Structural stability and electronic properties of Au5Hn (n=1–10) clusters

A systematic study on the geometrical structures, electronic and magnetic properties of Au₅H _n (n=1–10) clusters has been performed by using the all-electron scalar relativistic density functional theory with generalized gradient approximation at the PW91 level. It is found that all Au₅H _n clusters prefer to keep the planar structures like pure Au₅ cluster, the Au₅ structures in Au₅H₄, Au₅H₅ and Au₅H₆ clusters are distorted obviously. The adsorption of a number of hydrogen atoms enhances the stability of Au₅ cluster and all Au₅H _n clusters are more stable than pure Au₅ cluster energetically. The odd-even alteration of magnetic moment is observed in Au₅H _n clusters and may be served as the material with tunable code capacity of “0” and “1” by adsorbing odd or even number of H atoms. It seems that the most favorable adsorption between Au₅ cluster and a number of hydrogen atoms takes place in the case that the odd number of hydrogen atoms is adsorbed onto Au₅ cluster and becomes Au₅H _n cluster with even number of valence electrons.     

Ionization and fragmentation of clusters evaporated from the surface of a metal by accelerated ions

A mechanism is proposed for the ionization of clusters evaporated from the surface of a metal subjected to fast-ion bombardment. According to this mechanism, part of the internal energy of the cluster nuclei is transferred to its electronic subsystem, with subsequent ionization. Within the framework of this approach, the dependence of the degree of ionization on n (the number of atoms in the cluster) is calculated for the evaporated clusters. The results obtained are in agreement with experimental data. Zh. Tekh. Fiz. 67, 1–5 (December 1997)     

On the Theory of Neutral and Charged Cluster Ion Sputtering of Metal

A calculation method for large neutral and/or charged cluster (with number of atoms N > 5) elastic sputtering of a metal during ion bombardment is proposed. The result is presented as a simple assymptotic formula for the probability of cluster ejection and cluster charge state. A conclusion is made on the exponential nature of the dependence of the total cluster yield on the number of atoms the cluster consists of.     

Mechanisms for long-range forces in the “three atoms+electron” system

The Born-Oppenheimer approximation is used to obtain an equation for the effective interaction of three atoms bound by a single electron. For low binding energies long-range forces arise between the atoms in an “electron + atom pair” that lead to bound states when the size of the three-atom cluster is several tens of angstroms. A system made up of alkali metal atoms is considered as an example. Zh. éksp. Teor. Fiz. 111, 1229–1235 (April 1997)     

Electron spectroscopy of objects for nanoelectronics

An electron-spectroscopic analysis is made of layered nanostructures and clusters at the surface and in the bulk of a solid. A new method of forming metal/insulator/semiconductor (superconductor) nanostructures is proposed based on ion-stimulated metal segregation effects at the surface of low-temperature gallium arsenide and a 123 high-temperature superconductor. The geometric parameters and electronic structure of these nano-objects are studied. It is shown that their electronic properties can be controllably varied in situ by acting on the surface. The dimensional transformation of the electronic properties of metal clusters is studied for clusters in the insulator SiO₂, in the superconductor LTMBE-GaAs, and on silicon and graphite surfaces. The nature of this transformation is clarified. A diagnostics for cluster ensembles is developed by which one can determine the parameters needed to describe singleelectron transport: the average number of atoms per cluster, the average distance between clusters and isolated atoms, and the chemical state of the atoms. Ensembles of silver clusters with specified parameters are obtained on a silicon surface. It is shown that these ensembles are potentially useful for developing single-electron devices. Zh. Tekh. Fiz. 69, 85–89 (September 1999)     

Magnetization of optically polarized gas atoms by an optical pulse train

The properties of the density matrix and the multipole moments arising in oriented and aligned atoms with zero nuclear spin through the interaction with strong resonant ultrashort pulses with wave vector k ₀ and circular or linear polarization have been found. Calculations have been made for the time-dependent light-induced magnetization μ(t′) of a gas of pre-oriented and prealigned atoms following the passage of a weak resonant elliptically polarized pulse with frequency ω and wave vector k collinear with k ₀. It is shown that for oriented atoms, μ(t′) is an even function of the detuning from resonance, ω-ω _ba, and can be split into two terms whose directions are a consequence of symmetry and are determined by the vectors k ₀ and k as well as by the direction of rotation of the electric fields corresponding to the pulses. For aligned atoms the vector μ(t′) is collinear with k, and the first term is an even function of ω-ω _ba. However, the second term is an odd function of ω-ω _ba and reverses direction when the sign of ω-ω _ba changes, as well as when the orientation of the axes of the polarization ellipse is changed. It is shown that if a series of weak linearly polarized pulses pass through the gas, the light-induced magnetization of the oriented and aligned gas atoms can be decomposed into three factors: the first determines the direction and is a consequence of the symmetry; the second (with the dimensions of magnetic moment) depends on the characteristics of the resonant transitions; and the third is a universal function of t′ and ω-ω _ba that does not depend on the underlying characteristics of the resonant transition. These vector factors and the universal functions are in principle different for oriented and aligned atoms. Zh. éksp. Teor. Fiz. 111, 63–92 (January 1997)     

Structure and melting of dipole clusters

Two-dimensional microclusters made up of particles repelled by the dipole law and confined by an external quadratic potential are considered. The model describes a number of physical systems, in particular, electrons in semiconductor structures near a metallic electrode, indirect excitons in coupled semiconductor dots etc. Two competing types of particle ordering in clusters have been revealed: formation of a triangular lattice and of a shell structure. Equilibrium configurations of clusters with N=1–40 particles are calculated. Temperature dependences of the structure, potential energy, and mean-square radial and angular displacements are studied. These characteristics are used to investigate cluster melting. Melting occurs in one or two stages, depending on N. Melting of a two-shell microcluster takes place in two stages: at low temperatures—from the frozen phase to a state with rotationally reoriented “crystalline” shells with respect to one another, followed by a transition involving breakdown of radial order. Melting in a cluster made up of a larger number of shells occurs in one stage. This is due to the fact that the potential barrier to intershell rotation is substantially lower than that to particle jumping from one shell to another for small N, and of the same order of magnitude for large N. A method is proposed for predicting the character of melting in shell clusters by comparing the potential barriers for shell rotation and intershell particle jumping. Fiz. Tverd. Tela (St. Petersburg) 40, 1379–1386 (July 1998)     

Electron density of states and electrical conductivity of ordered alloys: Extension beyond the single-band approximation, taking into account scattering by clusters

A method is developed for going beyond the single-band approximation and taking into account scattering by clusters. The method is based on a cluster expansion of the averaged Green’s function of the alloy. It is shown that the contributions of scattering processes diminish with respect to a certain small parameter as the number of particles in the cluster increases. The prominent characteristics of the electronic structure and electrical conductivity of ordered alloys are analyzed numerically in the diagonal disorder approximation in the multiband s-d model. Fiz. Tverd. Tela (St. Petersburg) 39, 401–411 (March 1997)     

Diffusion and surface energy barrier for labile oxygen in YBa2Cu3O7−δ

A study is made of the isotope exchange kinetics of tracer oxygen atoms between the gas phase and the CuO_1−δ plane in the YBa₂Cu₃O_7−δ crystal under isochronous and isothermal annealing. The parameters of the surface energy barrier for oxygen atoms are found. Fiz. Tverd. Tela (St. Petersburg) 39, 42–48 (January 1997)     

Amorphization and shear microbands near grain boundaries in mechanically alloyed metallic materials

A theoretical model which effectively describes a novel micromechanism of solid-state amorphization in mechanically alloyed metallic materials is proposed. Within the model the amorphous phase nucleates as a result of the intersection of a grain boundary by a plastic-shear microband. It is shown that the growth of a nucleus of the amorphous α-β alloy near a site on a grain boundary intersected by a shear microband is effectively controlled by the rate of diffusion mixing of atoms of the mechanically alloyed metals α and β. Zh. Tekh. Fiz. 67, 35–38 (July 1997)     

Observation of oriented 23 S 1 helium atoms in a sodium-helium plasma irradiated by polarization-modulated laser radiation

An experiment on the observation of spin polarization of metastable helium atoms interacting with optically oriented sodium atoms under continuous rf discharge conditions is described. Laser radiation with alternating-sign circular polarization, tuned to the resonance excitation of the 3² S _1/2–3² P _1/2 transition in Na atoms, is used as the source for optical pumping of ground-state sodium atoms. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 385–387 (10 March 1997)     

Mechanism of three-dimensional polymerization of fullerite C60 at high pressures

A series of polycrystalline phases corresponding to different stages of three-dimensional polymerization and destruction of C₆₀ molecules has been synthesized by heating fullerite C₆₀ under a pressure P=12.5 GPa. The structure of the phases can be identified as fcc, and the lattice period decreases with increasing heating temperature. A model of three-dimensional polymerization in which the lattice parameter is a continuous function of the fraction of covalently bonded molecules is proposed. The model makes it possible to estimate the number of atoms in the sp ³ state. The hardness of the polymerized fcc phases is studied on the basis of percolation of rigidity. It is shown experimentally that the period a≈13.8 Å is the threshold for the formation of a three-dimensionally rigid C₆₀ polymer. It is found that the thermal stability of the strongly and weakly polymerized phases is qualitatively different. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 755–759 (10 December 1996)     

Lattice dynamics of impurity clusters. Application to pairs

A general solution is obtained for the lattice dynamics of a cluster ofn-impurity atoms using the double-time Green’s function formalism. The cluster is characterized byn-mass defect andm-force constant change parameters. It is shown that this general solution for the Green’s function for then-impurity cluster can also be expressed in terms of the Green’s function for the (n−1)-impurity cluster. As an application, the cluster impurity modes for a pair are calculated using the Debye model for the host lattice dynamics. The splitting of the high frequency local modes and nearly zero frequency resonant modes due to pairs show an oscillatory behaviour on varying the distance of separation between the two impurity atoms. These oscillations are most prominent for two similar impurities and get damped for two dissimilar impurities or if one of the impurities produces a force constant change. The predictions of the calculation provide qualitative explanation of the data obtained from the infrared measurements of the resonant modes in mixed crystal system of KBr_1−c Cl _c : Li⁺ and KBr_1−c I _c : Li⁺.     

Evolution of the electronic properties of transition metal nanoclusters on graphite surface

The electronic properties of nanoclusters of transition (Ni, Co, Cr) and noble (Au, Cu) metals deposited on the surface of highly oriented pyrolytic graphite (HOPG) are studied using the method of X-ray photoelectron spectroscopy. The laws of variation of a change ΔE _b in the binding energies of core-level electrons in the initial (ΔE _i) and final (ΔE _f) states of atoms in nanoclusters, the intrinsic widths γ of photoelectron lines, and their singularity indices α as functions of the metal cluster size d are determined. A qualitative difference in behavior of the ΔE _i(d) and α(d) values in metals of the two groups (Ni, Cr versus Co, Cu) is found. The values of the final-state energy (ΔE _f < 0) and the line width (Δγ > 0) in the clusters of all metals studied vary in a similar manner. It is shown that a significant contribution to E _i is due to a transfer of the valence-shell electrons at the cluster-substrate interface, which is caused by the contact potential difference. The value of an uncompensated charge per nanocluster is determined as a function of the cluster size and the number of atoms in the cluster. The behavior of ΔE _f(d) is controlled by the Coulomb energy of a charged cluster and by a decrease in the efficiency of electron screening, which is different in the metals studied. The broadening of photoelectron lines is determined by a spread of the cluster sizes and by lower electron screening in the final Fermi system. An asymmetry of the core-level electron spectra of nanoclusters can be explained using notions about the electron-hole pair excitation near the Fermi level. The effect of the structure of the density of electron states in the d band of transition metals on the asymmetry of photoelectron lines is considered and it is concluded that this structure near the Fermi level qualitatively changes with a decrease in the nanocluster size. The obtained results indicate that the behavior of the electron subsystem of clusters of the d-metals in a size range of 2–10 nm under consideration is close to the behavior of a normal Fermi system.     

Surface coverage studies of the icosahedron by Li using density based molecular dynamics

Density based molecular dynamics has been used to investigate the ground state structures of heterogeneous binary clusters Al₁₃Li_n, n = 1, 2, 3, 4, 10, 19, 20, 21. Some of these structures have also been investigated by full Kohn-Sham based calculations. Our earlier investigations have shown that in the Al-Li cluster, the ground state configurations are the ones where the Al atoms form a core around which the Li atoms form a “cage”. In the present work, we have chosen the well-known Al₁₃ icosahedron as the surface over which we study the evolution of the surface coverage as the number of Li atoms increases. On the basis of the earlier work, we expect that the Al₁₃Li₂₀ cluster would be the most stable and indeed our simulations do yield such a novel fivefold symmetric stable structure formed out of purely metal atoms. This icosahedral substrate is also used to study the coverage of the aluminum surface by lithium atoms. For a small number of Li atoms, these studies suggest that the Li-Li dimerisation is not particularly favored. Received: 24 October 1997 / Revised: 7 April 1998 / Accepted: 29 June 1998     

Electronic structure of Ce and Sm in hydrides and the 4f collapse in YbHx

The population of the 4f, 5d, and 6s shells of rare-earth atoms in RH_x hydrides (R=Ce, Sm, Yb; x≈2–3) has been studied by the x-ray line-shift method. The population of the 5d and 6s shells of Ce and Sm atoms, and the charge on them in metals and hydrides, were determined from experiment and calculated within the Hartree-Fock-Dirac (Koopmans) model. The decrease of the charge on Ce and Sm revealed upon transition from the metal to the hydride argues unambiguously for the anionic (hydride) model. In YbH_x with x⩾2, the structural transition is accompanied by a strongly pronounced electronic transition from divalent to a noninteger-valence state . Fiz. Tverd. Tela (St. Petersburg) 40, 1393–1396 (August 1998)     

Quantum-nondemolition measurement of the number of photons in a microcavity

A new scheme for quantum-nondemolition measurement of the number and statistics of photons in a microwave cavity on the basis of interferometry of strongly-controlled V atoms which interact by a dispersion interaction with a cavity mode at a different transition is proposed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 58–63 (10 July 1997)     

Quasiclassical description of electronic supershells in simple metal clusters

A quasiclassical method for calculating shell effects, which has been used previously in atomic and plasma physics, is used to describe electronic supershells in metal clusters. An analytical expression is obtained, in the spherical jellium model, for the oscillating part of the binding energy of electrons of a cluster as a sum of contributions from supershells with quantum numbers 2n _r +l, 3n _r +l, 4n _r +l,... This expression is written in terms of the classical characteristics of the motion of an electron with the Fermi energy in a self-consistent potential. The conditions under which a new supershell appears and the relative contribution of this shell are investigated as a function of the cluster size and form of the potential. Specific calculations are performed for a “square well” of finite depth. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 333–337 (10 September 1999)     

Density excitations of a harmonically trapped ideal gas

The dynamic structure factor S(q, ω) of a harmonically trapped Bose gas has been calculated well above the Bose-Einstein condensation temperature by treating the gas cloud as a canonical ensemble of non-interacting classical particles. The static structure factor is found to vanish s8 q ² in the long-wavelength limit. We also incorporate a relaxation mechanism phenomenologically by including a stochastic friction force to study S(q, ω). A significant temperature dependence of the density fluctuation spectra is found. The Debye-Waller factor has been calculated for the trapped thermal cloud as a function of q and the number N of atoms. A substantial difference is found for small- and large-N clouds.     

New model for systems of mesoscopic Josephson junctions

Quantum fluctuations of the phases of the order parameter in two-dimensional arrays of mesoscopic Josephson junctions and their effect on the destruction of superconductivity in the system are investigated by means of a quantum-cosine model that is free of the incorrect application of the phase operator. The proposed model employs trigonometric phase operators and makes it possible to study arrays of small superconducting granules, pores containing superfluid helium, or Josephson junctions in which the average number of particles n ₀ (effective bosons, He atoms, and so on) is small, and the standard approach employing the phase operator and the particle number operator as conjugate operators is inapplicable. There is a large difference in the phase diagrams between arrays of macroscopic and mesoscopic objects for n ₀<5 and U<J (U is the characteristic interaction energy of the particles per granule and J is the Josephson coupling constant). Re-entrant superconductivity phenomena are discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 649–654 (25 November 1997)