Comments on reconstruction of Fermi surface from high-resolution Compton data

A few comments are made on reconstruction of three-dimensional (3D) momentum density and Fermi surface mapping from high-resolution Compton data. Equivalence of LCW-folding and Schuelke's method in converting observed momentum density to occupation number density is explicitly shown.     

Strong anisotropy in the low temperature Compton profiles of electron momentum distribution in α-Ga metal

Compton profiles of momentum distribution of conduction electrons in the orthorhombic phase of α-Ga metal at low temperature are calculated in the band model for the three crystallographic directions (100), (010), and (001). Unlike the results at room temperature, previously reported by Lengeler, Lasser and Mair, the present results show strong anisotropy in the Compton profiles with the momentum distribution along (001) direction being substantially different from the other two directions. While experimental data on Compton profiles at low temperatures are not available for comparison with theory, the resistivity data in α-Ga at low temperature strongly support this anisotropic behaviour. Besides, the electronic heat capacity constant γ available from both experiment and present calculation suggests that the conduction electron distribution at low temperature in the orthorhombic phase is markedly different from the free-electron-like-distribution at room temperature, thus lending additional support to anisotropic behaviour of Compton profiles. It would be nice to have Compton profiles data from experiment at low temperature for direct comparison with theory. It is hoped that the present work would stimulate enough interest in that direction.     

A magnetic Compton scattering study of Ga rich Co-Ni-Ga ferromagnetic shape memory alloys

The temperature dependent spin momentum densities of Co(1.8)NiGa(1.2) and Co(2)Ni(0.76)Ga(1.24) alloys have been measured using the magnetic Compton scattering technique. The individual contributions of constituents in the formation of the total spin moment are also calculated using Compton line shape analysis. The magnetic Compton data when compared with the magnetization data obtained using a vibrating sample magnetometer show a negligible orbital contribution. The spin moments deduced from the experimental Compton data are compared with the theoretical results obtained from the full potential linearized augmented plane wave method and are found to be in good agreement. The origin of the magnetism in both alloys is also described in terms of the e(g) and t(2g) contributions of Ni and Co.     

Derivation of the radial distribution function from experimental Compton profiles

The analysis of experimental Compton profiles in position rather than momentum space provides a useful method of interpreting Compton data. A density matrix approach is employed to establish the relationship between the Compton profile J(p), and the Fourier transform of the momentum density B(r), and, for a homogeneous system, the radial distribution function g(r). An earlier Compton profile measurement on sodium provides the data for a demonstration of the long range charge correlations in a metal, in analogy with the Friedel oscillations in a screening charge.     

New reconstruction method of electron momentum densities from Compton profiles

Recent advances in Compton scattering experiments have led to renewed interest in reconstructing three-dimensional electron momentum densities from Compton profiles. We present a new reconstruction method based on the inversion of the Radon transform in terms of spherical harmonics and Jacobi polynomials. This technique is demonstrated on model projections having the hexagonal symmetry. Reconstructed densities, being in very good agreement with the model ones, are a very good performance of this new algorithm.     

Electron momentum density, band structure, and structural properties of SrS

The electron momentum density, the electronic band structure, and the structural properties of SrS are presented in this paper. The isotropic Compton profile, anisotropies in the directional Compton profiles, the electronic band structure and density of states are calculated using the ab initio periodic linear combination of atomic orbitals method with the CRYSTAL06 code. Structural parameters of SrS—lattice constants and bulk moduli in the B1 and B2 phases—are computed together with the transition pressure. The computed parameters are well in agreement with earlier investigations. To compare the calculated isotropic Compton profile, measurement on polycrystalline SrS is performed using 5Ci-²⁴¹Am Compton spectrometer. Additionally, charge transfer is studied by means of the Compton profiles computed from the ionic model. The nature of bonding in the isovalent SrS and SrO compounds is compared on the basis of equal-valenceelectron-density profiles and the bonding in SrS is found to be more covalent than in SrO.     

High-energy photons from passage of jets through Quark-Gluon plasma

We calculate the production of high-energy photons from Compton scattering and annihilation of a quark jet passing through a quark gluon plasma produced in a relativistic heavy ion collision. The contributions are large and reflect the momentum distribution of the jets and the initial conditions of the plasma.     

Performance of 20 Ci137Cs γ-ray Compton spectrometer for the study of momentum densities

In this paper, we present the design and construction of a 20 Ci γ-ray Compton spectrometer that employs a¹³⁷ Cs source with a strong line at 661.65 keV. The total resolution of the spectrometer in momentum scale is 0.40 a.u., which is much better than the conventional²⁴¹Am Compton spectrometers. The in-house¹³⁷Cs spectrometer is very useful for the measurement of momentum densities of heavy materials. The performance of the machine is assessed using aluminum, terbium and mercury samples and the experimental data from comparable apparatus.     

Momentum distribution in benzene: A comparison of compton scattering and positron annihilation

Probabilities in momentum space for the benzene molecule are calculated for two cases: An ordinary benzene molecule, and benzene with a bound positron which is coalesced with one of the electrons. The former probability is related to the Compton scattering profile; the latter, to angular correlation measurements made in positron annihilation experiments. In this work we make two comparisons on the basis of quantum mechanical calculations; a) between Compton scattering and positron annihilation results, and b) among the several possible positron annihilation results associated with different symmetries for the positronic molecular orbital (PMO). The Compton scattering results are found to be similar to the positron annihilation results for the more symmetrical PMOs; and all these are quite different from the positron annihilation results for the less symmetrical PMOs. A suggestion for a crucial experiment is made.     

Anisotropy in the compton profile of copper

Directional Compton profiles of high statistical accuracy are measured by means of a 412 keV gamma-ray Compton spectrometer. The experimental anisotropies are in very satisfactory agreement with earlier measurements and there is good qualitative agreement between the experimental data and a recent band structure calculation. Quantitatively, however, the experimental anisotropy is significantly smaller than predicted by theory.A simple model calculation based on the Seitz approximation, in which higher order Fermi surface volumes all of spherical shape are taken into account in obtaining the momentum density, demonstrates that the major contribution to the Compton profile anisotropy in copper is due to the hybridization which has mixedd-electrons and nearly-free electrons in the top band. Any fine structure in the theoretical anisotropy due to the detailed shape of the Fermi surface cannot be resolved in the present experiment.The potential of analysing the electronic structure of transition metals in terms ofB(r), the Fourier transform of the momentum density, is discussed in detail. The major contribution to the anisotropy arises from localised peaks inB(r) centered on the sites of the translational lattice. Within the Seitz approximation it could be shown that these secondary maxima are caused by the presence of localisedd-electrons in the highest partly occupied band. In conclusion we anticipate that a proper treatment of electron correlation would produce a marked quantitative improvement in the agreement between the present experimental data and the Compton profiles obtained from current band structure calculations.     

GW approximation study of the Compton profile of ZnSe

We have obtained the Compton profile of ZnSe from the first principles GW approximation (GWA) method and ground-state density functional theory (DFT) method. We observe that between 0 and 1.5?a.u., there is better agreement to previous studies via the GWA difference profile compared to the ground-state difference profile. Above 1.5?a.u., both cases do not agree with the trend of the previous study; however, the application of the GWA is seen to improve the agreement compared to localized density approximation. Previous studies have reported that discrepancies from experiment are related to pseudopotential calculations which have been observed to overestimate momentum density between 0 and 1.5?a.u., while the reverse trend is seen above 1.5?a.u. We thus conclude that improvement to the pseudopotential technique to obtain the Compton profile is possible if the sharp Fermi break of the momentum distribution between high and low momenta becomes more smeared. Using the broadened spectral functions via the contour deformation method to obtain the momentum distributions, the GWA is a natural tool to achieve this via the contribution from the dielectric screening to the quasiparticle energies.     

Neutron compton spectroscopy of pyrolitic graphite

The anisotropic momentum distribution of carbon atoms in pyrolitic graphite has been determined by neutron scattering at high momentum transfer with a resonance detector spectrometer. Our experiments demonstrate the capability of the RDS technique at a spallation neutron source for neutron Compton spectroscopy.     

Experimental and theoretical Compton profiles of Be, C and Al

The results of Compton profile measurements, Fermi momentum determinations, and theoretical values obtained from a linear combination of Slater-type orbital (STO) for core electrons in beryllium; carbon and aluminium are presented. In addition, a Thomas-Fermi model is used to estimate the contribution of valence electrons to the Compton profile. Measurements were performed using monoenergetic photons of 59.54 keV provided by a low-intensity Am-241 γ-ray source. Scattered photons were detected at 90° from the beam direction using a p-type coaxial high-purity germanium detector (HPGe). The experimental results are in good agreement with theoretical calculations.     

Electron momentum distributions and compton profiles of some molecules with FSGO model

The electron momentum distributions and the Compton profiles (within the impulse approximation) of H₂, LiH, methane, water, acetylene, ethylene, ethane cyclopropane and cyclobutane have been calculated using the floating spherical Gaussian orbital (FSGO) wavefunctions. The agreement of the single-FSGO Compton profiles with the corresponding experimental or the Hartree-Fock (HF-SCF) theoretical ones is fairly good in most of the cases examined. The advantages and drawbacks of using the FSGO model for the calculation of Compton profiles are discussed.     

Investigation of the Compton profiles for C,Si, and Ge

The Compton profiles of C, Si, and Ge are calculated for free atoms, and also taking into account their valence states in crystals. In order to obtain the distribution functions of the electrons with respect to momentum, a study is made of generalized hydrogen-like analytic orbitals. It is shown that the excitation of the electrons into the valence state leads to decrease of the Compton profiles and to the best agreement with their measured values.     

Compton profile of x rays and the momentum distribution functions of valence electrons in ZnSe

The momentum distribution functions of valence electrons are constructed from the measured Compton profile of cubic ZnSe. The Compton profile of ZnSe was measured using x rays (=0.071 nm, h=17.4 keV). The state of the valence electrons in ZnSe was investigated by comparing the experimental Compton profiles and the momentum distribution functions with those calculated theoretically with allowance for the change of the wave functions of the valence electrons in the crystal.Translated from Izvestiya Vysshikh Uchebnykh, Zavedenii, Fizika, No. 7, pp. 25–28, July, 1980.     

Compton scattering of 59.5 keV gamma rays from p-Si sample in an external electric field

This study is related to Compton scattering of photons from a p-Si sample whose surface charge density distributions are changed by an external electric field. The external electric field intensity in the range 0-75 kV/m was used to change the surface charge density distributions of the sample. The sample surface perpendicular to the electric field was selected as the scattering surface. The p-Si sample was bombarded by 59.5 keV γ-photons emitting from an Am-241 point source. The Compton scattered photons at an angle of 90^o were detected by an Si(Li) detector. The Compton scattering intensity suddenly increased with the application of the electric field since the applied electric field distorts both the negatively charged scattering center (free electron, bound electron, ionized acceptor) and the positively charged scattering center (hole) and their momentum distribution in the sample. There is a good third-order polynominal relation between the Compton scattering intensity and the increasing (or decreasing) electric field intensity. The results show that the positively charged scattering centers behave like negatively charged scattering centers, but the latter are slightly more effective than the former in the Compton scattering of γ-rays from the sample in the electric field.     

Perturbative and non-perturbative QCD corrections to wide-angle Compton scattering

We investigate corrections to the handbag approach for wide-angle Compton scattering off protons at moderately large momentum transfer: the photon–parton subprocess is calculated to next-to-leading order in and contributions from the generalized parton distribution E are taken into account. Photon and proton helicity flip amplitudes are non-zero due to these corrections, which leads to a wealth of polarization phenomena in Compton scattering. Thus, for instance, the incoming photon asymmetry or the transverse polarization of the proton is non-zero, although small. Received: 31 October 2001 / Revised version: 11 December 2001 / Published online: 8 February 2002