We present first principles charge-and spin-selfconsistent electronic structure computations on the Heusler-type disordered alloys Fe 3−x V x X for three different metalloids X= (Si, Ga and Al). In these calculations we use the methodology based on the Korringa-Kohn-Rostoker formalism and the coherent-potential approximation (KKR-CPA), generalized to treat disorder in multi-component complex alloys. Exchange correlation effects are incorporated within the local spin density (LSD) approximation. Total energy calculations for Fe 3−x V x Si show that V substitutes preferentially on the Fe(B) site, not on the Fe(A,C) site, in agreement with experiment. Furthermore, calculations have been carried out for Fe 3−x V x X alloys (with, x = 0.25, 0.50 and 0.75), together with the end compounds Fe 3 X and Fe 2 VX, and the limiting cases of a single V impurity in Fe 3 X and a single Fe(B) impurity in Fe 2 VX. We delineate clearly how the electronic states and magnetic moments at various sites in Fe 3−x V x X evolve as a function of the V content and the 1 metalloid valence. Notably, the spectrum of Fe 3−x V x X (X=Al and Ga) develops a pseudo-gap for the majority as well as minority spin states around the Fermi energy in the V-rich regime which, together with local moments of Fe(B) impurities, may play a role in the anomalous behavior of the transport properties. The total magnetic moment in Fe 3−x V x Si is found to decrease non-linearly, and the Fe(B) moment to increase with increasing x; this is in contrast to expectations of the 'local environment' model, which holds that the total moment should vary linearly while the Fe(B) moment should remain constant. The common-band model which describes the formation of bonding and antibonding states with different weights on the different atoms, however, provides insight into the electronic structure of this class of compounds.
Spin-polarized measurements of the two-dimensional angular correlation of annihilation radiation in NiMnSb are presented. By making use of the inherent partial polarization of the positron beam and the alignment of the magnetic domains with the aid of an external magnetic field, the sum and difference of the spin-dependent contributions to the angular correlation have been obtained for integration directions (100), (110),and (111).The results are compared with calculated distributions. A least-squares analysis of the data yields a value of ( -8.4+0. 1)X 10 ' for the three-photon difference effect in NiMnSb and establishes the half-metallic character of the band structure with an accuracy of -+0 02 electrons per formula unit.
Nanosized inorganic particles are of great interest because their electronic properties can be easily tailored, providing a tremendous potential for applications in optoelectronic devices, light-emitting diodes, solar cells and hydrogen storage. Confinement of electrons and holes to dimensions comparable to their wavelength leads to quantum-well states with modified wavefunctions and density of states. Surface phenomena are crucial in determining nanoparticle properties in view of their large surface-to-volume ratio. Despite a wealth of information, many fundamental questions about the nature of the surface and its relationship with the electronic structure remain unsolved. Ab initio calculations on CdSe nanocrystals suggest that passivating the ligands does not produce the ideal wurtzite structure and that Se atoms relax outwards irrespective of passivation. Here we show that implanted positrons are trapped at the surface of CdSe nanocrystals. They annihilate mostly with the Se electrons, monitor changes in composition and structure of the surface while hardly sensing the ligand molecules, and we thus unambiguously confirm the predicted strong surface relaxation.
The high-temperature superconducting cuprate La(2-x)Sr(x)CuO(4) (LSCO) shows several phases ranging from antiferromagnetic insulator to metal with increasing hole doping. To understand how the nature of the hole state evolves with doping, we have carried out high-resolution Compton scattering measurements at room temperature together with first-principles electronic structure computations on a series of LSCO single crystals in which the hole doping level varies from the underdoped (UD) to the overdoped (OD) regime. Holes in the UD system are found to primarily populate the O 2p(x)/p(y) orbitals. In contrast, the character of holes in the OD system is very different in that these holes mostly enter Cu d orbitals. High-resolution Compton scattering provides a bulk-sensitive method for imaging the orbital character of dopants in complex materials.
We report high-resolution Compton profiles ͑CP's͒ of Al along the three principal symmetry directions at a photon energy of 59.38 keV, together with corresponding highly accurate theoretical profiles obtained within the local-density approximation ͑LDA͒ based band-theory framework. A good accord between theory and experiment is found with respect to the overall shapes of the CP's and their first and second derivatives, as well as the anisotropies in the CP's defined as differences between pairs of various CP's. There are, however, discrepancies in that, in comparison to the LDA predictions, the measured profiles are lower at low momenta, show a Fermi cutoff that is broader, and display a tail that is higher at momenta above the Fermi momentum. A number of simple model calculations are carried out in order to gain insight into the nature of the underlying 3D momentum density in Al and the role of the Fermi surface in inducing fine structure in the CP's. The present results when compared with those on Li show clearly that the size of discrepancies between theoretical and experimental CP's is markedly smaller in Al than in Li. This indicates that, with increasing electron density, the conventional picture of the electron gas becomes more representative of the momentum density and that shortcomings of the LDA framework in describing the electron correlation effects become less important.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.