| 1. |
- Alling, Björn, 1980-, et al.
(författare)
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Energetics and magnetic impact of 3d-metal doping of the half-metallic ferromagnet NiMnSb
- 2008
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 77:14, s. 144414-
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Tidskriftsartikel (refereegranskat)abstract
- We have performed a theoretical study of the effect of doping the half-Heusler alloy NiMnSb with the magnetic 3d metals Cr, Mn, Fe, Co, and Ni, with respect to both energetics and magnetic properties. Starting from the formation energies, we discuss the possibility of placing the dopant on different crystallographic positions in the alloy. We calculate total and local magnetic moments, effective exchange interactions, and density of states and also outline strategies to tune the magnetic properties of the alloy. Doping of NiMnSb with Cr as well as substituting some Ni with extra Mn have the largest impact on magnetic interactions in the system while preserving its half-metallic property. Therefore, we suggest the possibility that these dopants increase the thermal stability of half-metallicity in NiMnSb, with implications for its possible usage in spintronics applications.
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| 2. |
- Andersson, David A., et al.
(författare)
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Modeling of Ce2, Ce2O3 ,and CeO2-x in the LDA+U formalism
- 2007
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 75, s. 035109-1-035109-6
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and thermodynamic properties of CeO2 and Ce2O3 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method, as implemented in the ab initio total-energy and molecular-dynamics program VASP (Vienna ab initio simulation package). The local density approximation (LDA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized Ce 4f electrons. We discuss how the properties of CeO2 and Ce2O3 are affected by the choice of U as well as the choice of exchange-correlation potential, i.e., the local density approximation or the generalized gradient approximation. Further, reduction of CeO2, leading to formation of Ce2O3 and CeO2-x, and its dependence on U and exchange-correlation potential have been studied in detail. Our results show that by choosing an appropriate U it is possible to consistently describe structural, thermodynamic, and electronic properties of CeO2, Ce2O3, and CeO2-x, which enables modeling of redox processes involving ceria-based materials.
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| 3. |
- Andersson, David A., et al.
(författare)
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Theoretical study of CeO(2) doped with tetravalent ions
- 2007
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 76, s. 1741191-17411910
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Tidskriftsartikel (refereegranskat)abstract
- 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).
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| 4. |
- Andersson, Ove, et al.
(författare)
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Tricritical Lifshitz point in the temperature-pressure-composition diagram for (PbySn1-y)2 P2(SexS1-x)6 ferroelectrics
- 2009
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 80, s. 174107-
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Tidskriftsartikel (refereegranskat)abstract
- The heat capacity of Sn2P2S6 ferroelectric crystals has been measured under quasihydrostatic pressures up to 0.7 GPa. The analysis of the heat-capacity and literature data for the birefringence shows that the tricritical point of Sn2P2S6 is in the 0.20–0.25 GPa range. Moreover, in the approximation of a linear change in the free-energy expansion coefficients, with respect to concentration and pressure, thermodynamic trajectories have been constructed for (PbySn1−y)2 P2(SexS1−x)6 solid solutions. We have thereby identified the region of the T-p-y-x diagram for (PbySn1−y)2 P2(SexS1−x)6 showing the tricritical Lifshitz point.
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| 5. |
- Andreasson, Jakob, et al.
(författare)
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Franck-Condon higher order lattice excitations in the LaFe1-xCrxO3 (x=0, 0.1, 0.5, 0.9, 1.0) perovskites due to Fe-Cr charge transfer effects
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75
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Tidskriftsartikel (refereegranskat)abstract
- First and higher order lattice excitations in the B-site disordered perovskites LaFe1-xCrxO3 (x=0, 0.1, 0.5, 0.9, and 1) and La0.835Sr0.165Fe0.5Cr0.5O3-delta are investigated using temperature dependent and polarized inelastic light scattering [lambda=515 nm (2.41 eV) and 676 nm (1.83 eV)] on oriented crystallites. A peak at approximately 2.4 eV in the imaginary part of the dielectric function of LaFe0.5Cr0.5O3 is assigned to a charge transfer from Fe3+ (d(5)) to Cr3+ (d(3)) ions, coupled with the appearance of an intense A(g)-like mode at approximately 700 cm(-1) in the Raman data. This excitation is identified as a symmetric oxygen breathing mode activated by the Fe-Cr charge transfer through an orbital coupling mechanism. Higher order scattering (up to seventh order) of the intrinsic Raman active symmetric breathing mode is also explained by an orbital-mediated electron-phonon coupling, similar to the Franck-Condon effect observed in the Jahn-Teller active-perovskite-structured manganite LaMaO(3). These results show that the Franck-Condon mechanism is a more common mechanism for resonant higher order scattering in solids than previously believed and propose the LaFe1-xCrxO3 system as a model system for electron-phonon coupling and higher order Raman scattering in solids.
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| 6. |
- Belonoshko, Anatoly, et al.
(författare)
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Properties of the fcc Lennard-Jones crystal model at the limit of superheating
- 2007
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 76:6, s. 064121-
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Tidskriftsartikel (refereegranskat)abstract
- The face-centered-cubic (fcc) Lennard-Jones (LJ) model can be considered as a representative model of a simple solid. We investigate the mechanism of melting at the limit of superheating in the fcc LJ solid by means of the procedure recently developed by us [Phys. Rev. B 73, 012201 (2006)]. Insight into the mechanism of melting was gained by studying diffusion and defects in the fcc LJ solid by means of molecular dynamics simulations. We found that the limit of superheating achieved by us is likely to be the highest so far. We also found that the size of the cluster which ignites the melting is very small (down to five to six atoms, depending on the size of the supercell) and closely correlates with the linear size of a supercell when the number of atoms varies between 500 and 13 500.
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| 7. |
- Belonoshko, Anatoly, et al.
(författare)
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Xenon melting: Density functional theory versus diamond anvil cell experiments
- 2006
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 74:5, s. 054114-
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Tidskriftsartikel (refereegranskat)abstract
- We performed two-phase ab initio density functional theory based molecular dynamics simulations of Xe melting and demonstrated that, contrary to claims in the recent literature, the pressure dependence of the Xe melting curve is consistent with the corresponding-states theory as well as with the melting curve obtained earlier from classical molecular dynamics with a Xe pair potential. While at low pressure the calculated melting curve is in perfect agreement with reliable experiments, our calculated melting temperatures at higher pressures are inconsistent with those from the most recent diamond anvil cell experiment. We discuss a possible explanation for this inconsistency.
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| 8. |
- Bergholtz, Emil Johansson, et al.
(författare)
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Quantum Hall hierarchy wave functions: From conformal correlators to Tao-Thouless states
- 2008
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 77:16, s. 165325-1-165325-9
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Tidskriftsartikel (refereegranskat)abstract
- Laughlin’s wave functions, which describe the fractional quantum Hall effect at filling factorsν=1/(2k+1), can be obtained as correlation functions in a conformal field theory, and recently, this construction was extended to Jain’s composite fermion wave functions at filling factors ν=n/(2kn+1). Here, we generalize this latter construction and present ground state wave functions for all quantum Hall hierarchy states that are obtained by successive condensation of quasielectrons (as opposed to quasiholes) in the original hierarchy construction. By considering these wave functions on a cylinder, we show that they approach the exact ground states, which are the Tao-Thouless states, when the cylinder becomes thin. We also present wave functions for the multihole states, make the connection to Wen’s general classification of Abelian quantum Hall fluids, and discuss whether the fractional statistics of the quasiparticles can be analytically determined. Finally, we discuss to what extent our wave functions can be described in the language of composite fermions.
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| 9. |
- Bergqvist, Lars, 1976-, et al.
(författare)
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Magnetic properties and disorder effects in diluted magnetic semiconductors
- 2005
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society (APS). - 0163-1829 .- 1095-3795. ; 72:19, s. 195210-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Bergqvist, Lars, 1976-, et al.
(författare)
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Volume dependence of the Curie temperatures in diluted magnetic semiconductors
- 2008
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society (APS). - 0163-1829 .- 1095-3795. ; 77:1, s. 014418-
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Tidskriftsartikel (refereegranskat)abstract
- Using electronic structure methods and statistical methods, we have studied theoretically the volume dependence of the exchange interactions and Curie temperatures in the diluted magnetic semiconductors. In both Mn-doped GaAs and Mn-doped InAs, the calculated Curie temperatures from numerical exact Monte Carlo simulations are more or less constant for a large volume interval. We have compared the exchange mechanisms in Mn-doped GaAs using both the local density approximation (LDA) and the LDA+U method. It is demonstrated that the magnetic properties are understood within Zener’s p-d exchange model for the LDA+U, while in LDA they reflect a mixture between double and p-d exchange mechanisms.
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