Planck spectroscopy and quantum noise of microwave beam splitters

We use a correlation function analysis of the field quadratures to characterize both the blackbody radiation emitted by a 50 Ω load resistor and the quantum properties of two types of beam splitters in the microwave regime. To this end, we first study vacuum fluctuations as a function of frequency in a Planck spectroscopy experiment and then measure the covariance matrix of weak thermal states. Our results provide direct experimental evidence that vacuum fluctuations represent the fundamental minimum quantum noise added by a beam splitter to any given input signal.     

Fluorescence Spectroscopic Studies on Plasma-Chemically Modified Polymer Surfaces with Fluorophore-Labeled Functionalities

Molecular engineering of polymer surfaces that includes the attachment of functional molecules to existing or previously generated reactive chemical groups like e.g., − OH, − NH₂, or − CHO requires simple strategies and tools for the controlled generation of surface functionalities and their derivatization as well as for their identification and eventually quantification. Here, we systematically investigate the plasma-chemical surface modification of polypropylene films in combination with dansyl labeling chemistry and conventional, yet costly, XPS and highly sensitive fluorescence spectroscopy for the detection of surface groups. Based on these results, the potential of and requirements on the fluorometric characterization and quantification of surfaces functionalities are discussed.     

Anchoring of Fluorophores to Plasma-chemically Modified Polymer Surfaces and the Effect of Cucurbit[6]uril on Dye Emission

Polypropylene supports were functionalized by plasma-deposition of polymeric allylamine layers. The surface amino groups generated were wet-chemically reacted with xanthene dyes resulting in fluorescent polymer films. The effect of polymer-attachment of the dyes on their emission features was studied fluorometrically and different methods were tested to improve the fluorescence properties of the films. Modification with cucurbit[6]uril (CB6) yields a moderately enhanced fluorescence as well as an improved photostability. The observed effect is most likely due to CB6-induced rigidization of the linker molecules which seems to reduce fluorescence quenching dye–dye and fluorophore–surface interactions.     

Solitary Roll Vortices in the Mixed Convection of a Vertical Fluid Layer

Some earlier experimental and numerical findings from convection in vertical channels suggest that localized convection rolls could play a role in the transition process of free or mixed convection. In the present work solitary convection roll vortices for a vertical fluid layer with stable stratification, differential shear and differentially heated side-walls have been obtained numerically for a fluid of unit Prandtl number. The solutions appear through saddle-node bifurcations and in certain parameter ranges they do also exist for linearly stable basic flow. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect-Dipole Formation in Copper-Doped PbTiO3 Ferroelectrics

The defect structure of hard copper-modified polycrystalline PbTiO3 ferroelectrics is investigated by means of electron paramagnetic resonance and hyperfine sublevel correlation spectroscopy, as well as density functional theory calculations. Special emphasis is put on the 207Pb-hyperfine couplings, which are resolved up to the third coordination sphere. The results prove that copper is incorporated at the octahedrally coordinated Ti site, acting as an acceptor. Because of charge compensation the formation of Cu impurity-oxygen vacancy pairs is energetically very favorable. The corresponding (CuTi'-VO)x defect dipole is found to be orientated along the [001] axis.     

Superconducting vortex pinning with artificial magnetic nanostructures

This review is dedicated to summarizing the recent research on vortex dynamics and pinning effects in superconducting films with artificial magnetic structures. The fabrication of hybrid superconducting/magnetic systems is presented together with the wide variety of properties that arise from the interaction between the superconducting vortex lattice and the artificial magnetic nanostructures. Specifically, we review the role that the most important parameters in the vortex dynamics of films with regular array of dots play. In particular, we discuss the phenomena that appear when the symmetry of a regular dot array is distorted from regularity towards complete disorder including rectangular, asymmetric, and aperiodic arrays. The interesting phenomena that appear include vortex-lattice reconfigurations, anisotropic dynamics, channeling, and guided motion as well as ratchet effects. The different regimes are summarized in a phase diagram indicating the transitions that take place as the characteristic distances of the array are modified respect to the superconducting coherence length. Future directions are sketched out indicating the vast open area of research in this field.     

Generation of high average power 1 kHz shaped THz pulses via optical rectification

Generation of near single-cycle THz pulses from lithium niobate with 3.3 μJ energy, 3.3 mW average power, 1.2 THz central frequency and 4 MW peak power was demonstrated by tilting the intensity front of the pump pulses from a 1 kHz Ti:sapphire laser. THz pulse intensity as high as 200 MW/cm² was achieved. The energy conversion efficiency was 7 × 10⁻⁴. The capability of the present scheme to generate high energy shaped THz pulses was also demonstrated by using a sequence of optical pump pulses.     

Thinning and thickening effects induced by shearing in lecithin solutions of polymer-like micelles

 The effects of shear flow on the lecithin organogels consisting of reverse polymer-like micelles have been investigated by dynamic rheology. It was established that the shear effects depended on the molar ratio of water to lecithin that determined the micellar type in the system. For an organogel with linear flexible polymer-like micelles, thinning was observed. The main features of the rheological behavior bore a resemblance to previously investigated aqueous systems made up of similar but direct polymer-like surfactant micelles. The thinning effects are explicable on the basis of alignment of micellar aggregates along the flow direction and their disentanglement. An organogel with branched micelles did not demonstrate any notable response to the shearing. Unusual behavior was noticed in the case of a jelly-like phase that included the highest amounts of water. The applied steady shear flow induced a thickening effect. This was followed by restructuring of the micellar system at the level of polymer-like micelles and their network. The shearing effects were characterized by slow kinetics. In addition, the system did not revert to the original state after the cessation of steady shear flow even within 8 h. Measurements performed in an oscillation regime on this system showed that shearing should promote a substantial growth of the polymer-like micelles and affect their alignment. Received: 18 May 1999 Accepted: 27 March 2000     

Phase and aggregation behaviour of double-chain cationic surfactants from the class of N-alkyl-N-alkyl′-N, N-dimethylammonium bromide surfactants

We present the phase diagrams and the properties of newly synthesised double-chain cationic N-alkyl-N-alkyl′-N,N-dimethylammonium bromide surfactants [C _x C _y DMABr (x = 12, 14 and 16; y = 10, 11, 12, 14 and 16)]. All the systems studied form liquid-crystalline lamellar phases but with different morphologies: unilamellar vesicles at low surfactant concentrations, multilamellar vesicles and tubular aggregates for surfactant concentrations between 2 and 10 wt% and at even higher concentrations planar bilayers of surfactant molecules in the classical L_α phase. The phase diagrams were determined with macroscopic and microscopic methods (polarisation microscopy, freeze-fracture transmission electron microscopy, scanning electron microscopy and differential interference contrast microscopy). The properties of the surfactant solutions were determined with differential scanning calorimetry measurements for Krafft point determination and small-angle neutron scattering measurements for interlamellar spacing and bilayer thickness. Finally, conductivity and viscosity measurements for phase characterisation were carried out. Received: 7 April 1999 Accepted in revised form: 30 April 1999     

Determination of the neutron electric form factor in the D(e,e′n)p reaction and the influence of nuclear binding

The electric form factor of the neutron G _E,n has been determined at the Mainz Microtron MAMI at the low momentum transfer Q ²= 0.15 (GeV/c)² in a measurement of the recoil polarisation ratio P _x/P_z in the quasifree reaction D(e,e′n)p. At this Q ² the influence of the nuclear binding is strong. A purely kinematical model is used to get some insight into the effect of the initial Fermi momentum distribution of the neutron. The influence of the final state interaction is determined quantitatively by a model of Arenh?vel et al.. After the corresponding corrections a value of G _E,n(0.15 (GeV/c)²) = 0.0481±0.0065_stat±0.0053_syst is obtained. Received: 12 April 1999