120 research outputs found
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 and density 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
Unrestricted Hartree-Fock theory of Wigner crystals
We demonstrate that unrestricted Hartree-Fock theory applied to electrons in
a uniform potential has stable Wigner crystal solutions for in
two dimensions and in three dimensions. The correlation energies
of the Wigner crystal phases are considerably smaller than those of the fluid
phases at the same density.Comment: 4 pages, 5 figure
Crystallization of a classical two-dimensional electron system: Positional and orientational orders
Crystallization of a classical two-dimensional one-component plasma
(electrons interacting with the Coulomb repulsion in a uniform neutralizing
positive background) is investigated with a molecular dynamics simulation. The
positional and the orientational correlation functions are calculated for the
first time. We have found an indication that the solid phase has a
quasi-long-range (power-law) positional order along with a long-range
orientational order. This indicates that, although the long-range Coulomb
interaction is outside the scope of Mermin's theorem, the absence of ordinary
crystalline order at finite temperatures applies to the electron system as
well. The `hexatic' phase, which is predicted between the liquid and the solid
phases by the Kosterlitz-Thouless-Halperin-Nelson-Young theory, is also
discussed.Comment: 3 pages, 4 figures; Corrected typos; Double columne
Roto-vibrational spectrum and Wigner crystallization in two-electron parabolic quantum dots
We provide a quantitative determination of the crystallization onset for two
electrons in a parabolic two-dimensional confinement. This system is shown to
be well described by a roto-vibrational model, Wigner crystallization occurring
when the rotational motion gets decoupled from the vibrational one. The Wigner
molecule thus formed is characterized by its moment of inertia and by the
corresponding sequence of rotational excited states. The role of a vertical
magnetic field is also considered. Additional support to the analysis is given
by the Hartree-Fock phase diagram for the ground state and by the random-phase
approximation for the moment of inertia and vibron excitations.Comment: 10 pages, 8 figures, replaced by the published versio
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
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