The boundary condition integration technique: results for the Hubbard model in 1D and 2D

We study models of strongly correlated electrons in one-and two dimensions. We exactly diagonalize small clusters with general boundary conditions (BC) and integrate over all possible BC. This technique recovers the kinetic energy part of the (extended lattice) Hamiltonianexactly in a grand-canonical formulation. A continuous range of particle densities may be described with this technique and the momentum space can be probed for arbitrary momenta. For the Hubbard Hamiltonian we recover details of the Mott-insulating behaviour for the momentum distribution function at half filling, both in 1D and 2D. Off half-filling the shape of thecanonical Fermi surface is strongly distorted in 2D with respect to thegrand canonical Fermi surface. The shape of the grand canonical Fermi surface obtained by this finite-size technique reduces in the weak-coupling limit exactly to that of the infinite-lattice Fermi sea.email: UPH 301 at DDOHR Z11     

Darstellung und Struktur der Verbindungen Ba2SiTe4 und Ba2SiSe4

On Chalcogenidosilicates: Ba₂SiTe₄ and Ba₂SiSe₄ The new compounds Ba₂SiSe₄ and Ba₂SiTe₄ crystallize in the monoclinic system, space group: P2₁/m (No. 11) with the lattice constants s. “Inhaltsübersicht”. Both compounds are isotypic to the Sr₂GeS₄ structure. Ba₂SiTe₄ is the first o-telluridosilicate with discrete SiTe₄^4? anions.     

Facile asymmetric synthesis of the core nuclei of xanthanolides,guaianolides, and eudesmanolides

[reaction: see text] Bicyclic and tricyclic gamma-butyrolactones with 5,7-, 5,6,5-, 5,6,6-, or 5,7,5-fused ring systems, being found in xanthanolides, eudesmanolides, and guaianolides, were readily synthesized from methyl furan-2-carboxylic acid. Key steps were a copper(I)-catalyzed asymmetric cyclopropanation, Sakurai allylations, intramolecular ene reactions, and ring-closing metathesis reactions.     

Conformation control of triplet state diffusion in platinum containing polyfluorene copolymers

The spectral diffusion of singlet and triplet excitons in 9,9-dioctylfluorene-based conjugated copolymers is investigated using photoluminescence spectroscopy at both low (5 K) and room temperature (300 K). Inclusion of a N,N-diphenyl-4-(pyridin-2-yl)aniline moiety into the polymer backbone allows subsequent cyclometalation with platinum acetylacetonate to increase the spin-orbit coupling and yield radiative decay from the triplet state. For suitably low fractions (≤5%) of bulky ligand inclusion, cyclometalated or not, the resulting longer sequences of fluorene units are able to adopt the chain-extended β-phase conformation. Comparison between the phosphorescence spectral diffusion in glassy- and β-phase Pt-copolymer samples provide insight into the triplet exciton transfer in more- or less-disordered conjugated polymer films. It is found in the glassy-phase samples with shorter conjugation lengths that the triplet exciton relaxation becomes frustrated at low temperature due to a freezing out of the thermally activated hops required to move from one conjugated segment to another. In contrast, for films containing β-phase chain segments, with increased conjugation lengths, this frustration is lifted as more hopping sites remain accessible through intra-segment motion. This work demonstrates controlled use of changes in molecular conformation to optimize triplet diffusion properties in a member of the widely deployed fluorene-based conjugated copolymers.     

Scalable Hybrid Synthetic/Biocatalytic Route to Psilocybin

Psilocybin, the principal indole alkaloid of Psilocybe mushrooms, is currently undergoing clinical trials as a medication against treatment-resistant depression and major depressive disorder. The psilocybin supply for pharmaceutical purposes is met by synthetic chemistry. We replaced the problematic phosphorylation step during synthesis with the mushroom kinase PsiK. This enzyme was biochemically characterized and used to produce one gram of psilocybin from psilocin within 20 minutes. We also describe a pilot-scale protocol for recombinant PsiK that yielded 150 mg enzyme in active and soluble form. Our work consolidates the simplicity of tryptamine chemistry with the specificity and selectivity of enzymatic catalysis and helps provide access to an important drug at potentially reasonable cost.     

Enantioselective synthesis of paraconic acids

The development of a new method for the enantioselective synthesis of disubstituted gamma-butyrolactones is reported. Based on this strategy, the total synthesis of three paraconic acids, that is (-)-roccellaric acid, (-)-nephrosteranic acid and (-)-protopraesorediosic acid, and the formal total synthesis of (-)-methylenolactocin and (-)-protolichesterinic acid is described, which are important because of their antibiotic and antitumor properties. Key steps of the synthesis are copper(I)-catalyzed asymmetric cyclopropanations of furans, highly diastereoselective Sakurai allylations, Lewis acid or Lewis base catalyzed retroaldol/lactonization cascades, and ruthenium(II)-catalyzed, intermolecular cross metathesis reactions.     

On the evaluation of the specific heat and general off-diagonal n-point correlation functions within the loop algorithm

We present an efficient way to compute diagonal and off-diagonal n-point correlation functions for quantum spin-systems within the loop algorithm. We show that the general rules for the evaluation of these correlation functions take an especially simple form within the framework of directed loops. These rules state that contributing loops have to close coherently. As an application we evaluate the specific heat for the case of spin chains and ladders. Recieved 10 January 2000 and Received in final form 9 February 2000     

A self-consistent cluster study of the Emery model

We calculate Fermi-surface properties of the Cuprate superconductors within the three band Hubbard model (also called the Emery model) using a cluster expansion for the proper selfenergy. The Fermi-surface topology is in agreement with angular-resolved photoemission data for dopings ～ 20%. We discuss possible violations of the Luttinger sum-rule for smaller dopings and the role of van-Hove singularities in the density of states of the Zhang-Rice singlets. We calculate the shift in the chemical potential upon doping and find quantitative agreement with recent experiments.     

Perovskites in high dimensions

We study the (D+1) band Hubbard model on generalizedD-dimensional perovskite structures. We show that in the limit of high dimensions the possible scaling behaviour is uniquely determined via the bandstructure and that the model without direct oxygen-oxygen hopping necessarily scales to the cluster limit. A 1/dimension expansion then leads to at-J like Hamiltonian and the Zhang-Rice analysis becomes rigorous. The large dimension fixed point, in general, still remains the cluster model even when a hopping term between n.n. oxygensites is included. Only for a unique ratio of the oxygen onsite energies to the oxygen-oxygen hopping amplitude is a new fixed point possible, corresponding to a heavy-Fermion Hamiltonian.