Metastability of a circular o-ring due to intrinsic curvature

An o-ring takes spontaneously the shape of a chair when strong enough torsion is applied in its tangent plane. This state is metastable, since work has to be done on the o-ring to return to the circular shape. We show that this metastable state exists in a Hamiltonian where curvature and torsion are coupled via an intrinsic curvature term. If the o-ring is constrained to be planar (2d case), this metastable state displays a kink-anti-kink pair. This state is metastable if the ratio is less than , where C and A are the torsion and the bending elastic constants [#!landau!#]. In three dimensions, our variational approach shows that . This model can be generalized to the case where the bend is induced by a concentration field which follows the variations of the curvature. Received: 27 August 1997 / Revised: 23 October 1997 / Accepted: 12 November 1997     

Effective tortuosity and different acoustic waves in porous media

Summary A simple method has been suggested to estimate the acoustic characteristics of porous structure from a hybrid model—a hybridisation of Biot's phenomenological model and the microscopic multiple-scattering theory which introduces the idea of an effective tortuosity. Without using any adjustable parameter this model may be used to provide rough estimates of the tortuosity, the fast, the shear and the slow sound speeds. The predictions are compared with observation on water-saturated glass bead samples. The author of this paper has agreed to not receive the proofs for correction.     

Zeroth and second laws of thermodynamics simultaneously questioned in the quantum microworld

A new and rather trivial model is suggested with mechanism that implies simultaneous violation of the zeroth and the second laws of thermodynamics. Mathematically rigorous quantum theory reduces to a trivial application of the Golden rule formula. It yields exciton on-energy-shell diffusion caused by bath-nonassisted excitation hopping between tails of different exciton site levels ε₁ < ε₂ broadened by bath-assisted finite life-time effects. The elastic character of the hopping implies 1 ↔ 2-symmetric transfer rate W. Thus the net diffusion exciton flow W(P ₁ - P ₂) and also, as argued, the net energy flow are possible due to different near-to-equilibrium exciton populations P ₁ > P ₂. As the sites are provided with two different baths, the population imbalance and the flows survive even for slightly different local bath temperatures T ₁ < T ₂ < T ₁ε₂/ε₁. Thus spontaneous exciton and also energy flows against temperature step become possible, in contradiction with the Clausius form of the second law. Violations of both the laws disappear in the high-temperature, i.e. classical limit Received 16 May 2001 and Received in final form 20 September 2001     

Effect of magnetic-field-induced torsion on neutrinos and supernova collapse

Summary The neutrino emission from supernova 1987A is being studied by taking into consideration the effect of the magnetic field present in the star. The magnetic field generates an anisotropic feature in the local space-time, which in turn leads to the cancellation of the axial-vector part of the neutrino current. The interaction cross-section of these neutrinos in the course of their interaction with matter on Earth has an inverse square energy dependence. The present work presents the calculations based on this changed cross-section that relates to predictions of the expected number of events at KAMIOKANDE on the basis of the Mont Blanc signal. It might be interesting to take a look into the implications of these results. The author of this paper has agreed to not receive the proofs for correction.     

On some natural and model 2D bimodal random cellular structures

The topological and metric properties of a few natural 2D random cellular structures, namely an armadillo shell structure and young soap froths, which are formed from two classes of cells, large and small, have been characterized. The topological properties of a model generated from a Kagome tiling, which mimics such random binary structures, have also been exactly calculated. The distribution of the number of cell sides is bimodal for the structures investigated. In contrast to the classical Aboav-Weaire law for homogeneous 2D random cellular structures, nm(n), the mean total number of edges of neighbouring cells of cells with n sides does not vary linearly with n. Only the nm(i, n) (i=1,2) determined separately for every class of cells are linear in n for all investigated structures. Topological properties and correlations between metric and topological properties are finally compared with the predictions of various literature models. Received: 24 December 1997 / Revised: 7 April 1998 / Accepted: 20 April 1998     

Orbital contribution to the magnetic properties of nanowires: is the orbital polarization ansatz justified?

We show that considerable orbital magnetic moments and magneto-crystalline anisotropy energies are obtained for a Fe monatomic wire described in a tight-binding method with intra-atomic electronic interactions treated in a full Hartree Fock (HF) decoupling scheme. Even though the use of the orbital polarization ansatz with simplified Hamiltonians leads to fairly good results when the spin magnetization is saturated this is not the case of unsaturated systems. We conclude that the full HF scheme is necessary to investigate low dimensional systems.     

Shock wave structure in a two-components gas mixture

Summary The nonlinear differential equations describing the problem of shock wave structure in a two-component gas mixture are reduced to a system of two coupled nonlinear differential equations. This system is solved numerically using the finite-difference method. It is found that the relative shock wave thickness of the mixture increases with the increase of the ratio between the viscosity coefficients when the ratio between the mass fractions is constant. It is also shown that the relative shock wave thickness of the mixture decreases with the increase of the ratio between the mass fractions when the ratio between the viscosity coefficients is constant.

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Riassunto Si riducono le equazioni differenziali non lineari che descrivono il problema della struttura dell’ onda d’urto in una miscela di gas a 2 componenti a un sistema di due equazioni differenziali accoppiate non lineari. Questo sistema si risolve numericamente usando il metodo delle differenze finite. Si trova che il relativo spessore dell’onda d’urto della miscela aumenta con l’aumento del rapporto tra i coefficienti di viscosità, quando il rapporto tra le frazioni di massa è costante. Si mostra anche che lo spessore relativo dell’onda d’urto della miscela diminuisce con l’aumento del rapporto tra le frazioni della massa, quando il rapporto tra i coefficienti di viscosità è costante.

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Резюме Нелинейные дифференциальные уравнения, описывающие проблему структуры ударной в двух-компоентной газовой смеси, сводятся к системе двух связанных нелинейыых дифференциальных уравнений. Получается, что относительная толщина ударной волны в смеси увеличвается с увеличением отношения между коэффиыентами вязкоти, когда отнощение между массовыми доями остается постоянным. Также показывается. что относительная толщина ударной волны уменьшается с увеличением отношения между массовыми долями. когда отношение между коэффициентами вязкости остается постоянным.

     

Stochastic self-oscillations in an extrinsic semiconductor under intense illumination

Summary We show that an extrinsic semiconductor (n-type), containing a deep level from impurity centres, may produce stochastic self-oscillations in the electron density and in the electric field, under intense illumination which ionizes the deep traps and heats the electrons. The feedback mechanism is the dependence of the capture coefficient of the deep centres on the electron temperature and the dependence of the deep energy level on electron temperature and concentration. The conditions for the occurrence of periodic and stochastic self-oscillations are explicitly given.

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Riassunto Si mostra che in un semiconduttore estrinseco (di tipon), contenente impurezze con un livello profondo, si possono produrre auto-oscillazioni stocastiche nella densità di elettroni in modo da ionizzare le impurezze e riscaldare gli elettroni. Il meccanismo responsabile è la dipendenza della sezione di cattura degli elettroni nel livello profondo della temperatura degli elettroni e la dipendenza dell’energia del livello dalla temperatura degli elettroni e dalla loro densitá. Si danno le esplicite condizioni per l’insorgere di auto-oscillazioni periodiche e stocastiche.

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Резюме Мы показываем, что собственный полупроводник (n-типа), содержащий глубокий уровень, связанный с примесными центрами, может производить стохастические автоколебания в электронной плотности и в электрическом поле при интенсивном облучении, которое ионизует глубокие ловушки и нагревает электроны. Этот механизм обратной связи представляет зависимость коэффициента захвата для глубоких центров от электронной температуры и зависимость глубокого энергетического уровня от температуры и концентрации электронов. В явном виде приводятся условия для возникновения периодических и стохастических автоколебаний.

     

On some measurements of the propagation properties of thermal waves in the Nb-H and Ta-H systems

Summary By means of an experimental technique based on a nonsteady-state method,i.e. on the propagation of thermal waves, we have measured at room temperature for oscillation frequencies between 6 and 30 mHz the thermal diffusivity, the lateral thermal-loss coefficient and the ?fractional heat loss? in Nb and Ta wires as a function of hydrogen doping. The appearance of hydride formation and precipitation notably changes the behaviour of these quantities. From these measurements it was also possible to calculate the propagation velocity and attenuation coefficient of thermal waves. It was found that the increase of the hydrogen concentration in the solid solution produces a gradual decrease in the velocity and increase in the attenuation coefficient until the solubility limit is reached. For larger hydrogen concentrations, the velocity showed a tendency to increase towards the value of the pure metal, while the attenuation coefficient decreased below the value of the pure metal. These results as a whole appear quite promising for studying the properties of hydrogenated systems. This work was supported by the Gruppo Nazionale di Struttura della Materia of the Consiglio Nazionale delle Ricerche and by the Centro Interuniversitario di Struttura della Materia of the M.P.I., Roma (Italia).     

Applications of laser for ceramic alloying on metallic surfaces

Summary Ceramic and metallic materials were alloyed and cladded on a C40 steel by means of a CO₂ laser heat source. The main objectives of the research were to obtain a surface modification of the C40 steel in order to increase its wear resistance and to develop a repeatable coating process of potential industrial application. In the following sections, the laser coating process (including experimental equipment and working parameters) and the most important metallurgical results are described.