J(y), to flight-path parameters and sample position are also examined, focusing on the comparability across experiments of results for H, D and He. We show that positioning the sample to within a few mm of the ideal position is required to ensure good comparability between experiments if data from detectors at high forward angles are to be reliably interpreted.

Electronic properties of the magnetocaloric compound GdAl 2: A Compton scattering study

We present Compton profiles of the GdAl 2 compound and its constituents using a 20Ci 137Cs Compton spectrometer. The experimental Compton data have been analysed using theoretical data obtained from the spin polarised relativistic Korringa–Kohn–Rostoker (SPR-KKR) method and also the charge transfer on the formation of the compound. Both the experimental and the SPR-KKR theoretical Compton data support a charge transfer from Al→Gd in GdAl 2, which is in accordance with the conclusions drawn from the partial, total and integrated density of states of GdAl 2 and its constituents.

Electronic structure of CaX (X = O, S, Se) compounds using Compton spectroscopy

The electronic structure of CaO, CaS and CaSe through Compton spectroscopy is reported in this work. Both directional as well as spherically averaged Compton profiles are calculated for all the three compounds employing the CRYSTAL code within the framework of density functional theory (DFT). The anisotropy [100]–[110] for CaO is in agreement with the earlier values. The spherically averaged theoretical values are compared with the first ever measurement made on polycrystalline samples using 59.54keV gamma-rays from Am241 source. The calculations are in good agreement with measurements in all cases. Charge transfer in the three compounds has also been estimated following the ionic model. The present study suggests charge transfer from Ca to X (O, S, Se) atom. On the basis of equal-valence-electron-density (EVED) profiles, it is found that CaO is more ionic compared to CaS and CaSe. From the DFT based calculations, we have also determined the cohesive energies, which are compared with other investigations.

Size dependent electron momentum density distribution in ZnS

In this paper, we present size dependent electron momentum density distribution in ZnS. ZnS nanoparticles of size 3.8nm and 2.4nm are synthesized using the chemical route and characterized by X-ray diffraction (XRD). The Compton profile measurements are performed on both the nano-sized as well as bulk ZnS samples employing 59.54keV gamma-rays from 241Am source. The results reveal that the valence electron density in momentum space becomes narrower with reduction of particle size. To evaluate the charge transfer on compound formation, the ionic model based calculations for a number of configurations of Zn+xS−x (0.0≤x≤2) are also performed utilizing free atom Compton profiles. These results suggest different amounts of charge transfer in these materials varying from 1.2 to 2.0 electron from Zn to S atom.

Spin-wise decomposed Compton profiles

Spin-wise decomposed Compton profiles

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.

Electron momentum density and band structure calculations of α- and β-GeTe

We have measured isotropic experimental Compton profile of α-GeTe by employing high energy (662 keV) γ-radiation from a 137Cs isotope. To compare our experiment, we have also computed energy bands, density of states, electron momentum densities and Compton profiles of α- and β-phases of GeTe using the linear combination of atomic orbitals method. The electron momentum density is found to play a major role in understanding the topology of bands in the vicinity of the Fermi level. It is seen that the density functional theory (DFT) with generalised gradient approximation is relatively in better agreement with the experiment than the local density approximation and hybrid Hartree–Fock/DFT.

Fermi surface topology of deuterium-doped vanadium: Compton scattering study

The deuteration-induced effect on the Fermi surface (FS) topology of V was investigated by the synchrotron-based Compton scattering technique with 115 keV X-rays. The three-dimensional occupation number density (OND) of α-VD 0.64 single crystal and V single crystal was reconstructed by the directional Compton profiles along 13–18 directions. The observed OND shows that the FS topology of α-VD 0.64.differs from that of V at Γ, N, and H points in the crystal momentum space, which are hole position for V. This indicates that electrons that originate from deuterium modify the upper part of the host metal d bands (3rd and 4th bands) at the Fermi level E F, and forms the metal–hydrogen bonding states with the lower part of the host metal d band (1st band).