12 research outputs found

    GW method applied to localized 4f electron systems

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    We apply a recently developed quasiparticle self-consistent GWGW method (QSGW) to Gd, Er, EuN, GdN, ErAs, YbN and GdAs. We show that QSGW combines advantages separately found in conventional GWGW and LDA+UU theory, in a simple and fully \emph{ab initio} way. \qsgw reproduces the experimental occupied 4f4f levels well, though unoccupied levels are systematically overestimated. Properties of the Fermi surface responsible for electronic properties are in good agreement with available experimental data. GdN is predicted to be very near a critical point of a first-order metal-insulator transition.Comment: 5 pages,3 figures, 2 table

    Ab-initio calculations of spin tunneling through an indirect barrier

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    We use a fully relativistic layer Green's functions approach to investigate spin-dependent tunneling through a symmetric indirect band gap barrier like GaAs/AlAs/GaAs heterostructure along [100] direction. The method is based on Linear Muffin Tin Orbitals and it is within the Density Functional Theory (DFT) in the Local Density Approximation (LDA). We find that the results of our {\it ab-initio} calculations are in good agreement with the predictions of our previous empirical tight binding model [Phys. Rev. {\bf B}, 075313 (2006)]. In addition we show the k∣∣k_{||}-dependence of the spin polarization which we did not previously include in the model. The {\it ab-initio} calculations indicate a strong k∣∣k_{||}-dependence of the transmission and the spin polarization due to band non-parabolicity. A large window of 25-50 % spin polarization was found for a barrier of 8 AlAs monolayers at k∣∣k_{||} = 0.03 2π/a2\pi/a. Our calculations show clearly that the appearance of energy windows with significant spin polarization depends mostly on the location of transmission resonances and their corresponding zeros and not on the magnitude of the spin splitting in the barrier.Comment: 10 pages, 3 figure

    Detection of the spin character of Fe(001) surface states by scanning tunneling microscopy: A theoretical proposal

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    We consider the magnetic structure on the Fe(001) surface and theoretically study the scanning tunneling spectroscopy using a spin-polarized tip (SP-STM). We show that minority-spin surface states induce a strong bias dependence of the tunneling differential conductance which largely depends on the orientation of the magnetization in the SP-STM tip relative to the easy magnetization axis in the Fe(001) surface. We propose to use this effect in order to determine the spin character of the Fe(001) surface states. This technique can be applied also to other magnetic surfaces in which surface states are observed.Comment: 5 pages, 4 figure

    Many-body Electronic Structure of Metallic alpha-Uranium

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    We present results for the electronic structure of alpha uranium using a recently developed quasiparticle self-consistent GW method (QSGW). This is the first time that the f-orbital electron-electron interactions in an actinide has been treated by a first-principles method beyond the level of the generalized gradient approximation (GGA) to the local density approximation (LDA). We show that the QSGW approximation predicts an f-level shift upwards of about 0.5 eV with respect to the other metallic s-d states and that there is a significant f-band narrowing when compared to LDA band-structure results. Nonetheless, because of the overall low f-electron occupation number in uranium, ground-state properties and the occupied band structure around the Fermi energy is not significantly affected. The correlations predominate in the unoccupied part of the f states. This provides the first formal justification for the success of LDA and GGA calculations in describing the ground-state properties of this material.Comment: 4 pages, 3 fihgure

    Strain-Induced Conduction Band Spin Splitting in GaAs from First Principles Calculations

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    We use a recently developed self-consistent GW approximation to present first principles calculations of the conduction band spin splitting in GaAs under [110] strain. The spin orbit interaction is taken into account as a perturbation to the scalar relativistic hamiltonian. These are the first calculations of conduction band spin splitting under deformation based on a quasiparticle approach; and because the self-consistent GW scheme accurately reproduces the relevant band parameters, it is expected to be a reliable predictor of spin splittings. We also discuss the spin relaxation time under [110] strain and show that it exhibits an in-plane anisotropy, which can be exploited to obtain the magnitude and sign of the conduction band spin splitting experimentally.Comment: 8 pages, 4 figures, 1 tabl

    Prediction of large linear-in-k spin splitting for holes in the 2D GaAs/AlAs system

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    The spin-orbit interaction generally leads to spin splitting (SS) of electron and hole energy states in solids, a splitting that is characterized by a scaling with the wavevector k\bf k. Whereas for {\it 3D bulk zincblende} solids the electron (heavy hole) SS exhibits a cubic (linear) scaling with kk, in {\it 2D quantum-wells} the electron (heavy hole) SS is currently believed to have a mostly linear (cubic) scaling. Such expectations are based on using a small 3D envelope function basis set to describe 2D physics. By treating instead the 2D system explicitly in a multi-band many-body approach we discover a large linear scaling of hole states in 2D. This scaling emerges from hole bands coupling that would be unsuspected by the standard model that judges coupling by energy proximity. This discovery of a linear Dresselhaus k-scaling for holes in 2D implies a different understanding of hole-physics in low-dimensions.Comment: 5 figures and 1 Tabl

    Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: First-principles calculations

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    A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin density approximation we calculate spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions

    Tunneling anisotropic magnetoresistance driven by resonant surface states: First-principles calculations of Fe(001) surface

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    Fully-relativistic first-principles calculations of the Fe(001) surface demonstrate that resonant surface (interface) states may produce sizeable tunneling anisotropic magnetoresistance in magnetic tunnel junctions with a single magnetic electrode. The effect is driven by the spin-orbit coupling. It shifts the resonant surface band via the Rashba effect when the magnetization direction changes. We find that spin-flip scattering at the interface is controlled not only by the strength of the spin-orbit coupling, but depends strongly on the intrinsic width of the resonant surface states.Comment: 5 pages, 3 figure

    Questaal: A package of electronic structure methods based on the linear muffin-tin orbital technique

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    This paper summarises the theory and functionality behind Questaal, an open-source suite of codes for calculating the electronic structure and related properties of materials from first principles. The formalism of the linearised muffin-tin orbital (LMTO) method is revisited in detail and developed further by the introduction of short-ranged tight-binding basis functions for full-potential calculations. The LMTO method is presented in both Green's function and wave function formulations for bulk and layered systems. The suite's full-potential LMTO code uses a sophisticated basis and augmentation method that allows an efficient and precise solution to the band problem at different levels of theory, most importantly density functional theory, LDA+U, quasi-particle self-consistent GW and combinations of these with dynamical mean field theory. This paper details the technical and theoretical bases of these methods, their implementation in Questaal, and provides an overview of the code's design and capabilities. Program summary: Program Title: Questaal Program Files doi: http://dx.doi.org/10.17632/35jxxtzpdn.1 Code Ocean Capsule: https://doi.org/10.24433/CO.3778701.v1 Licensing provisions: GNU General Public License, version 3 Programming language: Fortran, C, Python, Shell Nature of problem: Highly accurate ab initio calculation of the electronic structure of periodic solids and of the resulting physical, spectroscopic and magnetic properties for diverse material classes with different strengths and kinds of electronic correlation. Solution method: The many electron problem is considered at different levels of theory: density functional theory, many body perturbation theory in the GW approximation with different degrees of self consistency (notably quasiparticle self-consistent GW) and dynamical mean field theory. The solution to the single-particle band problem is achieved in the framework of an extension to the linear muffin-tin orbital (LMTO) technique including a highly precise and efficient full-potential implementation. An advanced fully-relativistic, non-collinear implementation based on the atomic sphere approximation is used for calculating transport and magnetic properties.</p
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