Stresses and strains arising from the mesoscopic mismatch at interfaces may affect in diverse ways the properties of surfaces and deposited nanostructures. The reasons for the occurrence of mesoscopic mismatch on metal surfaces and its consequences are briefly reviewed. It is shown how this mismatch affects the growth, structure, and morphology of thin films and nanostructures in the early stages of epitaxy.

In this paper, the segregation of the Ni impurity on open surfaces of the doped strontium titanate perovskite is investigated by means of ab initio molecular dynamics method based on the density functional theory and applied to a model periodic cell with stoichiometry La 0.5 Sr 0.5 TiO 3 (LST).The performed studies are based on recent experimental observations on the segregation of Ni impurity atoms and their tendency to form clusters at the boundaries of defect structure of La 0.2 Sr 0.7 Ni 0.1 Ti 0.9 O 3-δ (LSNT) perovskite. The results of the first-principles calculations of segregation energy showed that Ni does actively segregate toward the open surfaces. It was found that during segregation, nickel atoms leave the crystal volume to the perovskite surface and rise above its upper layer. Thus, the obtained results confirm the experimental data on the segregation and formation of nickel clusters on open LSNT surfaces.

First-principles calculations in the framework of the generalized gradient approximation together with U on-site Coulomb corrections in the GGA + U approach to density functional theory (DFT) are performed to investigate the structural stability, magnetic order, and magnetocrystalline anisotropy of one-dimensional (1D) cobalt-oxide chains on Rh(553) step-surfaces. We found that the chains' magnetic and structural stability strongly depends on the oxygen concentration, eta. It is determined that there exist competing direct ferromagnetic and indirect antiferromagnetic exchange interactions in the doped-oxygen 1D linear chains, and in general, the oxygen doping stabilizes the antiferromagnetism. For pure Co linear chains and low oxygen concentrations, in which eta <= 0.1 monolayers (ML), the ferromagnetic solution is the ground-state magnetic configuration. For eta > 0.2 ML, the antiferromagnetic arrangement stabilizes through superexchange interactions. The strong influence regarding the oxygen-doping on the Co linear chains' structural properties is evidenced when a small dimerization between the Co atoms at low O concentrations emerges. In contrast, dimerization in the Co chains is suppressed when the system is "oxygen-free" or eta > 0.2 ML. The increase of oxygen concentration strengthens the pd hybridization between Co d-states and O p-states, leading to an electronic redistribution of the majority and minority bands of the Co d-states. Such a redistribution yields to the formation of more localized bands. A significant reduction of the local magnetic moment in the Co atoms is followed. The robustness of the DFT + U results is also discussed to some extent. Throughout a perturbative analysis, we also investigate the oxygen dependence on the magnetocrystalline anisotropy energy (MAE) for the Co-oxide chains, which ranges from 0.4 to 1.2 meV. Interestingly, magnetization directions canted to the wires' direction or perpendicular to the Rh terrace are determined. Their origins are discussed in terms of the local contributions to the MAE.

Ab initio theoretical study of the quantum magnetic properties of Co nanowires on the pure and oxygen-reconstructed (1 × 2)/Au(110) and (1 × 2)/Pt(110) surfaces is performed. Their structures and electronic configurations are calculated using the electron density functional theory. High values of magnetic moment and magnetic anisotropy energies of Co atoms are found on both pure and oxygen-reconstructed (1 × 2)/Au(110) and (1 × 2)/Pt(110) surfaces. The adsorption of oxygen atoms on the (1 × 2)/Au(110) substrate is shown to affect the structural arrangement of Co nanowire atoms on this substrate and to increase the magnetic anisotropy energy (by 1.91 meV per nanowire atom). The adsorption of oxygen on the Pt(110) substrate substantially decreases the magnetic anisotropy energy of the Co nanowire on it (by 5.98 meV per atom). The origin of these changes is revealed by analyzing the local densities of states of the d electrons of nanowire atoms. The temperature ranges of the states with the lowest free surface energy are determined using the atomistic thermodynamics methods. These data and the available experimental data are used to predict the possibility of observing the structures under study in experiments.

Tailoring the magnetic properties at atomic-scale is essential in the engineering of modern spintronics devices. One of the main concerns in the novel nanostructured materials design is the decrease of the paid energy in the way of functioning, but allowing to switch between different magnetic states with a relative low-cost energy at the same time. Magnetic anisotropy (MA) energy defines the stability of a spin in the preferred direction and is a fundamental variable in magnetization switching processes. Transition-metal wires are known to develop large, stable spin and orbital magnetic moments together with MA energies that are orders of magnitude larger than in the corresponding solids. Different ways of controlling the MA have been exploited such as alloying, surface charging, and external electrical fields. Here we investigate from a first-principle approach together with dynamic calculations, the surface strain driven mechanism to tune the magnetic properties of deposited nanowires. We consider as a prototype system, the monoatomic Co wires deposited on strained Pt(111) and Au(111) surfaces. Our first-principles calculations reveal a monotonic increase/decrease of MA energy under compressive/tensile strain in supported Co wire. Moreover, the spin dynamics studies based on solving the Landau-Lifshitz-Gilbert equation show that the induced surface-strain leads to a substantial decrease of the required external magnetic field magnitude for magnetization switching in Co wire.

We present a study of the magnetic states and exchange coupling in transition-metal Mn, Fe, and Co atomic chains deposited on a self-corrugated Cu3N-Cu(110) molecular network by means of first-principles calculations based on the density functional theory. The various adsorption sites on a bumping area of a self-corrugated Cu3N layer are investigated where the atomic chains are formed at the initial stage of nanowire growth. We demonstrate, by calculating the ground-state magnetic configurations, that the exchange coupling, magnetic order, and anisotropies in atomic chains depend sensitively on their chemical composition and adsorption sites on the Cu3N network. We find that the exchange interactions in atomic chains could lead to ferromagnetic or antiferromagnetic coupling of atomic spins depending on the position of the chain on the surface. The classical spin dynamics is investigated by means of the kinetic Monte Carlo method based on transition-state theory. Moreover we evaluate the Heisenberg-Dirac-Van Vleck quantum spin Hamiltonian for calculations of the magnetic susceptibility, in order to demonstrate the existence of quantum entanglement in the antiferromagnetic atomic chains at low temperatures.

Studying the electronic and structural properties of AlN thin films is an important problem because such films are widely used as a buffer layer when growing GaN-based semiconductor heterostructures on Si substrates. In this paper, we carry out a theoretical investigation of the properties of an Al-terminated AlN(0001) surface in the framework of the density functional theory. Ab initio calculations allow us to analyze the effect of the in-plane lattice strain on the energy of this surface. It is shown that compressive strain causes a decrease in the AlN(0001) surface energy, while tensile strain leads to its increase. Knowing the surface energy values allows us to evaluate the stress of the surface under investigation. In addition, the curvature of the AlN surface is calculated for various AlN film thicknesses in the case of free growth. The obtained values of the surface curvature are in close agreement with the known experimental results.

Point defects play a key role in many microelectronics technologies. Knowledge of the properties of point defects and characteristics of their behavior during ion-beam synthesis of microstructures for use in silicon devices allows one to optimize the conditions of their production, improve their quality and the electronic properties. In this situation, of valuable help in studying the properties of point defects is numerical modeling, especially with the use of quantum mechanical methods based on density functional theory approach. The paper describes a systematic study of the effect of various quantum–mechanical simulation approximations on the calculated energy parameters of defects as applied to simple point defects in silicon. We demonstrate that the choice of the form of the exchange–correlation functional has the strongest effect on the predicted defect formation energy, whereas the variation of the other considered approximations is of secondary importance for simulation predictions.

The magnetic properties of Co nanowires on pure and oxygen-reconstructed Cu(210) surfaces have been investigated using ab initio density functional theory methods. The performed calculations have demonstrated that the studied nanowires on a pure Cu(210) surface have a high magnetic moment and a low magnetic anisotropy energy. However, the analysis of the influence of the oxygen reconstruction of the copper surface has revealed that Co nanowires on the oxidized (2 × 1)O-Cu(210) surface are characterized by a significant increase in the magnetic anisotropy energy due to the change in the projection of the orbital angular momentum of Co atoms after the oxygen adsorption on the surface. In this case, the oxygen reconstruction of the copper surface leads to a change of the magnetization easy axis direction, which is consistent with the experimental data on the self-organizing growth of Co nanowires on the nitrided Cu(210) surface.

We report on the results of ab initio calculations of vacancy and hydrogen-vacancy complexes in palladium and palladium hydride. Comparative analysis of the energies of the formation of defect complexes in palladium and its hydride has revealed that the formation of vacancy clusters is easier in the palladium hydride structure. Investigation of hydrogen-vacancy complexes in bulk crystalline palladium has shown that a hydrogen atom and a vacancy interact to form a stable hydrogen-vacancy (H-Vac) defect complex with a binding energy of E b = −0.21 eV. To investigate the initial stage in the formation of hydrogen-vacancy complexes (H n -Vac m ), we consider the clusterization of defects into clusters containing H-Vac and H2-Vac complexes as a structural unit. It is found that hydrogen-vacancy complexes form 2D defect structures in palladium in the (100)-type planes.

An ab initio DFT study of atomic and electronic structure of carbyne crystals was carried out. The influence of hydrogen impurities on carbyne structure was investigated. Calculations with atomic relaxations showed that carbon chains in the carbyne crystal structure are bow-like curved; free-energy calculations showed that the most probable lengths of those chains are four and six atoms, which is in a good agreement with experiments. Carbyne-crystal electronic-structure analysis showed that there is a small gap of 0.09 eV near the Fermi level in four-atomic carbyne, while there is no such gap in six-atomic carbyne. In studying of the hydrogen impurity influence on the atomic and electronic structure of carbyne crystals, hydrogen atoms were embedded in two directions: across and along carbon chains in the crystal. As a result we found that the crystal structure is not distorted in the case of hydrogen embedded across the chains, while the type of bonding between carbon atoms in carbon chains in the carbyne crystal structure depended on the impurity concentration. The crystal structure was distorted when hydrogen was embedded along the chains. The concentration of impurities influences the conductivity of a carbyne crystal.

The results of a theoretical study based on ab initio calculations of the polarization properties of AlN, GaN, and AlGaN semiconductors with the wurtzite structure are presented. The values of the spontaneous and piezoelectric polarizations, as well as the piezoelectric constants, are calculated for these nitride compounds. With the aim of further considering prospective heterostructures based on (Al,Ga,AlGa)N compounds, the charge densities at the AlN/GaN, AlGaN/AlN, and AlGaN/GaN interfaces and carrier concentration at the AlGaN/GaN heterointerface is estimated and compared with the experimental data.

Проведено компьютерное моделирование процесса нитридизации сапфира, используемого при получении нитридных гетероструктур GaN на подложке Al2O3. Применялось исследование из первых принципов адгезии атомарного азота на поверхности (0001) сапфира. Была изучена возможность замещения атомов кислорода поверхностного слоя атомами азота. Полученные результаты свидетельствуют о том, что при различных концентрациях азота наиболее стабильная позиция атомов азота при адсорбции находится над поверхностным атомом кислорода. Проведены расчеты изменения полной энергии системы при замещении поверхностных атомов кислорода атомами азота. Оказалось, что для одиночного адатома азота замещение кислорода энергетически невыгодно, однако при увеличении концентрации азота этот процесс становится энергетически выгодным. Этот результат получен впервые и позволяет улучшить понимание механизма процесса нитридизации сапфира на атомном уровне.

Computer simulation of sapphire nitridation used to obtain nitride-based heterostructures (GaN) on an Al2O3 substrate has been performed. The adhesion of atomic nitrogen to the sapphire (0001) surface is investigated ab initio. The possibility of replacing surface-layer oxygen atoms with nitrogen atoms has been examined. The calculated results indicate that adsorbed nitrogen atoms occupy the most stable positions above surface oxygen atoms at different nitrogen concentrations. The changes in the total system energy after replacement of surface oxygen atoms with nitrogen atoms have been calculated. It turns out that oxygen replacement is energetically unfavorable for a single nitrogen adatom. However, this process becomes energetically favorable if the concentration of nitrogen atoms increases. This outcome, obtained for the first time, enables better understanding of the atomic-scale mechanism of sapphire nitridation.