Density and stability in ultracold dilute boson-fermion mixtures

We analyze in detail recent experiments on ultracold dilute ⁸⁷Rb–⁴⁰K mixtures in Hamburg and in Florence within a mean-field theory. To this end we determine how the stationary bosonic and fermionic density profiles in this mixture depend in the Thomas-Fermi limit on the respective particle numbers. Furthermore, we investigate how the observed stability of the Bose-Fermi mixture with respect to collapse is crucially related to the value of the interspecies s-wave scattering length.     

Quantum phase diagram for homogeneous Bose‐Einstein condensate

We calculate the quantum phase transition for a homogeneous Bose gas in the plane of s‐wave scattering length a_s and temperature T. This is done by improving a one‐loop result near the interaction‐free Bose‐Einstein critical temperature T_c⁽⁰⁾ with the help of recent high‐loop results on the shift of the critical temperature due to a weak atomic repulsion based on variational perturbation theory. The quantum phase diagram shows a nose above T_c⁽⁰⁾, so that we predict the existence of a reentrant transition above T_c⁽⁰⁾, where an increasing repulsion leads to the formation of a condensate.     

3D ToF-SIMS Analysis of Peptide Incorporation into MALDI Matrix Crystals with Sub-micrometer Resolution

The analytical sensitivity in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is largely affected by the specific analyte-matrix interaction, in particular by the possible incorporation of the analytes into crystalline MALDI matrices. Here we used time-of-flight secondary ion mass spectrometry (ToF-SIMS) to visualize the incorporation of three peptides with different hydrophobicities, bradykinin, Substance P, and vasopressin, into two classic MALDI matrices, 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (HCCA). For depth profiling, an Ar cluster ion beam was used to gradually sputter through the matrix crystals without causing significant degradation of matrix or biomolecules. A pulsed Bi₃ ion cluster beam was used to image the lateral analyte distribution in the center of the sputter crater. Using this dual beam technique, the 3D distribution of the analytes and spatial segregation effects within the matrix crystals were imaged with sub-μm resolution. The technique could in the future enable matrix-enhanced (ME)-ToF-SIMS imaging of peptides in tissue slices at ultra-high resolution.

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Thermal transitions and dynamics in nanocomposite hydrogels

Hydrogels based on nanocomposites of statistical poly(hydroxyethyl acrylate-co-ethyl acrylate) and silica, prepared by simultaneous copolymerization and generation of silica nanoparticles by sol?Cgel process at various copolymer compositions and silica contents, characterized by a fine dispersion of filler, were investigated with respect to glass transition and polymer dynamics by dielectric techniques. These include thermally stimulated depolarization currents and dielectric relaxation spectroscopy, covering together broad ranges of frequency and temperature. In addition, equilibrium water sorption isotherms were recorded at room temperature (25?°C). Special attention was paid to the investigation of effects of silica on glass transition, polymer dynamics (secondary ?? and ?? _sw relaxations and segmental ?? relaxation), and electrical conductivity in the dry systems (xerogels) and in the hydrogels at various levels of relative humidity/water content. An overall reduction of molecular mobility is observed in the nanocomposite xerogels, in particular at high silica contents. Analysis of the results and comparison with previous work on similar systems enable to discuss this reduction of molecular mobility in terms of constraints to polymeric motion imposed by interfacial polymer?Cfiller interactions and by the formation of a continuous silica network interpenetrated with the polymer network at filler contents higher than about 15?wt%.     

Nicotinamide Adenine Dinucleotide Oxidation Studies at Multiwalled Carbon Nanotube/Polymer Composite Modified Glassy Carbon Electrodes

NADH oxidation has previously been investigated at carbon nanotube surfaces, although studies into the effect of the polymer binders are needed to fully understand whether the polymer binder affects the electrochemistry. This work details NADH oxidation at glassy carbon electrodes modified with composites containing multiwalled carbon nanotubes and selected polymer binders. NADH is shown to be oxidized at a lower potential than at glassy carbon electrodes and the oxidation potential is a function of the polymer binder. Hydrophobically modified Nafion, Nafion, linear poly(ethylenimine) (LPEI), octyl‐modified LPEI, and poly(vinylpyridine) binders were studied. Experiments showed the peak current and electrochemically assessible electrode area are dependent on the polymer binder. Overall, this paper shows that polymer binders affect NADH oxidation potential at carbon nanotube modified electrodes.     

A theory of finite-temperature Bose-Einstein condensates in neutron stars

We investigate the possible occurrence of a Bose-Einstein condensed phase of matter within neutron stars due to the formation of Cooper pairs among the superfluid neutrons. To this end we study the condensation of bosonic particles under the influence of both a short-range contact and a long-range gravitational interaction in the framework of a Hartree-Fock theory. We consider a finite-temperature scenario, generalizing existing approaches, and derive macroscopic and astrophysically relevant quantities like a mass limit for neutron stars.     

Nanodispersions of conducting particles: preparation, microstructure and dielectric properties

The properties of nanodispersions depend sensitively on their microstructure. Precise preparation techniques are needed to ensure controlled and well-defined spatial distribution of particles. We report on the correlation between synthesis, microstructure and dielectric properties. We conclude that temperature-dependent broadband dielectric spectroscopy up to microwave frequencies allows an insight into the microstructure of nanodispersions. Received: 4 June 1998 Accepted: 18 August 1998