G. Giaquinta

Re-entrant spin susceptibility of a superconducting grain

We study the spin susceptibility chi of a small, isolated superconducting grain. Because of the interplay between parity effects and pairing correlations, the dependence of chi on temperature T is qualitatively different from the standard BCS result valid in the bulk limit. If the number of electrons on the grain is odd, chi shows a re-entrant behavior as a function of temperature. This behavior persists even in the case of ultrasmall grains where the mean level spacing is much larger than the BCS gap. If the number of electrons is even, chi(T) is exponentially small at low temperatures.

Quasi-classical descendants of disordered vertex models with boundaries

We study descendants of inhomogeneous vertex models with boundary reflections when the spin–spin scattering is assumed to be quasi-classical. This corresponds to consider certain power expansion of the boundary-Yang–Baxter equation (or reflection equation). As final product, integrable su(2)-spin chains interacting with a long range with XXZ anisotropy are obtained. The spin–spin coupling constants are non-uniform, and a non-uniform tunable external magnetic field is applied; the latter can be obtained when the boundary conditions are assumed to be quasi-classical as well. The exact spectrum is achieved by algebraic Bethe ansatz. Having realized the su(2) operators in terms of fermions, the class of models we found turns out to describe confined fermions with pairing force interactions. The class of models presented in this paper is a one-parameter extension of certain Hamiltonians constructed previously. Extensions to su(n)-spin open chains are discussed.

Superconducting tunnel junctions with a non conventional insulator.

Abstract A method in performing tunnel junctions is discussed, employing a thin film of plastic material as insulator. Lead energy gap measurements exhibit good agreement with B.C.S. theory prevision.

Infrared study and theoretical model for the K-T-B phase transition of phosphatidylcholine from natural lipids

Abstract We present a study of the thermal behaviour of the intensity and bandwidth of two characteristic methylenic infrared absorptions of the hydrophobic chains of the phosphatidylcholine fraction from natural lipids. A theoretical model, based on the anisotropic XY approach for two-dimensional systems in which the phase diagram predicts also a spin-like system region of stability, has been proposed. In a qualitative view, the model explains the Kosterlitz-Thouless-Berezinskii shape of the first phase transition (Tc = 47°C) of this important lipidic component of biological membranes, around the critical temperature.

Restoration of the symmetries broken by reversible growth in hyperelastic bodies

In this paper, we interpret the development of material in homogeneities in continuum, hyper elastic bodies in the presence of reversible growth in terms of broken symmetries [1]. By applying Noethers Theorem [1, 2, 3, 4], we find a set of equations yielding the fields necessary to compensate for the broken symmetry. As growth occurs, these fields provide for an instantaneously updated reference configuration of the body, and are responsible for the dynamical restoring of the body symmetries. In addition, we propose to use these compensating fields in order to generalize the definition of the transplant operator given in [5,6]. This work has been motivated by the current theoretical investigations on the biomechanical aspects of growth in particular cartilage.

Low-frequency phonon spectroscopy by tunnel effect

SummaryLow-energy structures, observed in tunnelling characteristics of Al/formvar/Sn and Al/formvar/Al junctions are reported and discussed. Their connection to tbe vibrational excitation spectrum of the insulator is indicated. Arguments are given to interpret them as excited by an inelastic tunnel mechanism of quasi-holes. A spectroscopic classification of these structures is attempted according to some results previously published. The relevance of these results, obtained for an energy range in which no data from current molecular spectroscopy are available, is stressed.RiassuntoSi riportano e si discutono strutture a lassa energia osservate nelle caratteristiehe « tunnel » delle giunzioni Al/formvar/Sn e Al/formvar/Al. Si indica la loro connessione con lo spettro di eccitazione vibrazionale dell’isolante. Si forniscono delle argomentazioni per interpretarle come eccitate da un meccanismo di « tunnel » anelastico di quasi buche. Si tenta sulla base di alcuni risultati precedentemente pubblicati una classificazione spet. troscopica di queste strutture. Si sottolinea infine la rilevanza di questi risultati ottenuti per un’intervallo di energia in cui non sono disponibili dati dalla corrente spettroscopia molecolare.РезюмеОбсуждаются структу ры при низких энергия х, обнаруженные в туннельных характер истиках Аl/Fогmvaг/Sn и Аl/Fогmvа г/Аl соединений. Отмечает ся связь обнаруженны х структур со спектро м вибрацоинных возбу ждений со спектром вибрацои нных возбуждений изоляторов. Приводят ся аргументы, указыва ющие, что эти возбуждения связаны с неупругим туннельн ым механизмом квази-д ырок. Предлагается сп ектроскопичес механизмом квази-дыр ок. Предлагается спек троскопическая спектроскопическая классификация этих с труктур на основе ран ее опубликованных резу льтатов. Подчеркивае тся важность получен ных результатов для э нергерической полученных результа тов для энергерическ ой области, где отсутс твуют данные из молек улярной спектроскоп ии. где отсутствуют данн ые из молекулярной сп ектроскопии. спектроскопии.

Logarithmic-derivative curves of A1-Formvar-Pb junctions and trap-assisted current transport

Abstract Two types of structures in (d/d V >)[ln ( J / V )] are observed for A1-Formvar-Pb sandwiches. One occurs at high voltages and is expected for tunneling through trapezoidal barriers. The other occurs around zero bias and is interpreted as being due to localized traps in the insulator.

Fluctuation effects in granular superconductors of intermediate paracoherent transition temperature

Abstract Starting from a two order parameter free energy functional, we describe the properties of a granular superconductor accounting for both amplitude and phase fluctuations. Besides giving an effective function for describing the coherent transition, we discuss, in particular, the shift in the transition temperature due to fluctuations in the amplitude.

Diffusion dynamics of laser-ablated noble-metal nanoparticles in liquids

The diffusion dynamics in water of Brownian particles constituted by noble-metal nanoclusters has been treated by a transport model across a limiting surface that separates two layers of the same fluid, having different values of density and temperature. This model, developed by Parrondo and adapted by Streater, has recently been shown to be useful in several situations like ours by the work of Grillo et al. By this approach, which introduces a one-dimensional vertical adiabatic potential produced by density gradients governing the net current of particles across the limiting surface, we are able to explain the mass and thermal energy transport, which rapidly restores the equilibrium conditions when they are perturbed by laser ablation of a noble-metal target vertically submerged in water. Experimentally, the laser ablation of noble-metal targets has been performed by irradiating pure metal sheets submerged in water with a pulsed Nd:YAG laser at 500 mJ cm−2 fluency for 5 min and pulses of duration on the nanosecond timescale.

Zero temperature phase diagram of a small metallic junction

We discuss the phase diagram for a metal tunnel junction with quasiparticle dissipation. We present some evidences of aT=0 phase transition induced by dissipation, by means of Monte Carlo simulation and studying the problem by means of a selfconsistent harmonic approximation. When the nominal conductance of the junctioin is large the predictions of the spin wave theory turn out to be correct and the algebraic decay of correlations implies quasi-long range order (phase with infinite susceptibility), this situation corresponds to the absence of a Coulomb blockade voltage threshold. The critical value of the nominal junction resistance is estimated to beRt≈0.6 kΩ.