Compton Profiles Research Articles

Measurement of proton momentum distributions using a direct geometry instrument

We report the results of inelastic neutron scattering measurements on bulk water and ice using the direct geometry SEQUOIA chopper spectrometer at the Spallation Neutron Source (USA), with incident energy Ei= 6 eV. In this set up the measurements allow to access the Deep Inelastic Neutron Scattering regime. The scattering is centred at the proton recoil energy given by the impulse approximation, and the shape of the recoil peak conveys information on the proton momentum distribution in the system. The comparison with the performance of inverse geometry instruments, such as VESUVIO at the ISIS source (UK), shows that complementary information can be accessed by the use of direct and inverse geometry instruments. Analysis of the neutron Compton profiles shows that the proton kinetic energy in ice at 271 K is larger than in room temperature liquid water, in agreement with previous measurements on VESUVIO.

Static corrections versus dynamic correlation effects in the valence band Compton profile spectra of Ni

We compute the Compton profile of Ni using the Local Density Approximation of Density Functional Theory supplemented with electronic correlations treated at different levels. The total/magnetic Compton profiles show not only quantitative but also qualitative significant differences depending weather Hubbard corrections are treated at a mean field +U or in a more sophisticated dynamic way. Our aim is to discuss the range and capability of electronic correlations to modify the kinetic energy along specific spatial directions. The second and the fourth order moments of the difference in the Compton profiles are discussed as a function of the strength of local Coulomb interaction $U$.

Calculating electron momentum densities and Compton profiles using the linear tetrahedron method

A method for computing electron momentum densities and Compton profiles from ab initio calculations is presented. Reciprocal space is divided into optimally-shaped tetrahedra for interpolation, and the linear tetrahedron method is used to obtain the momentum density and its projections such as Compton profiles. Results are presented and evaluated against experimental data for Be, Cu, Ni, Fe3Pt, and YBa2Cu4O8, demonstrating the accuracy of our method in a wide variety of crystal structures.

Compton scattering and charge transfer in Er substituted DyAl2

A unique applicability of Compton spectroscopy in probing the electronic states of rare earth aluminides using high energy (662keV) γ-rays is reported. We have measured first-ever Compton profiles of Dy1-xErxAl2 (x=0, 0.2) using 20Ci 137Cs Compton spectrometer. The charge reorganization in Dy1−xErxAl2 (x=0, 0.2), on the formation of compound, has been discussed using the valence band Compton profile data. The experimental Compton profile data unambiguously establish charge transfer from Al to Dy (Dy and Er) on formation of x=0.0 (0.2) compound, which is in tune with spin polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) calculations. A reasonable agreement between SPR-KKR based Compton profiles and the experimental data show applicability of the Green function method in predicting the electronic properties of rare earth compounds.

Magnetic switching behaviors of orbital states with different magnetic quantum numbers in Au/Fe/MgO multilayer system

A spin specific magnetic hysteresis (SSMH) curve and an orbital specific magnetic hysteresis (OSMH) curve are obtained for Fe/Au/Fe/MgO multilayers by magnetic Compton scattering and SQUID magnetometer measurements. The SSMH curve with each contribution of magnetic quantum number |m| = 0, 1, and 2 states is obtained by decomposition analyses of magnetic Compton profiles. Residual magnetization is observed for the SSMH curve with magnetic quantum number |m| = 0, 2 and the OSMH curve. Although the SQUID magnetometer measurements do not show perpendicular magnetic anisotropy (PMA) in the present Fe/Au/Fe/MgO multilayer film, the SSMH curve with magnetic quantum number |m| = 0, 2 and OSMH curve show switching behaviors of PMA.

Spin-resolved momentum densities: probing orbitals in magnetic oxides

Studies of spin-resolved electron momentum densities involve the measurement of the so-called magnetic Compton profile. This is a one-dimensional projection of the electron momentum distribution of only those electrons that contribute to the spin moment of a sample. The technique is applicable to ferri- and ferromagnetic materials. Since electrons originating from different atomic orbitals have specific momentum densities, it is often possible to determine the origin of the magnetism present. Typically, interpretation requires the use of electronic structure calculations using molecular orbital and band structure approaches. The profile is obtained experimentally via the inelastic "Compton" scattering of high energy X-rays. For the experiments discussed here, the high energy beamlines at the ESRF and SPring-8 synchrotron X-ray sources were used, where we have a cryomagnet which can provide a sample environment with applied magnetic fields up to 9 Tesla, at temperatures from 1.3K to 600K. In this talk, we discuss our combined experimental and theoretical study of the spin density of the low-dimensional frustrated metamagnet Ca3Co2O6. The spin moment, measured using magnetic Compton scattering, confirms the existence of a large unquenched Co orbital moment (1.310.1 μB). With regards to the orbital occupation, we have performed molecular orbital calculations on the active trigonal CoO6cluster in order to determine which Co 3d orbitals are responsible for the observed electronic and magnetic behaviour and the observed orbital moment, and revealing the existence a oxygen spin moment of approximately 0.9 μB. Electronic structure calculations with a Hubbard U energy term give Compton profiles which are in good agreement with our experimental data. The magnetic Compton profile exhibits oscillations, which are well described, and their frequency in momentum space corresponds to the real-space inter-cobalt site bond length.

Redox orbitals in LixMn2O4(0 < x < 2) studied by X-ray Compton scattering

LixMn2O4 is attracting much interest as a positive electrode material for Li-ion rechargeable batteries. Redox orbitals of LixMn2O4 under the charge or discharge process are not fully understood yet. Some band calculations have pointed out that intercalated Li 2s electrons occupy Mn sites or down-spin Mn 3d bands [1,2]. On the other hand molecular orbital calculation has reported the Li 2s electrons occupy O sites [3]. To clarify the redox orbital is important to understand the electrochemical reaction in the electrodes. In this study we have investigated the redox orbitals in LixMn2O4 by X-ray Compton scattering. Compton profiles were measured at BL08W of SPring-8, Japan. The energy of incident X-rays were 115keV and the scattering angle was 165 degrees. Energy spectra of Compton scattered X-rays were measured using a two-dimensional position sensitive detector. The measurements were performed under room temperature and vacuum conditions. Samples are polycrystalline of LixMn2O4 (x=0.5, 1.1, 1.2, 1.8 and 2.0). In order to clarify the redox orbitals of LixMn2O4, we obtained difference Compton profiles which represent the incremental electronic states on Li intercalation. Comparing the results with KKR-CPA and DFT calculations, we found that the O 2p bands play an important role for the redox process in LixMn2O4 with 0<x<2.

Temperature-dependent spin magnetization density in Mn-rich Ni-Mn-Sn shape memory alloy by magnetic Compton scattering

Temperature-dependent spin momentum densities in Ni2Mn1.4Sn0.6 shape memory alloy have been measured using 182 keV circularly polarized synchrotron radiations. Near the martensitic phase transition , experimental magnetic Compton profiles (MCPs) unravel a dominating spin moment contribution from Mn sites. First-principles calculations are found to produce higher magnetic moment for orthorhombic structure than that for the L21 (cubic) structure, which is in tune with the present MCP data. It is also concluded that the spin moments of excess Mn atoms lying at Sn sites are antiferromagnetically coupled with Mn local spin moment in both the phases.

Molecular size from moments of the momentum density

Qualitative arguments and examination of numerical data for 1641 organic molecules are used to establish semiquantitative relationships between the size of a molecule and some of the moments of its electron momentum density. The peak height of the Compton profile correlates well with molecular size and volume. Surprisingly, the molecular volume enclosed by the 0.001a0-3 isodensity contour of the electron number density can be predicted, with a mean absolute error of 2.1%, from 〈1/p2〉.

Electronic properties and Compton scattering studies of monoclinic tungsten dioxide

We present the first ever Compton profile measurement of WO2 using a 20 Ci 137Cs γ-ray source. The experimental data have been used to test different approximations of density functional theory in linear combination of atomic orbitals (LCAO) scheme. It is found that theoretical Compton profile deduced using generalized gradient approximation (GGA) gives a better agreement than local density approximation and second order GGA. The computed energy bands, density of states and Mulliken׳s populations (MP) data confirm a metal-like behavior of WO2. The electronic properties calculated using LCAO approach are also compared with those obtained using full potential linearized augmented plane wave method. The nature of bonding in WO2 is also compared with isoelectronic WX2 (X=S, Se) compounds in terms of equal-valence-electron-density profiles and MP data, which suggest an increase in ionic character in the order WSe2→WS2→WO2.

Electronic structure of lanthanum sesquioxide: A Compton scattering study

We present the first-ever experimental and theoretical momentum densities of La2O3. The Compton line shape is measured using a 20 Ci 137Cs Compton spectrometer at an intermediate resolution with full width at half maximum of 0.34a.u. The experimental Compton profile is compared with the theoretical electron momentum densities computed using linear combination of atomic orbitals (LCAO) method with density functional theory (DFT). It is seen that the generalized gradient approximation (GGA) within DFT reconciles better with the experiment than other DFT based approximations, validating the GGA approximation for rare-earth sesquioxides. The energy bands and density of states computed using LCAO calculations show its wide band gap nature which is in tune with the available reflectivity and photo-absorption data. In addition, Mulliken׳s population and charge density are also computed and discussed.

Momentum space anisotropy of electronic correlations in Fe and Ni: An analysis of magnetic Compton profiles

The total and magnetically resolved Compton profiles are analyzed within the combined density functional and dynamical mean field theory for the transition metal elements Fe and Ni. A rather good agreement between the measured and computed magnetic Compton profiles (MCPs) of Fe and Ni is obtained with the standard Local Spin Density Approximation (LSDA). By including local but dynamic many-body correlations captured by Dynamical Mean Field Theory (DMFT), the calculated magnetic Compton profile is further improved when compared with experiment. The second moment of the difference of the total Compton profiles between LSDA and DMFT, along the same momentum direction, has been used to discuss the strength of electronic correlations in Fe and Ni.

Study of dielectronic recombination and resonance transfer and excitation with X-ray emission for Fe24++H2 collision

Based on the theory of Hartree-Fock with relativistic correction, the theoretical study is carried out on the resonance strength of dielectronic recombination (DR) of the resonance double-excited states (i.e., KLL, KLM, KLN, KLO, KLP) of Fe24+. The resonance strength and colliding cross section of KLL are investigated. By using the experimental results of Compton profiles for H2, the resonance transfer and excitation with X-ray emission cross sections during collision between Fe24+ and target molecule H2 in an energy range of 300-800 MeV are studied. Our results are compared with the recent experimental and theoretical studies. It is found that for the double-excited states of Fe24+, the Kα decay tunnel is the major decay tunnel, and the wavelengths of X-ray in this process range from 1.850 to 1.880 Å. For other decay tunnels, the wavelengths of decay wave range from 1.460 to 1.601 Å. There is no overlap between the wavelengths for two cases. Our results are in reasonable agreement with the experimental results within an estimated uncertainty.

Study of Perpendicular Magnetic Anisotropy in Co/Au Multilayer Probed by Magnetic Compton Profile

Magnetic Compton profiles (MCPs) of Co/Au multilayers have been measured and analyzed by DV-Xαcluster model calculations from a viewpoint of perpendicular magnetic anisotropy (PMA). The PMA and the MCPs are discussed for the presently obtained results for Co/Au, along with the previously obtained results for Co/Pd and Co/Pt. A Co/Au multilayer shows a weak PMA which is caused by |m|=1 states of Co 3d electrons at a smooth Co/Au interface. The increase of the interface-to-volume ratio plays the main role in determining the perpendicular anisotropy in Co/Au multilayers. The strain of a Co layer can have a secondary role in determining the magnetic anisotropy in Co/Au multilayers, although the strain dominates PMA in the case of Co/Pd and Co/Pt multilayers.

Combined study of the ground and unoccupied electronic states of graphite by electron energy-loss spectroscopy

Both the unoccupied and ground electronic states of graphite have been studied by electron energy-loss spectroscopy in a transmission electron microscope. Electron energy-loss near-edge structures of the K-edge of carbon have been investigated in detail for scattering angles from 0 to 2.8 mrad. The π* and σ* components were separated. The angular and energy dependences of the π* and σ* structures were in fair agreement with theory. Electron energy loss Compton spectra of graphite were recorded at scattering angles from 45 to 68 mrad. One Compton scattering spectrum was obtained in 1 min compared with several hours or days using photons. The contributions of core electrons were calculated by the exact Hartree-Slater method in the Compton scattering region. The electron Compton profile for graphite is in good agreement with other conventional Compton profile measurements, as well as with theory, thus establishing the validity of the technique.

Voigt profile characterization of copper Kα

We report a characterization of the Cu Kα profile and a transferable determination of the 2p satellite line using a new Voigt methodology which generates improved fits, smaller residuals and details of Compton profile features. The Kα1, 2 emission of Cu was obtained from a rotating anode through a monolithic Si channel-cut monochromator. Least-squares fitting of a minimum set of Voigt profiles reached a noise limit. Sufficient statistical information and resolution permits the determination of major and minor peak components in a fully-free least-squares analysis rather than the previous constrained single peak-by-peak method. Relative energies of the component Voigts within each profile, linewidths and Kα1/Kα2 peak intensity ratios, are compared to the previous best empirical sum of Lorentzian-slit peaks, clearly demonstrating that a sum of Voigt profiles provides a superior fit to the observed profile. 104 profiles at accelerating voltages from 20 kV through 50 kV provided a stable unique profile across the broad range of 2.5 − 6.25 times the characteristic energy. This robustness proves the stability of Cu Kα for use in high accuracy calibration, and supports the validity of the impulse approximation across this range of energy. The lineshape, contributions to noise broadening, the quantum yield and the Fano factor, relevant to spectral profiling, are discussed.

ChemInform Abstract: Magnetic Compton Scattering: A Reliable Probe to Investigate Magnetic Properties

AbstractReview: 8 refs.

Compton Profiles and Nature of Bonding in Tantalum Chalcogenides

Abstract. We report the Compton profiles of tantalum chalcogenides (TaS2 and TaSSe) using Hartree–Fock and hybridization of Hartree–Fock and density functional theories within linear combination of atomic (Gaussian) orbitals. To interpret the theoretical data on Compton line shapes, we have measured the Compton profiles using our in-house 100 mCi 241Am γ-ray Compton spectrometer. To understand the relative nature of bonding, we have obtained the equal-valence-electron-density (EVED) profiles. The EVED profiles shows that charge in TaSSe is more localized than TaS2 in the bonding direction which confirms that TaSSe is more covalent than TaS2, which is in agreement with the Mulliken’s population analysis.

Compton Scattering and Electronic Properties of Tungsten Ditelluride

We report the first ever isotropic experimental Compton profile of tungsten ditelluride using 20 Ci 137Cs Compton spectrometer. To compare our experimental data, we have also computed the Compton profiles, energy bands, density of states and band gap using Hartree-Fock and density functional theory within linear combination of atomic orbitals. The measured data is found to be in better accordance with the generalised gradient approximation of density functional theory than Hartree-Fock and local density approximation. We have discussed the nature of bonding in WTe2 using energy bands and density of states.

Electronic Properties of ZnO: Band Structure and Directional Compton Profiles

The electronic band structure and directional Compton profiles (DCPs) of ZnO are studied in this work. Calculations are performed considering a set of three schemes based on density functional theory (DFT), the Hartree–Fock (HF) method, and a hybrid scheme. All band structures predict direct bandgaps. The best agreement with experiment is, however, shown by the hybrid scheme. The three schemes are also applied to compute DCPs along [100], [110], and [001] directions. These are compared with measurements made on single crystals of ZnO employing a 59.54 keV gamma-ray Compton spectrometer. Calculations overestimate the momentum density in the low-momentum region while underestimate the anisotropies. Positions of extremes in anisotropies deduced from calculations are well reproduced by the measured anisotropies in some cases. Within the experimental limits, the DCPs from the HF method are in better agreement with the measurements compared with DFT.