Dithiolene transfer to the molybdenum nitrosyl complex [CpMo(CO)2(NO)]: Formation of bimetallic complexes

The reaction of the nitrosyl complex [CpMo(CO)2(NO)] (Cp = η-C5H5) with the nickel dithiolene complex [Ni(S2C2Ph2)2] produces the expected dimolybdenum complex [Mo2(NO)2(μ-S2C2Ph2)2Cp2], but only as a minor product (13% yield). The major product (41%) consists of two separable isomers of the tetranuclear complex [Mo2Ni2(NO)2(μ-S2C2Ph2)4Cp2], which comprises two CpMo(NO)Ni(S2C2Ph2)2units joined through bridging sulfur atoms. The isomers differ in the orientation of one dimeric unit in relation to the other. All three compounds have been structurally characterised

Preequilibrium Neutron Emission in (p, xn) Reactions with 80-160 MeV Projectiles

This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440

High Temperature Electron Localization in dense He Gas

We report new accurate mesasurements of the mobility of excess electrons in high density Helium gas in extended ranges of temperature $[(26\leq T\leq 77) K ]$ and density $[ (0.05\leq N\leq 12.0) {atoms} \cdot {nm}^{-3}]$ to ascertain the effect of temperature on the formation and dynamics of localized electron states. The main result of the experiment is that the formation of localized states essentially depends on the relative balance of fluid dilation energy, repulsive electron-atom interaction energy, and thermal energy. As a consequence, the onset of localization depends on the medium disorder through gas temperature and density. It appears that the transition from delocalized to localized states shifts to larger densities as the temperature is increased. This behavior can be understood in terms of a simple model of electron self-trapping in a spherically symmetric square well.Comment: 23 pages, 13 figure

Intersubband plasmons in quasi-one-dimensional electron systems on a liquid helium surface

The collective excitation spectra are studied for a multisubband quasi-one-dimensional electron gas on the surface of liquid helium. Different intersubband plasmon modes are identified by calculating the spectral weight function of the electron gas within a 12 subband model. Strong intersubband coupling and depolarization shifts are found. When the plasmon energy is close to the energy differences between two subbands, Landau damping in this finite temperature system leads to plasmon gaps at small wavevectors.Comment: To be published as a Rapid Communication in Phys. Rev.

Wigner Crystals in the lowest Landau level at low filling factors

We report on results of finite-size numerical studies of partially filled lowest Landau level at low electron filling factors. We find convincing evidence suggesting that electrons form Wigner Crystals at sufficiently low filling factors, and the critical filling factor is close to 1/7. At nu=1/7 we find the system undergoes a phase transition from Wigner Crystal to the incompressible Laughlin state when the short-range part of the Coulomb interaction is modified slightly. This transition is either continuous or very weakly first order.Comment: 5 papges RevTex with 8 eps figures embedded in the tex

Topological Defects and Non-homogeneous Melting of Large 2D Coulomb Clusters

The configurational and melting properties of large two-dimensional clusters of charged classical particles interacting with each other via the Coulomb potential are investigated through the Monte Carlo simulation technique. The particles are confined by a harmonic potential. For a large number of particles in the cluster (N>150) the configuration is determined by two competing effects, namely in the center a hexagonal lattice is formed, which is the groundstate for an infinite 2D system, and the confinement which imposes its circular symmetry on the outer edge. As a result a hexagonal Wigner lattice is formed in the central area while at the border of the cluster the particles are arranged in rings. In the transition region defects appear as dislocations and disclinations at the six corners of the hexagonal-shaped inner domain. Many different arrangements and type of defects are possible as metastable configurations with a slightly higher energy. The particles motion is found to be strongly related to the topological structure. Our results clearly show that the melting of the clusters starts near the geometry induced defects, and that three different melting temperatures can be defined corresponding to the melting of different regions in the cluster.Comment: 7 pages, 11 figures, submitted to Phys. Rev.

Tunneling from a correlated 2D electron system transverse to a magnetic field

We show that, in a magnetic field parallel to the 2D electron layer, strong electron correlations change the rate of tunneling from the layer exponentially. It results in a specific density dependence of the escape rate. The mechanism is a dynamical Mossbauer-type recoil, in which the Hall momentum of the tunneling electron is partly transferred to the whole electron system, depending on the interrelation between the rate of interelectron momentum exchange and the tunneling duration. We also show that, in a certain temperature range, magnetic field can enhance rather than suppress the tunneling rate. The effect is due to the magnetic field induced energy exchange between the in-plane and out-of-plane motion. Magnetic field can also induce switching between intra-well states from which the system tunnels, and a transition from tunneling to thermal activation. Explicit results are obtained for a Wigner crystal. They are in qualitative and quantitative agreement with the relevant experimental data, with no adjustable parameters.Comment: 16 pages, 9 figure

Topological Defects, Orientational Order, and Depinning of the Electron Solid in a Random Potential

We report on the results of molecular dynamics simulation (MD) studies of the classical two-dimensional electron crystal in the presence disorder. Our study is motivated by recent experiments on this system in modulation doped semiconductor systems in very strong magnetic fields, where the magnetic length is much smaller than the average interelectron spacing $a_0$, as well as by recent studies of electrons on the surface of helium. We investigate the low temperature state of this system using a simulated annealing method. We find that the low temperature state of the system always has isolated dislocations, even at the weakest disorder levels investigated. We also find evidence for a transition from a hexatic glass to an isotropic glass as the disorder is increased. The former is characterized by quasi-long range orientational order, and the absence of disclination defects in the low temperature state, and the latter by short range orientational order and the presence of these defects. The threshold electric field is also studied as a function of the disorder strength, and is shown to have a characteristic signature of the transition. Finally, the qualitative behavior of the electron flow in the depinned state is shown to change continuously from an elastic flow to a channel-like, plastic flow as the disorder strength is increased.Comment: 31 pages, RevTex 3.0, 15 figures upon request, accepted for publication in Phys. Rev. B., HAF94MD