Hubbard model on the triangular lattice: spiral order and spin liquid

We investigate the half-filled Hubbard model on an isotropic triangular lattice with the variational cluster approximation. By decreasing the on-site repulsion U (or equivalently increasing pressure) we go from a phase with long-range, three-sublattice, spiral magnetic order, to a nonmagnetic Mott insulating phase--a spin liquid--and then, for U less or similar to 6.7t, to a metallic phase. Clusters of sizes 3, 6, and 15 with open boundary conditions are used in these calculations, and an extrapolation to infinite size is argued to lead to a disordered phase at U = 8t, but to a spiral order at U greater or similar to 12.     

Direct and Reverse Limiting Series of Transition Metal Phosphides with Ordered Defects and Metal/Non Metal Ratio Close to 2

Two new limiting series of ternary compounds with ordered defects have been evidenced, which crystallize with hexagonal symmetry, space group P6m2. The first one (direct series) shows one metal vacancy and corresponds to the chemical formula □R T X . The compounds α‐UCr₆P₄ (n = 0) and Zr₄Co₁₃Si₉ (n = 1) are the first members of the series. X‐ray single crystal determination and/or electron microprobe analysis confirm the ternary phosphides Ce₉Ni₂₅P₁₃ and Ce₁₆Ni₃₆P₂₂ to be the following members. The second family (reverse series) with chemical formula R□ T X comprises the ternaries α‐UCr₆P₄ (in fact member n = 0 in both series) and UMo₁₃P₉ (n = 1), the structure of which has been determined on a single crystal. The limiting structures to which the two series converge have been found to be YbPtP (direct) and WC (reverse). The structural relationships between the direct and reverse series have been discussed in terms of metal vacancies and coordination polyhedra. Moreover, a general crystal chemical rule has been established that permits prediction of the different members for the two series and their structural definition in terms of lattice parameters, atomic coordinates and theoretical X‐ray diffraction patterns. Finally, this rule permits to give for each member the number of metal vacancies as well as the distribution of the metalloid polyhedra occupied by the metal atoms (trigonal prisms, pyramids, tetrahedra, triangles).     

Step change of counterion--a new option in capillary zone electrophoresis

In contrast to common capillary zone electrophoresis, with a constant composition of the background electrolyte, a new method is described, based on a change of the ionic matrix composition during the migration of the sample components. The proposed method expands the scale of pK's of analyzed compounds in a single run and affords new potential for optimizing the separation. The step change of the ionic matrix composition was achieved by displacing the primary matrix by the modified ionic matrix (electrolyte) which migrated against the movement of the sample components. The separation thus proceeded in the primary ionic matrix (electrolyte) while the detection was performed in the modified matrix with resultant shortening of the analysis time and convenient quantitation. A model mixture of five compounds, with a difference of four units in pK could be well resolved.     

Quantum theory of light propagation I. General theory

We present a particular approach to quantum theory of light propagation in nonlinear medium using space and time dependent modal operators. Spatial and temporal evolutions of this space and time dependent modal operators are given by the Heisenberg-like equation involving the momentum operator and Heisenberg equation, respectively, which can be justified from point of view of quantum electrodynamics. This useful concept can be applied to an arbitrary nonlinear interaction.The author would like to express his sincere thanks to Professor J. Peina for advice, comments and stimulating discussions.This work was partially supported by the grant PV202/1994 of Czech Ministry of Education and by an internal grant of the Palacký University.     

Synthesis and characterization of poly(butadiene-b-sulphone) by block-copolycondensation—I. Synthesis from α,ω-dichlorocarbonyl oligobutadienes and α,ω-diphenol oligoarylethersulphones

Synthesis of poly(butadiene-b-sulphone) has been carried out by two methods. Reaction of α,ω-di(sodium phenolate) oligosulphone with α,ω-di(chlorocarbonyl)oligobutadiene or reaction of α,ω-diphenol oligosulphone with α,ω-di(chlorocarbonyl)oligobutadiene in the presence of magnesium. For both methods, the reaction was carried out between two models (bisphenol A and adipoyl chloride) or between a model and an oligomer or between two oligomers. For the first reaction a two-step process, involving the separation and the purification of the phenolate, is described: it is more efficient than the classical one-step process, where acid chloride is added in an alkaline solution of phenol. The second method, which seems never to have been used in polymer synthesis, gives better results than the first, with higher $\text{DP}{}_{\mathrm{n}}$ and less interference from side-reactions.     

Amine/anhydride reaction versus amide/anhydride reaction in polyamide/anhydride carriers

Polyamide 6 (PA6) and poly(metaxylene adipamide) (PAmXD6) were blended in a batch mixer with anhydrides such as phthalic anhydride, n-octadecyl succinic anhydride, and anhydride-grafted ethylene propylene rubber. The melt viscosity, the solution viscosity, and chain end concentration were studied during the mixing. PA was first mixed 5 min to get an homogeneous melt prior to the anhydride addition. The introduction of the anhydride to the molten polyamide resulted in large decreases of melt and solution viscosities and of amine chain end concentrations. The anhydride units react with amine chain ends to form imide groups. The resulting low amine chain end concentration causes hydrolysis reaction to maintain the condensation equilibrium. As a consequence an increased carboxylic chain end concentration is observed. The imide concentration was studied by IR. It was shown that when most of the amine chain ends are consumed, the remaining anhydride reacts with amino groups formed by polyamide hydrolysis. © 1995 John Wiley & Sons, Inc.     

The dark side of disulfide-based dynamic combinatorial chemistry

During the last two decades, disulfide-based dynamic combinatorial chemistry has been extensively used in the field of molecular recognition to deliver artificial receptors for molecules of biological interest. Commonly, the nature of library members and their relative amounts are provided from HPLC-MS analysis of the libraries, allowing the identification of potential binders for a target (bio)molecule. By re-investigating dynamic combinatorial libraries generated from a simple 2,5-dicarboxy-1,4-dithiophenol building block in water, we herein demonstrated that multiple analytical tools were actually necessary in order to comprehensively describe the libraries in terms of size, stereochemistry, affinity, selectivity, and finally to get a true grasp on the different phenomena at work within dynamic combinatorial systems.

We show that multiple analytical tools are necessary in order to describe the different phenomena within disulfide-based dynamic combinatorial libraries in terms of size, stereochemistry, affinity and selectivity.     

A magnetic lens for cold atoms controlled by a rf field

We report on a new type of magnetic lens that focuses atomic clouds using a static inhomogeneous magnetic field in combination with a radio-frequency (rf) field. The experimental study is performed with a cloud of cold cesium atoms. The rf field adiabatically deforms the magnetic potential of a coil and therefore changes its focusing properties. The focal length can be tuned precisely by changing the rf frequency value. Depending on the rf antenna position relative to the dc magnetic profile, the focal length of the atomic lens can be either decreased or increased by the rf field. PACS 39.25.+k; 37.10.Gh