具有聚集诱导发光行为的环状多烯类分子

研究了ΔH-pyrans,fulvenes,siloles等环状多烯结构的小分子荧光染料在溶液,固态及薄层层析板上的荧光发射行为.与大多数传统的荧光染料小分子不同,这些多烯类荧光染料分子在稀溶液中基本没有荧光,而在聚集态下呈现非常明亮的荧光发射,同时伴随着荧光量子效率的大幅提高(聚集诱导发光).它们在薄层层析板上也具有很强的荧光发射.但当薄层层析板暴露于有机溶剂气氛下,荧光消失,离开有机溶剂气氛,荧光恢复,这一可逆过程并可多次重复.在固体状态,它们的荧光发射与聚集态结构密切相关.通过从无定型态到结晶态以及从一种结晶态到另一种结晶态的变化,可以有效的调节它们的固体荧光发射.     

Phase diagram of a model for diluted magnetic semiconductors beyond mean-field approximations

A lattice spin-fermion model for diluted magnetic semiconductors (DMS) is investigated numerically, improving on previously used mean-field approximations. Curie temperatures are obtained varying the Mn spin x and hole n densities, and the impurity-hole exchange J in units of the hop-ping t. Optimal values are found in the subtle intermediate regime between itinerant and localized carriers. At intermediate and large J/t, a "clustered" state is observed above the Curie temperature and ferromagnetism is suppressed. Formal analogies between DMS and manganites are discussed.     

Double-quantum filtered MAS NMR in the presence of chemical shielding anisotropies and direct dipolar and J couplings

Double-quantum filtered MAS NMR spectra of an isolated homonuclear spin-1/2 pair are considered, at and away from rotational resonance conditions. The pulse sequence used is the solid-state NMR equivalent of double-quantum filtered COSY, known from solution-state NMR. The 119Sn spin pair in [(chex3Sn)2S] is characterized by a difference in isotropic chemical shielding smaller than the two chemical shielding anisotropies and by direct dipolar and isotropic J-coupling constants of similar magnitudes. At rotational resonance, one-dimensional double-quantum filtered 119Sn lineshapes yield the relative orientation of the two 119Sn chemical shielding tensors. Good double-quantum filtration efficiencies are found at and away from rotational resonance conditions, despite the presence of large chemical shielding anisotropies. Numerical simulations illustrate the interplay of the direct dipolar and J-coupling pathways and identify the latter as the main pathway even at rotational resonance conditions.     

MC-simulation of the 3D,q=3 Potts model

We consider theq=3 Potts model in three dimensions by Monte Carlo simulations. The microcanonical density of states is calculated as a function of the internal energy of the system. We extrapolate the data for the simulated finite systems to the thermo-dynamic limit and find a discontinuous phase transition. This method is checked in the two-dimensional case, where exact results are known.     

Development of a very low energy μ+ beam at PSI

At PSI we are investigating the technique of decelerating an existing very intense secondary beam of surface ⁺ (4 MeV) to an energy of 10 eV using appropriate moderators. These ⁺ can then be used as a source of a tertiary beam of low energy muons with tunable kinetic energy between 10 eV and 10 keV.With a 1000 A layer of solid Argon deposited on an Al substrate we obtain a moderation efficiency (with respect to the number of incoming surface ⁺) of the order of 10^–4.Results of our investigations and the present status of the project are presented together with future plans and possibilities.     

Regularization of reaction progress variable for application to flamelet-based combustion models

Many combustion models that are based on the flamelet paradigm employ a reaction progress variable. While such a progress variable is well defined for one-step reaction kinetics, this is typically not the case for complex chemical mechanisms. Consequently, several expressions for a progress variable have been utilized. In this paper a formal method for the generation of a reaction progress variable is proposed that is optimal with respect to a set of constraints. The potential of the method is demonstrated in applications to partially premixed and diffusion flames, and the extension to premixed combustion is discussed. It is shown that the proposed method can lead to significant improvements in the definition of an optimal progress variable over conventional formulations, essentially eliminating the expert knowledge previously required in identifying such quantities.     

Cluster calculations of the electronic structure of Fe4C

The electronic structure and magnetic properties of Fe₄C are investigated with the first-principles discrete variational method (DV) in spin-polarized case. It is used for the local spin density approximation of the density functional theory. Clusters of 21 and 27 atoms were adopted to represent the two non-equivalent iron sites in this compound. The calculated magnetic moments for the corner and face centered iron sites are 2.63\mu_B and 1.77\mu_B, respectively, which are in very good agreement with the experimental results. A collapse of the magnetic moment for both inequivalent iron sites was verified above 12% of contraction of the lattice spacing. The C atom acts as an acceptor for electrons in this host. This revised version was published online in August 2006 with corrections to the Cover Date.     

Monstrous Branes

 We study D-branes in the bosonic closed string theory whose automorphism group is the Bimonster group (the wreath product of the Monster simple group with ℤ₂). We give a complete classification of D-branes preserving the chiral subalgebra of Monster invariants and show that they transform in a representation of the Bimonster. Our results apply more generally to self-dual conformal field theories which admit the action of a compact Lie group on both the left- and right-moving sectors. Received: 20 February 2002 / Accepted: 17 August 2002 Published online: 19 December 2002 Communicated by R.H. Dijkgraaf