87 research outputs found
(η5âCyclopentadienyl)Tricarbonylmanganese(I) Complexes
Since the initial preparation of (η5âC5H5)Mn(CO)3 by Fischer and Jira in 1954, many methods have been developed for the synthesis of this compound (often referred to as âcymantreneâ) and its C5 substituted derivatives. Interestingly, each of these preparations has at least one significant drawback from a preparative perspective (thallium reagents, high pressures, poor yields, or reactants that require significant preparative investment). Surprisingly missing from this collection of preparative methods is the direct reaction between commercially available Mn(CO)5Br and NaC5H5. In 1988, however, Smart and coworkers found MnBr(CO)3(pyridine)2 to be an effective reagent in making a substituted indacenylmanganese tricarbonyl complex. As this pyridine complex also serves as an effective manganese starting material, this chapter discusses the highâyield syntheses of cymantrene and its derivatives such as (η5âC5Me5)Mn(CO)3
Theory for Phase Transitions in Insulating Vanadium Oxide
We show that the recently proposed S=2 bond model with orbital degrees of
freedom for insulating VO not only explains the anomalous magnetic
ordering, but also other mysteries of the magnetic phase transition. The model
contains an additional orbital degree of freedom that exhibits a zero
temperature quantum phase transtion in the Ising universality class.Comment: 5 pages, 2 figure
RR Lyrae Stars in the Bootes dSph
We present a catalog of 15 RR Lyrae variable stars in the recently discovered
Bootes galaxy -- the most metal-poor simple stellar population with measured RR
Lyrae stars. The pulsational properties of the RR Lyrae conform closely to
period-abundance trends extrapolated from more metal-rich populations and we
estimate the distance of Bootes to be (m-M)_0=18.96+-0.12. The average period
(0.69 days), the ratio of type c to type ab pulsators (0.53) and the RRab
period shift (-0.07) indicate an Oosterhoff II classification for Bootes, a
marked contrast to the other dSph galaxies, which are Oosterhoff intermediate.
This supports the contention that the Oosterhoff dichotomy is a continuum --
that RR Lyrae properties, to first order, vary smoothly with abundance. The
dSph galaxies are not distinct from the Galactic globular clusters, but bridge
the Oosterhoff gap. The absence of any anomalous Cepheids in Bootes could
indicate the lack of an intermediate age population.Comment: 16 pages, submitted to Astrophysical Journal Letter
Bundles over Nearly-Kahler Homogeneous Spaces in Heterotic String Theory
We construct heterotic vacua based on six-dimensional nearly-Kahler
homogeneous manifolds and non-trivial vector bundles thereon. Our examples are
based on three specific group coset spaces. It is shown how to construct line
bundles over these spaces, compute their properties and build up vector bundles
consistent with supersymmetry and anomaly cancelation. It turns out that the
most interesting coset is . This space supports a large number of
vector bundles which lead to consistent heterotic vacua, some of them with
three chiral families.Comment: 32 pages, reference adde
Ground State and Excitations of Spin Chain with Orbital Degeneracy
The one dimensional Heisenberg model in the presence of orbital degeneracy is
studied at the SU(4) symmetric viewpoint by means of Bethe ansatz. Following
Sutherland's previous work on an equivalent model, we discuss the ground state
and the low-lying excitations more extensively in connection to the spin
systems with orbital degeneracy. We show explicitly that the ground state is a
SU(4) singlet. We study the degeneracies of the elementary excitations and the
spectra of the generalized magnons consisting of these excitations. We also
discuss the complex 2-strings in the context of the Bethe ansatz solutions.Comment: Revtex, 9 pages, 3 figures; typos correcte
Synthesis, Molecular Structure, and 1H NMR Analysis of Bis(tetraphenylcyclopentadienyl)ruthenium(II)
Reaction of [Ru(p-cymene)Cl2]2 with K(η5-C5HPh4) in refluxing diglyme yields (η5-C5Ph4)2Ru in ca 50% yield. The complex was not susceptible to oxidation or reduction. (C5HPH4)2Ru crystallizes in the triclinic P1 space group with a = 8.549(4), b = 10.793(4), c = 12.842(5) Ă
, α = 65.98(3), ÎČ = 73.10(3), Îł = 83.49(3)° and Z = 1. The least-squares data refined to R(F) = 3.53% and R(wF = 3.82% for the 3952 independent observed reflections with Fo â„ 5Ï(Fo). The metal-centroid distance is 1.832(2) Ă
and all other bond lengths and angles are similar to other octaphenylmetallocenes. 1H NMR analysis employing 2D J-resolved, COSY and low temperature techniques allowed assignment of all protons in the molecule. The motional processes of the phenyl groups are discussed
Do interactions increase or reduce the conductance of disordered electrons? It depends!
We investigate the influence of electron-electron interactions on the
conductance of two-dimensional disordered spinless electrons. By using an
efficient numerical method which is based on exact diagonalization in a
truncated basis of Hartree-Fock states we are able to determine the exact
low-energy properties of comparatively large systems in the diffusive as well
as in the localized regimes. We find that weak interactions increase the d.c.
conductance in the localized regime while they decrease the d.c. conductance in
the diffusive regime. Strong interactions always decrease the conductance. We
also study the localization of single-particle excitations close to the Fermi
energy which turns out to be only weakly influenced by the interactions.Comment: final version as publsihed, 4 pages REVTEX, 6 EPS figures include
Plaquette Ordering in SU(4) Antiferromagnets
We use fermion mean field theory to study possible plaquette ordering in the
antiferromagnetic SU(4) Heisenberg model. We find the ground state for both the
square and triangular lattices to be the disconnected plaquette state. Our mean
field theory gives a first order transition for plaquette ordering for the
triangular lattice. Our results suggest a large number of low lying states.Comment: 16 pages, 5 figure
Wave Functions, Quantum Diffusion, and Scaling Exponents in Golden-Mean Quasiperiodic Tilings
We study the properties of wave functions and the wave-packet dynamics in
quasiperiodic tight-binding models in one, two, and three dimensions. The atoms
in the one-dimensional quasiperiodic chains are coupled by weak and strong
bonds aligned according to the Fibonacci sequence. The associated d-dimensional
quasiperiodic tilings are constructed from the direct product of d such chains,
which yields either the hypercubic tiling or the labyrinth tiling. This
approach allows us to consider rather large systems numerically. We show that
the wave functions of the system are multifractal and that their properties can
be related to the structure of the system in the regime of strong quasiperiodic
modulation by a renormalization group (RG) approach. We also study the dynamics
of wave packets to get information about the electronic transport properties.
In particular, we investigate the scaling behaviour of the return probability
of the wave packet with time. Applying again the RG approach we show that in
the regime of strong quasiperiodic modulation the return probability is
governed by the underlying quasiperiodic structure. Further, we also discuss
lower bounds for the scaling exponent of the width of the wave packet and
propose a modified lower bound for the absolute continuous regime.Comment: 25 pages, 13 figure
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