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  • 1. Andersson, D. A.
    et al.
    Simak, S. I.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Abrikosov, I. A.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Theoretical study of CeO2 doped with tetravalent ions2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 17, p. 174119-Article in journal (Refereed)
    Abstract [en]

    We have used density functional theory calculations within the LDA+U formulation to investigate how small amounts of dissolved SiO2, GeO2, SnO2, or PbO2 affect the redox thermodynamics of ceria (CeO2). Compared to pure ceria, reduction is facilitated and the reducibility increases in the sequence of CeO2-SnO2, CeO2-GeO2, and CeO2-SiO2, which correlates with the decrease of the ionic radii of the solutes. For low solute concentrations, there is an inverse relation between high reducibility and the solution energy of tetravalent solutes. CeO2-PbO2 is unique in the sense that the initial reduction occurs by Pb(IV)double right arrow Pb(II) instead of the usual Ce(IV)double right arrow Ce(III) reaction. Among the investigated ceria compounds, CeO2-PbO2 has the lowest reduction energy and rather low solution energy. We have studied how the solution and reduction energies depend on the concentration of Si, Ge, Sn, Pb, Ti, Zr, Hf, and Th solute ions. While the solution energy increases monotonously with concentration, the reduction energy first decreases, as compared to pure ceria (except for Th, which exhibits a small increase), and with further increase of solute concentration, it either remains almost constant (Zr, Hf, and Th) or slightly increases (Ti, Si, Ge, and Sn).

  • 2.
    Andersson, P. H.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    The effect of hydrogenation on the crystal structure and magnetic state in Pd3Mn2001In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 226-230, no Part 1, p. 1040-1041Article in journal (Refereed)
    Abstract [en]

    The effect of hydrogenation on the crystalstructure and magneticstate of Pd3Mn has been calculated using a full-potential linear augmented plane wave method in the local spin density approximation.When hydrogenated the system is observed to transform from the tetragonal Al3Zr structure to the cubic Cu3Au structure. When hydrogen is removed the system remains in a meta-stable cubic phase for an extended period of time. The structural transformations are associated with a change in the magneticstate. Our theoretical results of magnetic as well as crystalstructure stability are in agreement with experiments, especially the destabilization of the tetragonal structure for increased hydrogen concentration and complex magneticstructure of the cubic phases. The latter is analyzed in terms of Fermi surface nesting.

  • 3.
    Arapan, Sergiu
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    De Almeida, Jailton Souza
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Formation of sp(3) hybridized bonds and stability of CaCO3 at very high pressure2007In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 98, no 26, p. 268501-Article in journal (Refereed)
    Abstract [en]

    By performing ab initio electronic structure calculations, we observed a new high-pressure phase transition within the Pmcn structure of CaCO3. This transition is characterized by the change of the carbon's sp hybridization state and is driven by the intrinsic property of the carbon atom to form tetrahedral covalent bonds at high pressure. The formation of sp(3) hybridized bonds explains the stability of MgCO3 and CaCO3 at Earth's lower mantle pressure conditions and may serve as a criterion for searching new possible high-pressure phases of carbon bearing minerals.

  • 4.
    Araujo, Carlos Moyses
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Osorio Guillén, Jorge Mario
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Jena, P
    Role of titanium in hydrogen desorption in crystalline sodium alanate2005In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 86, no 25, article id 251913Article in journal (Refereed)
    Abstract [en]

    The role of Ti in improving the thermodynamics of hydrogen desorption in crystalline sodium alanate (NaAlH4) has been investigated by using the density functional theory. The total energy calculations reveal that Ti prefers to occupy the Na site over that of the Al site when the atomic energies are used as the reference. However, the use of the cohesive energies of Al, Na, and Ti leads to the Al site being the least unfavourable site. Irrespective of whether Ti occupies the Na or the Al site, the energy necessary to remove a hydrogen atom from Ti substituted sodium alanate is significantly lowered from that of the pure alanate. The understanding gained here may help in designing hydrogen storage materials suitable for industrial applications.

  • 5.
    Araújo, C. Moysés
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Blomqvist, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ti-induced destabilization of NaBH4 from first-principles theory2008In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, no 12, p. 122202-Article in journal (Refereed)
    Abstract [en]

    In this work, we have employed first-principles total energy calculations and ab initio molecular dynamics simulations to investigate the Ti doping of NaBH4. We show that Ti destabilizes the BH4 cages, which in turn increases the mobility of hydrogen atoms. Such an effect is shown to be due to the formation of B-Ti bonds, rather than the lowering of the BH4 charge state as expected. These results indicate that Ti may catalyse the dehydrogenation process in NaBH4 as it does for NaAlH4.

  • 6.
    Araújo, C. Moysés
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Thermodynamic analysis of hydrogen sorption reactions in Li-Mg-N-H systems2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 2, p. 021907-Article in journal (Refereed)
    Abstract [en]

    We report a comprehensive first-principles study of the thermodynamics of the hydrogen release reaction from xLiH-yMg(NH2)(2) mixtures involving the composition ratios (x=2, y=1), (x=8, y=3), and (x=12, y=3), with special emphasis on the effect of the different intermediate steps. For all three mixing ratios of LiH/Mg(NH2)(2) we find that the hydrogen release is initiated by the same reaction with an enthalpy of 46.1 kJ/mol of H-2 in excellent agreement with recent experimental results. Additionally, we also investigated the substitution of LiH by MgH2 as reaction partner of Mg(NH2)(2) in the fully hydrogenated state.

  • 7. Belonoshko, A. B.
    et al.
    Burakovsky, L.
    Chen, S. P.
    Johansson, B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Mikhaylushkin, A. S.
    Preston, D. L.
    Simak, S. I.
    Swift, D. C.
    Molybdenum at high pressure and temperature: Melting from another solid phase2008In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 100, no 13, p. 135701-Article in journal (Refereed)
    Abstract [en]

    The Gibbs free energies of bcc and fcc Mo are calculated from first principles in the quasiharmonic approximation in the pressure range from 350 to 850 GPa at room temperatures up to 7500 K. It is found that Mo, stable in the bcc phase at low temperatures, has lower free energy in the fcc structure than in the bcc phase at elevated temperatures. Our density-functional-theory-based molecular dynamics simulations demonstrate that fcc melts at higher than bcc temperatures above 1.5 Mbar. Our calculated melting temperatures and bcc-fcc boundary are consistent with the Mo Hugoniot sound speed measurements. We find that melting occurs at temperatures significantly above the bcc-fcc boundary. This suggests an explanation of the recent diamond anvil cell experiments, which find a phase boundary in the vicinity of our extrapolated bcc-fcc boundary.

  • 8.
    Bergman, Anders
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Holmström, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Niklasson, A.M.N.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Frota-Pessôa, Sonia
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Magnetism of Fe clusters embedded in a Co matrix from first-principles theory2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 17, p. 174446-Article in journal (Refereed)
    Abstract [en]

    We have calculated spin and orbital moments for Fe clusters of sizes up to 700 atoms embedded as impurities in a bcc Co matrix. The calculations have been carried out using relativistic first-principles real-space density functional theory, and we have made a comparison with earlier experimental studies. For Fe atoms close to theFeCo interface, the spin moments are found to increase while atoms far from the interface exhibit bulklike moments. The Co moments remain essentially unchanged and close to the moment of bulk bcc Co. With increasing cluster size, the average moments of the cluster atoms decrease due to the decreased surface to volume ratio. The orbital moments of both Fe and Co are calculated to be small and they stay almost constant regardless of cluster size. Our results for spin moments agree with experimental data but the calculated orbital moments are slightly underestimated. A simplified model indicates that a compound of close-packed Fe clusters surrounded by Co show higher average total moments compared to bulk and multilayer systems with a similar concentration. This increase seems to disappear when cluster-cluster interactions are taken into account. The general trend is that for a given alloy concentration of FexCo1−x, clustering tends to lower the average magnetic moment compared to that of ordered structures and random alloys.

  • 9.
    Bergqvist, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Kudrnovsky, Josef
    Drchal, Vaclav
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Turek, Ilja
    Magnetic properties and disorder effects in diluted magnetic semiconductors2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 19, p. 195210-Article in journal (Refereed)
    Abstract [en]

    We present calculations of the exchange interactions and critical temperatures for several diluted magnetic semiconductor systems. It is shown that the exchange interactions are dominated by short-ranged interactions that have a strong directional dependence. Using a combination of first-principles calculations of the exchange interactions together with Monte Carlo simulations of the classical Heisenberg model, in which the positional disorder and spin fluctuations are properly included, the calculated critical temperatures are in good agreement with experimantal observations. It is shown that agreement between theory and experiment, as regards ordering temperatures, is obtained only when the magnetic atoms are randomly positioned in a simulation cell which proves that disorder effects play a very important role. The effect of strong electron-electron interaction has been studied by means of the LSDA+U scheme. We investigate in detail the nature of the anisotropic exchange interactions by means of a Fermi surface analysis.

  • 10.
    Bergqvist, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Korzhavyi, Pavel
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Mirbt, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Abrikosov, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Smirnova, E.
    Mohn, Peter
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Magnetic and electronic structure of (Ga1-xMnx)As2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 20, p. 205201-Article in journal (Refereed)
  • 11.
    Blomqvist, Andreas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Araújo, C. Moysés
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Srepusharawoot, P.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Li-decorated metal-organic framework-5: A route to achieving a suitable hydrogen storage medium2007In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 104, no 51, p. 20173-Article in journal (Refereed)
    Abstract [en]

    A significant improvement in molecular hydrogen uptake properties is revealed by our ab initio calculations for Li-decorated metal-organic framework 5. We have found that two Li atoms are strongly adsorbed on the surfaces of the six-carbon rings, one on each side, carrying a charge of +0.9e per Li atom. Each Li can cluster three H-2 molecules around itself with a binding energy of 12 kJ (mol H-2)(-1). Furthermore, we show from ab initio molecular dynamics simulations with a hydrogen loading of 18 H2 per formula unit that a hydrogen uptake of 2.9 wt % at 200 K and 2.0 wt % at 300 K is achievable. To our knowledge, this is the highest hydrogen storage capacity reported for metal-organic framework 5 under such thermodynamic conditions.

  • 12. Borman, V. D.
    et al.
    Borisyuk, P. V.
    Vasiliev, O. S.
    Pushkin, M. A.
    Tronin, V. N.
    Tronin, I. V.
    Troyan, V. I.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Observation of electron localization in rough gold nanoclusters on the graphite surface2007In: JETP Letters: Journal of Experimental And Theoretical Physics Letters, ISSN 0021-3640, E-ISSN 1090-6487, Vol. 86, no 6, p. 393-397Article in journal (Refereed)
    Abstract [en]

    The results of scanning tunneling spectroscopy of the electronic states of Au nanoclusters on the graphite surface are presented. The tunneling current is found to be different at different points of a rough-surface nano-cluster. The measured differential current-voltage curve of the clusters is nonmonotonic near the Fermi energy, and the tunneling conductance decreases by almost a factor of two as the cluster volume changes from I to 0.1 mm(3). This decrease can be associated with the change in the density of the electronic states near the Fermi energy. The observed features are qualitatively described within the framework of the mechanism of electron localization in disordered systems.

  • 13. Borman, V. D.
    et al.
    Tronin, I. V.
    Tronin, V. N.
    Troyan, V. I.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Formation of high- and low-density clusters in a 1D system2008In: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 40, no 3, p. 643-648Article in journal (Refereed)
    Abstract [en]

    The properties of monoatomic chains have been studied theoretically by means of statistical mechanics methods. The applied approach can be used to evaluate the interatomic distances and lifetimes of one-dimensional (1D) and quasi-1D systems. In particular, we show that the 1D clusters of gold atoms can exist in two states with different lattice parameters (similar to 3.6 and similar to 2.8 angstrom) that can explain the whole variety of experimental observations on monoatomic gold chains without assuming any wire contamination.

  • 14.
    Burkert, Till
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    James, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Simak, Sergei I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Calculation of uniaxial magnetic anisotropy energy of tetragonal and trigonal Fe, Co, and Ni2004In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 69, no 10, p. 104426-Article in journal (Refereed)
  • 15.
    Burkert, Till
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Simak, Sergei I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ruban, Andrei V.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Wills, John M.
    Magnetic anisotropy of L10 FePt and Fe1-xMnxPt2005In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 71, no 13, p. 134411-Article in journal (Refereed)
    Abstract [en]

    The uniaxial magnetic anisotropy energy (MAE) of L10 FePt and Fe1−xMnxPt, x=0−0.25, was studied from first principles using two fully relativistic computational methods, the full-potential linear muffin-tin orbitals method and the exact muffin-tin orbitals method. It was found that the large MAE of 2.8 meV/f.u. is caused by a delicate interaction between the Fe and Pt atoms, where the large spin-orbit coupling of the Pt site and the hybridization between Fe 3d and Pt 5d states is crucial. The effect of random order on the MAE was modeled by mutual alloying of the sublattices within the coherent potential approximation (CPA), and a strong dependence of the MAE on the degree of chemical long-range order was found. The alloying of FePt with Mn was investigated with the virtual crystal approximation and the CPA as well as supercell calculations. The MAE increases up to 33% within the concentration range studied here, an effect that is attributed to band filling. Furthermore, the dependence of the MAE on the structural properties was studied.

  • 16.
    Burkert, Till
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Heinonen, Olle
    Giant Magnetic Anisotropy in Tetragonal FeCo Alloys2004In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 93, no 2, p. 027203-Article in journal (Refereed)
    Abstract [en]

    The magnetic anisotropy energy (MAE) of hcp Gd was calculated from first principles using the full-potential linear muffin-tin orbital method. It was found that the principal contributions to the MAE are the dipole-dipole interaction between the localized 4f spins and the spin-orbit interaction of the valence band states. The dipole contribution has the form 1/2K(1)(d)(1-cos 2 theta), where theta is the angle between the magnetization direction and the c axis. The contribution of the spin-orbit interaction is shown to arise from the polarization of the conduction band that becomes exchange split due to exchange interaction with the localized 4f electrons. We argue that this leads to significant contributions from higher order anisotropy constants. An imposed reduced 4f moment leads to a repopulation of the electronic states at the Fermi level and a reduced exchange splitting of the valence states, which we demonstrate leads to a modification of the MAE. This modification is in qualitative agreement with the observed temperature dependence of the MAE. In addition, the dependence of the MAE on the c/a ratio has been studied.

  • 17.
    Colarieti-Tosti, M.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Brooks, Michael S. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Wills, J. M.
    Crystal field levels in lanthanide systems2001In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 226-230, no Part 1, p. 1027-1028Article in journal (Refereed)
    Abstract [en]

    Crystalfieldlevels in lanthanide compounds have been calculated by first-principles theory, combining the strength of the standard model for rare earths in treating the localized 4f electrons and the power of Density Functional Theory in describing the, weakly correlated, delocalized, electrons. The stability of our calculations has been checked against changes of the constraints in the Full Potential-LMTO implementation. The assumption of weak 4f–4f inter-atomic interactions has also been checked by means of supercell calculations. Suggestions for the treatment of magnetic systems on a similar basis are given.

  • 18.
    Colarieti-Tosti, M.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Simak, S. I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Åberg, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Edvardsson, S.
    Brooks, Michael S. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Origin of Magnetic Anisotropy of Gd Metal2003In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 91, no 15, p. 157201-157204Article in journal (Refereed)
    Abstract [en]

    Using first-principles theory, we have calculated the energy of Gd as a function of spin direction, θ, between the c and a axes and found good agreement with experiment for both the total magnetic anisotropy energy and its angular dependence. The calculated low temperature direction of the magnetic moment lies at an angle of 20° to the c axis. The calculated magnetic anisotropy energy of Gd metal is due to a unique mechanism involving a contribution of 7.5  μeV from the classical dipole-dipole interaction between spins plus a contribution of 16  μeV due to the spin-orbit interaction of the conduction electrons. The 4f spin polarizes the conduction electrons via exchange interaction, which transfers the magnetic anisotropy of the conduction electrons to the 4f spin.

  • 19.
    Colarieti-Tosti, Massimiliano
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Burkert, Till
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Brooks, Michael S. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Theory of the temperature dependence of the easy axis of magnetization in hcp Gd2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 9, p. 094423-Article in journal (Refereed)
    Abstract [en]

    The magnetic anisotropy energy (MAE) of hcp Gd was calculated from first principles using the full-potential linear muffin-tin orbital method. It was found that the principal contributions to the MAE are the dipole-dipole interaction between the localized 4f spins and the spin-orbit interaction of the valence band states. The dipole contribution has the form 1/2K(1)(d)(1-cos 2 theta), where theta is the angle between the magnetization direction and the c axis. The contribution of the spin-orbit interaction is shown to arise from the polarization of the conduction band that becomes exchange split due to exchange interaction with the localized 4f electrons. We argue that this leads to significant contributions from higher order anisotropy constants. An imposed reduced 4f moment leads to a repopulation of the electronic states at the Fermi level and a reduced exchange splitting of the valence states, which we demonstrate leads to a modification of the MAE. This modification is in qualitative agreement with the observed temperature dependence of the MAE. In addition, the dependence of the MAE on the c/a ratio has been studied.

  • 20. Dantas, N. Souza
    et al.
    de Almeida, J. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, R.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Persson, C.
    da Silva, A. Ferreira
    Novel semiconducting materials for optoelectronic applications: Al1-xTlxN alloys2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 12, p. 121914-Article in journal (Refereed)
    Abstract [en]

    We proprose the ternary semiconducting Al1-xTlxN alloys as new material for optoelectronic applications. Ab initio calculations have been performed to study structural, electronic, and optical properties of the theoretically designed thallium-aluminum based nitride alloys. We found that the lattice constants vary linearly with thallium composition whereas the band gap and absorption edge span from ultraviolet to infrared energy region by increasing thallium content which make the predicted material interesting for infrared optical devices among other technological applications.

  • 21. Davis, Sergio M.
    et al.
    Belonoshko, Anatoly B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    van Duin, Adri C. T.
    High-pressure melting curve of hydrogen2008In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 129, no 19, p. 194508-Article in journal (Refereed)
    Abstract [en]

    The melting curve of hydrogen was computed for pressures up to 200 GPa, using molecular dynamics. The inter- and intramolecular interactions were described by the reactive force field (ReaxFF) model. The model describes the pressure-volume equation of state solid hydrogen in good agreement with experiment up to pressures over 150 GPa, however the corresponding equation of state for liquid deviates considerably from density functional theory calculations. Due to this, the computed melting curve, although shares most of the known features, yields considerably lower melting temperatures compared to extrapolations of the available diamond anvil cell data. This failure of the ReaxFF model, which can reproduce many physical and chemical properties (including chemical reactions in hydrocarbons) of solid hydrogen, hints at an important change in the mechanism of interaction of hydrogen molecules in the liquid state.

  • 22.
    Dziekan, Thomas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Zahn, Peter
    Meded, Velimir
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Mirbt, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Theoretical calculations of mobility enhancement in strained silicon2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 19, p. 195213-Article in journal (Refereed)
    Abstract [en]

    The conductivity enhancement of n -doped Si due to strain has been calculated by solving the Boltzmann equation within the relaxation-time approximation. For T=0 , we determined the Fermi surface from first principles, whereas for T>0 , we modeled the Fermi surface using the effective-mass approximation. We explain the saturation of the conductivity enhancement with increasing strain in terms of the topology of the Fermi surface. We find in accordance with experiment a decreasing enhancement with increasing doping concentration.

  • 23. Edvardsson, S.
    et al.
    Åberg, D.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Materials Science, Condensed Matter Theory.
    An atomic program for energy levels of equivalent electrons: lanthanides and actinides2001In: Comput. Phys. Commun., ISSN 0010-4755, Vol. 133, p. 396-406Article in journal (Refereed)
  • 24. Edvardsson, S.
    et al.
    Åberg, D.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Materials Science, Condensed Matter Theory.
    The energy matrix using determinantal product states applied to Ho:YAG2000In: J. Alloys Compd., ISSN 0925-8388, Vol. 303, p. 280-284Article in journal (Refereed)
  • 25.
    Edvardsson, Sverker
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Åberg, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Uddholm, Per
    A program for accurate solutions of two-electron atoms2005In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 165, no 3, p. 260-270Article in journal (Refereed)
    Abstract [en]

    We present a comprehensible computer program capable of treating non-relativistic ground and excited states for a two-electron atom having infinite nuclear mass. An iterative approach based on the implicitly restarted Arnoldi method (IRAM) is employed. The Hamiltonian matrix is never explicitly computed. Instead the action of the Hamiltonian operator on discrete pair functions is implemented. The finite difference method is applied and subsequent extrapolations gives the continuous grid result. The program is written in C and is highly optimized. All computations are made in double precision. Despite this relatively low degree of floating point precision (48 digits are not uncommon), the accuracy in the results can reach about 10 significant figures. Both serial and parallel versions are provided. The parallel program is particularly suitable for shared memory machines such as the Sun Starcat series. The serial version is simple to compile and should run on any platform.

  • 26. Enkovaara, Jussi
    et al.
    Ayuela, Andres
    Jaklanen, J.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Nieminen, Risto
    First-principles calculations of spin spirals in Ni2MnGa and Ni2MnAl2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 5, p. 054417-Article in journal (Refereed)
    Abstract [en]

    We report here noncollinear magnetic configurations in the Heusler alloys Ni2MnGa and Ni2MnAl which are interesting in the context of the magnetic shape memory effect. The total energies for different spin spirals are calculated and the ground-state magnetic structures are identified. The calculated dispersion curves are used to estimate the Curie temperature which is found to be in good agreement with experiments. In addition, the variation of the magnetic moment as a function of the spiral structure is studied. Most of the variation is associated with Ni, and symmetry constraints relevant for the magnetization are identified. Based on the calculated results, the effect of the constituent atoms in determining the Curie temperature is discussed.

  • 27.
    Fang, Changming
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Local structure and electronic-spin transition of Fe-bearing MgSiO3 perovskite under conditions of the Earth's lower mantle2008In: Physics of the Earth and Planetary Interiors, ISSN 0031-9201, E-ISSN 1872-7395, Vol. 166, no 1-2, p. 77-82Article in journal (Refereed)
    Abstract [en]

    We report first-principles electronic structure calculations on the structural and electronic-spin behaviours of Fe-bearing MgSiO3 crystals up to the pressure of Earth's mantle. The transition pressure of the Fe-bearing MgSiO3 from the orthorhombic perovskite (OPv) to the orthorhombic post-perovskite (OPPv) phase decreases with increasing Fe concentration. The lattice distortion has impacts on the electronic-spin behaviour of the Fe ions in the PVs. The spin-polarizations of the Fe ions in the (Fe,Mg)SiO3 OPvs and OPPvs keep unchanged up to the pressures in the lower mantle. Meanwhile, the Fe-bearing MgSiO3 OPV Containing Fe-Mg-Fe-Si pairs exhibits multiple-magnetic moments co-existing in a large pressure range (from about 78 to 110 GPa), and finally becomes non-magnetic at pressure higher than 110 GPa. These results provide a mechanism to understand the recent experimental results about Fe valence states and the electronic transitions of the Fe-bearing MgSiO3 under high pressure.

  • 28. Gowtham, S.
    et al.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Pandey, Ravindra
    Karna, Shashi P.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    First-principles study of physisorption of nucleic acid bases on small-diameter carbon nanotubes2008In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 19, no 12, p. 125701-Article in journal (Refereed)
    Abstract [en]

    We report the results of our first-principles study based on density functional theory on the interaction of the nucleic acid base molecules adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U), with a single-walled carbon nanotube (CNT). Specifically, the focus is on the physisorption of base molecules on the outer wall of a (5, 0) metallic CNT possessing one of the smallest diameters possible. Compared to the case for CNTs with large diameters, the physisorption energy is found to be reduced in the high-curvature case. The base molecules exhibit significantly different interaction strengths and the calculated binding energies follow the hierarchy G>A>T>C>U, which appears to be independent of the tube curvature. The stabilizing factor in the interaction between the base molecule and CNT is dominated by the molecular polarizability that allows a weakly attractive dispersion force to be induced between them. The present study provides an improved understanding of the role of the base sequence in deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) in their interactions with carbon nanotubes of varying diameters.

  • 29. Gupta, Amita
    et al.
    Owens, Frank J.
    Rao, K. V.
    Iqbal, Zafar
    Guille, J. M. Osorio
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    High-temperature ferromagnetism in Cu-doped GaP by SQUID magnetometry and ferromagnetic resonance measurements2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 22, p. 224449-Article in journal (Refereed)
    Abstract [en]

    The search for ferromagnetism above room temperature in semiconductors doped with paramagnetic ions has intensified in recent years because of the potential of combining magnetic information storage and electronic switching in one spintronic device. Here we report an observation of ferromagnetism well above room temperature in gallium phosphide doped with Cu2+ detected by ferromagnetic resonance and SQUID magnetometry. Other important features of the results in this p-type Cu-doped GaP are the high Curie temperature above 700 K significantly higher than previous observations, the relatively simple low-temperature bulk sintering process used to synthesize the material, which will significantly reduce the cost of large-scale production, and the use of copper as the dopant rather than manganese, which precludes ferromagnetic clusters or magnetic alloy impurities as the origin of the ferromagnetism. Ab initio calculations also show the existence of ferromagnetism in Cu-doped GaP. When the spin-orbit coupling is included, the total moment is enhanced and we get a total magnetic moment of 0.31 mu(B) with a local moment on Cu 0.082 and on P 0.204 mu(B).

  • 30.
    Hallstedt, Bengt
    et al.
    Materials Chemistry, RWTH Aachen University, Aachen, Germany .
    Music, Denis
    Materials Chemistry, RWTH Aachen University, Aachen, Germany .
    Sun, Zhimei
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Thermodynamic evaluation of the Al-Cr-C system2006In: International Journal of Materials Research - Zeitschrift für Metallkunde, ISSN 1862-5282, E-ISSN 2195-8556, Vol. 97, no 5, p. 539-542Article in journal (Refereed)
    Abstract [en]

    The system Al-Cr-C contains one ternary phase, Cr2AlC. By combining a Calphad assessment with ab initio calculations and differential thermal analysis, the Gibbs energy of Cr2AlC could be determined and the complete phase diagram calculated. Cr2AIC melts incongruently at about 1773 K to form Cr7C3 (or Cr3C2) and Al4C3 in addition to liquid. According to the ab initio calculations the equilibrium composition of Cr2AIC is very nearly stoichiometric, but can dissolve some Al metastably by replacing Cr. The agreement between the thermodynamic calculation and the various sources of information is excellent.

  • 31.
    Holmström, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Abrikosov, Igor A.
    Dugdale, S.B.
    Györffy, B.L.
    Fermi-surface effect on magnetic interlayer coupling2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 6, p. 064408-Article in journal (Refereed)
    Abstract [en]

    The oscillating magnetic interlayer coupling of Fe over spacer layers consisting of CuxPd1−x alloys is investigated by first principles density functional theory. The amplitude, period, and phase of the coupling, as well as the disorder-induced decay, are analyzed in detail and the consistency to the Ruderman-Kittel-Kasuya-Yoshida theory is discussed. An effect of the Fermi surface nesting strength on the amplitude is established from first principles calculations. An unexpected variation of the phase and disorder-induced decay is obtained and the results are discussed in terms of asymptotics.

  • 32.
    Holmström, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Bergqvist, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Skubic, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Structural and magnetic aspects of multilayer interfaces2004In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 272-276, no Suppl., p. E941-E942Article in journal (Refereed)
  • 33. Hu, Q. M.
    et al.
    Yang, R.
    Lu, J. M.
    Wang, L.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Effect of Zr on the properties of (TiZr)Ni alloys from first-principles calculations2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 22, p. 224201-Article in journal (Refereed)
    Abstract [en]

    The effect of Zr on the martensitic transformation (MT) behavior and mechanical properties of (Ti0.5-xZrx)Ni-0.5 alloys is investigated by calculating the elastic constants and elastic moduli in the B2 phase as a function of x for 0 <= x <= 0.2. The calculations are performed using the coherent potential approximation implemented within the framework of the exact muffin-tin orbitals method. We find that the theoretical elastic properties correlate well with the behavior of the MT. With increasing Zr concentration, the anisotropy of the alloy decreases, indicating that the nonbasal plane shear on which the modulus C-44 plays an important role, dominates and, therefore, a monoclinic martensitic phase should result. The experimental Zr content dependence of the MT temperature is paralleled with the calculated C-44 versus Zr content. The theoretical elastic moduli demonstrate that the (TiZr)Ni alloys, with Zr distributed randomly on the Ti sublattice, are intrinsically ductile, which suggests that the poor ductility of these alloys may be ascribed to some other factors, for example, impurities, precipitation, and grain boundaries.

  • 34. Hu, Qing-Miao
    et al.
    Kádas, Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Hogmark, Sture
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Yang, R.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Predicting hardness of covalent/ionic solid solution from first-principles theory2007In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, no 12, p. 121918-Article in journal (Refereed)
    Abstract [en]

    We introduce a hardness formula for the multicomponent covalent and ionic solid solutions. This expression is tested on nitride spinel materials A3N4 (A=C,Si,Ge) and applied to titanium nitrogen carbide (TiN1-xCx with 0<=x<=1), off-stoichiometric transition-metal nitride (TiN1-x and VN1-x with x<=0.25), and B-doped semiconductors (C1-xBx, Si1-xBx, and Ge1-xBx with x<=0.1). In all cases, the theoretical hardness is in good agreement with experiments.

  • 35. Hu, Qing-Miao
    et al.
    Kádas, Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Hogmark, Sture
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Yang, Rui
    Johansson, Börje
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Hardness and elastic properties of covalent/ionic solid solutions from first-principles theory2008In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 103, no 8, p. 083505-Article in journal (Refereed)
    Abstract [en]

    Most of the engineering materials are alloys (solid solutions) and inevitably contain some impurities or defects such as vacancies. However, theoretical predictions of the hardness of this kind of materials have rarely been addressed in literature. In this paper, a hardness formula for multicomponent covalent solid solution is proposed based on the work of Simunek and Vackar [Phys. Rev. Lett. 96, 085501 (2006)]. With this formula, the composition dependence of the hardness is investigated for titanium nitrogencarbide (TiN1-xCx), off-stoichiometric transition-metal nitrides (TiN1-x and VN1-x), and B-doped semiconductors. The predicted hardness is in good agreement with experiments. To investigate the most frequently quoted correlation between hardness and elastic modulus, the elastic moduli of the systems involved in this paper have also been calculated. The results show that the elastic moduli cannot be used for rigorous predictions of the hardness of the solid solutions.

  • 36.
    Hugosson, Håkan W.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Theoretical studies of substitutional impurities in molybdenum carbide1999In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 60, no 22, p. 15123-15130Article in journal (Refereed)
    Abstract [en]

    First-principles full-potential linear muffin-tin orbital calculations have been used to study the effect on the cohesion and electronic structure of cubic δ-MoC when 25% of the carbon is substituted for boron, nitrogen, or oxygen and when 25% of the molybdenum is substituted for niobium, tungsten, or ruthenium. A thorough study of the changes in the electronic structure and the effect of these on the properties of the compounds is made. Special attention is paid to the character (ionic, covalent, or metallic) of the states becoming occupied (or unoccupied) due to the substitution. A study is also made on the properties of the quaternary alloy Mo0.75W0.25C0.75N0.25. This substitution is shown to harden δ-MoC.

  • 37.
    Höglund, Andreas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Castleton, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
    Mirbt, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Diffusion mechanism of Zn in InP and GaP from first principles2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 11, p. 113201-Article in journal (Refereed)
    Abstract [en]

    The diffusion mechanism of Zn in GaP and InP has been investigated using first-principles computational methods. It is found that the kickout mechanism is the favored diffusion process under all doping conditions for InP, and under all except n-type conditions for GaP. In n-type GaP the dissociative mechanism is probable. In both p-type GaP and InP, the diffusing species is found to be Zn. The activation energy for the kickout process is 2.49 eV in GaP and 1.60 eV in InP, and therefore unintentional diffusion of Zn should be a larger concern in InP than in GaP. The dependence of the activation energy both on the doping conditions of the material and on the stoichiometry is explained, and found to be in qualitative agreement with the experimentally observed dependencies. The calculated activation energies agree reasonably with experimental data, assuming that the region from which Zn diffuses is p type. Explanations are also found as to why Zn tends to accumulate at pn junctions in InP and to why a relatively low fraction of Zn is found on substitutional sites in InP.

  • 38.
    Höglund, Andreas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Castleton, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
    Mirbt, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Increasing the equilibrium solubility of dopants in semiconductor multilayers and alloys2008In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 100, no 10, p. 105501-Article in journal (Refereed)
    Abstract [en]

    We have theoretically studied the possibility to control the equilibrium solubility of dopants in semiconductor alloys, by strategic tuning of the alloy concentration. From the modeled cases of C-0 in SixGe1-x, Zn- and Cd- in GaxIn1-xP it is seen that under certain conditions the dopant solubility can be orders of magnitude higher in an alloy or multilayer than in either of the elements of the alloy. This is found to be due to the solubility's strong dependence on the lattice constant for size mismatched dopants. The equilibrium doping concentration in alloys or multilayers could therefore be increased significantly. More specifically, Zn- in a GaxIn1-xP multilayer is found to have a maximum solubility for x=0.9, which is 5 orders of magnitude larger than that of pure InP.

  • 39.
    Isaev, Eyvaz I.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Vekilov, Yuri K.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Dynamical stability of Fe-H in the Earth's mantle and core regions2007In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 104, no 22, p. 9168-9171Article in journal (Refereed)
    Abstract [en]

    The core extends from the depth of 2,900 km to the center of the Earth and is composed mainly of an iron-rich alloy with nickel, with 10% of the mass comprised of lighter elements like hydrogen, but the exact composition is uncertain. We present a quantum mechanical first-principles study of the dynamical stability of FeH phases and their phonon densities of states at high pressure. Our free-energy calculations reveal a phonon-driven stabilization of dhcp FeH at low pressures, thus resolving the present contradiction between experimental observations and theoretical predictions. Calculations reveal a complex phase diagram for FeH under pressure with a dhcp -> hcp -> fcc sequence of structural transitions.

  • 40.
    Kochetov, A. E.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Mikhaylushkin, A. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Interaction of Au nanowires with impurities2008In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 61, no 4, p. 441-444Article in journal (Refereed)
    Abstract [en]

    We report the results of our first-principles studies of the interaction between an infinite monoatomic gold nanowire and a carbon-monoxide molecule. We show that the gold monoatomic nanowire is capable of absorbing the CO molecule at the distances of about 1.8 angstrom and forms a bond with the carbon atom. Further, we find that dissociation of the CO molecule as the source of gold nanowire contamination with carbon, which is widely discussed in literature as the possible reason for the striking stability of gold nanowires under stretching, is thermodynamically unfavored.

  • 41. Kokko, K.
    et al.
    Ropo, M.
    Punkkinen, M. P. J.
    Laukkanen, P.
    Alatalo, M.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Kollar, J.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Surface core-level shift of Pd at the AgcPd1-c(111) surface: Nonlinear subsurface effects2007In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 601, no 23, p. 5419-5423Article in journal (Refereed)
    Abstract [en]

    The surface core-level binding-energy shift (SCLS) of Pd at the AgcPd1-c(111) surface is calculated as a function of bulk concentration of the alloy. The equilibrium volume and the surface concentration profile used in the calculations refer to the 0 K case. The SCLSs are evaluated within the Z + 1 approximation. The results are analysed using the mixing enthalpy of the alloy and the bulk and surface chemical potentials. A relation of the SCLS to the bulk concentration is considered. This relation is shown to be mediated by the surface concentration profile which induces the observed nonlinear behaviour. The results are interpreted using a simple model for the alloy electronic structure.

  • 42. Kollár, J.
    et al.
    Vitos, Levente
    Osorio-Guillén, J. M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Calculation of surface stress for fcc transition metals2003In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 68, no 24, p. 245417-Article in journal (Refereed)
    Abstract [en]

    Using the density functional theory, formulated within the framework of the exact muffin-tin orbitals method, we have calculated the surface stress for the (111) free surfaces of the fcc 4d and 5d transition metals. Good agreement is obtained with the available ab initio data for Pd, Ir, and Au, while for Pt we predict a surface stress, which is about 33% lower compared to former theoretical results. The present surface stress values for the 4d and 5d fcc metals show the typical trend characteristic for the cohesive or surface energies of d series.

  • 43.
    Koči, L.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, R.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Belonoshko, A. B.
    The impact of system restriction in molecular dynamics applied to the melting of Ne at high pressure2008In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 44, no 2, p. 605-610Article in journal (Refereed)
    Abstract [en]

    There are two major ways to perform molecular dynamics (MD) calculations, namely classical and ab initio MD. As ab initio techniques require considerably longer calculation times, it is of interest to compare the results of the two methods. Furthermore, when melting is studied with MD, the use of coexistent solid and liquid structures (two-phase) in the calculations, instead of only a solid structure (one-phase), can have a substantial impact on the results   obtained. In this work, comparisons have been made between classical and ab initio methods applied to one- and two-phase systems for the melting of Ne at high pressure. The temperatures needed to melt the classical one-phase system are somewhat higher compared to the two-phase temperatures, evaluated at the same pressure. Furthermore,   there is a significant discrepancy comparing the one-phase ab initio curve to previously reported classical predictions. At 150 GPa, the calculations in this work show a melting temperature approximately 1000 K above the estimate based on an exponential-6 potential. However, there is a close match between the one-phase ab initio curve and the classical one-phase results in this work. This suggests a possible agreement between a two-phase ab initio and classical two-phase melting curve.

  • 44.
    Koči, L.
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Materials Science, Condensed Matter Theory.
    Belonoshko, A. B.
    Ahuja, R.
    Ab initio calculations of hcp and bcc Fe at extreme conditionsManuscript (Other academic)
  • 45.
    Koči, L.
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Materials Science, Condensed Matter Theory.
    Kim, D. Y.
    de Almeida, J. S.
    Mattesini, M.
    Isaev, E.
    Ahuja, R.
    Mechanical stablity of TiO2 polymorphs under pressure: ab initio calculationsIn: Journal of Applied PhysicsArticle in journal (Refereed)
  • 46.
    Koči, Love
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Materials Science, Condensed Matter Theory.
    Studies of Material Properties using Ab Initio and Classical Molecular Dynamics2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, material properties have been examined under extreme conditions in computer-based calculations.

    The research on iron (Fe), nickel (Ni), and ferropericlase (Mg1-xFexO) are not only important for our understanding of the Earth, but also for an improved knowledge of these materials per se.

    An embedded-atom model for Fe demonstrated to reproduce properties such as structure factors, densities and diffusion constants, and was employed to evaluate temperature gradients at Earth core conditions. A similar interaction together with a two-temperature method was applied for the analysis of shock-induced melting of Ni. For Mg1-xFexO, the magnetic transition pressure was shown to increase with iron content. Furthermore, the C44 softening with pressure and iron composition supports the experimentally observed phase transition for Mg0.8Fe0.2O at 35 GPa.

    The properties of high density helium (He) is of great interest as the gas is one of the most abundant elements in the solar system. Furthermore, He and neon (Ne) are often used as pressure media in diamond anvil cells. The melting of He showed a possible fcc-bcc-liquid transition starting at T=340 K, P=22 GPa with a Buckingham potential, whereas the bcc phase was not seen with the Aziz form. For Ne, Monte Carlo calculations at ambient pressure showed very accurate results when extrapolating the melting temperatures to an infinite cluster limit. At high pressure, a one-phase ab initio melting curve showed a match with one-phase L-J potential results, which could imply a correspondence between ab initio/classical one-phase/two-phase calculations.

    In the search for hard materials, ab initio calculations for four TiO2 phases were compared. Just as imposed by experiment, the cotunnite phase was found to be very hard. The anomalous elastic behavior of the superconducting group-V metals V, Nb, Ta was found to be related to shrinking nesting vectors and the electronic topological transition (ETT).

    List of papers
    1. The impact of system restriction in molecular dynamics applied to the melting of Ne at high pressure
    Open this publication in new window or tab >>The impact of system restriction in molecular dynamics applied to the melting of Ne at high pressure
    2008 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 44, no 2, p. 605-610Article in journal (Refereed) Published
    Abstract [en]

    There are two major ways to perform molecular dynamics (MD) calculations, namely classical and ab initio MD. As ab initio techniques require considerably longer calculation times, it is of interest to compare the results of the two methods. Furthermore, when melting is studied with MD, the use of coexistent solid and liquid structures (two-phase) in the calculations, instead of only a solid structure (one-phase), can have a substantial impact on the results   obtained. In this work, comparisons have been made between classical and ab initio methods applied to one- and two-phase systems for the melting of Ne at high pressure. The temperatures needed to melt the classical one-phase system are somewhat higher compared to the two-phase temperatures, evaluated at the same pressure. Furthermore,   there is a significant discrepancy comparing the one-phase ab initio curve to previously reported classical predictions. At 150 GPa, the calculations in this work show a melting temperature approximately 1000 K above the estimate based on an exponential-6 potential. However, there is a close match between the one-phase ab initio curve and the classical one-phase results in this work. This suggests a possible agreement between a two-phase ab initio and classical two-phase melting curve.

    Keywords
    Classical MD, Ab initio MD, Melting, Ne
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97006 (URN)10.1016/j.commatsci.2008.05.011 (DOI)000261731700052 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    2. Mechanical stablity of TiO2 polymorphs under pressure: ab initio calculations
    Open this publication in new window or tab >>Mechanical stablity of TiO2 polymorphs under pressure: ab initio calculations
    Show others...
    In: Journal of Applied PhysicsArticle in journal (Refereed) Submitted
    Identifiers
    urn:nbn:se:uu:diva-97007 (URN)
    Available from: 2008-04-04 Created: 2008-04-04Bibliographically approved
    3. Dynamical stability of the hardest known oxide and the cubic solar material: TiO2
    Open this publication in new window or tab >>Dynamical stability of the hardest known oxide and the cubic solar material: TiO2
    2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, no 17, p. 171903-Article in journal (Refereed) Published
    Abstract [en]

    The authors have studied dynamical stability of different polymorphs of TiO2 using ab initio phonon calculations based on density functional theory in conjunction with force-constant method. Rutile TiO2 was found stable at ambient pressure, but unstable at high pressure. The calculated Raman frequency and phonon density of states (PDOS) of rutile TiO2 are in a good agreement with experiment. Concerning two cubic phases (solar materials), fluorite stabilized under pressure, whereas pyrite showed instability throughout the whole pressure range. Furthermore, the PDOS of cotunnite (the hardest known oxide) phase confirmed that it exists at high pressure and can be quenched down to a low pressure limit.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97008 (URN)10.1063/1.2731522 (DOI)000246568600028 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    4. Elasticity study of the superconducting metals V, Nb, Ta, Mo and W at high pressure
    Open this publication in new window or tab >>Elasticity study of the superconducting metals V, Nb, Ta, Mo and W at high pressure
    Show others...
    2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Physical Review B, Vol. 77, no 21, p. 214101-Article in journal (Refereed) Published
    Abstract [en]

    First-principles calculations have been performed for V, Nb, Ta, Mo, and W. The recently discovered bcc -> , rhombohedral transition for vanadium [Phys. Rev. Lett. 98, 085502 (2007)] was confirmed as the mechanical instability of c(44) was found at P=80 GPa. Furthermore, the c(11), c(12), and c(44) constants for the group-V elements showed erratic behaviors whereas the constants for the group-VI elements were monotonically increasing with pressure. The metals were analyzed with Fermi surface calculations, showing shrinking nesting vectors with pressure for V, Nb, and Ta but were not seen for Mo and W. From electronic topological transition contributions, a critical energy closely situated to the Fermi level for vanadium could be the reason why the elastic constants of V and Nb were difficult to reproduce at ambient pressure.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97009 (URN)10.1103/PhysRevB.77.214101 (DOI)000257288900022 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    5. Study of the high-pressure helium phase diagram using molecular dynamics
    Open this publication in new window or tab >>Study of the high-pressure helium phase diagram using molecular dynamics
    2007 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 1, p. 016206-Article in journal (Refereed) Published
    Abstract [en]

    The rich occurrence of helium and hydrogen in space makes their properties highly interesting. By means of molecular dynamics ( MD), we have examined two interatomic potentials for He-4. Both potentials are demonstrated to reproduce high-pressure solid and liquid equation of state (EOS) data. The EOS, solid - solid transitions and melting at high pressures ( P) were studied using a two-phase method. The Buckingham potential shows a good agreement with theoretical and experimental EOS, but does not reproduce experimental melting data. The Aziz potential shows a perfect match with theoretical melting data. We conclude that there is a stable body-centred-cubic (bcc) phase for He-4 at temperatures ( T) above 340 K and pressures above 22 GPa for the Buckingham potential, whereas no bcc phase is found for the Aziz potential in the applied PT range.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97010 (URN)10.1088/0953-8984/19/1/016206 (DOI)000243136300018 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    6. Towards accurate melting temperatures from Monte Carlo simulations for neon and argon: from clusters to the bulk
    Open this publication in new window or tab >>Towards accurate melting temperatures from Monte Carlo simulations for neon and argon: from clusters to the bulk
    Show others...
    Manuscript (Other academic)
    Identifiers
    urn:nbn:se:uu:diva-97011 (URN)
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2010-01-13Bibliographically approved
    7. Ab initio and classical molecular dynamics of neon melting at high pressure
    Open this publication in new window or tab >>Ab initio and classical molecular dynamics of neon melting at high pressure
    2007 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 21, p. 214108-Article in journal (Refereed) Published
    Abstract [en]

    First-principles and classical molecular dynamics calculations have been performed to study the high-pressure melting curve of Ne. In the low temperature region, simulations with solid and liquid in coexistence (two-phase) with a Lennard-Jones interatomic potential well reproduce experimental findings. As anticipated, there is a melting temperature overestimation when heating a crystal (one-phase) compared to the two-phase results. Furthermore, there is a significant discrepancy comparing the one-phase ab initio curve to previously reported classical predictions: at 150GPa , the calculations in this work show a melting temperature approximately 1000K above the estimate based on an exponential-6 potential. However, there is a close match between the one-phase ab initio curve and the classical one-phase results in this work. This could also imply an agreement between a two-phase ab initio and classical two-phase melting curve. Therefore, considering the documented accuracy of the coexistence method, the classical two-phase melting in this work could well indicate the most probable melting behavior. In conjunction with recent theoretical results for Xe, no significant melting slope decrease was observed for Ne in this study.

    Keywords
    Solid-liquid transitions
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97012 (URN)10.1103/PhysRevB.75.214108 (DOI)000247624700034 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    8. Melting of Na at high pressure from ab initio calculations
    Open this publication in new window or tab >>Melting of Na at high pressure from ab initio calculations
    2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 13, p. 132101-Article in journal (Refereed) Published
    Abstract [en]

    The alkali metals continue to attract attention due to their enigmatic melting behavior at high pressure. In this study, ab initio molecular dynamics (AIMD) calculations for Na using the projector augmented waves method and the generalized gradient approximation as exchange-correlation energy were performed. The melting temperature (T-m) results from the AIMD at extreme conditions confirm the recent findings of a T-m maximum and show a decrease of similar to 200 K between 45 and 120 GPa. Furthermore, from elastic constant calculations using the exact muffin-tin orbitals method, a Lindemann model curve was calculated. The model reproduces the trend of the experimental results of Gregoryanz [Phys. Rev. Lett. 94, 185502 (2005)] who reported an extraordinarily big melting temperature drop of similar to 700 K between 31 and 118 GPa.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97013 (URN)10.1103/PhysRevB.77.132101 (DOI)000255457200001 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    9. Simulation of shock-induced melting of Ni using molecular dynamics coupled to a two-temperature model
    Open this publication in new window or tab >>Simulation of shock-induced melting of Ni using molecular dynamics coupled to a two-temperature model
    Show others...
    2006 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 1, p. 012101-Article in journal (Refereed) Published
    Abstract [en]

    Using nonequilibrium molecular dynamics (MD) simulations we study shock-induced melting in Ni with an embedded atom method (EAM). Dynamic melting is probed by the pair correlation function, and we find a melting lattice temperature of T-melt=6400 +/- 300 K for a melting pressure of P-melt=275 +/- 10 GPa. When a combined MD+TTM (two-temperature model) approach is used to include electronic heat conduction and electron-phonon coupling, P-melt and T-melt change. For a given pressure, the temperature behind the shock decreases due to electronic heat diffusion into the cold, unshocked material. This cooling of the material behind the shock slightly increases the melting pressure compared to simulations without electronic heat conduction and electron-phonon coupling. The decrease in the temperature behind the shock front is enhanced if the electron-phonon coupling is artificially made larger. We also explore the feasibility of using x-ray diffraction to detect melting.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97014 (URN)10.1103/PhysRevB.74.012101 (DOI)000239426400001 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    10. Ab initio calculations of the elastic properties of ferropericlase Mg(1-x)Fe(x)O (x<=0.25)
    Open this publication in new window or tab >>Ab initio calculations of the elastic properties of ferropericlase Mg(1-x)Fe(x)O (x<=0.25)
    2007 (English)In: Physics of the Earth and Planetary Interiors, ISSN 0031-9201, E-ISSN 1872-7395, Vol. 164, no 3-4, p. 177-185Article in journal (Refereed) Published
    Abstract [en]

    Ferropericlase Mg1-xFexO is believed to be the second most abundant mineral in the Earth's mantle. Therefore, the electronic and elastic properties of ferropericlase are important for the understanding of the Earth's interior. Ab initio total energy calculations have been performed for Fe concentrations x ≤ 0.25. The equation of state (EOS) clearly shows a volume expansion as a function of Fe concentration, consistent with experimental data. Magnetic moment calculations as a function of pressure show a high-spin to low-spin transition of Fe2+, and the theoretical transition pressure increases with iron composition. At ambient pressure, we have found that the shear constant C44 reproduces well the experimental data as a function of Fe concentration. The MgO and Mg0.9Fe0.1O minerals show an increasing C44 with pressure, whereas the Δ C44 / Δ P is slightly negative after 26 GPa for Mg0.8Fe0.2O. The C44 softening could be related to the transition from the cubic to a rhombohedrally distorted phase, recently found by experiment.

    Keywords
    Ferropericlase, Elasticity, Pressure
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97015 (URN)10.1016/j.pepi.2007.06.012 (DOI)000250473900004 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    11. Molecular dynamics calculation of liquid iron properties and adiabatic temperature gradient in the Earth's outer core
    Open this publication in new window or tab >>Molecular dynamics calculation of liquid iron properties and adiabatic temperature gradient in the Earth's outer core
    2007 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 168, no 2, p. 890-894Article in journal (Refereed) Published
    Abstract [en]

    The knowledge of the temperature radial distribution in the Earth's core is important to understand the heat balance and conditions in the Earth's interior. Molecular dynamics (MD) simulations were applied to study the properties of liquid iron under the pressure-temperature conditions of the Earth's outer core. It is shown that the model used for the MD simulations can reproduce recent experimentally determined structure factor calculations to the highest pressure of 58 GPa. Applying this model for higher pressures, the calculated densities and diffusion parameters agree well with the results of first-principles. The MD calculations indicate that a reasonable estimate of the adiabatic temperature profile in the Earth's outer core could be evaluated.

    Keywords
    adiabat, core, iron, molecular dynamics
    National Category
    Physical Sciences Earth and Related Environmental Sciences
    Identifiers
    urn:nbn:se:uu:diva-97016 (URN)10.1111/j.1365-246X.2006.03256.x (DOI)000243754500031 ()
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    12. Molecular dynamics study of liquid iron under high pressure and high temperature
    Open this publication in new window or tab >>Molecular dynamics study of liquid iron under high pressure and high temperature
    2007 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 22, article id 224113Article in journal (Refereed) Published
    Abstract [en]

    Unlike ab initio calculations, classical potentials in molecular dynamics (MD) allow the treatment of very large atomic systems. However, the quality of the potentials has to be carefully assessed, making the evaluation of the models important. By means of MD simulations, we have studied the properties of liquid iron (Fe) with an embedded-atom method (EAM) under extreme pressure-temperature conditions, to resemble the conditions found in the Earth's outer core. The model used for the MD simulations is demonstrated to reproduce recent experimentally determined structure factor calculations to the highest pressure of 58 GPa. The calculations of densities and diffusion parameters at the various temperatures and pressures agree well with first-principles calculations. The results indicate the model's validity for the physical description of liquid iron at extreme conditions.

    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:uu:diva-97017 (URN)10.1103/PhysRevB.73.224113 (DOI)000238696300028 ()
    External cooperation:
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-14Bibliographically approved
    13. Ab initio calculations of hcp and bcc Fe at extreme conditions
    Open this publication in new window or tab >>Ab initio calculations of hcp and bcc Fe at extreme conditions
    Manuscript (Other academic)
    Identifiers
    urn:nbn:se:uu:diva-97018 (URN)
    Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2010-01-13Bibliographically approved
  • 47.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Pinsook, U.
    Melting of Na at high pressure from ab initio calculations2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 13, p. 132101-Article in journal (Refereed)
    Abstract [en]

    The alkali metals continue to attract attention due to their enigmatic melting behavior at high pressure. In this study, ab initio molecular dynamics (AIMD) calculations for Na using the projector augmented waves method and the generalized gradient approximation as exchange-correlation energy were performed. The melting temperature (T-m) results from the AIMD at extreme conditions confirm the recent findings of a T-m maximum and show a decrease of similar to 200 K between 45 and 120 GPa. Furthermore, from elastic constant calculations using the exact muffin-tin orbitals method, a Lindemann model curve was calculated. The model reproduces the trend of the experimental results of Gregoryanz [Phys. Rev. Lett. 94, 185502 (2005)] who reported an extraordinarily big melting temperature drop of similar to 700 K between 31 and 118 GPa.

  • 48.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Belonoshko, A. B.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Molecular dynamics calculation of liquid iron properties and adiabatic temperature gradient in the Earth's outer core2007In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 168, no 2, p. 890-894Article in journal (Refereed)
    Abstract [en]

    The knowledge of the temperature radial distribution in the Earth's core is important to understand the heat balance and conditions in the Earth's interior. Molecular dynamics (MD) simulations were applied to study the properties of liquid iron under the pressure-temperature conditions of the Earth's outer core. It is shown that the model used for the MD simulations can reproduce recent experimentally determined structure factor calculations to the highest pressure of 58 GPa. Applying this model for higher pressures, the calculated densities and diffusion parameters agree well with the results of first-principles. The MD calculations indicate that a reasonable estimate of the adiabatic temperature profile in the Earth's outer core could be evaluated.

  • 49.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Belonoshko, A. B.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Molecular dynamics study of liquid iron under high pressure and high temperature2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 22, article id 224113Article in journal (Refereed)
    Abstract [en]

    Unlike ab initio calculations, classical potentials in molecular dynamics (MD) allow the treatment of very large atomic systems. However, the quality of the potentials has to be carefully assessed, making the evaluation of the models important. By means of MD simulations, we have studied the properties of liquid iron (Fe) with an embedded-atom method (EAM) under extreme pressure-temperature conditions, to resemble the conditions found in the Earth's outer core. The model used for the MD simulations is demonstrated to reproduce recent experimentally determined structure factor calculations to the highest pressure of 58 GPa. The calculations of densities and diffusion parameters at the various temperatures and pressures agree well with first-principles calculations. The results indicate the model's validity for the physical description of liquid iron at extreme conditions.

  • 50.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ma, Y.
    Oganov, A. R.
    Souvatzis, Petros
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Elasticity study of the superconducting metals V, Nb, Ta, Mo and W at high pressure2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Physical Review B, Vol. 77, no 21, p. 214101-Article in journal (Refereed)
    Abstract [en]

    First-principles calculations have been performed for V, Nb, Ta, Mo, and W. The recently discovered bcc -> , rhombohedral transition for vanadium [Phys. Rev. Lett. 98, 085502 (2007)] was confirmed as the mechanical instability of c(44) was found at P=80 GPa. Furthermore, the c(11), c(12), and c(44) constants for the group-V elements showed erratic behaviors whereas the constants for the group-VI elements were monotonically increasing with pressure. The metals were analyzed with Fermi surface calculations, showing shrinking nesting vectors with pressure for V, Nb, and Ta but were not seen for Mo and W. From electronic topological transition contributions, a critical energy closely situated to the Fermi level for vanadium could be the reason why the elastic constants of V and Nb were difficult to reproduce at ambient pressure.

12 1 - 50 of 87
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