arXiv subjects

M. Samsel-Czekala

Publications and source records attributed to M. Samsel-Czekala.

11 recordsLinked to original sources

arXiv2007

On the reliability of linear band structure methods

We discuss an efficiency of various band structure algorithms in determining the Fermi surface (FS) of the paramagnetic ErGa3. The linear muffin-tin orbital (LMTO) in the atomic sphere approximation (ASA) method and three full potential (FP) codes: FP-LMTO, FP linear augmented plane wave (FLAPW), and FP local orbitals (FPLO) methods are employed. Results are compared with electron-positron (e-p) momentum densities reconstructed from two dimensional angular correlation of annihilation radiation (2D ACAR). Unexpectedly, none of used modern FP codes is able to give satisfying description of the experimental data that are in perfect agreement with LMTO-ASA results. We suspect that it can be connected with a different choice of the linearization energy.

cond-mat.mtrl-sci↗

arXiv2006

Observation of a strongly nested Fermi surface in the shape-memory alloy Ni[0.62]Al[0.38]

The Fermi surface topology of the shape-memory alloy Ni[0.62]Al[0.38] has been determined using Compton scattering. A large area of this Fermi surface can be made to nest with other areas by translation through a vector of ~ 0.18 [1,1,0] (2pi/a), which correponds to the wavevector associated with martensitic precursor phenomena such as phonon softening and diffuse streaking in electron diffraction patterns. This observation is compelling evidence that these phenomena are driven by the enhanced electron-lattice coupling due to the Fermi surface nesting.

cond-mat.mtrl-sci↗

arXiv2005

Influence of experimental noise on densities reconstructed from line projections

The influence of experimental noise on densities rho(p) reconstructed by the Cormack method from their line projections, e.g. 2D ACAR spectra, is investigated. Simulations of statistical noise are performed for various sets of 2D spectra for two model rho(p) having the cubic symmetry. For the reconstructed densities propagation of the statistical error in terms of standard deviations, sigma[rho(p)], is estimated. We observe that the distribution of sigma[rho(p)] has its extremes along the main symmetry directions and also a tendency to accumulate for small p. Moreover, the more density components, rho_n(p), have to be taken to description of rho(p) the less anisotropic is the distribution of sigma[rho(p)]. Additionally, the error generated by the reconstruction method itself is discussed.

cond-mat.mtrl-sci↗

arXiv2005

Different ways of dealing with Compton scattering and positron annihilation experimental data

Different ways of dealing with one-dimensional (1D) spectra, measured e.g. in the Compton scattering or angular correlation of positron annihilation radiation (ACAR) experiments are presented. On the example of divalent hexagonal close packed metals it is shown what kind of information on the electronic structure one can get from 1D profiles, interpreted in terms of either 2D or 3D momentum densities. 2D and 3D densities are reconstructed from merely two and seven 1D profiles, respectively. Applied reconstruction techniques are particular solutions of the Radon transform in terms of orthogonal Gegenabauer polynomials. We propose their modification connected with so-called two-step reconstruction. The analysis is performed both in the extended p and reduced k zone schemes. It is demonstrated that if positron wave function or many-body effects are strongly momentum dependent, analysis of 2D densities folded into k space may lead to wrong conclusions concerning the Fermi surface. In the case of 2D ACAR data in Mg we found very strong many-body effects. PACS numbers: 71.18.+y, 13.60.Fz, 87.59.Fm

cond-mat.mtrl-sci↗

arXiv2004

Band structure of LaB6 by an algorithm for filtering reconstructed electron-positron momentum densities

A new method (NM) for filtering three-dimensional reconstructed densities is proposed. The algorithm is tested with simulated spectra and employed to study the electronic structure of the rare-earth compound LaB6. For this system, momentum densities are reconstructed from theoretical and experimental two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) spectra. The experimental results are in good agreement with the band structure calculated with the full-potential linearized augmented-plane-wave (FLAPW) method within the local-density approximation (LDA), apart from the detection of small electron pockets in the 15th band. It is also shown that, unlike the electron-positron enhancement, the electron-electron correlations affect noticeably the momentum density.

cond-mat.mtrl-sci↗

arXiv2003

Fermi-surface mapping from Compton profiles: Application to beryllium

The two-dimensional momentum density of Be on the basal GMK plane, i.e. the line integral of the three-dimensional momentum density along the c-axis, is reconstructed via the Cormack method from both experimental and theoretical Compton profiles. It is shown that in the case of Be, despite the momentum density is highly anisotropic, merely two Compton profiles are sufficient to reproduce the main features of the momentum density. The analysis of the reconstructed densities is performed both in the extended and reduced zone schemes.

cond-mat.mtrl-sci↗

arXiv2002

Electron momentum density in Cu0.9Al0.1

A reconstruction technique based on the solution of the Radon transform in terms of Jacobi polynomials is used to obtain the 3D electron momentum density rho(p) from nine high-resolution Compton profiles (CPs) for a Cu0.9Al0.1 disordered alloy single crystal. The method was also applied to theoretical CPs computed within the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) first-principles scheme for the same nine orientations of the crystal. The experimental density rho(p) is in satisfactory agreement with the theoretical density and shows most details of the Fermi surface (FS) and exhibits electron correlation effects. We comment on the map of the FS obtained by folding the reconstructed rho(p) into the first Brillouin zone which yields the occupation number density, rho(k). A test of the validity of data via a consistency condition (within our reconstruction algorithm) as well as the propagation of experimental noise in the reconstruction of both rho(p) and rho(k) are investigated.

cond-mat.mtrl-sci↗

arXiv2002

Electron momentum density in yttrium

A simultaneous analysis of high-resolution directional Compton profiles and two-dimensional angular correlation of positron annihilation experimental data has been performed by studying both a directional anisotropy of measured spectra and reconstructed densities. The results were compared with theoretical fully-relativistic augmented plane-wave calculations with and without including correlation effects. Estimated symmetry selection rules have allowed us to establish some values of Fermi momenta. Both experiments show exactly the same shape of the anisotropy of the momentum densities, in agreement with the band structure results. In the positron annihilation data electron-positron correlations are not seen while in both experiments electron-electron correlations are observed.

cond-mat.mtrl-sci↗

arXiv2002

Special directions in the Brillouin zone

Rules are given for determining special directions in the Brillouin zone which optimize the descrip-tion of various physical quantities with Gamma1 type symmetry. We consider the cubic, hexagonal, tetragonal and trigonal (e.g. Bi) lattice. These rules allow us to construct, in all of momentum space, quantities which have the full symmetry of the Brillouin zone such as the Fermi surface, momentum density and others, from the knowledge of these quantities for a limited number of spe-cial directions. These results can also be used for determining the projections which should be measured either in Compton scattering or in positron annihilation experiments in order to recon-struct properly the electron momentum density. Such a treatment is approximately equivalent to applying Gaussian quadrature in calculations of the radial functions that occur in the expansion of data into lattice harmonics.

cond-mat.mtrl-sci↗

arXiv2002

Fermi surface of Mg and Cd

We present three-dimensional (3D) electron-positron momentum densities in Mg and Cd, reconstructed from 2D angular correlation of annihilation radiation data. By folding densities from the extended momentum space into the first Brillouin zone, we separated Fermi surfaces in various bands. Our results are in good agreement with band structure LMTO calculations. In order to examine how many 1D profiles give us a possibility to reproduce the same details of the Fermi surfaces and Umklapp components of electron densities, various sets of 1D spectra were created for special directions in the Brillouin zone. Finally, corresponding 3D densities were reconstructed to show how many details one can get depending on the number of 1D profiles.

cond-mat.mtrl-sci↗

arXiv2002

Reconstructed Momentum Density and Fermi Surface in Cu0.9Al0.1

A reconstruction technique based on Radon transforms is used to obtain 3D electron momentum density rho(p) using nine recently measured high-resolution Compton profiles (CPs) from a Cu_{0.9}Al_{0.1} disordered alloy single crystal. The method was also applied to nine corresponding theoretical CPs computed within the KKR-CPA first-principles scheme in order to show that our reconstruction procedure reproduces rho(p) reasonably. We comment briefly on how well a map of the Fermi surface (FS) can be obtained by folding the reconstructed rho(p) into the first Brillouin zone.

cond-mat.mtrl-sci↗