Superconducting diamagnetic fluctuations in ropes of carbon nanotubes

We report low-temperature magnetisation measurements on a large number of purified ropes of single wall carbon nanotubes. In spite of a large superparamagnetic contribution due to the small ferromagnetic catalytical particles still present in the sample, at low temperature ($T < 0.5K$) and low magnetic field ($H < 80 Oe$), a diamagnetic signal is detectable. This low temperature diamagnetism can be interpreted as the Meissner effect in ropes of carbon nanotubes which have previously been shown to exhibit superconductivity from transport measurements.Comment: 10 pages 3 figure

Accelerated convergence method for fast Fourier transform simulation of coupled cavities

Fast Fourier transform (FFT) simulation was used to calculate the power and spatial distribution of resonant fields in optical cavities. This is an important tool when characterizing the effect of imperfect geometry and mirror aberrations. This method is, however, intrinsically slow when the cavities are of relatively high finesse. When this is the case, an accelerated convergence scheme may be used to calculate the steady-state cavity field with a speed that is orders of magnitude faster. The rate of convergence of this method, however, is unpredictable, as many different factors may detrimentally affect its performance. In addition, its use in multiple cavity configurations is not well understood. An in-depth study of the limitations and optimization of this method is presented, together with a formulation of its use in multiple cavity configurations. This work has not only resulted in consistent improvement in performance and stability of the accelerated convergence method but also allows the simulation of optical configurations, which would not previously have been possible

Study of cationic N-isopropylacrylamide-styrene copolymer latex particles using fluorescent probes

Monodisperse cationically charged core-shell poly[styrene/N-isopropylacrylamide] latexes, differing in their shell structure, were studied at temperatures around the lower critical solution temperature (LCST) of poly[N-isopropylacrylamide]. Near the LCST, a transition on the latex dimensions was observed by quasi-elastic light scattering measurements. The same transition could also be detected using the intensity ratio of the pyrene fluorescence vibronic bands, I1/I3, and the excimer to monomer fluorescence intensity ratio of 1,10-bis(1-pyrenyl)decane. The fluorescence spectra and decay curve measurements of 1,10-bis-(1-pyrenyl)decane provided a better understanding of both the hydrophilic-hydrophobic variation and the conformational changes occurring in the poly[N-isopropylacrylamide] shell of the latex particles upon temperature variation.J. M. G. Martinho is grateful for the support of this work from FCT. E. M. S. Castanheira acknowledges FCT for a grant (PRAXIS XXI/BPD/9968/96)

Ultra-Wideband Microwave Imaging of Heterogeneities

International audienceThe technique of time-reversal acoustics was applied to image a bottle filled with saline, using an eight element Vivaldi antenna array with frequency bandwidth 2 to 8 GHz. At these short length scales, a smooth three-dimensional image of the bottle was obtained, with the usual limitations imposed by limited offset and frequency. Time snapshots of the wavefield evolution in reversed time are presented for two real data sets. The first, shows the focusing for the single target of the bottle, while the second demonstrates the principle for two targets

Pressure dependence of Raman modes in double wall carbon nanotubes filled with α-Fe.

The preparation of highly anisotropic one-dimensional (1D) structures confined into carbon nanotubes (CNTs) in general is a key objective in CNTs research. In this work, the capillary effect was used to fill double wall carbon nanotubes with iron. The samples are characterized by Mössbauer and Raman spectroscopy, transmission electron microscopy, scanning area electron diffraction, and magnetization. In order to investigate their structural stability and compare it with that of single wall carbon nanotubes (SWNTs), elucidating the differences induced by the inner-outer tube interaction, unpolarized Raman spectra of tangential modes of double wall carbon nanotubes (DWNTs) filled with 1D nanocrystallin α-Fe excited with 514 nm were studied at room temperature and elevated pressure. Up to 16 GPa we find a pressure coefficient for the internal tube of 4.3 cm−1 GPa−1 and for the external tube of 5.5 cm−1 GPa−1. In addition, the tangential band of the external and internal tubes broadens and decreases in amplitude. All findings lead to the conclusion that the outer tube acts as a protection shield for the inner tubes (at least up 16 GPa). Structural phase transitions were not observed in this range of pressure