Showing papers by "University of Trento published in 1999"
TL;DR: In this article, the authors reviewed the Bose-Einstein condensation of dilute gases in traps from a theoretical perspective and provided a framework to understand the main features of the condensation and role of interactions between particles.
Abstract: The phenomenon of Bose-Einstein condensation of dilute gases in traps is reviewed from a theoretical perspective. Mean-field theory provides a framework to understand the main features of the condensation and the role of interactions between particles. Various properties of these systems are discussed, including the density profiles and the energy of the ground-state configurations, the collective oscillations and the dynamics of the expansion, the condensate fraction and the thermodynamic functions. The thermodynamic limit exhibits a scaling behavior in the relevant length and energy scales. Despite the dilute nature of the gases, interactions profoundly modify the static as well as the dynamic properties of the system; the predictions of mean-field theory are in excellent agreement with available experimental results. Effects of superfluidity including the existence of quantized vortices and the reduction of the moment of inertia are discussed, as well as the consequences of coherence such as the Josephson effect and interference phenomena. The review also assesses the accuracy and limitations of the mean-field approach.
4,782 citations
TL;DR: A set of general guidelines for structure refinement using the Rietveld (whole profile) method has been formulated by the International Union of Crystallography Commission on Powder Diffraction.
Abstract: A set of general guidelines for structure refinement using the Rietveld (whole-profile) method has been formulated by the International Union of Crystallography Commission on Powder Diffraction. The practical rather than the theoretical aspects of each step in a typical Rietveld refinement are discussed with a view to guiding newcomers in the field. The focus is on X-ray powder diffraction data collected on a laboratory instrument, but features specific to data from neutron (both constant-wavelength and time-of-flight) and synchrotron radiation sources are also addressed. The topics covered include (i) data collection, (ii) background contribution, (iii) peak-shape function, (iv) refinement of profile parameters, (v) Fourier analysis with powder diffraction data, (vi) refinement of structural parameters, (vii) use of geometric restraints, (viii) calculation of e.s.d.'s, (ix) interpretation of R values and (x) some common problems and possible solutions.
1,808 citations
TL;DR: In this paper, it was shown that normal boiling is subject to a major kinetic obstacle in the process of bubble diffusion, such motion being sufficiently slow that it will simply not occur for t < 100 ns.
Abstract: Three kinds of thermal processes may lead to material loss from a laser-irradiated surface: 1) vaporization, 2) normal boiling, and 3) explosive boiling. The latter is equivalent to phase explosion. It is appropriate, at this point, to exclude “subsurface heating”, as there are strong doubts about its existence. The relevance of the three processes depends on the laser pulse duration as well as on the temperature attained in the irradiated zone. We revisit the three thermal processes by noting that: 1) vaporization is not important for the shortest time-scales (<1 ns). 2) Normal boiling is subject to a major kinetic obstacle in the process of bubble diffusion, such motion being sufficiently slow that it will simply not occur for t<100 ns. This is because the value of the bubble diffusion coefficient leads to distances traveled which are atomically small for both 1 ns and 100 ns, and for both T=Tm and T=2Tm, with Tm being the melting temperature. 3) Phase explosion, notwithstanding the unfavorable time-scale (1–100 ns) advocated by Martynyuk, as carefully analyzed in this paper, is found to be the most efficient mechanism in the ablation process when looking at thermal processes. Here it should be recognized that a new field in the physics of condensed matter may be emerging when looking at physical properties near the thermodynamic critical temperature, Ttc. In fact, laser irradiation experiments probably represent a unique tool to investigate matter under extreme thermodynamic conditions and on very short time-scales (ps or fs).
332 citations
TL;DR: In this paper, a semi-implicit finite difference model for non-hydrostatic, free-surface flows is analyzed and discussed, and it is shown that the present algorithm is generally more accurate than recently developed models for quasi-Hydrostatic flows.
Abstract: In this paper a semi-implicit finite difference model for non-hydrostatic, free-surface flows is analyzed and discussed. It is shown that the present algorithm is generally more accurate than recently developed models for quasi-hydrostatic flows. The governing equations are the free-surface Navier–Stokes equations defined on a general, irregular domain of arbitrary scale. The momentum equations, the incompressibility condition and the equation for the free-surface are integrated by a semi-implicit algorithm in such a fashion that the resulting numerical solution is mass conservative and unconditionally stable with respect to the gravity wave speed, wind stress, vertical viscosity and bottom friction. Copyright © 1999 John Wiley & Sons, Ltd.
305 citations
TL;DR: In this article, the supersymmetry properties of four-dimensional asymptotically anti-de Sitter (AdS) dyonic black holes in the context of gauged N = 2 supergravity were investigated.
249 citations
TL;DR: In this paper, the authors explore, from the perspective of the sociology of knowledge, the implicit assumptions underlying the Organizational Learning literature, and to look for alternative ways of conceptualizing learning-working-innovating as non-distinct activities.
Abstract: The phrase `learning in the face of mystery' is borrowed in homage from Barry Turner, who coined it in contrast to `learning in the face of problems', which assumes learning to be voluntaristic, always functional to the organization, synonymous with change. The shift from the one to the other resides in an aesthetic choice between rationality and relationality. In fact, in spite of the huge amount of literature on organizational learning, there is still very little understanding about organizing as a practice based on a distributed knowledge system. At the same time, the normative/prescriptive discourse on Organizational Learning and the Learning Organization is mobilized as a further means of organizational control. The aim of this paper is to explore, from the perspective of the sociology of knowledge, the implicit assumptions underlying the Organizational Learning literature, and to look for alternative ways of conceptualizing learning-working-innovating as non-distinct activities. The term `learning-i...
248 citations
TL;DR: In this paper, the dry sliding wear behavior of the Ti-6Al-4V alloy sliding against itself and AISI M2 steel was investigated at different sliding velocities (between 0.3 and 0.8 m/s).
230 citations
TL;DR: The goal of this paper is to provide an introduction, with various elements of novelty, to the Planning as Model Checking paradigm.
Abstract: The goal of this paper is to provide an introduction, with various elements of novelty, to the Planning as Model Checking paradigm.
228 citations
TL;DR: A survey of the results obtained in the study of isochronous centers of vector fields in the plane can be found in this paper, where the authors give an overview of the state of the art at the moment this review was written.
Abstract: In this survey we give an overview of the results obtained in the study of isochronous centers of vector fields in the plane. This paper consists of two parts. In the first one (sections 2--8), we review some general techniques that proved to be useful in the study of isochronicity. In the second one (sections 9--16), we try to give a picture of the state of the art at the moment this review was written. In section 2, we give some basic definitions about centers, isochronous centers, first integrals, integrating factors, particular algebraic solutions, and other related concepts. In this sections we also give some general theorems about centers and isochronous centers, and we give a brief account of the evolution of the researches in this field. In the successive sections we focus on various methods that have been used in attacking the isochonicity problem. We start with linearizations in section 3, stating Poincar\'e's classical theorem and some of its consequences. Section 4 is devoted to describe the procedure that leads to define and compute isochronous constants. In section 5, commutators are introduced, and basic facts about couples of commuting systems are described. Classical theorems about systems obtained from complex ordinary differential equations are collected in section 6. Hamiltonian systems are considered in section 7, where their connection to the study of the Jacobian Conjecture is showed, too. Section 8 is concerned with systems having constant angular speed with respect to some coordinate system. The second part starts with section 9, that is devoted to recent results about second order differential equations not immediately reducible to hamiltonian systems. This section also contains the characterization of isochronous centers of reversible Li\'enard systems. In section 10 we list all fundamental results about isochronous centers of quadratic systems. Next section contains results about cubic systems with homogeneous nonlinearities. Sections 12 is devoted to cubic reversible systems. In section 13 we collect results about quartic and quintic systems with homogeneous nonlinearities. A class of particular cubic systems, with degenerate infinity is considered in section 14. Finally, section 15 is devoted to Kukles system. All the sections of the second part, and some of the first part, contain tables, where the main features of the considered systems are collected. When possible, for every class of systems we have written the system in rectangular and polar coordinates, and we have reported a first integral, a commutator, a linearization and a reciprocal integrating factor. The bibliography contains references both to papers devoted to the study of isochronicity and to papers concerned with integrability of plane systems and the study of the period function of centers. We have tried to make the bibliography so complete as possible for what is concerned with isochronicity. We have made no effort to make it complete for papers about integrability and the study of the period function. We apologize for possible mistakes and encourage the readers to communicate us any corrections.
222 citations
06 Jul 1999
TL;DR: The experimental results show that the state-of-the-art algorithm for obtaining an automaton from a linear temporal logic formula outperforms the previous one, with respect to both the size of the generated automata and computation time.
Abstract: We improve the state-of-the-art algorithm for obtaining an automaton from a linear temporal logic formula. The automaton is intended to be used for model checking, as well as for satisfiability checking. Therefore, the algorithm is mainly concerned with keeping the automaton as small as possible. The experimental results show that our algorithm outperforms the previous one, with respect to both the size of the generated automata and computation time. The testing is performed following a newly developed methodology based on the use of randomly generated formulas.
219 citations
TL;DR: In this article, the ideal trend of coating capacitance is compared with some real experimental results, discussing the reasons of disagreement and the limits of the model which seem to be in many cases insufficient and too simple to explain the actual water uptake processes.
TL;DR: A processing approach has been identified and reduced to practice in which a residual stress profile can be designed such that cracks in a brittle material are arrested or grow in a stable manner.
Abstract: A processing approach has been identified and reduced to practice in which a residual stress profile can be designed such that cracks in a brittle material are arrested or grow in a stable manner. In the approach, cracks in the body encounter an increase in the magnitude of residual compression as the crack propagates. If correctly designed, the process increases strength and significantly decreases strength variability. This approach was demonstrated for a silicate glass, and multiple cracking was observed as a forewarning of the final failure. Normally, such glasses would fail catastrophically with the propagation of a dominant crack.
TL;DR: The results indicate the predominance of CD4+ T cells producing both the pro inflammatory cytokine IFN-γ and the anti-inflammatory cytokine IL-10 in BAL of patients with active TB.
TL;DR: In this article, a simple model based on bounding surface and kinematic hardening plasticity was proposed to represent the general multiaxial stress-strain behavior of granular materials over the full range of void ratios and stress level.
Abstract: This paper explores the possibility of using well-accepted concepts—Mohr-Coulomb-like strength criterion, critical state, existence of a small strain elastic region, hyperbolic relationship for representing global plastic stress–strain behaviour, dependence of strength on state parameter and flow rules derived from the Cam-Clay Model—to represent the general multiaxial stress–strain behaviour of granular materials over the full range of void ratios and stress level (neglecting grain crushing). The result is a simple model based on bounding surface and kinematic hardening plasticity, which is based on a single set of constitutive parameters, namely two for the elastic behaviour plus eight for the plastic behaviour, which all have a clear and easily understandable physical meaning.
In order to assist the convenience of the numerical implementation, the model is defined in a ‘normalized’ stress space in which the stress–strain behaviour does not undergo any strain softening and so certain potential numerical difficulties are avoided.
In the first part the multiaxial formulation of the model is described in detail, using appropriate mixed invariants, which rationally combine stress history and stress. The model simulations are compared with some experimental results for tests on granular soils along stress paths lying outside the triaxial plane over a wide range of densities and mean stresses, using constitutive parameters calibrated using triaxial tests. Furthermore, the study is extended to the analysis of the effects induced by the different shapes of the yield and bounding surfaces, revealing the different role played by the size and the curvature of the bounding surface on the simulated behaviour of completely stress- and partly strain-driven tests. Copyright © 1999 John Wiley & Sons, Ltd.
TL;DR: An iteration-by-subdomain procedure is proven to converge, showing that the preconditioner implicitly defined by the iterative procedure is optimal, and proving a regularity theorem for Dirichlet and Neumann harmonic fields.
Abstract: The time-harmonic Maxwell equations are considered in the low-frequency case. A finite element domain decomposition approach is proposed for the numerical approximation of the exact solution. This leads to an iteration-by-subdomain procedure, which is proven to converge. The rate of convergence turns out to be independent of the mesh size, showing that the preconditioner implicitly defined by the iterative procedure is optimal. For obtaining this convergence result it has been necessary to prove a regularity theorem for Dirichlet and Neumann harmonic fields.
TL;DR: The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements as discussed by the authors.
Abstract: The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements. Temperature induced changes include structural relaxation and densification from 1000 to 1200°C, and crystallization to form a fine and homogeneous β-SiC/glass-matrix nanocomposite with 2.5 nm large crystals above 1200°C. Young's modulus measurements clearly reveal a consolidation of the material upon annealing below 1200°C. Crystallization is almost complete after few hours at 1300°C and results in a significant increase in Young's modulus. The viscosity of the oxycarbide glass is much higher than that of fused silica, with two orders of magnitude difference at 1200°C, and the glass transition temperature ranges from 1320 to 1370°C.
01 Jan 1999
TL;DR: In these notes, systems of hyperbolic equations and their reformulation in the framework of multi-domain partition of the computational domain are considered.
Abstract: In these notes we consider systems of hyperbolic equations and their reformulation in the framework of multi-domain partition of the computational domain.
TL;DR: In this article, the sensitivity of extended x-ray-absorption fine structure (EXAFS) to thermal expansion has been studied by temperature-dependent measurements on germanium.
Abstract: The sensitivity of extended x-ray-absorption fine structure (EXAFS) to thermal expansion has been studied by temperature-dependent measurements on germanium. The first cumulant does not reproduce the thermal expansion owing to vibrations normal to the bond. The perpendicular relative displacement $〈\ensuremath{\Delta}{u}_{\ensuremath{\perp}}^{2}〉$ has been for the first time experimentally obtained; the ratio $〈\ensuremath{\Delta}{u}_{\ensuremath{\perp}}^{2}〉/〈\ensuremath{\Delta}{u}_{\ensuremath{\Vert}}^{2}〉$ is in agreement with vibrational model calculations. Low-temperature quantum effects on the 3rd cumulant have been for the first time observed. The possibility of measuring thermal expansion from the 3rd cumulant is demonstrated, provided that quantum effects are taken into account.
TL;DR: In this paper, a set of yttria partially stabilized zirconia coatings with different thickness was deposited on flat nickel-base alloy coupons by air plasma spray (APS) under uncontrolled temperature conditions.
Abstract: A set of yttria partially stabilized zirconia coatings with different thickness was deposited on flat nickel-base alloy coupons by air plasma spray (APS) under uncontrolled temperature conditions. In this way, the length of the spraying process (and consequently the coating thickness) had a direct effect on phase composition as well as on the thermal properties of the material. In particular, both the monoclinic phase percentage and thermal diffusivity increased considerably with the thickness. Because this trend was observed together with a slight but clearly visible increase in the total porosity, the interpretation of the results was not straightforward, but required a detailed discussion of the thermal transport mechanism. Considering the complex microstructure typical of APS coatings and the relevant role of porosity, it was shown how a modest reduction in the fraction of closed pores can account for the observed increase in diffusivity. It was then proposed that the volume change associated with the progressive tetragonal to monoclinic phase transformation can be responsible for the reduction of the closed porosity of lenticular shape oriented parallel to the surface, in spite of the observed increase in the total porosity.
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TL;DR: In this article, a three dimensional formulation of the problem is presented along with a discussion on the relevant closure relationships for free bars under bedload dominated conditions and an account is given on the effect on bar instability induced by suspended load, grain sorting and width variations.
Abstract: In the paper we review some recent work on the mechanics of formation and development of river bars. The emphasis is placed on the instability process which leads to the spontaneous development of bars in almost straight reaches of alluvial rivers. A three dimensional formulation of the problem is presented along with a discussion on the relevant closure relationships. Results of linear and non linear theories for free bars under bedload dominated conditions are summarised. Furthermore, account is given on the effect on bar instability induced by suspended load, grain sorting and width variations. Some as yet unpublished results are also presented.
TL;DR: In this paper, the optimal control problem for a Gurtin-MacCamy type system describing the evolution of an age-structured population was studied and necessary optimality conditions were established in the form of an Euler-Lagrange system and existence of an optimal control was proved using the Ekeland principle.
Abstract: The present paper is concerned with the optimal control problem for a Gurtin–MacCamy type system describing the evolution of an age-structured population. Necessary optimality conditions are established in the form of an Euler–Lagrange system and existence of an optimal control is proved using the Ekeland principle.
TL;DR: In this paper, Ni-P composite coatings containing SiC particles and PTFE, MoS 2 and BN particles, to reduce friction, were produced on a steel substrate by means of electroless deposition, and the dry sliding behavior against a steel counterface (made of AISI M2) was found to develop through two stages (indicated with stage I and II).
TL;DR: In this article, a new model of line-profile broadening due to the effect of linear and planar lattice defects has been incorporated into the conventional Rietveld algorithm for the structural refinement and whole-pattern fitting of powder data.
Abstract: A new model of line-profile broadening due to the effect of linear and planar lattice defects has been incorporated into the conventional Rietveld algorithm for the structural refinement and whole-pattern fitting of powder data. The proposed procedure, applied to face-centred cubic (f.c.c.) structure materials, permits better modelling, even in the case of anisotropic line broadening and other hkl-dependent effects that can be related to the presence of dislocations and planar defects (stacking faults and twinning). Besides better quality of the profile fitting, detailed information on the defect structure can be produced: dislocation density and cut-off radius, stacking- and twin-fault probabilities can be refined together with the structural parameters. For each phase (in different samples or in multi-phase samples) the appropriate size–strain model can be selected. The Fourier formalism, which is the basis of the line-profile modelling, also permits an easy adaptation to different lattice-defect models. New approaches can be easily introduced and tested against or together with the existing ones. Finally, the devised program can also be used for the simulation of powder patterns for materials with different types and amounts of line and plane lattice defects.
TL;DR: New heuristic algorithms are proposed for the Graph Partitioning problem and detailed experimental results are presented on benchmark suites used in the previous literature, consisting of graphs derived from parametric models and of "real-world" graphs of large size.
Abstract: New heuristic algorithms are proposed for the Graph Partitioning problem. A greedy construction scheme with an appropriate tie-breaking rule (MIN-MAX-GREEDY) produces initial assignments in a very fast time. For some classes of graphs, independent repetitions of MIN-MAX-GREEDY are sufficient to reproduce solutions found by more complex techniques. When the method is not competitive, the initial assignments are used as starting points for a prohibition-based scheme, where the prohibition is chosen in a randomized and reactive way, with a bias towards more successful choices in the previous part of the run. The relationship between prohibition-based diversification (Tabu Search) and the variable-depth Kernighan-Lin algorithm is discussed, Detailed experimental results are presented on benchmark suites used in the previous literature, consisting of graphs derived from parametric models (random graphs, geometric graphs, etc.) and of "real-world" graphs of large size. On the first series of graphs, a better performance for equivalent or smaller computing times is obtained, while, on the large "real-world" instances, significantly better results than those of multilevel algorithms are obtained, but for a much larger computational effort.
TL;DR: The polymorphism of WO3 powder samples, resulting from mild mechanical treatments and from temperature changes between 30 K and room temperature, has been investigated by using Raman spectroscopy and X-ray di⁄raction.
TL;DR: In this paper, the authors investigate the oscillation of a dilute atomic gas generated by a sudden rotation of the confining trap (scissors mode), which reveals the effects of superfluidity exhibited by a Bose-Einstein condensate.
Abstract: We investigate the oscillation of a dilute atomic gas generated by a sudden rotation of the confining trap (scissors mode). This oscillation reveals the effects of superfluidity exhibited by a Bose-Einstein condensate. The scissors mode is also investigated in a classical gas above ${T}_{c}$ in various collisional regimes. The crucial difference with respect to the superfluid case arises from the occurrence of low frequency components, which are responsible for the rigid value of the moment of inertia. Different experimental procedures to excite the scissors mode are discussed.
TL;DR: In this paper, a superfluid hydrodynamic model for the increase in moment of inertia of molecules rotating in liquid was presented, where the static inhomogeneous He density around each molecule was assumed to adiabatically follow the rotation of the molecule.
Abstract: We present a superfluid hydrodynamic model for the increase in moment of inertia, $\Delta I$, of molecules rotating in liquid $^4$He. The static inhomogeneous He density around each molecule (calculated using the Orsay-Paris liquid $^4$He density functional) is assumed to adiabatically follow the rotation of the molecule. We find that the $\Delta I$ values created by the viscousless and irrotational flow are in good agreement with the observed increases for several molecules [ OCS, (HCN)$_2$, HCCCN, and HCCCH$_3$ ]. For HCN and HCCH, our model substantially overestimates $\Delta I$. This is likely to result from a (partial) breakdown of the adiabatic following approximation.
TL;DR: In this article, the block-effective macrodispersivity tensor is proposed to model macrodisperivities in spatially variable velocity fields, such as exist in geologically heterogeneous formations, and closed-form expressions are developed for small variability in the log-conductivity and unidirectional mean flow, and tested numerically.
Abstract: We present a new approach for modelling macrodispersivity in spatially variable velocity fields, such as exist in geologically heterogeneous formations. Considering a spectral representation of the velocity, it is recognized that numerical models usually capture low-wavenumber effects, while the large-wavenumber effects, associated with subgrid block variability, are suppressed. While this suppression is avoidable if the heterogeneity is captured at minute detail, that goal is impossible to achieve in all but the most trivial cases. Representing the effects of the suppressed variability in the models is made possible using the proposed concept of block-effective macrodispersivity. A tensor is developed, which we refer to as the block-effective macrodispersivity tensor, whose terms are functions of the characteristic length scales of heterogeneity, as well as the length scales of the model's homogenized areas, or numerical grid blocks. Closed-form expressions are developed for small variability in the log-conductivity and unidirectional mean flow, and are tested numerically. The use of the block-effective macrodispersivities allows conditioning of the velocity field on the measurements on the one hand, while accounting for the effects of unmodelled heterogeneity on the other, in a numerically reasonable set-up. It is shown that the effects of the grid scale are similar to those of the plume scale in terms of filtering out the effects of portions of the velocity spectrum. Hence it is easy to expand the concept of the block-effective dispersivity to account for the scale of the solute body and the pore-scale dispersion.
TL;DR: In this paper, a diffusion Monte Carlo method was used to calculate the lowest energy state of a uniform gas of bosons interacting through different model potentials, both strictly repulsive and with an attractive well.
Abstract: We use a diffusion Monte Carlo method to calculate the lowest-energy state of a uniform gas of bosons interacting through different model potentials, both strictly repulsive and with an attractive well. We explicitly verify that at low density the energy per particle follows a universal behavior fixed by the gas parameter ${\mathrm{na}}^{3}.$ In the regime of densities typical for experiments in trapped Bose-condensed gases, the corrections to the mean-field energy greatly exceed the differences due to the details of the potential.
TL;DR: In this article, the high temperature stability of sol-gel-derived SiOC glasses has been investigated in order to understand the role played by the presence of a free-carbon phase in the amorphous oxycarbide network.
Abstract: The high temperature stability of sol-gel-derived SiOC glasses has been investigated in order to understand the role played by the presence of a free-carbon phase in the amorphous oxycarbide network. Therefore, pure and excess free-carbon containing SiOC glasses have been prepared from hybrid gels and used for the high temperature stability experiments. Moreover, the possible effects of the sample size with respect to local kinetics and thermodynamics and of the testing environment have also been investigated. Results show that all these parameters play a crucial role in defining the high temperature behavior of SiOC glasses.