Experimental studies of phase transitions in isolated metal clusters

The reactions of isolated, neutral transition metal clusters with small molecules are used to probe cluster structure and to identify changes in structure with cluster size. Examples are presented of reactivity, adsorbate uptake, and product composition studies. The general conclusion is that transition metal clusters seem to have structure (are “solid”) under typical experimental conditions, and that their structure, i.e., the way the atoms pack, can change many times in the growth sequence from small clusters to bulk metal. These phase changes are often accompanied by dramatic changes in both chemical and physical properties. Evidence is presented for the existence of isomers of certain cluster sizes for some metals. In a few cases, the chemical evidence can be used to propose possible cluster structure; this is illustrated for iron and nickel clusters.     

Configurational probabilities of spinel octahedral site clusters

Graphs are presented from which one can read directly the probability that in octahedral sites of the spinel lattice (containing two types of atomsC andD randomly distributed over the octahedral sites) a typeD atom will be in a cluster of one, two, three or fourD atoms. The results are presented as a function of the concentration ofD atoms in octahedral sites.     

Phase transitions in simple clusters

Formation of the liquid state of clusters with pairwise interactions between atoms is examined within the framework of the void model, in which configurational excitation of atoms results from formation of voids. Void parameters are found from computer simulation by molecular dynamics methods for Lennard-Jones clusters. From that standpoint, phase transitions are analyzed in terms of two aggregate states. This information allows us to divide the entropy jump during a solid-liquid phase transition into two parts: one corresponds to configurational excitation at zero temperature and the other arises from thermal vibrations of atoms. The latter part contributes approximately 40% for Lennard-Jones clusters consisting of 13 and 55 atoms, increasing to 56% for bulk inert gases. These magnitudes explain the validity of melting criteria based on thermal motion of atoms, even though the distinctive mechanism of this phase transition results from configurational excitations. It is shown that the void concept allows analyzing various aspects of the liquid state of clusters including the existence of a limiting freezing temperature below which no metastable liquid state exists, as well as the existence and properties of glassy states that may exist below the freezing limit.     

Radiative transitions involving impurities

A many-electron development of effective-mass theory is given, based on a Hartree-Fock viewpoint. The procedure provides a basis for the calculation of polarization effects other than those produced by single point charges. An exact effective-mass differential equation is obtained, taking into account non-parabolicities of the band and gradients of the perturbation. Results are obtained for the matrix-element for dipole emission between two impurity-states in a semiconductor. When applied to the problem of donor-acceptor transfer, the results offer an explanation as to why the no-phonon process is considerably stronger in Ge than in Si, even though dielectric constant and effective-mass magnitude effects predict the opposite. The highly non-parabolic conduction band of Ge is responsible.     

Structural transitions in small nickel clusters

The processes of a thermal impact on Ni nanoclusters with a radius of up to 0.8 nm have been studied by means of molecular dynamics with the use of a tight-binding potential. The simulation indicates that the structural transition from the initial fcc phase to the icosahedral modification occurs under the influence of temperature. The transition temperature is shifted towards the cluster melting temperature with an increase in the cluster size. A similar behavior has been observed for copper and gold nanoparticles. A conclusion has been drawn that 200–250 atoms is presumably the limiting size of a metallic cluster, below which the initial fcc modification cannot be kept under realistic industrial conditions. The adequacy of the results is checked in the computer experiments with Lennard-Jones nanoparticles. The results for the Lennard-Jones and metallic nanoparticles have been shown to agree with each other.     

Optical transitions involving excitons in quantum wires

Il Nuovo Cimento D - We present a comprehensive discussion of the excitonic properties of V-shaped GaAs and InGaAs quantum wires grown on patterned substrates. Systematic linear and non-linear...     

Electron states in metal clusters

Radiative transitions in metal clusters are analyzed in terms of quantum transitions of valence electrons that interact with surrounding valence electrons and ion cores. The analysis is based on the solution of the Thomas-Fermi equation for valence electrons in a spherical cluster. The quantum states of valence electrons and the energy and the dipole moments of transitions are determined in the quasiclassical approximation. It is shown that the frequencies of dipole oscillations and the dipole moments of the transitions strongly depend on the size of a cluster.     

Electron-phonon scattering in metal clusters

Electron-lattice energy exchanges are investigated in gold and silver nanoparticles with sizes ranging from 30 to 2.2 nm embedded in different environments. Femtosecond pump-probe experiments performed in the low-perturbation regime demonstrate a strong increase of the intrinsic electron-phonon interaction for nanoparticles smaller than 10 nm due to a confinement effect.     

Relativistic two-body processes in axial-charge transitions

We study the contribution of two-body meson-exchange processes to axial-charge transitions using relativistic models of the nucleus. We conduct calculations at the mean-field and oneloop levels for nuclei in the lead region and also for tin and oxygen. The aim is to provide a comprehensive one-body plus two-body treatment within the relativistic framework. The results indicate that one- and two-body processes enhance the matrix elements of the axial-charge operator by some (70 ± 20) % in all three regions studied, in good qualitative agreement with the data and more conventional calculations. We also discuss the sensitivities of the calculation to the change in some of the couplings and the parameters involved.     

Magnetic clusters in amorphous metal alloys

The effect of thermal treatment in combination with an external magnetic field and/or elastic stress on the magnetic characteristics of amorphous metal alloys of the 2NSR type is studied. The complex behavior of the magnetization and coercivity values in dependence on the length of annealing at temperatures below crystallization is described. It is assumed that the observed changes in the macroscopic magnetic characteristics are associated with the formation of clusters with different degrees of exchange interaction.