K. Kokko

Assessing the Perdew-Burke-Ernzerhof exchange-correlation density functional revised for metallic bulk and surface systems

We test the accuracy of the revised Perdew-Burke-Ernzerhof exchange-correlation density functional (PBEsol) for metallic bulk and surface systems. It is shown that, on average, PBEsol yields equilibrium volumes and bulk moduli in close agreement with the former generalized gradient approximation (PBE) and two gradient level functionals derived from model system approach (LAG and AM05). On the other hand, for close-packed metal surfaces, PBEsol has the same performance as AM05, giving significantly larger surface energies than PBE and LAG.

within the LSDA + U approach

In the present work we concentrate on the electronic structure of haematite which shows two competing phenomena: correlation and hybridization. Using the tight-binding linearized muffin-tin orbital method with the on-site Coulomb interaction we show that, while haematite has considerable hybridization between different symmetries of electronic states, its on-site Coulomb interaction is also significant. The electronic structure is calculated by using LSDA and LSDA + U approximations to take into account the exchange-correlation effects. The LSDA + U potential describes many basic electronic properties of haematite better than the LSDA potential.

Molecular dynamics simulation of Co thin films growth on Cu(001)

Abstract Results of MD simulations of the structure of ultrathin films of Co on Cu(001) are presented. Growth conditions corresponding to vacuum evaporation as well as laser ablation are considered. The dynamics of the growth process and the structure of the as-deposited films are investigated as a function of the kinetic energy of adatoms. The effect of fast interdiffusion due to a high impact energy is observed. Tight-binding potentials in the second-moment approximation are used. Co–Cu interaction parameters are determined from ab-initio electronic structure calculations.

First-principles calculations for work function and surface energy of thin lithium films

Abstract Surface relaxation, electronic work function and surface energy for (100), (110) and (111) surfaces of bcc Li are calculated using a first-principles molecular dynamics methods. The obtained relaxations for three-layer Li slabs are outward for the (100) and inward for the (110) and (111) surface layers. The results show that the work function increases in the order (100), (111) and (110) in the surface orientation. Furthermore, the results suggest that, with increasing number of atomic layers, the various energy quantities converge rather fast to values that are within only a few percents of their semi-infinite bulk results, although the surfaces of the slabs, with less than ten atomic layers, still interact.

First-principles atomistic study of surfaces of Fe-rich Fe–Cr

The surface properties of Fe-rich ferromagnetic Fe-Cr alloys are investigated using a first-principles quantum-mechanical method. In dilute alloys, the surfaces are dominated by Fe, whereas the Cr-containing surfaces become favorable when the bulk Cr concentration exceeds the limit of ∼ 10 atomic per cent. The abrupt change in the surface behavior is the consequence of complex competing magneto-chemical interactions between the alloying atoms. Considering the quantities of various features: equilibrium surface profiles, chemical potentials, segregation energies, surface energies, magnetic moments, mixing energies and pair interactions, within a wider range of bulk and surface concentrations enables us to build a comprehensive picture of the physics of Fe-Cr surfaces. Using the present achievements many previously controversial results can now be merged into a consistent model of Fe-rich Fe-Cr alloys.

Does Bi form clusters in GaAs1-xBix alloys?

GaAs1 - xBix alloys attract significant interest due to their potentiality for several applications, including solar cells. Recent experiments link the crucial optical properties of these alloys to ...

The effect of long-range order on the elastic properties of Cu3Au

Ab initio calculations, based on the exact muffin-tin orbitals method are used to determine the elastic properties of Cu-Au alloys with Au/Cu ratio 1/3. The compositional disorder is treated within the coherent potential approximation. The lattice parameters and single-crystal elastic constants are calculated for different partially ordered structures ranging from the fully ordered L1(2) to the random face centered cubic lattice. It is shown that the theoretical elastic constants follow a clear trend with the degree of chemical order: namely, C(11) and C(12) decrease, whereas C(44) remains nearly constant with increasing disorder. The present results are in line with the experimental findings that the impact of the chemical ordering on the fundamental elastic parameters is close to the resolution of the available experimental and theoretical tools.

First-Principles prediction of the deformation modes in austenitic Fe-Cr-Ni alloys

First-principles alloy theory is used to establish the γ-surface of Fe-Cr-Ni alloys as function of chemical composition and temperature. The theoretical stacking fault energy (SFE) versus chemistry and temperature trends agree well with experiments. Combining our results with the recent plasticity theory based on the γ-surface, the stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective stacking fault energy below ∼18 mJ m−2. Alloys with SFE above this critical value show both twinning and full slip at room temperature. Interestingly, twinning remains a possible deformation mode in addition to full slip even at elevated temperatures, in line with observations.

Flexibility of the quasi-non-uniform exchange-correlation approximation

In our previous study [Phys. Rev. B 86, 201104 (2012).] we introduced the so-called quasi-non-uniform gradient-level exchange-correlation approximation (QNA) and demonstrated its strength in produc ...

Quasi-non-uniform gradient-level exchange-correlation approximation for metals and alloys

The flexibility of the common generalized gradient approximation for the exchange-correlation energy is investigated by monitoring the equilibrium volume of transition metals. It is shown that no universal gradient-level approximation yielding consistent errors for all metals exists. Based on an element-specific optimization, the concept of quasi-non-uniform gradient-level approximation is introduced. The strength of the scheme is demonstrated on several transition-metal alloys.