Compton Profiles Research Articles

Quantum Monte Carlo Compton profiles of solid and liquid lithium

We computed the Compton profile of solid and liquid lithium using quantum Monte Carlo (QMC) and compared with recent experimental measurements obtaining good agreement. Importantly, we find it crucial to account for proper core-valence orthogonalization and to address density differences when comparing with experiment. To account for disorder effects, we sampled finite-temperature configurations using molecular dynamics (MD), then performed diffusion Monte Carlo (DMC) simulations on each configuration. We used Slater-Jastrow wavefunctions and grand-canonical twist-averaged boundary conditions. A QMC pseudopotential correction, derived from an all-electron DMC simulation of the perfect crystal was also used. Our calculations provide the first all-electron QMC benchmark for the Compton profile of lithium crystal and pseudopotential-corrected QMC Compton profiles for both the liquid and solid.

Structural, magnetic and electronic properties of nickel ferrites: Experiment and LCAO calculations

Abstract Structural and magnetic response of Ni1-xCrxFe2O4 (x = 0 and 0.05) are presented using X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and SQUID magnetometer. The single phase of both the compositions is confirmed using Rietveld refinement method. The absence of any impurity is further cinched using structural sensitive techniques, namely FTIR and Raman spectroscopy. Interestingly, a pinched shaped M − H behaviour is observed for the Cr doped ferrite. In addition, we have computed magnetic moment, Mulliken’s population, density of states (DOS) and Compton profiles (CPs) for NiFe2O4 using linear combination of atomic orbitals (LCAO) scheme with and without hybrid functional for exchange and correlation potentials. Further, theoretical CPs have been validated using isotropic CP measurement with 137Cs radio-isotope for NiFe2O4. Among the considered exchange-correlation potentials within LCAO, the hybrid B3LYP scheme based momentum densities give better agreement with the experimental CP. Majority- and minority-spin DOS have confirmed the insulating nature of NiFe2O4. Peculiarities of presently deduced MP data and magnetic moments are also discussed.

Relativistic impulse approximation in Compton scattering

Relativistic impulse approximation (RIA) has been widely used in atomic, condensed matter, nuclear, and elementary particle physics. In former treatments of RIA formulation, differential cross sections for Compton scattering processes were factorized into atomic Compton profiles by performing further simplified approximations in the integration. In this study, we develop an ‘exact’ numerical method without using any further simplified approximations or factorization treatments. The validity of the approximations and factorizations used in former RIA treatments can be tested using our approach. Calculations for C, Cu, Ge, and Xe atomic systems are carried out using Dirac–Fock (DF) wavefunctions, and comparisons between the proposed approach and former treatments of RIA are performed and discussed in detail. Numerical results indicate that these simplified approximations work reasonably in the Compton peak region, and our results have little difference with the best of the former RIA treatments in the entire energy region. Meanwhile, in regions far from the Compton peak, the RIA results become inaccurate, even when our ‘exact’ numerical treatment is used.

Electronic properties of lithium tantalate using Compton spectroscopy

We have measured the electron momentum density (EMD) of LiTaO3 using 100 mCi 241Am Compton spectrometer. The experimental Compton profile (CP) is compared with theoretical EMDs deduced using linear combination of atomic orbitals (LCAO) method with different exchange and correlation potentials namely PBE, PWGGA and VBH. On the basis of χ2 fit (which scales goodness of fitting) and difference profiles between experimental and theoretical CPs, LCAO-PWGGA potential based CP is found close to experimental results. Electronic properties like energy bands and density of states of LiTaO3 using LCAO-PWGGA potential show a band gap of 4.85 eV of LiTaO3, thereby depicting its semiconductor nature with wide band gap. It is observed that upper valence band region is mainly dominated by 2p states of O and 5d states of Ta, although there is small contribution of 2s states of Li in this region.

Electronic response of hydrogen storage intermetallics LaNi5 and LaNi4.5Co0.5: inelastic scattering experiment and ab initio calculations

Electron momentum densities of intermetallic compounds LaNi5 and LaNi4.5Co0.5 using inelastic scattering of 662 keV γ-rays are presented. To check the usefulness of different exchange and correlation potentials for such type of hydrogen storage compounds, the resulting experimental Compton profiles (CPs) have been compared with the Compton lines deduced using linear combination of atomic orbitals (LCAO) scheme. It is seen that the LCAO with HISS scheme (middle range separated hybrid approach) leads to a better agreement with the experimental CPs than other considered schemes based on LDA, GGA, SOGGA, B3LYP and short range separated hybrid HSE06. In addition, energy bands and density of states of these systems derived using LCAO and the full potential linearized augmented plane wave methods are discussed. Anisotropies in momentum densities with respect to [001] direction namely (J001−J110 and J001–J100) show almost similar trend for both the intermetallics. It is noticed that the energy states corresponding to d states of Ni (2c), Ni (3 g) and Co atoms dominate near Fermi level (EF). The Fermi surface (FS) topology and real space CPs of LaNi5 and LaNi4.5Co0.5 show non-spherical character of FS and failure of free electron theory for such intermetallics.

Accurate atomic momentum integrals and Compton profiles

Analytical expressions for integrals involving momentum density have been developed, and an algorithm for the efficient computation of Compton profiles (CPs) based in those equations is reported. The performance of the algorithm is tested by computing the CPs of atoms from helium to xenon for a wide range of incident photon energies. Calculations have been made at Hartree–Fock level (HF), using three different Slater basis sets, showing a good agreement with the experimental results.

Performance of hybrid functional in linear combination of atomic orbitals scheme in predicting electronic response in spinel ferrites ZnFe2O4 and CdFe2O4

Pure and hybrid density functional theory (DFT) schemes within linear combination of atomic orbitals (LCAO) have been employed to compute Mulliken population (MP), energy bands, density of states (DOS) and electron momentum densities (EMDs) of TMFe2O4 (TM = Zn and Cd). Pure DFT calculations were performed within local density and generalized gradient approximations, while Hartree–Fock exchange contribution is added to DFT for hybrid calculations (B3LYP and PBE0). To validate the performance of hybrid functionals, we have also performed EMD measurements using 661.65 keV γ-rays from 137Cs source. Chi-square test predicts an overall better agreement of experimental Compton profile data with LCAO–B3LYP scheme-based momentum densities leading to usefulness of hybrid functionals in predicting electronic and magnetic response of such ferrites. Further, LCAO–B3LYP-based majority- and minority-spin energy bands and DOS for ZnFe2O4 and CdFe2O4 predict semiconducting nature in both the compounds. In addition, MP data and equal-valence-electron-density scaled EMDs show more covalent character of ZnFe2O4 than that of CdFe2O4. A reasonable agreement of magnetic moments of both the ferrites with available data unambiguously promotes use of Gaussian-type orbitals in LCAO scheme in exploring magnetic properties of such ferrites.

Compton profile of few-layer graphene investigated by electron energy-loss spectroscopy

In this paper, acquisition of the valence Compton profile of few-layer graphene using electron energy-loss spectroscopy at large scattering angle is reported. The experimental Compton profile is compared with the corresponding theoretical profile, calculated using the full-potential linearized augmented plane wave method based on the local-density approximation. Good agreement exists between the theoretical calculation and experiment. The graphene profile indicates a substantially greater delocalization of the ground state charge density compared to that of graphite.

The charge transfer in AgBr compound in terms of Compton profile and by adopting the ionic model

In this paper the Compton profile J(p z ) for AgBr compound is calculated by using the superposition model and compared with available experimental value ,while the values of ( Cp) J(p z ) for the covsitituent namely Ag were taken from our previous study on AgCl to check whether there is any charge transfer between Ag metal and the Br , The Ionic model was applied for this purpose and found a fraction of charge is transfer from the d-state of the metal Ag to p-state of Br ,the best electron configuration for the AgBr compound is found to be Ag (4d 9.1 5s 1 ) Br(4s 2 4p 5.1 ) Key Word : Ionic model, Superposition model, RFA model, Compton profile (CP) J(p z ) ,Free electron (FE) model ,Free atom (FA) model . DOI : 10.7176/APTA/80-06 Publication date :October 31 st 2019

Signatures of the differential Klein‐Nishina electronic cross section in Compton's quantum theory of scattering of radiation

Here, we consider the problem of separating the relative contributions of kinematics and dynamics to the differential Klein‐Nishina electronic cross section using graphical and numerical analysis. We show that the values of the energy of scattered photons, and hence the kinetic energy of recoiled electrons calculated from Compton's quantum theory of scattering of radiation, show a degree of matching that increases with the increase in incident photon energy as quantified by chi-square goodness of fit test, with the calculated differential Klein‐Nishina electronic cross section per electron per unit solid angle for the scattering of an unpolarized photon by a stationary free electron, when appropriate normalization procedures are invoked. There is a high degree of matching in a regime where the total electronic Klein‐Nishina cross section for the Compton scattering on a free stationary electron scales as the inverse of the incident photon energy and the contribution of the electro-magnetic interaction to differential electronic cross section diminishes. Hence the third level explanation of Compton effect by quantum electrodynamics has a degree of matching with the first level of Compton's quantum theory. The degree of mismatch is an indicator of the relative contribution of dynamics to differential Klein‐Nishina electronic cross section compared to kinematics. For incident photon energies less than 1 MeV, we obtain the values of the scattering angles at which calculated differential cross section is nonzero but is kinematically limited which may lead to broadening of Compton profile. At the scattering angle where the differential cross section value is minimum for a given incident photon energy, we obtain the relative contribution of dynamics to the differential cross section compared to kinematics. Therefore, these predictions which need to be confirmed experimentally have significance to the understanding of the mechanisms of photon‐electron interactions in the Compton scattering.

Investigation of Electron Momentum Density in Carbon Nanotubes Using Transmission Electron Microscopy.

Valence Compton profiles (CPs) of multiwall (MWCNTs) and single-wall carbon nanotubes (SWCNTs) were obtained by recording electron energy-loss spectra at large momentum transfer in the transmission electron microscope, a technique known as electron Compton scattering from solids (ECOSS). The experimental MWCNT/SWCNT results were compared with that of graphite. Differences between the valence CPs of MWCNTs and SWCNTs were observed, and the SWCNT CPs indicate a greater delocalization of the ground-state charge density compared to graphite. The results clearly demonstrate the feasibility and potential of the ECOSS technique as a complementary tool for studying the electronic structure of materials with nanoscale spatial resolution.

Optimization of detection strategies for epithermal neutron spectroscopy using photon-sensitive detectors.

In this work, we discuss an improved detection procedure for the photon-sensitive yttrium-aluminum-perovskite detectors installed on the VESUVIO spectrometer at the ISIS pulsed neutron and muon source. By decreasing the low-level energy threshold of detected photons, we observe an increased count rate up to a factor ∼3, and a decrease of relative error bars and noise of ∼40% and 35%, respectively, for deep inelastic neutron scattering measurements. In addition, we demonstrate how the reported optimization may increase the accuracy in the line shape analysis of neutron Compton profiles, as well as in the application of the mean-force approach to detect the anisotropy and anharmonicity in the single-particle local potential. We envisage that such an upgrade of the detection procedure would have a substantial impact on the VESUVIO scientific programme based on deep inelastic neutron scattering investigations.

Angular distribution of multiple Compton scattered gamma rays of 662 keV in tin

In multiple Compton scattering, the incident gamma photon is scattered more than once due to finite dimensions of the sample. It serves as a noise for the exact evaluation of electron momentum distribution in Compton profile, Compton cross section and non-destructive testing of in-assessable large samples. To overcome these difficulties, there is a need to characterise the contribution of multiple scattering as a function of atomic number, scattering angle and incident gamma photon energy. In the present work, measurements are carried out to investigate the multiple scattering of 662 keV photons with tin sample of different thickness in the forward and backward hemispheres. The scattered photon flux is recorded by a NaI(Tl) scintillation detector and with an HPGe solid-state detector to check for the trend of saturation thickness as a function of resolution of the detector. The number of multiply scattered photons is found to be increasing with the sample thickness up to a particular thickness value called saturation thickness beyond which it remains constant with increase in target thickness. The effect of detector collimator size on multiply scattered photons is also studied. Both the detectors show the same trend of saturation thickness as a function of thickness. Angular distribution of multiply scattered photons shows symmetry around 90° scattering angle. The low energy side of multiple scattered photon spectrums is also characterised as a function of sample thickness and order of scattering. Monte Carlo calculations support the present experimental results.

Mindfulness-based cognitive therapy for irritable bowel syndrome: Reducing symptoms and increasing quality of life

We show how to calculate isotropic Compton profiles from sets of profiles along so-called “special directions” in the Brillouin zone computed from density-functional theory calculations, with reference to a recent paper by Bhatt et al. We present the correct formula and demonstrate the power of special directions, highlighting the importance of carefully choosing directions and using the correct weights in obtaining accurate isotropic profiles.

Temperature dependence of the microscopic magnetization process of Tb12Co88 using magnetic Compton scattering

Abstract Tb-Co perpendicular magnetization films are mother materials for perpendicular magnetic recording media. Since the magnetic recording technique is based on magnetic switching of a material, investigating a magnetic the response of the material to a magnetic field is important. The magnetic response of the entire system is the net of each microscopic response of the spin magnetic moment and orbital magnetic moment of the elements. Herein we study the magnetization processes of the spin, orbital, and the element-specific magnetic moments by measuring the magnetic field dependence of the magnetic Compton profile of a Tb12Co88 film. The spin and orbital magnetic moments have opposite orientations. Similarly, Tb and Co magnetic moments have opposite directions. When the total magnetic moment reverses, the orientation of spin and orbital magnetization as well as Tb and Co reverse. Decreasing the temperature, the fluctuation of the magnetic moment of Tb in the perpendicular direction decreases. So the cancelation between the Tb and Co moments reduces total magnetic moment.

Comment on “Magnetic Compton scattering study of Laves phase ZrFe2 and Sc doped ZrFe2: Experiment and Green function based relativistic calculations” by Bhatt et al.

Abstract We show how to calculate isotropic Compton profiles from sets of profiles along so-called “special directions” in the Brillouin zone computed from density-functional theory calculations, with reference to a recent paper by Bhatt et al. We present the correct formula and demonstrate the power of special directions, highlighting the importance of carefully choosing directions and using the correct weights in obtaining accurate isotropic profiles.

Magnetic properties and spin momentum densities of Ni excess Ni-Mn-Sn Heusler alloys

Abstract The role of excess Ni in Ni2+xMn1−xSn (x = 0.05, 0.1) Heusler alloys has been reported using temperature dependent magnetic Compton spectroscopy. The experimental Compton profiles from magnetic electrons have been analyzed to quantify the contribution of Mn 3d electrons, Ni sites and sp-electrons in building-up the total spin moments. The experimentally deduced spin momentum densities, total and element-specific spin moments are examined using theoretical investigations within the spin polarized relativistic Korringa-Kohn-Rostoker method. The build-up of spin moments has been explored in terms of total and projected density of states. Site-dependent Ni magnetic moment is also discussed. Heisenberg exchange coupling parameters are also computed to study the change in Ruderman-Kittel-Kasuya-Yosida type exchange interaction on increasing Ni concentration in the Ni-Mn-Sn system.

Electronic Structure in Heavy Fermion Compound UPd2Al3 through Directional Compton Profile Measurement

We have measured directional Compton profiles on the (0001) plane in the UPd2Al3 single crystal with hexagonal symmetry in order to investigate the change in electronic structure related to the cro...

Combined study of the ground and excited states in the transformation of nanodiamonds into carbon onions by electron energy-loss spectroscopy

The electron momentum density and sp2/sp3 ratio of carbon materials in the thermal transformation of detonation nanodiamonds (ND) into carbon nano-onions are systematically studied by electron energy-loss spectroscopy (EELS). Electron energy-loss near-edge structures of the carbon K-ionization in the electron energy-loss spectroscopy are measured to determine the sp2 content of the ND-derived samples. We use the method developed by Titantah and Lamoen, which is based on the ability to isolate the π* spectrum and has been shown to give reliable and accurate results. Compton profiles (CPs) of the ND-derived carbon materials are obtained by performing EELS on the electron Compton scattering region. The amplitude of the CPs at zero momentum increases with increasing annealing temperature above 500 °C. The dramatic changes occur in the temperature range of 900–1300 °C, which indicates that the graphitization process mainly occurs in this annealing temperature region. Our results complement the previous work on the thermal transformation of ND-derived carbon onions and provide deeper insight into the evolution of the electronic properties in the graphitization process.

Electron Localization Function and Compton Profiles of Cu2O.

The nature of bonding in the cubic cuprous oxide is studied by means of the theoretical tools, namely, the electron localization function and Compton profiles. The isotropic Compton profiles together with the anisotropies in the directional Compton profiles are presented. Taking free-atom Compton profiles, the charge-transfer model is also applied. The first-principles calculations based on the GGA are performed, and the self-interaction correction is incorporated, adopting the GGA+U approach. Both types of calculations are performed deploying the linearized augmented plane-wave (LAPW) method. The effect of self-interaction correction on the electron localization function, Compton profiles, and anisotropies is discussed. The electron localization function reveals ionic behavior in the (110) plane and covalent nature in the Cu-O bond intersecting plane. The GGA+U exhibits more covalent nature. The two LAPW calculations of the Compton profiles show better agreement with the available experimental data than the free-atom profiles. Among all of the calculations undertaken, the GGA+U shows the best agreement with the experiment. The GGA+U calculation shows more anisotropic behavior in directional Compton profiles.