1,721,919 research outputs found
Projector augmented wave method with spin-orbit coupling: Applications to simple solids and zincblende-type semiconductors
No full textUsing the fully relativistic projector augmented wave (PAW) approach, we address the relevance of spin-orbit coupling for the structural properties of several solids. Results available in the literature obtained either via an all-electron full potential linearized augmented plane wave (FLAPW) method with spin-orbit treated by a second-variation procedure or via a PAW method in which spin-orbit is added to a scalar relativistic picture are used to critically estimate the soundness of our data. We construct scalar and fully relativistic PAW data sets for 26 new elements and use them to calculate the lattice constants and the bulk moduli of 56 solids finding that relativistic effects beyond the scalar relativistic ones change slightly some lattice constants and bulk moduli in the presence of fifth- or sixth-row elements, while no change above the numerical threshold is found in the other cases. Moreover, we study the spin-orbit-induced splittings of the electron energy bands of a few zincbende-type semiconductors and we show that the fully relativistic PAW method remains accurate also in the presence of light elements such as boron or nitrogen: The splittings of the Γ8v and Γ_7v valence states and of the Γ_8c and Γ_7c conduction states agree with FLAPW within a few meV
Nonlinear piezoelectricity in CdTe
No full textNonlinear piezoelectricity has recently been detected in strained-layer superlattices where CdTe is pseudomorphically grown along (111) on different materials. We investigate this problem from first principles using density-functional theory. As a byproduct, we provide the ab initio calculation of several properties of this material, including the equilibrium lattice constant, Born effective charges, internal strain parameter, zone-center optical-phonon frequencies, dielectric constant, elastic constants, and the ''clamped-ions'' piezoelectric constant. Nonlinear effects on all of them are considered. Our major result is that piezoelectricity is accurately linear over a wide range of volume-conserving strains, while it displays strong nonlinearity whenever the strain is not volume conserving. This finding implies that the observed nonlinear effects can be accurately accounted for by the linear piezoelectric response of the cubic system at the strained volume
Temperature-dependent atomic B factor: an ab initio calculation
No full textThe Debye-Waller factor explains the temperature dependence of the intensities of X-ray or neutron diffraction peaks. It is defined in terms of the B matrix whose elements B αβ are mean-square atomic displacements in different directions. These quantities, introduced in several contexts, account for the effects of temperature and quantum fluctuations on the lattice dynamics. This paper presents an implementation of the B factor (8π2 B αβ) in the thermo-pw software, a driver of Quantum ESPRESSO routines that provides several thermodynamic properties of materials. The B factor can be calculated from the ab initio phonon frequencies and displacements or can be estimated, although less accurately, from the elastic constants, using the Debye model. The B factors are computed for a few elemental crystals: silicon, ruthenium, magnesium and cadmium; the harmonic approximation at fixed geometry is compared with the quasi-harmonic approximation where the B factors are calculated accounting for thermal expansion. The results are compared with the available experimental data
Lattice dynamics effects on the magnetocrystalline anisotropy energy: application to MnBi
No full textUsing a first-principles fully relativistic scheme based on ultrasoft pseudopotentials and density functional perturbation theory, we study the magnetocrystalline anisotropy free energy of the ferromagnetic binary compound MnBi. We find that differences in the phonon dispersions due to the different orientations of the magnetization (in plane and perpendicular to the plane) give a difference between the vibrational free energies of the high-temperature and low-temperature phases. This vibrational contribution to the magnetocrystalline anisotropy energy (MAE) constant, K-u, is non-negligible. When the energy contribution to the MAE is calculated by the PBEso1 exchange-correlation functional, the addition of the phonon contribution allows one to get a T = 0 K K-u and a spin-reorientation transition temperature in reasonable agreement with experiments
Quasi-harmonic temperature dependent elastic constants: applications to silicon, aluminum, and silver
Full text linkWe present ab-initio calculations of the quasi-harmonic temperature dependent elastic constants. The isothermal elastic constants are calculated at each temperature as second derivatives of the Helmholtz free energy with respect to strain and corrected for finite pressure effects. This calculation is repeated for a grid of geometries and the results interpolated at the minimum of the Helmholtz free energy. The results are compared with the quasi-static elastic constants. Thermodynamic relationships are used to derive the adiabatic elastic constants that are compared with the experimental measurements. These approaches are implemented for cubic solids in the thermo_pw code and are validated by applications to silicon, aluminum, and silver
Density functional perturbation theory for lattice dynamics with fully relativistic ultrasoft pseudopotentials: the magnetic case
No full textWe extend density functional perturbation theory for lattice dynamics with fully relativistic ultrasoft pseudopotentials to magnetic materials. Our approach is based on the application of the time-reversal operator to the Sternheimer linear system and to its self-consistent solutions. Moreover, we discuss how to include in the formalism the symmetry operations of the magnetic point group which require the time-reversal operator. We validate our implementation by comparison with the frozen phonon method in fcc Ni and in a monatomic ferromagnetic Pt wire
Spin-polarized electronic surface states of Re(0001): An ab-initio investigation
No full textWe study the electronic structure of the Re(0001) surface by means of ab-initio techniques based on the Fully Relativistic (FR) Density Functional Theory (DFT) and the Projector Augmented-Wave (PAW) method. We identify the main surface states and resonances and study in detail their energy dispersion along the main symmetry lines of the SBZ. Moreover, we discuss the effect of spin-orbit coupling on the energy splittings and the spin-polarization of the main surface states and resonances. Whenever possible, we compare the results with previously studied heavy metals surfaces. We find empty resonances, located below a gap similar to the L-gap of the (111) fcc surfaces, that have a downward dispersion and cross the Fermi level, similarly to the recently studied Os(0001) surface. Their spin polarization at the Fermi level is similar to that predicted by the Rashba model, but the usual level crossing at Γ ̄ is not found with our slab thickness. Moreover, for selected states, we follow the spin polarization along the high symmetry lines, discussing its behavior with respect to k ∥ , the wave-vector parallel to the surface
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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