Some considerations of fluid interfaces in two dimensions

Basic assumptions of the capillary wave theory of fluid interfaces are examined critically as a function of space dimensionalityd. When the predictions of capillary wave theory are compared with those of the nonclassical Maxwell-van der Waals theory, agreement is found ind=3 and 4, but strong disagreement occurs ind=2. It is shown that the total effective mass density obtained from the Hamiltonian describing the collective capillary wave excitations has a logarithmic divergence ind=2. This result suggests the possibility of anomalous behavior for fluid interfaces ind=2.     

Generation, propagation, and annihilation of metastable patterns

We study the full-time dynamics of the initial value problem, for u^ε=u^ε(x,t),

     

Preparation and characterization of Co80Nb20-xBx amorphous metallic ribbons

Co₈₀Nb₁₄B₆, Co₈₀Nb₁₂B₈ and Co₈₀Nb₁₀B₁₀ amorphous alloys were obtained through the melt-spinning process. The ribbons structure was investigated by X-ray diffractometry and the crystallization process and the thermal stability were studied by means of differential thermal analysis and thermomechanical technique. It was observed that the crystallization temperature depends on the alloy composition and occurs in a temperature range between 420 and 730°C. The coercive field of all alloys was determined by magnetic susceptibility measurements, the values of which range from 2.78 to 5.95 A m^-1. This revised version was published online in August 2006 with corrections to the Cover Date.     

Molecular organization and syn ? anti conformational equilibria in ethane-bridged bis(zinc porphyrin) floating films at the air-water interface

The behavior of the symmetrical ethane-bridged bis(Zn porphyrin) (1) has been investigated at the air-water interface. The molecular organization of floating films of pure 1 and its mixture with amphiphilic substances, such as arachidic acid and n-octadecylamine, was inspected by means of surface pressure-area isotherms, Brewster angle microscopy and reflection spectroscopy in the UV-Vis region. The overall results suggest the presence in all cases of mainly the anti conformer of 1 even when the dimer was spread on pure water, through the ligation of water molecules to the zinc atoms in the axial positions. It was also demonstrated for the first time that the syn-to-anti conformational transition of the porphyrin dimer can be accelerated by the ligation of suitable amphiphiles even at the liquid-air interface. In particular, it is noted that n-octadecylamine, and arachidic acid (to a lesser extent), added to the system as amphiphiles, drive the syn ? anti equilibrium of 1 towards the anti form through the axial ligation of the amino or carboxylate group to the zinc atoms.     

An Integrated CMOS Front-End for Optical Absolute Rotary Encoders

This paper presents a front-end circuit for optical rotary encoders. The light pulses modulated by the encoder disc are transduced into current signals, which are pre-processed and converted into digital waveforms related to the disc angular position information. The proposed front-end circuit is compensated against temperature drifts. Digitally programmable calibration is provided to account for spreads in impinging light pulse power. Measurement results on integrated prototypes are shown, demonstrating correct operation of the front-end with an optical input power from 0.5 W to 3 W up to a signal frequency of 500 kHz in a temperature range from 0 °C to 80 °C.     

Phase transitions in gallium nanodroplets detected by dielectric spectroscopy

Both theoretical and experimental works give evidence that gallium exhibits solid phases labelled , , , besides the stable phase strongly dependent both on the size and the confinement conditions. An experimental technique was used based on capacitance and conductance measurements vs. temperature in the audiofrequency range. This technique is particularly sensitive to the conditions of the investigated particle surface that plays a fundamental role in the melting and more generally in the phase transition processes. In particular the strict relation between the derivative of the capacitance with respect to the temperature, dC/dT, and the entropy of the system is considered. In gallium nanoparticles 20 nm in radius, only the phase is shown to occur. Further the transition to liquid phase was detected. The melting process was found to start about 65 K below the full melting temperature value. In the case of particles 10 nm in radius, where different metastable phases may occur, the capacitance vs. temperature curve was found to display abrupt changes of the slope. The singularities are associated to a well defined transition temperature.     

Optical Properties of Amorphous Co‐Containing Alloys in the IR Region of the Spectrum and Their Magnetoresistive Characteristics

The optical properties of amorphous and nanocrystalline tapes of CoFeNiSiB alloys and the ac magnetoresistive effect in them have been investigated. It is shown that the magnetoresistive effect in these alloys increases by about a factor of two after lowtemperature treatment and subsequent laser annealing of these tapes in an external magnetic field. Correlation between the anomalous behavior of the dielectric function of these alloys in the IR region of the spectrum and their magnetoresistive characteristics after thermal annealings has been established. The interrelation between the density of localized electronic states near the Fermi level and the value of the magnetoresistive effect has been revealed. The influence of the induced magnetic anisotropy on the transverse permeability in amorphous and nanocrystalline CoFeNiSiB alloys and, consequently, also on the magnetoresistive effect has been analyzed.     

Coherence transition in granular YBa2Cu3O7-δ, YBa2Cu2.95Zn0.05O7-δ, and YBa1.75Sr0.25Cu3O7-δ superconductors

We have studied experimentally the electrical magneto-conductivity near the superconducting transition of YBa₂Cu₃O_7-δ, YBa₂ (Cu_2.95Zn_0.05)O_7-δ and Y(Ba_1.75Sr_0.25)Cu₃O_7-δ polycrystalline samples. The measurements were performed in magnetic fields ranging from 0 to 400 Oe applied parallel to the current orientation. The results show that the resistive transition of these systems proceeds in two stages. The pairing transition occurs at the bulk critical temperature T_c, where superconductivity is stabilized within small and homogeneous regions of the sample generically called grains. The regime of approach to the zero resistance state reveals the occurrence of a coherence transition at a lower temperature T_c0. This transition is related to the connective nature of the granular samples and is controlled by fluctuations of the order-parameter phase of individual grains. Our experiments show that the Zn-doping, besides depressing the pairing critical temperature, strongly enlarges the temperature range dominated by effects related to the coherence transition. The substitution of Ba by Sr causes only a small reduction of T_c, but also enhances significantly the effects related to the grain coupling phenomenology. In general, our results indicate that these impurity substitutions in YBa₂Cu₃O_7-δ produce or magnify the granularity at a microscopic level, enhancing the effects of phase fluctuations in the conductivity near the transition.