We present a phenomenological description of the high-frequency vortex dynamics in YBa 2 Cu 3 O 7 and discuss the main parameters related to vortex motion, namely the viscous drag coefficient η, the pinning constant k p (Labusch parameter) and the depinning frequency ω 0 . We demonstrate experimental results on the angular and temperature dependence of η, k p and ω 0 in YBa 2 Cu 3 O 7 and compare these results with existing models. We show how studies of the vortex viscosity may yield information on the superclean limit. This limit corresponds to the formation of the discrete excitation spectrum in the vortex core due to quantum confinement and small coherence length. From the low-temperature viscosity data we conclude that the superclean limit in YBa 2 Cu 3 O 7 is reached for magnetic field perpendicular to the c-axis.
Multifunctional core–shell particles composed of magnetic particles covered with a gold nanoshell can be induced to align into conducting lines upon application of a magnetic field (see Figure). The formation of Au clusters and “streaky” gold nanoparticles on the surface of the PS beads is demonstrated and the preparation, characterization, and applications of magnetic and polystyrene beads featuring a gold shell are addressed.
The authors report on gold nanoparticle enhancement of the gain in dyes/nanoparticles/polymer film for random lasers. Importantly, they demonstrate a lower threshold for lasing and a stronger output intensity, which depend on the nanoparticle (NP) size. By calculating the “NP scattering resonance” using Mie theory and the measured surface-plasmon resonance, we can qualitatively explain the results, suggesting that the random lasing properties are dominated by an enhanced photon scattering mechanism induced by the surface plasmons.
We have used the parallel-plate resonator technique to study the microwave surface resistance R, and the penetration depth A, of YBazCu307 " thin epitaxial films at 5.5 GHz in the presence of magnetic field H &0.8 T and at 30 K & T & 80 K. The magnetic-field dependence of R, and A, allows determination of the vortex viscosity g, pinning constant a, and depinning frequency coo for dil'erent field orientations. To the best of our knowledge, this is among the first measurements of the angular dependence of g, a, and uo. We find that the angular dependences of g, a, and coo are fairly well described by the scaling model of Blatter, Geshkenbein, and Larkin. We demonstrate that {i)the temperature dependence of g is well described by the Bardeen-Stephen model with reduced normal-state resistivity, (ii} the pinning constant is close to its upper limit at lowest temperatures and decreases exponentially with increasing temperature, and (iii) the depinning frequency is of the order of 10 GHz, almost orientation independent and weakly depends on temperature. We analyze the values of the depinning frequency for difFerent type-II superconductors and demonstrate that it is directly related to the quasiparticle scattering rate.
We developed a novel surface plasmon resonance (SPR) method, based on Fourier transform infrared (FTIR) spectroscopy, as a label-free technique for studying dynamic processes occurring within living cells in real time. With this method, the long (micrometer) infrared wavelength produced by the FTIR generates an evanescent wave that penetrates deep into the sample. In this way, it enables increased depth of sensing changes, covering significant portions of the cell-height volumes. HeLa cells cultivated on a gold-coated prism were subjected to acute cholesterol enrichment or depletion using cyclodextrins. Cholesterol insertion into the cell plasma membrane resulted in an exponential shift of the SPR signal toward longer wavelengths over time, whereas cholesterol depletion caused a shift in the opposite direction. Upon application of the inactive analog alpha-cyclodextrin (alpha-CD), the effects were minimal. A similar trend in the SPR signal shifts was observed on a model membrane system. Our data suggest that FTIR-SPR can be implemented as a sensitive technique for monitoring in real time dynamic changes taking place in living cells.
We report stable ordered arrays of millimeter-size magnetic particles floating on a liquid surface. Self-assembly into a regular two-dimensional lattice results from lateral magnetic interactions between the particles. The lattice constant may be easily tuned by the application of external magnetic field. The array symmetry is designed by using different particle shapes, magnets, and magnet position inside the particle, so that complex symmetries may be achieved. Three-dimensional ordered arrays are obtained in a stack of troughs containing floating magnets. Computer simulations of electromagnetic wave propagation in such three-dimensional structures suggest an opening of a tunable photonic band gap in the microwave range.
Neutron and x-ray reflectivity studies of modulated heterostructures consisting of alternate layers of conjugated and nonconjugated polymers is described. Such heterostructures are currently being used to fabricate polymer-based light emitting diodes. The heterostructures were prepared by the layer-by-layer self-assembly technique using the precursor of the conjugated polymer, deuterated poly(phenylenevinylene) (D-PPV), and other polyelectrolyte spacers. Heat treatment after the layer assembly converted the pre-D-PPV to a conjugated semiconducting polymer. For the first time in such heterostructures, we were able to observe quasi-Bragg reflections (up to the third order) due to the formation of ordered modulated structures. Both the neutron and the x-ray data were analyzed using the same layer-by-layer model and the same fitting procedure with consistent results. Most importantly, the model and the fitting procedure yield the buried interlayer roughness at the D-PPV/spacer interface. This roughness parameter, of the order of 12±3 Å, was found to be smaller than the thickness of the D-PPV and the spacer layers, suggesting that the length over which interdigitation between neighboring polymer layers occurs is significantly smaller than the spacer layer. We demonstrate that the conversion to conjugated polymer by the heat treatment leads to ∼7% reduction of the repeat unit and the film thickness without significant changes of other structural properties; in fact, the interfacial roughness was somewhat improved. The fabrication of high quality modulated structures with controlled layer thickness and relatively small interfacial roughness may be a first step towards polymer-based multiquantum wells analogous to such devices in inorganic heterostructures.
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