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    Ab initio lattice dynamics of Ni2MnX (X = Sn, Sb) magnetic shape memory alloys
    (Elsevier Science Sa, 2012) Agduk, Savas; Gokoglu, Gokhan
    In this study, we present the results of first principles calculations of elastic constants and phonon properties of nickel-manganese based magnetic shape memory compounds Ni2MnSn and Ni2MnSb in stoichiometric composition. The plane wave basis sets and pseudopotential method within spin-polarized generalized gradient approximation (sigma-GGA) scheme of the density functional theory is applied. In investigation of the phonon dispersion spectra, linear response technique of the Density Functional Perturbation Theory is used. Phonon softening is observed in dispersion spectra at the transverse acoustic mode (TA(2)) in [zeta zeta 0] direction as an indication of the structural instability of these systems to shear deformation. The vibrational instability of Ni2MnSb system is larger than that of Ni2MnSn yielding negative phonon frequencies. This vibrational anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. The minority spin Fermi surfaces of both systems exhibit strong nesting features. (C) 2011 Elsevier B. V. All rights reserved.
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    Electronic structure of antiferromagnetic PbCrO3 (001) surfaces
    (Elsevier Science Sa, 2011) Yildirim, Hasan; Agduk, Savas; Gokoglu, Gokhan
    Surfaces of cubic perovksite PbCrO3 in (0 0 1) plane are investigated through density functional theory. The plane wave pseudopotential method is applied with generalized gradient approximation scheme. Hubbard U correction (GGA + U) is included in all calculations in order to simulate on-site Coulomb interactions between Cr-d states. Two types of terminations, namely, PbO-and CrO2-terminations are considered in construction of the surfaces. Surfaces of both terminations show convergence at 9-layer slab geometry. The density of states calculations on the converged slab geometry yield a metallic behavior for both PbO-and CrO2-terminations. Both metal atoms, Pb and Cr, in the uppermost layer of the respective terminations, have inward atomic relaxations much larger in magnitude than the oxygen atoms of the respective layer. However, Cr atoms which are labeled as up and down according to their spin orientation show different relaxations. The interlayer distance between the uppermost layer and the first one next to it decreases in both PbO- and CrO2-terminated surface geometries. The calculations of the relative movement of the oxygen atom with respect to the Pb or Cr atom in each terminations give a positive rumpling in the uppermost layer. (C) 2011 Elsevier B.V. All rights reserved.
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    High-pressure elasticity and lattice dynamics of Mg2La from first principles
    (Elsevier Science Sa, 2012) Agduk, Savas; Gokoglu, Gokhan
    In this study, we present the results of first principles calculations of elastic constants and structural properties of the Mg2La cubic Laves phase (C15) up to approximate to 150 GPa pressure together with vibrational properties at equilibrium geometry. projector augmented wave (PAW) potentials are used with generalized gradient approximation (GGA) scheme of the density functional theory. The linear response technique of the density functional perturbation theory is applied in investigation of the phonon dispersion spectra. The static equation of states of the system has been studied with Vinet formulation. The values of applied pressure have also been obtained from the Vinet formulation of equation of states. The cubic phase of the system remains stable within the studied pressure range. The elastic anisotropy ratio indicates an electronic topological transition (ETT) around approximate to 100 GPa. This situation is also confirmed by electronic band structures. (C) 2011 Elsevier B. V. All rights reserved.