| 771. |
- Pašti, Igor, et al.
(författare)
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Theoretical analysis of electrochromism of Ni-deficient nickel oxide - from bulk to surfaces
- 2023
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 25:11, s. 7974-7985
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Tidskriftsartikel (refereegranskat)abstract
- The development of new electrochromic materials and devices, like smart windows, has an enormous impact on the energy efficiency of modern society. One of the crucial materials in this technology is nickel oxide. Ni-deficient NiO shows anodic electrochromism, whose mechanism is still under debate. We use DFT+U calculations to show that Ni vacancy generation results in the formation of hole polarons localized at the two oxygens next to the vacancy. In the case of NiO bulk, upon Li insertion or injection of an extra electron into Ni-deficient NiO, one hole gets filled, and the hole bipolaron is converted into a hole polaron well-localized at one O atom, resulting from the transition between oxidized (colored) to reduced (bleached) state. In the case of the Ni-deficient NiO(001) surface, the qualitatively same picture is obtained upon embedding Li, Na, and K into the Ni surface vacancy, reinforcing the conclusion that the electron injection, resulting in the filling of the hole states, is responsible for the modulation of the optical properties of NiO. Hence, our results suggest a new mechanism of Ni-deficient NiO electrochromism not related to the change of the Ni oxidation states, i.e., the Ni2+/Ni3+ transition, but based on the formation and annihilation of hole polarons in oxygen p-states.
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| 772. |
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| 773. |
- Peil, Oleg E., et al.
(författare)
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Ab initio investigation of magnetic ordering and spin-glass transition in Cu-rich Cu-Mn systems
- 2007
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 310:2, s. 1561-1563
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Tidskriftsartikel (refereegranskat)abstract
- Manganese-noble metal alloys exhibit a large variety of magnetic structures as a function of Mn concentration. Ab initio studies of these structures are practically absent due to difficulties in handling simultaneously atomic and magnetic ordering effects in disordered alloys. In this work we use the generalized perturbation method (GPM) A.V.Ruban, S. Shallcross, S.I. Simak H.L. Skriver, Phys. Rev. B 70 (2004) 125115 in the framework of the KKR method and the coherent potential approximation (CPA) in order to determine both effective chemical interactions and magnetic exchange interaction parameters of the classical Heisenberg Hamiltonian in Cu-Mn alloys. The effective chemical interactions have been used in Ising Monte Carlo simulations to determine the equilibrium distribution of atoms on the FCC lattice at the experimental aging temperatures. The obtained atomic short-range order for Cu-Mn alloys is in excellent agreement with the existing experimental data. The underlying atomic distribution has subsequently been used in Heisenberg Monte Carlo simulations with the exchange interaction parameters calculated for the corresponding alloy composition by the GPM method. For alloys with low Mn content, up to 20 at%, we find a transition into a spin-glass state with a specific magnetic short order.
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| 774. |
- Peil, O. E., et al.
(författare)
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Ab initio study of atomic ordering and spin-glass transition in dilute CuMn alloys
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:2
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Tidskriftsartikel (refereegranskat)abstract
- An archetypical spin-glass metallic alloy, Cu0.83Mn0.17, is studied by means of an ab initio based approach. First-principles calculations are employed to obtain effective chemical, strain-induced, and magnetic exchange interactions, as well as static atomic displacements, and the interactions are subsequently used in thermodynamic simulations. It is shown that the calculated atomic and magnetic short-range order accurately reproduces the results of neutron-scattering experiments. In particular, it is confirmed that the alloy exhibits a tendency toward ordering and the corresponding ordered phase is revealed. The magnetic structure is represented by spin-spiral clusters accompanied by weaker ferromagnetic short-range correlations. The spin-glass transition temperature obtained in Monte Carlo simulations by a finite-size scaling technique, 57 K, is in reasonable agreement with experimental data, 78 K.
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| 775. |
- Peil, O. E., et al.
(författare)
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Detailed ab initio calculations of the structure and magnetic state of a metallic spin glass
- 2008
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- We present for the first time a long-sought atomic and magnetic structure of one of the most well-known and studied spin-glass systems: the Cu83Mn17 alloy, which is the prototype for a large class of metallic spin-glass systems. The structure has been determined from simulations based on atomic and magnetic interactions obtained from first-principles calculations. Being in perfect agreement with recent neutron scattering experiments, our results reveal the atomic and magnetic structure that exhibits a specific short-range order (SRO) and is believed to be responsible for a complicated magnetic cluster dynamics leading to the spin-glass behaviour in this alloy. The ordered phase underlying the atomic SRO is shown to have a Cu3Mn-type structure with 16 atoms per unit cell. The magnetic SRO is associated with incommensurate spin-spiral correlations.
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| 776. |
- Peil, O. E., et al.
(författare)
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Self-consistent supercell approach to alloys with local environment effects
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:16, s. 165140-
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Tidskriftsartikel (refereegranskat)abstract
- We present an efficient and accurate method for calculating electronic structure and related properties of random alloys with a proper treatment of local environment effects. The method is a generalization of the locally self-consistent Green's-function technique for the exact muffin-tin orbital method. An alloy system in the calculations is represented by a supercell with a certain set of atomic-distribution correlation functions. The Green's function for each atom in the supercell is obtained by embedding the cluster of neighboring atoms lying within a local interaction zone (LIZ) into an effective medium and solving the cluster Dyson equation exactly. The key ingredients of the method are locality, which makes it linearly scaling with the number of atoms in the supercell, and coherent-potential self-consistency of the effective medium, which results in a fast convergence of the electronic structure with respect to the LIZ size. To test the performance and accuracy of the method, we apply it to two systems: Fe-rich bcc-FeCr random alloy with and without a short-range order, and a Cr impurity on the Fe surface. Both cases demonstrate the importance of taking into account the local environment effects for correct description of magnetic and bulk properties.
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| 777. |
- Peil, Oleg E., 1981-
(författare)
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Theory of Disordered Magnets
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Studying magnetic properties of disordered alloys is important both for the understanding of phase transformations in alloys and from the point of view of fundamental issues of magnetism in solids. Disorder in a magnetic system can result in unconventional magnetic structures, such as spin glass, which have rather peculiar features.In this Thesis, a rather general approach to studying disordered magnetic alloys from first principles is presented. Phase transformations and magnetic behavior of crystalline substitutional alloys are considered. This approach is exemplified by calculations of an archetypical spin-glass material: the CuMn alloy.First, a general theoretical framework for the description of the thermodynamics of disordered magnetic alloys is given. It is shown that under certain conditions, a complex magnetic system can be reduced to an effective system containing no magnetic degrees of freedom. This substantially simplifies the investigation of phase transformations in magnetic alloys. The effective model is described in terms of material-specific interaction parameters.It is shown that interaction parameters can be obtained from the ground-state property of a disordered alloy which are in turn calculated from first principles by means of highly accurate up-to-date numerical techniques based on the Green's function method. The interaction parameters can subsequently be used in thermodynamic Monte-Carlo simulations to produce the atomic and magnetic structures of an alloy. An example of calculations for the Cu-rich CuMn alloy is given. It is demonstrated that the atomic and magnetic structure of the alloy obtained by the presented approach agrees very well with the results of neutron-scattering experiments for this system. Moreover, numerical simulations enable one to predict the ground state structure of the alloy, which is difficult to observe in experiment due to large atomic diffusion barriers at temperatures close to the temperature of the phase transformation.A general description of a spin glass is given, and difficulties of modeling this type of magnetic systems are discussed. To overcome the difficulties, improved Monte-Carlo methods, such as parallel tempering, overrelaxation technique, and finite-size scaling method of analysis, are introduced. The results for the CuMn alloy are presented.
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| 778. |
- Persson, Clas, et al.
(författare)
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Effective electron and hole masses in intrinsic and heavily n-type doped GaN and AlN
- 2001
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 13:40, s. 8915-8922
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the electronic structure near the band edges in intrinsic and heavily n-type doped GaN and AlN. Both the wurtzite and the zincblende polytypes have been considered. The electronic structures of the intrinsic materials were obtained from a full-potential linearized augmented plane wave calculation. We show the importance of performing a fully relativistic calculation. For instance, the hole mass in cubic AIN is a very large and negative quantity if spin-orbit coupling is excluded, whereas the fully relativistic calculation gives a relatively small and positive value. The electron-phonon coupling was taken into account according to the Frohlich Hamiltonian for large polarons, resulting in effective polaron masses. The effects on the effective electron masses due to doping were investigated by using a Green's function formalism within the random phase approximation and with a local-field correction according to Hubbard.
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| 779. |
- Persson, Clas, et al.
(författare)
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Effective electronic masses in wurtzite and zinc-blende GaN and AlN
- 2001
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Ingår i: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 231:3, s. 397-406
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Tidskriftsartikel (refereegranskat)abstract
- The effective electron and hole masses are fundamental quantities of semiconductors, used in numerous analyses of experiments and theoretical investigations. We present calculations of the band structure near the band edges in intrinsic GaN and AIN, both for the wurtzite and the zinc-blende polytypes. We have utilized a full-potential linearized augmented plane wave method within the density functional theory and with two different exchange-correlation potentials. The lattice parameters have been determined by a minimization of the total energy, whereupon the crystal-field splitting, the spin-orbit splitting, and the effective electron and hole masses have been calculated. The calculated effective masses are in good agreement with available experimental values. We show the importance of performing a fully relativistic calculation. For instance, the hole mass in cubic AIN is a very large and negative quantity if the spin-orbit coupling is excluded, whereas the fully relativistic calculation gives a relatively small and positive value.
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| 780. |
- Persson, Clas, et al.
(författare)
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First-principle calculations of optical properties of wurtzite AlN and GaN
- 2001
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Ingår i: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 231:3, s. 407-414
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Tidskriftsartikel (refereegranskat)abstract
- The imaginary part of the dielectric function of wurtzite AIN and GaN has been calculated in the long wavelength limit, using two different first-principle electronic structure methods. The first method is a full-potential linearized augmented plane wave method and the second is a full-potential linear muffin-tin orbital method. From the Kramers-Kronig dispersion relations the real part of the dielectric function has been obtained, taking into account a quasi-particle band-gap correction according to Bechstedt and Del Sole. Absorption due to optical phonons is treated as a delta function in the imaginary part of the dielectric function. Both the longitudinal as well as the transverse components of the dielectric function are presented, showing that the anisotropy is small in these materials. Although we use different correlation potentials in the two methods, the results are similar, We compare our calculated dielectric functions with spectroscopic ellipsometry and reflectance spectra measurements.
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