Numerical simulations with two flavours of twisted-mass Wilson quarks and DBW2 gauge action

Discretisation errors in two-flavour lattice QCD with Wilson quarks and DBW2 gauge action are investigated by comparing numerical simulation data at two values of the bare gauge coupling. Both non-zero- and zero-twisted-mass values are considered. The results, including also data from simulations using the Wilson plaquette gauge action, are compared to next-to-leading order chiral perturbation theory formulas.     

Robustness and nondegenerateness for linear complementarity problems

In this paper the main focus is on a stability concept for solutions of a linear complementarity problem. A solution of such a problem is robust if it is stable against slight perturbations of the data of the problem. Relations are investigated between the robustness, the nondegenerateness and the isolatedness of solutions. It turns out that an isolated nondegenerate solution is robust and also that a robust nondegenerate solution is isolated. Since the class of linear complementarity problems with only robust solutions or only nondegenerate solutions is not an open set, attention is paid to Garcia's classG _n of linear complementarity problems. The nondegenerate problems inG _n form an open set.     

Structure‐Directing Forces in Intercluster Compounds of Cationic [Ag14(CCtBu)12Cl]+ Building Blocks and Polyoxometalates: Long‐Range versus Short‐Range Bonding Interactions

Crystallization of [Ag₁₄(C?CtBu)₁₂Cl][BF₄] and different polyoxometalates in organic solvents yields a series of new intercluster compounds: [Ag₁₄(C?CtBu)₁₂Cl(CH₃CN)]₂[W₆O₁₉] ( 1 ), (nBu₄N)[Ag₁₄(C?CtBu)₁₂Cl(CH₃CN)]₂[PW₁₂O₄₀] ( 2 ), and [Ag₁₄(C?CtBu)₁₂Cl]₂[Ag₁₄(C?CtBu)₁₂Cl(CH₃CN)]₂[SiMo₁₂O₄₀] ( 3 ). Applying the same technique to a system starting from polymeric {[Ag₃(C?CtBu)₂][BF₄]?0.6 H₂O}_n and the polyoxometalate (nBu₄N)₂[W₆O₁₉] results in the formation of [Ag₁₄(C?CtBu)₁₂(CH₃CN)₂][W₆O₁₉] ( 4 ). Here, the Ag₁₄ cluster is generated from polymeric {[Ag₃(C?CtBu)₂][BF₄]?0.6 H₂O}_n during crystallization. In a similar way, [Ag₁₅(C?CtBu)₁₂(CH₃CN)₅][PW₁₂O₄₀] ( 5 ) has been obtained from {[Ag₃(C?CtBu)₂][BF₄]?0.6 H₂O}_n and (nBu₄N)₃[PW₁₂O₄₀]. The use of charged building blocks was intentional, because at these conditions the contribution of long‐range Coulomb interactions would benefit most from full periodicity of the intercluster compound, thus favoring formation of well‐crystalline materials. The latter has been achieved, indeed. However, as a most conspicuous feature, equally charged species aggregate, which demonstrates that the short‐range interactions between the “surfaces” of the clusters represent the more powerful structure direction forces than the long‐range Coulomb bonding. This observation is of significant importance for understanding the mechanisms underlying self‐organization of monodisperse and structurally well‐defined particles of nanometer size.     

A step toward direct fullerene synthesis: C60 fullerene precursors with fluorine in key positions

Several fluorine containing polycyclic aromatic hydrocarbons with exact carbon atom topology of the C₆₀ fullerene have been synthesized. Different numbers of fluorine atoms were introduced in the key positions, as needed for an efficient intramolecular condensation to the fullerene molecule. The polycyclic aromatic compounds obtained represent attractive precursors for rational, high-yield fullerene synthesis by flash vacuum pyrolysis.     

On the equivalence of ring-coupled cluster and adiabatic connection fluctuation-dissipation theorem random phase approximation correlation energy expressions

The correlation energy in the direct random phase approximation (dRPA) can be written, among other possibilities, either in terms of the interaction strength averaged correlation density matrix, or in terms of the coupled cluster doubles amplitudes obtained in the direct ring approximation (drCCD). Although the corresponding dRPA correlation density matrix on the one hand, and the drCCD amplitude matrix on the other hand, differ significantly, they yield identical energies. Similarly, the analogous RPA and rCCD correlation energies calculated from antisymmetrized two-electron integrals are identical to each other despite very different underlying working equations. In the present communication, a direct correspondence between amplitudes and densities is established and investigated with perturbation theory arguments. Our analysis also sheds some light on the properties of recently proposed RPA/rCCD variants which use antisymmetrized integrals in part of the equations and nonantisymmetrized integrals in others.     

High average power large-pitch fiber amplifier with robust single-mode operation

Ytterbium-doped large-pitch fibers with very large mode areas are investigated in a high-power fiber amplifier configuration. An average output power of 294?W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62?μm. Compared to previous active large-mode area designs, the threshold of mode instabilities is increased by a factor of about 3.