Showing papers on "Cluster (physics) published in 1968"

Concentration of Clusters in Nucleation and the Classical Phase Integral

Abstract: A classical phase integral treatment of the equilibrium concentration of clusters in homogeneous nucleation from the vapor is discussed. It is concluded that the motions of a stationary cluster, i.e., a cluster in a bulk liquid, are more nearly those of translational and torsional vibration than free translation and rotation in a large free volume. In this case one returns to the original Lothe‐Pound equation. Accordingly, the 1017 factor of correction to classical homogeneous nucleation theory in condensation still remains.

Correlation and Magnetic Effects in Narrow Energy Bands. II

Abstract: Correlation and magnetic effects in narrow bands are studied using a generalized self-consistent cluster treatment of the narrow-band Hamiltonian. Here the dynamics consists of electron hopping between nearest-neighbor sites and Coulomb interaction between electrons on the same site and nearest-neighbor sites. In the simpler case, where the neighbor interactions are neglected, i.e., the Hubbard model, the environment of the cluster is regarded as a particle reservoir, one for each spin. Hopping between the cluster and reservoirs is described by fermion source terms. It is required that the thermodynamic average of the particle currents of each spin within the cluster equal the corresponding particle exchange between cluster and reservoir. Electron motion in the system is examined for varying ratios of the strength of the hopping to the intrasite Coulomb repulsion. A discontinuous transition from an antiferromagnetic insulator to a metal is found under dynamical conditions close to those predicted by Hubbard and Kemeny. The effect of the intersite Coulomb interaction on the transition is next studied. These added terms are seen greatly to influence the transition and the magnetic properties of the cluster.

Kinetic Study of Vacancy Clustering and Annealing in Metals

Abstract: The rate equations governing the clustering and annealing of vacancies in metals have been studied. Single vacancies were considered to be immobile at temperatures at which small clusters migrate. Clusters with up to seven vacancies were explicitly treated, and approximations were made to account for larger clusters. A variety of cases for different cluster binding and migration energies were run. Since the annealing curves generally gave little indication of the actual processes occurring, the input data could not be inferred from standard analyses of isothermal and isochronal data. For small binding energies the annealing was small, as some clusters reached sinks, and the single-vacancy concentration increased. For large binding energies the annealing was also small, as clustering dominated the recovery and few clusters reached sinks. Between these limits, complex results were obtained; the total annealing could be quite large with an appreciable decrease in concentration of single vacancies, and anomalously low activation energies were often found.

Electron spin resonance in cadmium sulphide

Abstract: Electron spin resonance measurements on sixteen undoped single crystals of CdS are reported. Three resonance lines have been observed which are not associated with impurities. Of the lines, which are designated as A, B and D, A and D are discussed in some detail. It is concluded that line D which has a g value near 18 is associated with the donor centre formed by a sulphur vacancy. It is suggested that line A is associated with a cluster of four nearest-neighbour cadmium vacancies which form an acceptor centre with a level approximately 07 ev above the valence band. The g value of line A varies slightly from sample to sample and is about 208.

Cluster light support and control mechanism

Abstract: A number of light units are arranged in a cluster on a common support. Means is provided for tilting and aiming the cluster. The alignment of the light units relative to each other can be varied so as to create either converging or diverging beams.

Temperature Dependence of Isomer Shift and Hyperfine Field near the Curie Point in Iron

Abstract: Current theories of critical‐point phenomena predict that in the region below the Curie temperature TC, the spontaneous magnetization M of a ferromagnet varies with temperature T according to M/M0 = h(1 − T/TC)β for T→TC. Values of h and β have been calculated for theoretical models, and β is of special interest because of the possibility that it is a simple number such as ⅓ or ½. Measured values of β for various ferromagnets seem to cluster about these two values.Pure iron is a favorable case for determining h and β by the Mossbauer effect since the hyperfine field should be closely proportional to M. However, previous results for iron1 showed a rather sharp discontinuity in the isomer shift at TC, and this is difficult to reconcile with the accepted opinion that the magnetic transition in iron is of higher than first order. If this were a first‐order transition, then β would not be a meaningful parameter.A more careful study of the Mossbauer effect in the vicinity of this transition is now being made. T...

A simple theory of inhomogeneity linewidth broadening in MnxFe3−x,O4 ferrites

Abstract: A simple theory of linewidth broadening due to thermal treatment is given. The theory is based on the assumption that the manganese-rich regions — clusters — with local tetragonal symmetry appear in the sample due to the alignment of tetragonally distorted Mn3 + ions. This is a source of local anisotropy fields, leading to two-magnon scattering. The size of clusters and consequently their contribution to the linewidth depends on the thermal treatment. The function giving the dependence of the linewidth broadening on the cluster size is calculated using Schlomann's autocorrelation function. The recent experimental results are qualitatively discussed in terms of this theory. For the temperature dependence of the effect an effective compositionx = 1·8 inside the cluster is assumed, which gives a qualitative agreement with experiment. The discussion of the dependence of linewidth broadening on the composition has only an illustrative character and shows that the theory is compatible with experiment. A qualitative disagreement found for the angular dependence of δ(ΔH) is believed to be a consequence of the high field approximation used.

Stress fields around the edge multipole and around multipole arrays in anisotropic H.C.P crystals

Abstract: The stress field of the edge multipole (dipole cluster) in the basal plane is approximated by the superdipole stress field. The stress fields of different dipole (superdipole) arrays are given for anisotropic cadmium crystal. The stress field components into the secondary non-basal slip planes (pyramidal and prismatic) were computed for the same metal. It was shown that the magnitude of this component is comparable with both the value of flow stresses in the secondary systems and the same component of the pile-up stress field.

Experimental evidence concerning the electronic structure of the Nb6Cl122+ and Ta6Cl122+ clusters

Abstract: Irradiation in the region of the spectra of the M6Cl122+(M = Nb or Ta) cluster claimed to be due to metal–chlorine charge transfer does not increase the rate of chloride release from this cluster. This negative result is significant as it casts doubt on the validity of this assignment of these spectra.