No Arabic abstract
We study theoretically large metal clusters containing vacancies. We propose an approach, which combines the Kohn-Sham results for monovacancy in a bulk of metal and analytical expansions in small parameters $c_{v}$ (relative concentration of vacancies) and $R_{N,v}^{-1}$, $R_{N,v}$ being cluster radius. We obtain expressions of the ionization potential and electron affinity in the form of corrections to electron work function, which require only the characteristics of 3D defect-free metal. The Kohn-Sham method is used to calculate the electron profiles, ionization potential, electron affinity, electrical capacitance, dissociation, cohesion and monovacancy-formation energies of the small perfect clusters Na$_{N}$, Mg$_{N}$, Al$_{N}$ ($N leq 270$) and the clusters containing a monovacancy ($Ngeq 12$) in the stabilized-jellium model. The quantum-sized dependences for monovacancy-formation energies are calculated for the Schottky scenario and the bubble blowing scenario, and their asymptotic behavior is also determined. It is shown that the asymptotical behaviors of size dependences for these two mechanisms differ from each other and weakly depend on the number of atoms in the cluster. The contribution of monovacancy to energetics of charged clusters, the size dependences of their characteristics and asymptotics is discussed. It is shown that difference between the characteristics for the neutral and charged cluster is entirely determined by size dependences of ionization potential and electron affinity. Obtained analytical dependences may be useful for the analysis of the results of photoionization experiments and for the estimation of the size dependences of the vacancy concentration including the vicinity of the melting point.
An expansion of energy characteristics of wide thin slab of thickness L in power of 1/L is constructed using the free-electron approximation and the model of a potential well of finite depth. Accuracy of results in each order of the expansion is analyzed. Size dependences of the work function and electronic elastic force for Au and Na slabs are calculated. It is concluded that the work function of low-dimensional metal structure is always smaller that of semi-infinite metal sample. A mechanism for the Coulomb instability of charged metal clusters, different from Rayleighs one, is discussed. The two-component model of a metallic cluster yields the different critical sizes depending on a kind of charging particles (electrons or ions). For the cuboid clusters, the electronic spectrum quantization is taken into account. The calculated critical sizes of Ag_{N}^{2-} and Au_{N}^{3-} clusters are in a good agreement with experimental data. A qualitative explanation is suggested for the Coulomb explosion of positively charged Na_{N}^{n+} clusters at 3<n<5.
We study the influence of oxygen vacancies on the formation of charged 180$^circ$ domain walls in ferroelectric BaTiO$_3$ using first principles calculations. We show that it is favorable for vacancies to assemble in crystallographic planes, and that such clustering is accompanied by the formation of a charged domain wall. The domain wall has negative bound charge, which compensates the nominal positive charge of the vacancies and leads to a vanishing density of free charge at the wall. This is in contrast to the positively charged domain walls, which are nearly completely compensated by free charge from the bulk. The results thus explain the experimentally observed difference in electronic conductivity of the two types of domain walls, as well as the generic prevalence of charged domain walls in ferroelectrics. Moreover, the explicit demonstration of vacancy driven domain wall formation implies that specific charged domain wall configurations may be realized by bottom-up design for use in domain wall based information processing.
Arrays of identical and individually addressable qubits lay the foundation for the creation of scalable quantum hardware such as quantum processors and repeaters. Silicon vacancy centers in diamond (SiV) offer excellent physical properties such as low inhomogeneous broadening, fast photon emission, and a large Debye-Waller factor, while the possibility for all-optical ultrafast manipulation and techniques to extend the spin coherence times make them very promising candidates for qubits. Here, we have developed arrays of nanopillars containing single SiV centers with high yield, and we demonstrate ultrafast all-optical complete coherent control of the state of a single SiV center. The high quality of the chemical vapor deposition (CVD) grown SiV centers provides excellent spectral stability, which allows us to coherently manipulate and quasi-resonantly read out the state of individual SiV centers on picosecond timescales using ultrafast optical pulses. This work opens new opportunities towards the creation of a scalable on-chip diamond platform for quantum information processing and scalable nanophotonics applications.
We suggest a method for the self-consistent calculations of characteristics of metal films in dielectric environment. Within a modified Kohn-Sham method and stabilized jellium model, the most interesting case of asymmetric metal-dielectric sandwiches is considered, for which dielectric media are different from the two sides of the film. As an example, we focus on Na, Al and Pb. We calculate the spectrum, electron work function, and surface energy of polycrystalline and crystalline films placed into passive isolators. We find that a dielectric environment generally leads to the decrease of both the electron work function and surface energy. It is revealed that the change of the work function is determined only by the average of dielectric constants from both sides of the film.
We report a computational first-principles study of positron trapping at vacancy defects in metals and semiconductors. The main emphasis is on the energetics of the trapping process including the interplay between the positron state and the defects ionic structure and on the ensuing annihilation characteristics of the trapped state. For vacancies in covalent semiconductors the ion relaxation is a crucial part of the positron trapping process enabling the localization of the positron state. However, positron trapping does not strongly affect the characteristic features of the electronic structure, e.g., the ionization levels change only moderately. Also in the case of metal vacancies the positron-induced ion relaxation has a noticeable effect on the calculated positron lifetime and momentum distribution of annihilating electron-positron pairs.