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  • Springer  (352)
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  • 1995-1999  (173)
  • 1990-1994  (179)
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  • Springer  (352)
  • American Meteorological Society
  • Oxford University Press
  • Periodicals Archive Online (PAO)
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  • 1995-1999  (173)
  • 1990-1994  (179)
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  • 1
    Electronic Resource
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    Springer
    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1715-1721 
    ISSN: 0392-6737
    Keywords: Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.) ; Flux pinning ; flux motion ; fluxon-defect interactions ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary High-quality single-phase thin platelets of BSCCO-2223 have been produced by a novel method on thermal-gradient-enhanced KCl flux which allows the growth of the 2223-phase in an extremely short time interval compared to those reported in the literature. In-plane electrical resistivity measurements in zero applied magnetic field have been performed. Experimental data have been analysed and interpreted within the Aslamazov-Larkin theory of amplitude fluctuations and the Ginzburg-Landau Coulomb gas model of phase fluctuations. These theories seem to fit in a very reliable way the experimentalR(T) curves.
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  • 2
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1815-1820 
    ISSN: 0392-6737
    Keywords: Insulator-superconductor transitions ; unconventional mechanisms of high-T c superconductivity ; Other topics in superconductivity ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The temperature dependence of local Cu site conformations in single-domain crystals of Bi2Sr2CaCu2O8+y (Bi-2212) and La1.85Sr0.15CuO4 (La-214) has been determined by EXAFS (Extended X-ray Absorption Fine Structure), a fast (10−15 s) and local probe. Two sets of Cu site conformations characterized by the Cu−O(apical) bond distances and tilting of copper square plane have been found atT〈T*∼1.5T c . The difference between the long and short Cu−O(apical) distance appeared to be 0.17 Å and 0.12 Å atT〈T* in Bi-2212 and La-214 systems, respectively. The polarons are associated with the short Cu−O(apical) bond distance. The ratio between the number of short and the total number of Cu−O(apical) bonds (N short/N tot∼0.42 in both systems) is a measure of the relative area of distorted and undistorted domains. By using this result we have been able to show the polaronic nature of the one-dimensional structural anharmonic modulation of the CuO2 plane in Bi-2212 where the polarons have dimensionW=(11±1) Å atT〈T*≈1.4T c and are condensed into a one-dimensional generalized Wigner charge density wave (CDW). It appears that superconductivity is stabilized at thigh temperature by resonant quantum confinement in a superlattice of stripe widthL wherek Fx ∼k Fy ∼π/L.
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  • 3
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1843-1849 
    ISSN: 0392-6737
    Keywords: Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary This work is focused on how oxygen stoichiometry and distribution affects the pretransitional region in YBa2Cu3O7−δ pellets prepared by different thermal treatments starting from the same batch with oxygen content 6.94. We find that slightly deoxygenated samples, which show higherT c and narrower transition than the starting material, also exhibit a more pronounced rounding of the resistivity extending towards higher temperatures. We analyse the presence and the extension of regions with different power law behaviour of excess conductivityvs. temperature.
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  • 4
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1871-1876 
    ISSN: 0392-6737
    Keywords: Magnetization curves ; Meissner effect ; penetration depth ; Critical currents ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary BSCCO (2223) phase superconductor, prepared by the traditional solid-state reaction route, was densified by hot pressing, yielding high-density and textured-bulk materials. A correlation between processing parameters and inter-, intra-granular properties of the sample was performed through the evaluation of microstructural features and the study of electrical resistivity and magnetization behaviour as a function of different temperature and magnetic field.
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  • 5
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1925-1932 
    ISSN: 0392-6737
    Keywords: Flux pinning ; flux motion ; fluxon-defect interactions ; Critical currents ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The results of low-temperature magnetic-relaxation experiments in TlBaCaCuO superconductors (2212 and 2223 phase) are reported. They give evidence of a crossover from a thermally activated regime of flux motion to a nonthermally activated one. The magnetic-relaxation rate is found to become temperature independent below ≅7K and ≅3K, for the 2212 and 2223 phase, respectively. Its value compares well with that predicted by the theories of flux motion by quantum tunnelling.
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  • 6
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1917-1924 
    ISSN: 0392-6737
    Keywords: Flux pinning ; flux motion ; fluxon-defect interactions ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Measurements of fundamental and higher harmonics of a.c. susceptibility on polycrystalline high-T c superconductors were carried out using a new method, involving a 2-channel dynamic analyser, allowing 20 harmonics in phase and out of phase to be measured at the same time. The critical-state model, used for describing the higher-harmonics dependence on temperature at fixed d.c. and a.c. magnetic fields, did not explain the experimental results. A frequency analysis indeed suggested that an effect of the fluxon dynamics takes place. Simple considerations lead to the conclusion that really Thermally Activated Flux Flow (TAFF), Flux Creep (FC) and Flux Flow (FF) would have a big role in determining the higher harmonics of a.c. susceptibility. Measurements on low-T c superconductors showed the occurrence of the same effect.
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  • 7
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1933-1939 
    ISSN: 0392-6737
    Keywords: Flux pinning, flux creep, and flux-line lattice dynamics ; Critical currents ; Material effects onT c, κ, critical currents (including impurities, ion implantation, etc.) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Bulk slabs of sintered YBCO were irradiated with monoenergetic protons at different energies (3.5 and 6.5 MeV) and different fluences, ranging from 1.8·1016 to 8.6·1016 p/cm2. The effects on intergrain critical currents of protons crossing the sample and the effects of protons implanting in the specimen, has been studied by means of a.c. susceptibility characterizations. D.c. hysteresis cycles were measured in low fields in order to investigate the modifications in the intergrain critical current density due to proton implantation. The main result is the evidence that the intergrainJ c response to the applied field can be modulated by means of the proton irradiation. Resistive measurements were also performed in order to investigate the complex transport phenomena in polycrystalline materials. Irradiation-induced effects on both superconducting- and normal-state resistivity are studied. The addition of extrinsic defects can lead to enhanced transport properties, emerging especially when a magnetic field is applied.
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  • 8
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1671-1678 
    ISSN: 0392-6737
    Keywords: Effects of material synthesis ; crystal structure ; chemical composition ; Y-based compounds ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary YBCO with high granulometric perfection and low dispersion can be obtained by a low-temperature reaction (≈800°C) of atomic-scale dispersed powders, produced through the thermal decomposition of liquid precursors. As reacted such powders are tetragonal and not superconducting, but they transform into the high-T c phase after optimized annealing treatments. We describe the method we developed and the characteristics of the materials we achieved, and we discuss the main features of the process in the light of the current know-how.
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  • 9
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2019-2024 
    ISSN: 0392-6737
    Keywords: Proximity effects, tunneling phenomena, and Josephson effects ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary An analysis of superconducting transport properties and magnetic behaviour of d.c. SQUIDs employing YBCO bicrystal grain boundary junctions (GBJs) has been performed. GBJs have been obtained by deposition of ac-axis-oriented YBCO film on a SrTiO3 bicrystal substrate by ICM sputtering technique. Experimental measurements on a YBCO d.c. SQUID with a misorientation angle θ=20° are reported. The SQUID shows a critical temperatureT c∼89 K and a high critical current densityJ c∼3·106 A/cm2 atT=4.2 K. Current-voltage characteristics are close to the behaviour predicted by the resistively shunted junction (RSJ) model and the temperature dependenceJ c(T) shows a linear behaviour at small reduced temperatures and a depressedJ c value close toT c. High-quality flux-voltage curves have been found upT=87 K over a large range of magnetic field. The high reproducibility and the good control of transport properties by the variation of θ make YBCO bicrystal GBJs very useful for applications in electronics.
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  • 10
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1799-1802 
    ISSN: 0392-6737
    Keywords: Relaxation effects ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary High-temperature (T〉200 K)139La NMR relaxation rates in single crystals of La2CuO4+δ (δ∼0.015 and δ∼0.002) at two measuring frequencies are presented and compared with zero-field NQR data for a series of La2−x Sr x CuO4+δ powder samples with 0≤x≤0.08 and 0〈δ0.013. For δ≠0 an enhancement of the relaxation rate is observed which is related to the time dependence of the quadrupole interaction. Quantitative information on the oxygen diffusion is obtained from the analysis of the data with a simple diffusion model. The results are discussed in relation to the problem of macroscopic phase separation occurring for higher oxygen content.
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  • 11
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1701-1708 
    ISSN: 0392-6737
    Keywords: Superconducting perovskites and related structures ; Treatment of materials and its effects on microstructure and properties ; Conductivity phenomena in semiconductors and insulators ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The transformation of the 2223 (n=3) phase in the 2201 (n=1) phase by heating the sample in Ar atmosphere was studied. The subsequent cycling of the sample temperature and the oxygen enrichment of the gas phase convert the 2201 (n=1) phase in the 2223 (n=3) phase in the presence of the 2212 (n=2) and Ca2PbO4 compound. Thein situ resistivity measurements,T c values and XRD carried out on the sample confirm these phase changes. The experiment indicates that this process is oxygen dependent at low oxygen partial pressure.
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  • 12
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1735-1741 
    ISSN: 0392-6737
    Keywords: Response to electromagnetic fields ; nuclear magnetic resonance ; ultrasonic attenuation ; Scattering by phonons, magnons, and other nonlocalized excitations ; Superconducting perovskites and related structures ; Refractive index, reflectivity, critical angle, dispersive power ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Evidence of polaron formation in superconducting cuprates is provided by infrared reflectivity spectra. In the Nd−Ce−Cu−O withT c≈20 K, polaron formation corresponds to vibrational local modes in the far-infrared and to a band in the mid-infrared built up through overtones and combination bands of these modes. A comparison with infrared spectra taken by different groups on Y−Ba−Cu−O, and Tl−Sr−Ca−Cu−O suggests that polarons survive in the highest-T c superconductors.
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  • 13
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2005-2010 
    ISSN: 0392-6737
    Keywords: Superconducting films ; PACS 81.15.Cd ; Deposition by sputtering ; Microwave circuits (e.g., parametric, solid state) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Superconducting films of YBa2Cu3O7−x were depositedin situ on LaAlO3 substrates using single-target 90° off-axis sputtering. The obtained films have tipicalT c values of 91K. Surface resistance measurements on as-grown films reach 1.1 mΩ at 77K and 10GHz; whilst on ion-etched patterned resonant linesR s (77K, 10GHz) it is about 10mΩ.
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  • 14
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2113-2117 
    ISSN: 0392-6737
    Keywords: Superconducting magnets and magnetic levitation devices ; superconducting cables and conductors ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Ag/BSCCO-2223 phase tapes were prepared with a new plastic deformation cycle. Starting from the hot extrusion of composite billets, hundreds of metres of tape 0.15×3 mm were obtained by drawing and rolling the extruded bars. The cross-section, monofilamentary with a central Ag insert, is quite regular along all the length. Samples from 30 cm to 100 cm in length were heat-treated at different temperatures between 830°C and 480°C in Ar+10% O2 atmosphere. Electrical-transport properties were tested at 77K.J c of 3000 A/cm2 were usually achieved after first heat treatment. The superconducting properties were also tested by magnetic measurements using a vibrating sample magnetometer.
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  • 15
    ISSN: 0392-6737
    Keywords: Polymer reactions and polymerization ; Brillouin and Rayleigh scattering ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We analyse data from the dynamic light scattering of poly(N-isopropylacrilamide) in water solution as we cross the collapse transition. Experimental data are interpreted by the Gaussian self-consistent Zimm model that takes into account two- and three-body excluded-volume effects, and Oseen hydrodynamic interactions, as well as by the standard cumulant and Contin analyses. By fitting the dynamic structure factor we extract the temperature dependence of the diffusion coefficientD and the first relaxation time τ1 across the collapse transition for a range of scattering angles. The relaxation time τ1 possesses a characteristic peak at about 32.4 °C due to slowing of the internal motions of the polymer at the theta-point, and a minimum at 33.4 °C. We interpret this as a combination of collapse closely followed by the growth of critical correlations. At large scattering angles we reach the universalk 3 regime, and observe that this behaviour vanishes at the onset of collapse transition.
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  • 16
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 817-824 
    ISSN: 0392-6737
    Keywords: Computer simulation of static and dynamic behavior ; Liquid metals and liquid alloys ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Results of extensive computer simulations performed in liquid water are reported in order to clarify the main features of dynamical density correlations in this system. Puzzling aspects, such as the anomalous sound wave propagation at finite wave vectors, are related to the peculiar structural aspects of this hydrogen-bonded liquid. Additional features (such as the evolution of the relevant memory functions at increasing wave vectors and the emergency of a second mode in the longitudinal current spectra) are also discussed in some detail.
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  • 17
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 697-702 
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Field theory ; Spontaneous and radiative symmetry breaking ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Filamentary-surfaces randomly branched polymers can be represented byO(N) symmetric quantum field theories in the limit whereN→∞ and the coupling constantg→g c in a correlated manner. The statistical systems of randomly branched polymers are realized as dual diagrams of the Feynman graphs of the underlyingO(N) symmetric quantum field theory. The purpose of these studies is to gain intuition for the long-lasting search for a possible description of quantum field theory in terms of extended objects.
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  • 18
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 803-807 
    ISSN: 0392-6737
    Keywords: Conformation (statistics and dynamics) ; Polymers and plastics ; Heterogeneous liquids: suspensions, dispersions, emulsions, pastes, slurries, foams, block copolymers, etc ; Thermodynamics of surfaces and interfaces ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Simulation results as well as experimental data indicate that full stretching of flexible polyelectrolytes will not occur under experimentally realizable conditions. Using density-dependent swelling exponents ν(ϕ) as suggested by Stevens and Kremer from the results of a MD simulation study, we present an Alexander-de Gennes-like scaling picture for the behaviour of charged brushes. The brush height is found to become dependent on the grafting density as soon as internal stretching is incomplete. For a particular anchoring technique used in experimental studies the grafting density itself becomes dependent on the chain length. The resulting modified overall chain length dependence of the brush height is discussed.
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  • 19
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 835-842 
    ISSN: 0392-6737
    Keywords: Order-disorder and statistical mechanics of model systems ; Small-angle scattering, EDXD, EXAFS, and other techniques ; Layer structures, intercalation compounds, and superlattices: growth, structure, and nonelectronic properties ; Polymers, elastomers, and plastic ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A homologous series of nine symmetric poly(styrene)-block-poly(butadieme) diblock copolymers was synthesized and characterized. The scaling of the lamellar thickness,D, with the chain lengthN(DαN δ ) was investigated with small-angle X-ray scattering. Different sample preparation methods were used: annealing, solvent-casting and shear alignment. We find that the lamellar thickness and thus the value of the exponent δ depends on the preparation method used,i.e. care must be taken to ensure that the samples investigated are in thermodynamic equilibrium.
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  • 20
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 865-871 
    ISSN: 0392-6737
    Keywords: Disperse systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The phase diagram of the ternary system,i.e. poly(diallyldimethylammonium chloride), poly(acrylic acid, sodium salt), and water was investigated by varying the molecular weight of the components. In general, a range of phase separation is surrounded by a one-phase system. The range of phase separation can be subdivided into a region of flocculation and/or coacervation in contrast to stable symplex dispersions. By increasing the molecular weight of the components the range of phase separation is increased and phenomena of flocculation followed by coacervation were observed predominantly. Above a molecular weight of 10 000 g/mol the transition to concentrated macroscopic homogeneous one-phase systems was observed at a constant ionic strength of 2 mol/l coinciding with the «critical ionic strength» for Coulombic interactions in this system. That means the transition to concentrated one-phase systems can be understood by the disappearance of Coulombic forces above a system-specific ionic strength resulting in a quite additive behaviour of the mixtures with regard to the components.
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  • 21
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 895-900 
    ISSN: 0392-6737
    Keywords: Liquid crystals ; Interface elasticity, viscosity, and viscoelasticity ; ther topics in the structure and nonelectronic properties of surfaces and thin films ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The nematic planar anchoring is usually explained by using simple elastic models: the surface easy axis corresponds to the surface direction that minimizes the excess of nematic elastic energy. When anisotropic rough substrates are used to align nematic liquid crystals, due to the complex surface morphology, usual elastic models are not directly applicable. This paper presents quantitative topographical data of rough substrates, obtained with oblique SiO evaporation under vacuum for nematic planar anchoring. Experimental data are obtained by means of Atomic Force Microscopy and they are used to demonstrate the self-affine nature of these substrates and to relate the nematic anchoring with the anisotropy of the local fractal properties of the substrate itself.
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  • 22
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 637-640 
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Interface elasticity, viscosity and viscoelasticity ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We discuss the role of «connectors» between two rubber blocks or between a rubber and a solid. Early descriptions of these systems were restricted to very dilute connectors, working independently. In the present work, we investigate theoretically the role and the limits of the surface concentration of connectors.
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  • 23
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 923-931 
    ISSN: 0392-6737
    Keywords: Brillouin and Rayleigh scattering ; Ultrasonic relaxation processes in gases, liquids, and solids ; Structure of associated liquids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We have performed Brillouin and depolarized Rayleigh light scattering on water-methanol solutions as a function of methanol molar fraction and temperature. For this system, molecular-dynamics (MD) simulation predicts that a shell of water forms around a methanol molecule, prediction which has been recently confirmed by neutron diffraction experiments. The results obtained by light scattering and presented here confirm previous observations and show, in addition, an increase of the stability in time of the water structure in the shells when temperature decreases and methanol concentration approaches a particular value.
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  • 24
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 641-652 
    ISSN: 0392-6737
    Keywords: Macromolecular and polymer solutions (solubility, swelling, etc.) ; polymer melts ; Thermodynamics of surfaces and interfaces ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We consider the configurations and the various regimes when an alternating, symmetric, multiblock copolymer made of successive sequences of A and B monomers with equal lengthZ is at the interface between two immiscible solvents that are respectively good for one of the sequences and poor for the other one. We find two semi-dilute regimes, when the copolymers overlap. For monomer concentrations belowГ **, we find a first range where the thickness of the copolymers is constant and equal to the radius of a sequence. For concentrations larger thanГ **, a second regime appears, where the seqeunces stretch our and adopt a conformation similar to what was reported for grafted polymers. Finally at a concentrationГ sat, the interface is completely saturated, and the copolymers form micelles in the bulk: it becomes energetically more favourable to have copolymers in the bulk than at interface.
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  • 25
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1205-1216 
    ISSN: 0392-6737
    Keywords: Statistical theories of liquid structure ; Computer simulation of static and dynamic behavior ; General studies of phase transitions ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Binary-fluid mixtures show first-order phase transition phenomena which are either classified as mixing-demixing (phase separation), or as condensation-evaporation. The localization of the transition lines, the characterization of the new phases, and the analysis of the parameters which force model systems to choose one of the two transition mechanisms is the aim of this study. We performed both parallel-molecular-dynamics simulations and integral-equation theory calculations on binary mixtures of Lennard-Jones particles. By fixing the composition and by varying the interaction strength, we found a continuous change between the two kinds of transition mechanisms. Theoretical and simulation results are compared with each other and a quantitative agreement i8s found. However, the simultation analysis appears to be more promising since, in contrast to the integral-equation theory in which the spinodal lines can only be approched, information on the microscopic structure of the new phases can be obtained.
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  • 26
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1065-1089 
    ISSN: 0392-6737
    Keywords: Optical chaos ; Classical mechanics of continuous media: general methematical aspects ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In 1952 A. Turing introduced the concept of chemical morphogenesis. A medium with at least two interacting and diffusing components (activator and inhibitor) can be subjected to spontaneous pattern formation, with a scale length independent of the boundary conditions, and thus maintained even in the infinite volume limit. This is at variance with pattern formation in fluids (as,e.g., Rayleigh-Benard and Faraday instabilities) where the size is imposed by the boundary geometry. In non-linear optics, patterns emerge from the coupling of a diffractive equation describing electromagnetic propagation with a diffusion equation describing the local modification of the polarizability in a medium. As we adjust an extensive parameter (the so-called Fresnel numberF) corresponding to the optical aspect ratio, we observe a transition from a regime dominated by the boundary constraints to a Turing-type regime dominated by the bulk parameter. This is equivalent to saying that the preminent role is due to the diffractive equation in one case and to the diffusive one in the other. Morphogenesis for lowF arises from the non-linear competition among a small number of degrees of freedom, giving rise to a space-uniform excitation with a low-dimensional dynamics. This gives rise to the different scenarios of chaos. Their properties have been explored in the past decade. In the large-F case, the space-time instabilities rapidly evolve toward complex patterns, not reducible to a few indicators. In the case of two-dimensional fields, global characterization is achieved via the statistics of the topological defects.
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  • 27
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1127-1139 
    ISSN: 0392-6737
    Keywords: Colloids ; Solubility, segregation, and mixing ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We review the phase behaviour of mixtures of colloids and non-adsorbing polymers. The exclusion of polymer molecules from overlapping «depletion zones» between two neighbouring colloidal particles results in an unbalanced osmotic pressure pushing the particles together. This depletion potential is separately tunable in range and depth. Theory predicts that the resulting phase behaviour is sensitive to ξ=r g/R, the ratio of the radius of gyration of a polymer molecule, to the radius of the colloid. At large ξ, a stable colloidalliquid phase becomes possible. This has been confirmed by recent experiments. The formation of non-equilibrium «transient gel» states when the size ratio is small (≈0.08) is also introduced briefly.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1171-1176 
    ISSN: 0392-6737
    Keywords: Thermal expansion and thermomechanical effects ; Lattice theory and statistic ; Ising problems ; Polymer reactions and polymerization ; General studies of phase transitions ; Conference proceedings
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    Topics: Physics
    Notes: Summary The role of chemical reactions in the selection of patterns in phase-separating mixtures is presented. Linearized theory and computer simulation show that the initial long-wavelength instability characteristic of spinodal decomposition is suppressed by chemical reactions, which restrict domain growth to intermediate length scales even in the late stages of phase separation. Our findings suggest that chemical reactions may provide a novel way to stabilize and tune the steady-state morphology of phase-separating materials.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1373-1390 
    ISSN: 0392-6737
    Keywords: Mechanical properties of liquids ; Conference proceedings
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    Topics: Physics
    Notes: Summary A general method is presented for the preparation of viscoelastic surfactant phases that consist of densely packed multilamellar vesicles in water. The phases form spontaneously when ionic surfactants are added to Lα-or L3-phases from uncharged surfactants and cosurfactants. The dimensions and the structure of the vesicles were studied from FF-TEM micrographs for 100 mM surfactant solutions. The average diameter of the vesicles is in the range of 1 μm, the interlamellar spacing is around 800 Å. Under these conditions the vesicles are densely packed and cannot pass each other. The systems are highly viscoelastic and have a yield value. The storage and the loss moduli were determined by oscillating rheological measurements. The storage modulus was about one order of magnitude larger than the loss modulus and independent of frequency. Both parameters were determined as a function of the concentration and the chain length of the surfactant and cosurfactant, the charge density and ionic strength, the solubilization of hydrocarbon and temperature. The yield values and shear moduli increase linearly with the surfactant concentration. For constant surfactant concentration the moduli increase in a S-shaped form with the charge density and reach saturation for a mole fraction of about 7% of ionic surfactant. The storage moduli and yield values decrease with the addition of excess salt. The storage moduli depend strongly on the chain length of the surfactant. The values of the moduli are explained on the basis of a hard-sphere model in which the multilamellar vesicles are considered as hard-sphere particles.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1433-1437 
    ISSN: 0392-6737
    Keywords: Viscosity and diffusion: experimental ; Diffusion and thermal diffusion ; Conference proceedings
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    Topics: Physics
    Notes: Summary The dynamics of water molecules occupying very small volumes can be probed by incoherent quasi-elastic neutron scattering. We summarise some results obtained with wet samples of porous silica and hydrated proteins. The movements of the molecules in the vicinity of the substrate are extremely reduced as compared to those of bulk water at the same temperature. Almost only rotational movements subsist and the local diffusion is of the order of that observed in deeply supercooled water.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1471-1477 
    ISSN: 0392-6737
    Keywords: Photochemical reactions of biomolecules biochemical reaction mechanisms and kinetics, model reactions ; Conference proceedings
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    Topics: Physics
    Notes: Summary Photoinduced electron transfer (PET) was studied in phospholipid vesicles and in Langmuir-Blodgett bilayers in the attempt to produce a model for electron transfer processes in biological media. Spatial organization of the reaction centers in lipid membranes needs to be controlled in order to provide high efficiency of light-to-chemical energy conversion. We designed a phospholipid system where the donor is localized in the inner bilayer whereas the acceptor is at the polar groups-water interface. We used dipalmitoylphosphatidic acid vesicles containing low molar fractions of dipalmitoylphosphatidylcholine with pyrene (donor) bound to one of the alkyl chains. Methylviologen (acceptor) was added to the external aqueous phase; upon photoexcitation of the donor we observed the electron transfer to take place in a unidirectional manner from the inside of the bilayer to the interface. Information about the location of the donor was obtained studying the photophysical properties of the pyrene chromophore in vesicles and in LB layers. The photoinduced electron transfer reaction was evidenced by quenching of pyrene fluorescence in the presence of increasing concentrations of acceptor, the process was studied both with steady-state and time-resolved fluorescence emission. Fluorescence intensity was found to decrease with increasing concentration of methylviologen, similar results were obtained for vesicles and LB layers of analog composition immersed in a methylviologen solution. Lifetimes of the excited species were found to be of the same order of magnitude in vesicle and LB-layer systems.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1297-1303 
    ISSN: 0392-6737
    Keywords: Fluid surfaces and fluid-fluid interfaces ; Liquid thin films ; Conference proceedings
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    Topics: Physics
    Notes: Summary Stiff rod-like chain molecules with harmonic bond length potentials and trigonometric bond angle potentials are used to model Langmuir monolayers at high densities. One end of the rod-like molecules is strongly bound to a flat two-dimensional substrate which represents the air-water interface. A ground-state analysis is performed which suggests phase transitions between phases with and without collective uniform tilt. Large-scale off-lattice Monte Carlo simulations over a wide temperature range show in addition to the tilting transition the presence of a strongly constrained melting transition at high temperatures. The latter transition appears to be related to two-dimensional melting of the head group lattice. These findings show that the model contains both, two- and three-dimensional ergodicity breaking solidification transitions. We discuss our findings with respect to experiment.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1401-1410 
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    Keywords: Mechanical properties of liquids ; Conference proceedings
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    Topics: Physics
    Notes: Summary Both linear and non-linear viscoelasticity experiments have been performed in semi-dilute micellar aqueous solutions of ethanediyl-α,ω-bis (dodecylmethylammonium bromide) as a function of solution chloride (NaCl) concentration. The results suggest that both linear and non-linear properties are affected by the electrostatic interactions. In particular, the plateau modulusG′∞ and the ratioσ m/G′∞, whereσ m represents the plateau value in a stress-shear rate experiment, are enhanced as the electrostatic interactions are screened by addition of salt. The zero-shear viscosity is lowered upon addition of NaCl, due likely to an exchange of the Cl− ions with the Br− counterions.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1367-1372 
    ISSN: 0392-6737
    Keywords: Fluid surfaces and fluid-fluid interfaces ; Transitions in liquid crystals ; Defects in liquid crystals ; Liquid thin films ; Conference proceedings
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    Topics: Physics
    Notes: Summary Optical studies of Langmuir monolayers of simple amphiphiles such as fatty acids and esters by polarized fluorescence microscopy and Brewster-angle microscopy reveal a large variety of textures including stripes, stars, and spirals that closely resemble textures observed in liquid crystals. The textures represent large-scale (〉10 μm) spontaneous organization of the molecular tilt azimuth. Phase transitions within the monolayer can be directly observed by marked changes in texture, revealing a complex phase diagram. Many of the textures can be explained by a Landau-deGennes theory of tilted hexatic phases that takes into account the broken-symmetry characteristic of amphiphiles at an interface.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1411-1418 
    ISSN: 0392-6737
    Keywords: Physical chemistry of solutions of biological macromolecules ; Colloids ; Gels and sols ; Conference proceedings
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    Topics: Physics
    Notes: Summary Mucus is the viscoelastic secretion that lines, most epithelial surfaces forming a protective, lubricating barrier. The viscoelastic properties of mucus arise from mucin, a glycoprotein of molecular weight ranging from 2–10 million. In this paper we address two problems where the aggregation and interactions of mucin with colloidal particles is of physiological relevance. The first deals with gastric mucin and the question of the mechanisms that prevent the stomach from digesting itself. Using dynamic light scattering techniques we show that solutions of gastric mucin aggregate belowpH 4. Very large aggregates with 100-fold slower diffusion constants than the mucin monomer are observed atpH 2. Viscosity measurements indicate that mucin will eventually gel at lowpH, thus acting as a diffusional, barrier and protecting the stomach. The second problem is concerned with the role of mucin in the nucleation of cholesterol crystals which lead to gallstone formation. Using dynamic light scattering we have shown that mucin at relatively low concentrations (4 mg/ml) promotes the fusion of phospholipid + cholesterol vesicles. The time evolution of the fusion process was measured. No changes in the aggregation state of the gallbladder mucin were observed during the fusion process, suggesting that this phenomenon is related to physico-chemical interactions between the polymer (mucin) and the colloidal particle (vesicle).
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1551-1557 
    ISSN: 0392-6737
    Keywords: Disperse systems ; Conference proceedings
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    Topics: Physics
    Notes: Summary A simple computer model of defection process caused by interactions between melittin and phosphatidylcholine (PC) model membrane has been proposed. It leads to a scaling formula which connects the average diameter (or linear size parameter) of the affected domain with the reciprocity of the number of such domains. Some preliminary investigations show the scaling exponent to be a small fractional number between ca. 0.3 and 0.1, depending on the number of possible infection directions and the type of lattice taken as a lipid matrix. Such small values of the scaling exponent can roughly be interpreted in terms of random tesselations on a plane (like random Voronoi networks), and the whole process displays a statistical self-similarity. The values obtained seem to be, however, small comparable to the scaling exponents characteristic for the faulting process occurring during the earthquake propagation.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1579-1584 
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    Keywords: Fluid surfaces and fluid-fluid interfaces ; Conference proceedings
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    Notes: Summary Oriented systems such as monolayers and Langmuir-Blodgett multilayers constitute supramolecular structures appropriate for ion detection. In fact, they are sensitive to small quantities of ions since they have specifically oriented ligand head groups that can easily combine with the ions to be detected in solution. We investigated monolayers of dioctadecyldithiocarbamate (DODTC) specificially synthesized and we studied the interactions of the monolayers with transition metal ions such as Ni2+ dissolved in the subphase. The spreading isotherms, the surface potential-area isotherms and the spectroscopic measurements (UV-Vis absorption and ESCA spectroscopy) show that the metal ions can combine with the DODTC monolayers. The present findings show that it is possible to employ these systems to build chemical sensors for transition metal ions
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1585-1594 
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    Keywords: Molecular biophysics ; Defects in liquid crystals ; Conference proceedings
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    Topics: Physics
    Notes: Summary The effect of adding the defect-forming molecules melittin and C12E5 to DMPC membranes has been studied and the corresponding phase diagrams established. Light, X-ray and neutron small-angle scattering have been used to characterize the mixed membranes. Both systems show a melting of the lamellar Lα into an isotropic phase upon addition of the second membrane constituent. The molar ratio, where the melting occurs is the same in concentrated and dilute samples. For the DMPC/C12E5 system not only membrane composition, but also temperature can be used to induce a transition from an isotropic to a lamellar phase.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1619-1625 
    ISSN: 0392-6737
    Keywords: Brillouin and Rayleigh scattering ; Ultrasonic relaxation processes in gases, liquids, and solids ; Acoustical properties of liquids ; Order-disorder and statistical mechanics of model systems ; Conference proceedings
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    Topics: Physics
    Notes: Summary A water/butoxyethanol solution has been investigated by means of several techniques (viscosity, ultrasound and hypersound) as a function of concentration and temperature. The butoxyethanol molar fraction range investigated was situated between 0.015 and 0.09, while temperature ranged between −10°C and +45°C. We observed the building-up of large structures lowering the temperature, in particular below +15°C. All data coming from different techniques seem to confirm such a picture and indicate that this aggregation phenomenon is a percolation-like process.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1285-1293 
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    Keywords: Localized single-particle electronic states (excluding impurities) ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Conference proceedings
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    Topics: Physics
    Notes: Summary We discuss some specific properties of self-organized InAs-GaAs quantum dots grown by Molecular Beam Epitaxy. We report on the optical spectra obtained under resonant excitation, where both a Raman contribution and a resonant emission of the dots are evidenced. We show that the emission of single dots can easily be isolated by processing measas on samples with a graded indium content, and finally discuss the temporal and temperature behaviour of the emission.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1315-1322 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Time resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Conference proceedings
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    Topics: Physics
    Notes: Summary The concept of coherence due to optical excitation of an ensemble of two-level atoms is relatively simple and well established. For the laser-excited electron-hole pairs or excitions in a semiconductor, the Pauli exclusion and the Coulomb interaction lead to the necessary evolution of the theory of coherence to incorporate the spin degree of freedom, the filling of phase space, the excitons and particularly, their correlation. We have developed two theoretical approaches: one using the Feynman diagrams and one in terms of a force-force correlation function. The former gives us a pictorial way to describe the optical processes and the latter gives us a prescription for computation beyond the perturbative approach. In this paper, we will attempt to use simple physical pictures based on our theory to describe the time development of the coherence, the role of correlation, the dependence on the polarization of light, and the nonlinear time-resolved optical experiments which measure the coherence.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1401-1405 
    ISSN: 0392-6737
    Keywords: Electromagnetic wave propagation ; radiowave propagation ; Developments in band-structure calculation techniques ; Other topics in optical properties of condensed matter and other interactions of matter with particles and radiations ; Other topics in electromagnetic technology ; Conference proceedings
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    Topics: Physics
    Notes: Summary Recently, periodic dielectric structures have been proposed to inhibit spontaneous emission in semiconductors. From this suggestion, the new concepts of photonic band gap and photonic crystal have been developed. Zero-threshold lasers, wave guides and polarizers are promising applications. A new class of twodimensional periodic structures with hexagonal symmetry is investigated in order to obtain photonic band gap materials. This set has the hexagonal symmetry and contains, in particular, several structures previously discussed. The photonic band gap structure is related to the basic properties of the materials and some features of the opening of the gaps are explained. By varying the crystal pattern, we show how band gaps common toE andH polarizations appear for a new design of two-dimensional periodic dielectric structures. The dependence of the widths of these gaps on the filling patterns is studied and potential application for the creation of photonic crystals in the optical domain is discussed.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1429-1433 
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    Keywords: Excitons and related phenomena (including electron-hole drops) ; Localised single-particle electronic states (excluding impurities) ; Photoluminescence ; II–VI semiconductors ; Conference proceedings
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    Topics: Physics
    Notes: Summary Time-resolved photoluminescence experiments in a wide carrier density range up to and beyond the threshold for stimulated emission in ZnSe/ZnS x Se1−x superlattices are presented. Different localisation mechanisms of free excitons are identified giving rise to different radiative recombination channels. At high carrier density localised energy states merge into miniband that allows for exciton-exciton interaction, such as screening of the free-exciton absorption and inelastic exciton-exciton scattering.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1407-1412 
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    Keywords: Excitons and related phenomena (including electron-hole drops) ; Surface and interface electron states ; Conference proceedings
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    Notes: Summary We have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser arises from the recombination of the optically forbidden A-exciton. Radiative recombination occurs via a phonon-assisted virtual transition to theB-exciton state. The electron-hole exchange energy obtained from the experimental results strongly increases with decreasing nanocrystal size as expected from the increasing overlap of the electron and hole wave functions. The size dependence is in good agreement with the theoretical predictions.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1423-1427 
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    Keywords: Excitons and related phenomena (including electron-hole drops) ; Surface and interface electron states ; Conference proceedings
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    Notes: Summary Normalized reflection spectra in GaInAs/GaAs quantum wells are shown for two sets of samples with different alloy concentration (x=9% and 18.5%) and well thickness ranging from 1.5 nm to 25 nm. All samples were grown on (001) GaAs surface by Molecular Beam Epitaxy and characterized by RHEED and X-ray reflection diffraction. Exciton envelope function in effective mass approximation and optical response in polaritonic schema are computed. Normalized reflection spectroscopy has shown itself to be a well suited technique in order to study structural and electronic properties of confined quantum structures.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1447-1452 
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    Keywords: Excitons and related phenomena (including electron-hole drops) ; Optical properties of thin films, surfaces, and layer structures (superlattices, heterojunctions, and multilayers) ; Conference proceedings
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    Notes: Summary We investigate the absorption spectrum and the distribution of radiative decay times for two-dimensional excitons in semiconductor quantum wells, as affected by an adiabatic white-noise random potential. Such a potential could result from atomic-scale fluctuations in the composition of an alloy semiconductor, or in quantum well thickness. We find in general that the shortest radiative decay time is directly proportional to the inhomogeneous width of the exciton line in absorption, itself proportional to the correlation parameter for the random potential.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1473-1479 
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    Keywords: Surface and interface electron states ; Excitons and related phenomena (including electron-hole drops) ; Polaritons (including photon-phonon and photon-magnon interactions) ; Conference proceedings
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    Notes: Summary We report the evidence of exciton-polariton centre-of-mass quantization in II–VI quantum wells consisting of ternary alloy. Absorption and photoluminescence studies on Zn0.89Cd0.11Se/ZnSe quantum wells, covering the intermediate region between two- and three-dimensional exciton behaviour, exhibit distinct optical resonances in the high-energy tail of the free-exciton structure. We reproduce the well width dependence of the centre-of-mass states by modeling the optical response of the QWs with a thin-slab variational wave function, which takes into account non-adiabatic terms.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1499-1503 
    ISSN: 0392-6737
    Keywords: Optical properties of thin films, surfaces, and thin layer structures (including superlattices, heterostructures, and intercalation compounds) ; Conference proceedings
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    Notes: Summary We have studied modulation spectra related to the intersubband transitions at 300 K and 20 K from an In GaAs/GaAs-strained asymmetric triangular quantum well (ATQW) heterostructures fabricated by molecular beam epitaxy using the digital alloy compositional grading (DACG) method. A careful analysis of the spectra has led to the identification of various excitonic transitions,mnh(1), between them-th conduction band state to then-th, heavy (light)-hole band state. Comparison of the observed intersubband transitions with a theoretical calculation based on the envelope, function model, including the effects of strain provided a self-consistent verification that the DACG method produced the intended ATQW composition profile.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1519-1526 
    ISSN: 0392-6737
    Keywords: Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; III–V compounds and systems ; III–V semiconductors ; Polarimeters and ellipsometers ; Conference proceedings
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    Notes: Summary Luminescence investigations on MBE-grown AlAs/AlGaAs microcavities reveal position-dependent spectra from embedded GaAs quantum wells and from the reflector material. Because of their uniform energetic shifts they should be connected with the spatial distribution of strain. The spectra themselves are related to the radiative recombination of confinement-shifted free-electron-hole pairs, free and bound excitons and to the intrinsic luminescence of AlGaAs. Ellipsometric investigations in the range of the resonator stop band around 1.5 eV accentuate the one-dimensional behaviour as a 1D singularity within the effective joint density of states. The resonator mode and the stop band are dependent on the angle of incidence. The effective dielectric function is metal-like with the maximum imaginary part above 103,i.e. as it is typical for a pure metal in the FIR. Analogous investigations between 2.5 and 3.5 eV determined the critical pointsE 0 andE 1 of the energy band structure of AlAs and AlGaAs, respectively.
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    Keywords: Studies of specific magnetic materials ; Optical properties of thin films surfaces and layer structures (superlattices, heterojunctions and multilayers) ; Excitons and related phenomena (including electron-hole drops) ; Conference proceedings
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    Topics: Physics
    Notes: Summary Two qualitatively different e-h systems have been investigated in undoped CdTe/Cd0.88Mn0.12Te and Cd0.97Mn0.03 Te/Cd0.75Mn0.25 Te QWs at 2 K and magnetic fields up to 14 T, namely a strongly spin-depolarized system in the CdTe QW and a highly spin-polarized one in the Cd0.97Mn0.03 Te QW. The interaction of the spin-aligned excitons is repulsive. In contrast, in the spectra of a dense spin-depolarized excitonic system an additional well pronounced emission lineM was observed at approximately 5 meV below the excitonic line. This line is assigned to the emission of molecules consisting of excitons with opposite spins of both electrons and holes.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1573-1578 
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    Keywords: Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Excitons and related phenomena (including electron-hole drops) ; Conference proceedings
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    Notes: Summary We report on the observation of an electric-field-induced electric-dipole moment of biexcitonic molecules in a GaAs/Al0.3Ga0.7As superlattice. The macroscopic oscillating electric field, associated with the microscopic wave packet dipole moments, is monitored by time-resolved transmittive electro-optic sampling. The field dependence of the biexcitonic binding energy is detected by transient four-wave mixing. From these experiments, an ultrafast non-resonant excitation mechanism of biexcitons is inferred.
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    Keywords: Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Photoluminescence ; II–VI compounds and other chalcogenides ; Conference proceedings
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    Notes: Summary Resonant photoluminescence and excitation spectroscopy and time-resolved photoluminescence are performed on a 50 Å thick ZnSe/(Zn, Mn) Se strained quantum well. From these experiments, the existence of a relaxation from a type-II heavy-hole bound interface exciton to a type-I light-hole exciton is demonstrated. The time transfer from the heavy-hole exciton to the light-hole exciton is found to be of the same order of magnitude as the recombination time of the heavy-hole exciton. This transfer is enhanced when temperature increases.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1255-1264 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Polaritons (including photon-photon and photon-magnon interactions) ; Conference proceedings
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    Topics: Physics
    Notes: Summary We develop the bipolariton concept for a quasi-2D excitonic molecule (m) in quantum wells (QWLs). The bipolariton wave equation, which includes both the exciton-exciton (x-x) attraction and thex-photon (γ) coupling, is analysed. In the 2D bipolariton model, a quasi-2Dm exists in QWLs mainly as two quasi-boundsurface polaritons and, consequently, decays mainly into two outgoing surface polaritons. In quasi-2D GaAs QWLs the polariton renormalizations give rise to a «Mexican hat» structure in them dispersion at small momenta together with a considerable increase of both them effective binding energy ɛ m and the inversem radiative lifetimeГ m . A van Hove singularity in the joint density of polariton states is responsible for this effect.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1657-1662 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Polaritons (including photon-phonon and photon-magnon interactions) ; Dynamics of nonlinear optical systems ; optical instabilities, optical chaos, and optical spatio-temporal dynamics ; Conference proceedings
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    Notes: Summary The linewidth of the microcavity exciton polariton is studied by onresonant and off-resonant photoluminescences. The two main contributions to the linewidth are caused by radiative loss and phonon scattering. The oscillator strength is found to be unmodified by the exciton-phonon coupling. Comparison is made between theory and experiment.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1693-1697 
    ISSN: 0392-6737
    Keywords: High-field and nonlinear effects ; Theory of electronic transport ; scattering mechanism ; Ultrafast optical techniques ; Optical properties of thin films, surfaces and layer structures (superlattices, heterojunctions, and multilayers) ; Conference proceedings
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    Notes: Summary The effective dimensionality of excitons can be drastically changed by applying an alternating electric field. On the basis of a full three-dimensional description of both coherent and incoherent phenomena in anisotropic structures it is found that appropriate applied oscillating fields change the exciton wave function from anisotropic three dimensional to basically two dimensional. This effective-dimension change is caused by dynamic localization which leads to an increase of the exciton binding energy and of the corresponding oscillator strength.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1717-1722 
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    Keywords: Excitons and related phenomena (including electron-hole drops) ; Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Conference proceedings
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    Topics: Physics
    Notes: Summary The coupled dynamics of free carriers and excitons after non-resonant excitation with short laser pulses is investigated by means of an Ensemble Monte Carlo method which includes phonon-assisted formation and dissociation of free excitons. Simulations for bulk GaAs reveal that excitonic binding under participation of LO phonons is a very fast and effective process. The temporal rise of exciton luminescence depends on the particular excitation conditions and is determined by all interaction mechanisms in the system.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1743-1746 
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    Keywords: Wave propagation transmission and absorption ; Optical properties of thin films, surfaces, and layer structures (superlattices, heterojunctions, and multilayers) ; Polaritons (including photon-phonon and photon-magnon interactions) ; Conference proceedings
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    Notes: Summary An analysis of the optical properties of an array of quantum wires of cylindrical shape is presented. Effects of the electromagnetic coupling between the wires are found to be given by additional energy shifts and modified oscillator strengths of the reflectivity peaks with respect to the single-wire exciton polariton ones previously calculated. Numerical values for realistic structures show the importance of these effects. Changes of the lifetimes of the single-wire excitations produced by this electromagnetic coupling are also put into evidence. In particular, the short lifetimes of the two-dimensional exciton-polariton are recovered when the interwire distanceD is smaller than the wavelength λ of light. Stationary, or non-radiative electromagnetic modes are also expected in this close-packed case. These radiative and non-radiative modes therefore show some analogies with those found in two-dimensional systems such as quantum wells.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1747-1751 
    ISSN: 0392-6737
    Keywords: Polaritons (including photon-phonon and photon-magnon interactions) ; Excitons and related phenomena (including electron-hole drops) ; Quantum optics ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Conference proceedings
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    Notes: Summary A great interest in the study of exciton-photon coupling in resonant optical media such as semiconductor microcavities is currently developing both in experiments and in theory. We investigate the spectral response of a new semiconductor quantum system in which the cavity material itself is the active medium. If the Fabry-Perot quasi-mode is carefully tuned on the excitonic transition of the cavity bulk material, strong exciton-photon coupling takes place and produces new features such as a Rabi-like splitting as large as that observed in quantum well microcavities and comparable optical absorption. In addition, the polaritonic spatial dispersion and the quantization of the centre-of-mass motion introduce remarkable fine structures which are absent in the quantum well case. We demonstrate the above effects from a spectroscopic analysis of GaAs cavities, using standard reflectance and photoluminescence techniques. The experimental results are further clarified with theoretical calculations performed with an adapted transfer-matrix approach. The case of semi-cavities is finally considered and a new way to control the exciton-photon interaction in our system is achieved tailoring the photonic wave function with a change in the reflectivity phase of the mirrors.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1775-1779 
    ISSN: 0392-6737
    Keywords: Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Dielectric, piezoelectric, and ferroelectric materials ; Electrooptical effects ; Conference proceedings
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    Topics: Physics
    Notes: Summary We describe an experimental and theoretical investigation of Wannier-Stark ladders in the optical spectra of (111)B InGaAs/GaAs piezoelectric superlattices. A unique feature of these structures is that an external electric field can be used to produce a flat superlattice—that is zero overall potential drop per period—in which there are large, opposing fields in the well and barrier. These fields cause a spatial separation of the electron and hole wave functions in the axial direction, leading to Stark ladderss which are intermediate between Type I (electrons and holes together) and Type II (electrons and holes separated by half a period) in character.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1801-1805 
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    Keywords: Excitons and related phenomena (including electron-hole drops) ; Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Electron states in low-dimensional structures (including quantum wells ; superlattices ; layer structures ; and intercalation compounds ; Conference proceedings
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    Notes: Summary Disorder-induced Rayleigh scattering of excitons in quantum wells is investigated theoretically. For classical excitions (long-range spatial correlation), the disorder average can be performed exactly. For the full problem, a kinetic equation with «weak memory» for the excition distribution is derived which coincides with the classical result in the appropriate limit. The theoretical results are compared with recent experimental data on femtosecond time-resolved luminescence explaining well the finite rise time and the heavy-light-hole beating.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 733-741 
    ISSN: 0392-6737
    Keywords: General, theoretical, and mathematical biophysics (including logic of biosystems, quantum biology, and relevant aspects of thermodynamics, information theory, cybernetics, and bionics) ; Conference proceedings
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    Notes: Summary Recent experiments show that the neural codes at work in a wide range of creatures share some common features. At first sight, these observations seem unrelated. However, we show that all of these features of the code arise naturally in a simple threshold crossing model when we choose the threshold to maximize the transmitted information. This maximization process requires neural adaptation to not only the d.c. signal level, as in conventional light and dark adaptation (for example), but also to the statistical structure of the signal and noise distributions. Interestingly, if we fix the threshold level, we can observe a peak in the transmitted information at a finite value of the input signal-to-noise ratio. However, when we allow the threshold to adapt to the statistical structure of the signal and noise, the transmitted information is always monotonically increasing with increasing input signal-to-noise ratio.
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  • 62
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    Keywords: Optical bistability, multistability, and switching ; optical computers ; Fluctuation phenomena, random processes and Brownian motion ; Optical resonantors ; Probability theory, stochastic processes and statistics ; Conference proceedings
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    Notes: Summary Statistical distributions, power spectra and susceptibilities of an all-optical bistable system subject to noise are considered. In the presence of an additional small periodic signal, a stochastic-resonance phenomenon for additive noise is found to occur, and has been investigated theoretically and experimentally. A new form of optical heterodyning related to stochastic resonance, in which two high-frequency signals (an input signal and a reference one) are applied to an all-optical bistable system, is reported. A noise-induced enhancement of the heterodyne signal has been investigated theoretically, by means of analogue electronic simulation and in experiments in which a optical system was driven by two modulated laser beams at different wavelengths.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 725-731 
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    Keywords: General, theoretical, and mathematical biophysics (including logic of biosystems, quantum biology, and relevant aspects of thermodynamics, information theory, cybernetics, and bionics) ; Conference proceedings
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    Notes: Summary The effect of a weak periodic perturbation on the firing activity of two different types of neurons is investigated. Post-stimulus and interspike interval histograms are built up from the times of occurrence of the action potentials and a comparative analysis is performed between the two preparations. A discussion of how the leaky integrate-and-fire model can mimic the experimental data is included.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 653-660 
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    Keywords: Chaos ; Theory and models of chaotic systems ; Conference proceedings
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    Notes: Summary We discuss the complexity in dynamical systems with a random perturbation by considering the rateK of divergence of nearby orbits evolving under two different noise realization. This quantity has a clear meaning in the context of the information theory and it has physical relevance for the analysis of experimental data. Our definition of complexity becomes crucial for strongly intermittent systems whereK is very different from the Lyapunov exponent.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 685-697 
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    Keywords: Active transport processes ; Fluctuation phenomena, random processes, and Brownian motion ; Conference proceedings
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    Notes: Summary Ratchets are anisotropic periodic potentials. Particles subject to ratchet forces and simultaneously to thermal and nonthermal fluctuations can rectify the nonequilibrium noise, thereby extracting energy from incoherent and otherwise symmetric random forces. In this paper I describe some simple models which illustrate this phenomenon, highlighting two particular mechanisms leading to fluctuation-induced current.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 709-723 
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    Keywords: Nonlinear theories (including bifurcation theory) ; Fluctuation phenomena, random processes, and Brownian motion ; Theory and models of chaotic systems ; Computational modeling: simulation ; Conference proceedings
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    Notes: Summary The statistical properties of non-linear dynamical systems are studied using the master equation approach. Detailed analysis of a model system undergoing a Hopf bifurcation reveals a non-trivial interference between macroscopic behaviour and microscopic dynamics in spatially extended systems. The effect of intrinsic fluctuations in the regime of homogeneous deterministic chaos is also analysed and the robustness of the macroscopic description is confirmed.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 855-861 
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    Keywords: Chaos ; Theory and models of chaotic systems ; Instability and shear flows ; Conference proceedings
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    Notes: Summary Analysis is presented of a system, derived previously as a low-order model for the shear instability of convection, whose dynamics are dramatically simplified by tiny amounts of additive noise. The dynamics divide naturally into two phases. In the slower phase, trajectories are close to an invariant manifold; this allows small random disturbances to exert a controlling influence. A map is derived which provides an accurate description of the trajectories.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 883-891 
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    Keywords: General, theoretical, and mathematical biophysics (including logic of biosystems, quantum biology, and relevant aspects of thermodynamics, information theory, cybernetics, and bionics) ; Conference proceedings
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    Notes: Summary The contribution of chemical steps in trasduction chains to the output noise of a receptor cell (as opposed to the noise inherent in the stimulus,e.g., a dim light flash) has been evaluated both analytically and by a computer simulation study on a simple model, using a Monte Carlo technique. The noise deriving from chemical reactions (chemical shot noise, CSN) is shown to add to the source noise thereby increasing significantly the output variation coefficient. It is proposed that this noise contribution should be taken into account in interpreting the sensitivity of a sensory system and in particular the dose-response curves at low levels of the stimulus.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 941-947 
    ISSN: 0392-6737
    Keywords: Physiological optics: vision ; General, theoretical, and mathematical biophysics (including logic of biosystems, quantum biology, and relevant aspects of thermodynamics, information theory, cybernetics, and bionics) ; Conference proceedings
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    Topics: Physics
    Notes: Summary Both bottom-up sensory information and top-down influences contribute to the perception processes. We studied the perceptual alternations of a multistable ambiguous pattern. We observed that it is possible to interfere on the process of the perception alternance by means of subliminal visual stimuli, which either contrast or second the previous perception. We investigated also the effect of the top-down volitional factor on the perceptual alternation. By using a combination of such top-down factor and bottom-up stimulation, we ascertained that a non-linear type of interaction occurs between the two above factors.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1213-1223 
    ISSN: 0392-6737
    Keywords: Optoelectronic devices ; Other solid inorganic materials ; Electron states in low-dimensional structures (superlattices, quantum well-structures and multilayers) ; Quantum optics ; Conference proceedings
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    Topics: Physics
    Notes: Summary The properties and preliminar applications of porous-silicon (p-Si) microcavities are here reported. These structures are based on a planar resonator formed by two narrow-band high-reflectance distributed Bragg reflectors separated by a thin active optical layer, all of which are made of p-Si layers. The accurate control of the electrochemical dissolution of Si lets us realize p-Si multilayers with the desired refractive indices sequence. Large improvements of the emission properties of p-Si microcavities with respect to standard p-Si samples are observed: 1) increased emission intensity, 2) spectral narrowing of the emission band, and 3) high directionality in the emission pattern. Resonant-cavity light-emitting diodes with higher efficiencies and stabilities with respect to standard p-Si/metal devices are demonstrated. Reflectivity changes due to absorption saturation have been observed.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1121-1129 
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    Keywords: Optoelectronic devices ; Conference proceedings
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    Topics: Physics
    Notes: Summary We present a theoretical study of two infinite wires of Si with a different lateral size. The analysis is based on the linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA). We consider free, partially and totally H-covered [001] Si quantum wires with rectangular cross-section. The results of this investigation prove the quantum wire nature of porous Si and interpret many of its physical features. In particular we show thata) as expected quantum confinement originates the opening of the LDA gap;b) the gap opening effect is asymmetric: 1/3 of the widening is in the valence band, while 2/3 in the conduction band;c) the near band gap states originate from Si atoms located at the center of the wire;d) the confinement is enhanced in the case of free surfaces;e) the imaginary part of the dielectric function shows a low-energy side structure strongly anisotropic, identified as responsible of the luminescence transition;f) the presence of dangling bonds destroys the luminescence properties;g) in spite of featurec), all Si atoms are collectively involved in the luminescence transition;h) the shift detected by the Si L2, 3VV Auger signal is due to H-interaction effect and is not a measure of the quantum confinement effect;i) the Si atoms probed by the Si L2, 3VV Auger are bonded with H and H2.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1131-1148 
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    Keywords: Optoelectronic devices ; Conference proceedings
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    Notes: Summary In this work we demonstrate that efficient light emission at 1.54 μm can be achieved when Er ions are incorporated into crystalline Si or in heavily oxygen-doped amorphous and polycrystalline Si films (SIPOS). We have found that temperature quenching of photo- and electroluminescence, which is the major limitation towards the achievement of room temperature luminescence, can be strongly reduced by codoping these films with oxygen. This impurity is already present in as-prepared SIPOS and it is introduced by ion-implantation in crystalline Si. Er luminescence is obtained under both optical and electrical excitation and we demonstrate that excitation occurs through a carrier-mediated process. Electrical excitation is obtained by incorporating Er in properly designed device structures. It is found that this excitation can occur both through the recombination of hole-electron pairs and through impact excitation of the Er ions by hot electrons. These two mechanisms have different efficiencies and impact excitation is shown to prevail at room temperature. These data are presented and possible future developments are discussed.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1111-1119 
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    Keywords: Optoelectronic devices ; Conference proceedings
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    Notes: Summary The striking optical properties of porous silicon (PS) show a twofold aspect typical of an ordered and a disordered material, respectively. Raman, electron microscopy, and resonant photoluminescence studies indicate that the light emission originates from crystalline regions. On the contrary, several features, like the non-exponential decay of photoluminescence (PL), the broad emission spectrum, the photoluminescence fatigue under light exposure etc. are typical of a disordered material and reminiscent of similar effects founde.g. in amorphous semiconductors. These twoapparently conflicting aspects have for a long time hindered the understanding of the basic light emission mechanism. In this paper we report new optical data showing that disorder in porous silicon leads to strong carrier localisation. Light emission in PS is suggested to occur through transitions involving localized states.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1159-1165 
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    Keywords: Optoelectronic devices ; Conference proceedings
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    Topics: Physics
    Notes: Summary Porous silicon is an attractive material for silicon optoelectronics. The great advantage of porous silicon lies on the simple way of production which makes silicon nanostructures easily available. After several papers have been published on this topic, we are able to identify some disadvantages connected to the porous nature of the material and to the method of fabrication. Other dry processes can be used to produce Si nanostructures. In this paper we present a method fully compatible with the standard semiconductor technology. The optical and structural properties of the nanocrystalline films so far obtained are presented, together with some promising results indicating good electrical properties.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1167-1177 
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    Keywords: Optoelectronic devices ; Conference proceedings
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    Notes: Summary We report room temperature time-resolved photoluminescence (PL) and temperature dependence of continuous wave (cw) PL studies of high fluence (from 3·1016 to 3·1017 cm−2) Si+-implanted thermal SiO2 layers after annealing at high temperature (T=1000°C). Such measurements were related to TEM analysis of samples. Nancocrystals were observed at TEM only a samples implanted at higher fluence. In these samples a near infrared PL signal peaked at approximately 1.5 eV with decay time of about 100 μs is present. Besides, in all samples a light emission is present in the green region of the spectrum. The intensity of the emission shows large variations with ion fluence, and is characterized by 0.4, 2 and 7 ns decay times.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 337-340 
    ISSN: 0392-6737
    Keywords: Atoms and molecules ; Magnetic ordered materials: other intrinsic properties ; Magnetomechanical and magnetoelectric effects, magnetostriction ; Mössbauer effect: other γ-ray spectroscopy ; Conference proceedings
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    Topics: Physics
    Notes: Summary The high-energy heavy-ion irradiation of magnetic materials induces not only amorphous matter in the ion tracks but also improvement or even creation of new magnetic properties in a stressed shell surrounding the track core. Two examples are presented in this paper: magnetization creation in the spinel ZnFe2O4 after Kr, Xe and Pb irradiations and trapping of magnetic domains in another spinel, the magnetite Fe3O4, after lead irradiation.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 359-363 
    ISSN: 0392-6737
    Keywords: X- and γ-rays instruments and techniques ; γ-rays ; Mössbauer effect ; other γ-ray spectroscopy ; Conference proceedings
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    Topics: Physics
    Notes: Summary In the present work we report an investigation on cultures of the human osteosarcoma cell line «MG-63» and the human osteoblastic cell line «HOBIT». We have examined the combined effects of hematin and 14.4keV gamma-radiation from a Mössbauer source. Preliminary results seem to confirm the trend already observed for the bone marrow system. Different degrees of growth inhibition were observed when hematin alone and hematin plus gamma-rays were administered to the cultures. It was previously found that hematin has no toxic effects on normal cells up to a concentration of 10−4 M, but the same concentration produces various degrees of inhibition on cultures of tumor cells. While no significant effect could be attributed to irradiation alone, hematin plus irradiations show a larger inhibition than that expected for pure additive effects.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 381-384 
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    Keywords: Mössbauer effect ; other γ-ray spectroscopy ; Phonons and vibrations in crystal lattices ; Conference proceedings
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    Notes: Summary The phonon energy spectra of a polycrystalline α-Fe foil were observed at 150 K and 300 K by using the nuclear resonant scattering of synchrotron radiation. In each spectrum, inelastic scattering was observed at both sides of the elastic peak. It was found that the ratio of the elastic-scattering component and the asymmetry of the intensity of the side bands observed at 150 K are larger than those observed at 300 K, respectively. The observed temperature-dependent spectra are in good agreement with the spectra calculated from the phonon energy distribution function. One of the advantageous features of this method is that the excitation of only a specific element is possible. Our results show that this method is applicable to the study of lattice dynamics and opens a new field of the nuclear resonant scattering spectroscopy.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 163-180 
    ISSN: 0392-6737
    Keywords: Fine particle systems ; PACS 76.80 ; Mössbauer effect ; other γ-ray spectroscopy ; Conference proceedings
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    Topics: Physics
    Notes: Summary Mössbauer spectroscopy is well known to be a very sensitive technique for studying small magnetic particles: small variations in the dynamic properties often manifest themselves as rather large changes in the spectra. However, anay detailed investigation is limited by the complexity of the phenomena and also by the line shape problem. As a result, variations in Mössbauer features between samples can always be interpreted in several ways. Utilization of other techniques to support the conclusions in indispensable. This will be emphasized in the case of γ-Fe2O3 nanoparticles, the dynamic properties of which have been studied over a very wide time range using measurements of the zero-field-cooled magnetization, thermoremanent magnetization, and a.c. susceptibility at various frequencies, in addition to Mössbauer spectroscopy. After a survey of the basic laws of superparamagnetic relaxation and the effects of interparticle interactions, this paper will show how the approximations usually made for interpreting Mössbauer data can lead to misinterpretation of the phenomena.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 221-225 
    ISSN: 0392-6737
    Keywords: Radiolysis, dissociation and ionization by X-ray, γ-ray and particle radiation ; Conference proceedings
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    Topics: Physics
    Notes: Summary The final product of the gamma-radiolysis of tetralithium iron (III) trioxalate chloride nonahydrate has been identified by Mössbauer spectroscopy as FeC2O4·2H2O. The radiolytic decomposition proceeds as a first-order process due to the original compound depletion and to the radiolytic stability of the ferrous compound. Chemical calibration of the relative peaks areas of the two iron species indicates that the correspondingf-factors ratio is unaffected by the radiolysis.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 243-246 
    ISSN: 0392-6737
    Keywords: Mösbauer effect ; other γ-ray spectroscopy ; Fe and its alloys ; Conference proceedings
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    Topics: Physics
    Notes: Summary We report on low-temperature (4.2 K)57Fe Mössbauer studies of the ternary alloy system Fe(Sb1−x Te x )2 for 0.5≤x≤1.0. The Mössbauer spectra are electric-quadrupole doublets for all the specimens, indicating a non-magnetic character. The quadrupole splitting decreases with the increase in tellurium concentration. FeTe2 has also been studied in external magnetic fields up to 6T. These experiments show that the electric-field gradient in this alloy is negative and has an asymmetry parameter of η ≈0.7.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 227-230 
    ISSN: 0392-6737
    Keywords: Mössbauer spectra ; Mössbauer effect ; other γ-ray spectroscopy ; Crystalline state (including molecular motions in solids) ; Conference proceedings
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    Topics: Physics
    Notes: Summary C60 (Buckminsterfullerene) is isolated from organic solvents above 260 K as an f.c.c. solid with complete orientational disorder. The rotational dynamics are diffuse and isotropic, with a short (12 ps) correlation time at 300 K. Below 260 K, the solid forms in a simple cubic lattice withPa3 symmetry. The librational motion occurs by activated jumps with a correlation time of about 60 ns at 200 K. To gain further insight into the dynamics of C60, the compound C60Fe(CO)4 (I) (generously made available to us by A. Stephens and M. Green of Oxford University) has been studied by57Fe Mössbauer spectroscopy. As expected, the resonance at 90 K consists of a doublet with an isomer shift (relative to metallic iron) of 0.034(1) mm s−1 and a quadrupole splitting of 1.607(5) mm s−1. There is no significant temperature-dependent, intensity asymmetry of the doublet. The temperature dependence of the effect magnitude is well fit by a linear regression over the range 85〈T〈210 K. To elucidate the relationship of these results to the effect of the C60 moiety in I, further lattice dynamical Mössbauer studies have been carried out on (maleic anhydride) Fe(CO)4 (II), one of the few isolatable tetracarbonyl complexes of iron. For (II), the quadrupole splitting is 1.371(5) mm s−1 at 90 K, and the temperature, dependence of the isomer shift is smaller than it is for (I), while the temperature dependence of the recoil-free fraction is significantly larger in (I) than in (II).
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    Keywords: Mössbauer effect ; other γ-ray spectroscopy ; Electrochemical devices ; Crystal symmetry: models, space groups and crystalline systems and classes ; Electron microscopy and other methods ; Conference proceedings
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    Topics: Physics
    Notes: Summary We present here a study carried out on In16Sn4S32 and In16Fe8S32 spinels. Electronic and local environment changes of Sn and Fe are discussed from recorded Mössbauer parameters. In the case of the tin thiospinel, a reduction process from SnIV to SnII is obvious till an amount of 8 inserted Li. For the iron thiospinel a migration of iron atoms from the octahedral to the tetrahedral sites is observed. In the region of higher lithium amounts (10– Li), in both spinels a further environment appears indicating an increase of covalency.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1679-1683 
    ISSN: 0392-6737
    Keywords: Thermodynamic properties ; thermal conductivity ; Methods of crystals growth and purification ; Phase diagrams of other materials ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In this work the thermal conditions required for single-crystal growth in RBa2Cu3O7−x systems (R=Y or rare earths apart from Ce and Tb) have been systematically revised in order to improve the size of crystals obtainable by the flux method. Large single crystals of NdBCO, SmBCO and EuBCO have been actually obtained and characterized by XRD, oxygen content and electrical resistivity.
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  • 85
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1695-1699 
    ISSN: 0392-6737
    Keywords: Bi-based compounds ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A systematic study of yttrium substitution for calcium in the high-temperature superconductor Bi2Sr2CaCu2O8 compound has been performed. The samples of Bi2Sr2Y x Ca1−x Cu2O8 have been prepared by the sintering standard procedure using oxides and carbonates as precursors, weighted to obtain the nominal composition with 0≤x≤1. The real stoichiometric composition of the pellets and uniformity of the samples has been verified by EDS analysis. The grain dimension (≈1 μm) and the good pellet compactness have been established by SEM morphologic observation. By X-ray diffraction analysis the crystalline structure of orthorhombic type has been established. By increasing the yttrium concentration the resistivity dependence on temperature goes from metallic to insulating behaviour. For the superconductive samplesT c decreases for increasing doping percentage. «Thick» films on MgO substrates of Bi2Sr2CaCu2O8 have been obtained by melting-quenching-annealing procedure. These films showed critical temperatures higher than the pellets.
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  • 86
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1723-1728 
    ISSN: 0392-6737
    Keywords: Superconducting materials ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Two procedures of partial melting of bulk NdBa2Cu3O7−δ, in air and in reduced oxygen pressure, were developed. When the treatment was carried out in air, poor superconducting properties were observed. On the contrary, when the treatment was carried inP O 2=1.3 mbar, good properties were obtained, withT conset=93 K and 100% shielding at lower temperatures. Near full density (2.5×2.5×28) mm3 Nd1.07Ba1.89Cu3.04O7−δ bars, with domains extending in the mm size range, were obtained.
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  • 87
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1755-1760 
    ISSN: 0392-6737
    Keywords: La-based compounds ; Tl-based compounds ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Deterioration of the superconducting properties of HTC materials, induced by different substitutions, offers a key towards the understanding of the basic HTC interactions. Similar effects are obtained either by doping of layers of the perovskite not directly involved in the superconducting mechanism, or by direct substitution of the copper itself. However, this apparent similarity is misleading. In YBCO, for instance, in the first case the oxygen content or the substitution of Y with Pr4+ appear to control essentially the carrier density,n, while in the second case the direct substitution of Cu in the bilayers by Zn or Ni affects more extensively the overall electronic structure of the material. A sensitive experimental tool, such as μSR, detects correspondingly different behaviours.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2039-2044 
    ISSN: 0392-6737
    Keywords: Proximity effects, tunneling phenomena, and Josephson effects ; Amorphous, highly disordered, and granular superconductors ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We present a careful study of the behaviour of the below-gap photoresponse in 2D arrays of superconducting junctions. We have included the effects of the temperature, not only through the white-noise current term, but also through theT-dependence of the junction coupling energy. In order to render our model more realistic we also consider the correction due to the self-field generated by the screening currents.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2067-2072 
    ISSN: 0392-6737
    Keywords: Superconducting magnets and magnetic levitation devices ; superconducting cables and conductors ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary This paper reviews and assesses the progress in conductor development across the world in advancing high-temperature superconducting science and technology towards the fabrication of superconducting transmission cables operating at 77 K. Materials, fabrication routes and the feasibility of high-temperature superconducting cables will be discussed.
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  • 90
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2095-2102 
    ISSN: 0392-6737
    Keywords: Proximity effects, tunneling phenomena, and Josephson effects ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The current status of the electronic applications of high-T c Josephson junctions is briefly reviewed. Recent results obtained by the authors on devices employing step-edge junctions are reported. In particular the design of a microwave oscillator based on a parallel array of junctions is discussed and preliminary experimental results are presented.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1941-1946 
    ISSN: 0392-6737
    Keywords: Superconducting materials ; General properties ; Critical currents ; Material effects onT c,x, critical currents (including impurities, ion implantation etc.) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary High-resolution a.c. magnetic-susceptibility measurements have been performed on sintered samples of Mg-doped YBCO superconductors YBa2(Cu1−x Mg x )3O7−δ (δ≤0.08, 0≤x≤0.1). An estimation of the intergranular shielding currentsJ s (T, H d.c.) has been obtained by separating the grains and coupling-junctions contributions to the measured susceptibility. The results show that Mg doping increases the weak links weight and reduces the values ofJ s. AtH d.c.=0,J s (T, 0) follows a power law of the type (1−T/T c)β, with β varying from 2 (undoped sample) to about 5 (x=2.5%). The dependence ofJ s on the applied d.c. magnetic field can be fitted to a Lorentzian lineshape whose width appears to be independent of the substitution level.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2045-2051 
    ISSN: 0392-6737
    Keywords: Proximity effects, tunneling phenomena, and Josephson effects ; Amorphous, highly disordered, and granular superconductors ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In order to describe properly the magnetic status of a 2D superconducting junction array, one has to consider not only the effect of the screening currents but also that of the particular experimental protocol followed to measure the physical quantities of interest like, for example, the magnetization. We show that the value of the lower critical field,f c1, of the junction array depends strongly on the intensity of the screening currents,i.e. on the strength of the junction coupling,E j, and that reliable results can be obtained only by considering the full-inductance matrix. We also show that the magnetic configuration of the vortices may depend on the particular experimental approach followed (static or dynamic) and, even, in some cases, on the initial configuration of the phases.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1987-1991 
    ISSN: 0392-6737
    Keywords: Superconducting films ; Critical currents ; Material effects onT c, κ, critical currents (including impurities, ion implantation etc.) ; Deposition by sputtering ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Epitaxial films of YBa2Cu3O7−x were depositedin situ on LaAlO3 substrates using single-target 90° off-axis sputtering. The films were characterized by magnetization measurements (M vs. T, H), applying the field parallel toc-axis. The observed differences in theT c andJ c values are attributed to the different oxygen content in the superconducting films.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1973-1977 
    ISSN: 0392-6737
    Keywords: Josephson devices ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In this paper we report the in-plane fabrication and the characterization of artificially engineered biepitaxial grain boundaries (GB) obtained by partly interposing a MgO seed layer between a bare or even-buffered substrate and a CeO2 thin layer. The main peculiarity of this technique can be summarized by the capability to locate and engineer a single 45°-tilted GB at any stage of the device preparation. The junctions, realized by patterning the grain boundary occurring in the overhanging YBCO film, show Josephson current modulation in a large temperature range with anI c R n value of about 200 μV at 4.2K, theR n value being constant over the whole superconducting region. Under microwave irradiation, theI–V characteristics display several Shapiro steps while, according to the Resistively Shunted Junction (RSJ) behaviour, the step heights have the typical current biased junction dependence.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1793-1797 
    ISSN: 0392-6737
    Keywords: Response to electromagnetic fields ; nuclear magnetic resonance ; ultrasonic attenuation ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary 139La NQR spectra, μSR precessional frequencies and NQR relaxation ratesW driven by the Cu2+ spin fluctuations are used to study the static magnetic properties and the low-energy spin excitations upon substitution of Zn2+ (S=0) for Cu2+ (S=1/2) in the antiferromagnetic (AF) phases of La2 Cu1−x Zn x O4. It is found that the Neel temperaturesT N decrease withx almost linearly, with initial suppression rate close to 4, while the sublttice magnetization is only slightly affected, close to the behaviour expected for a 2DS=1/2 Heisenberg AF in the framework of dilution model. Also the spin wave spectrum appears slightly affected by Zn doping, with moderate effects possibly related to disorder in the AF interactions and/or to finite size. A novel and remarkable effect is observed forT≤100 K. The relaxation ratesW show large maxima, withx-dependentW max andT max. This phenomenon is attributed to the cooperative freezing of local magnetic moments induced by Zn on Cu orbitals. Information on the characteristic fluctuation frequencies and on the spin-glass state superimposed to the AF matrix are thus obtained.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1821-1826 
    ISSN: 0392-6737
    Keywords: Superconductivity: general properties ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Thec-axis transport of impure layered metals is discussed in the cases of clean and dirty limits. The crossover from the Drude regime to the hopping regime in transverse conductivity is treated qualitatively as the change from anisotropic diffusion to Zener oscillations of electrons. It is shown that the superconducting fluctuations in strongly anisotropic layered superconductors substantially affect the transport coefficients above the transition temperature. In a wide temperature range aboveT c the main effect of these fluctuations is to create a virtual gap in the electron spectrum, causing an increase of transverse electric and heat resistivities and decrease of thermoelectric power. This effect turns out to be specially competitive due to the suppression of paraconductivity along thec-axis direction due to the hopping character of electron motion.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2061-2065 
    ISSN: 0392-6737
    Keywords: Superconducting materials ; Superconducting magnets and magnetic levitation devices ; superconducting cables and conductors ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A Modified-Melt-Powder-Melt-Growth (MMPMG) technique has been developed for the preparation of melt-textured Y-Ba−Cu−O pellets. To test their performances, the samples were characterized with respect to their microstructure, pinning strength and interaction force with permanent magnets. A superconducting magnetic bearing was built and integrated in a levitating flywheel system for energy storage.
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    Keywords: Superconducting magnets and magnetic levitation devices ; superconducting cables and conductors ; Conference proceedings
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    Topics: Physics
    Notes: Summary This paper deals with a comparison of chemical, physical and mechanical characterisation of YBCO samples obtained with Temporary-Liquid-Densification Melt Growth (TLDMG). Ba2YCu3O7-x +50% excess BaY2CuO5 pellets were prepared from different Ba-Y-Cu-O system phases, suitable to produce during the heat treatment temporary liquids at temperatures lower than 1000°C. A mechanical characterisation was performed in zero-field-cooling and field-cooling condition, showing hysteretic behaviour and creep of the levitation force. Measurements of magnetic hysteresis cycles in low fields and a.c. susceptibility were also performed. Some mechanical applications (high-speed electric motor and flywheel energy accumulator) were built and tested. The work now proceeds on two new industrial applications of melt-processed HTSC materials.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 703-709 
    ISSN: 0392-6737
    Keywords: Theory and models of liquid structure ; Conference proceedings
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    Topics: Physics
    Notes: Summary We use the Sticky Electrolyte Model, which we solved before in PY/MSA, to obtain the equation of state of a charge colloid taking into account both sticky and charge contribution.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 727-736 
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Phase equilibria, phase transitions, and critical points of specific substances ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A queous solutions of highly charged flexible polyelectrolytes phase separate in the presence of multivalent counterions. We present a theoretical model which explains this behavior in terms of electrostatic bridging between monomers via the condensed counterions. The «Polymer Reference Interaction Site Model» with the Debye-Hückel closure (equivalent to the «Random Phase Approximation») is sufficient to understand the crucial role of the valency of the counterions, to predict a demixion above a given valency and a redissolution of the precipitate in excess of added salt. The model successfully reproduces the different experimental phase diagrams of polystyrene sulfonate suspensions with different added electrolytes.
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