(η5‐Cyclopentadienyl)Tricarbonylmanganese(I) Complexes

Since the initial preparation of (η5‐C5H5)Mn(CO)3 by Fischer and Jira in 1954, many methods have been developed for the synthesis of this compound (often referred to as “cymantrene”) 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 high‐yield syntheses of cymantrene and its derivatives such as (η5‐C5Me5)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 V$_{2}$O$_{3}$ 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 $SU(3)/U(1)^2$. 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) Å, α = 65.98(3), β = 73.10(3), γ = 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 ≥ 5σ(Fo). The metal-centroid distance is 1.832(2) Å 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