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  • Articles  (173)
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  • Physics  (173)
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  • Articles  (173)
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  • Springer  (173)
  • American Geophysical Union (AGU)
  • Annual Reviews
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  • 1995-1999  (173)
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  • Physics  (173)
  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1285-1293 
    ISSN: 0392-6737
    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
    Source: Springer Online Journal Archives 1860-2000
    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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  • 2
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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
    Source: Springer Online Journal Archives 1860-2000
    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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  • 3
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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
    Source: Springer Online Journal Archives 1860-2000
    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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  • 4
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1429-1433 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Localised single-particle electronic states (excluding impurities) ; Photoluminescence ; II–VI semiconductors ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    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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  • 5
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1407-1412 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Surface and interface electron states ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 6
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1423-1427 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Surface and interface electron states ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 7
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1447-1452 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Optical properties of thin films, surfaces, and layer structures (superlattices, heterojunctions, and multilayers) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 8
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1473-1479 
    ISSN: 0392-6737
    Keywords: Surface and interface electron states ; Excitons and related phenomena (including electron-hole drops) ; Polaritons (including photon-phonon and photon-magnon interactions) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 9
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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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 10
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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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 11
    ISSN: 0392-6737
    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
    Source: Springer Online Journal Archives 1860-2000
    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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  • 12
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1573-1578 
    ISSN: 0392-6737
    Keywords: Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Excitons and related phenomena (including electron-hole drops) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 13
    ISSN: 0392-6737
    Keywords: Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Photoluminescence ; II–VI compounds and other chalcogenides ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 14
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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
    Source: Springer Online Journal Archives 1860-2000
    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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  • 15
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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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 16
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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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 17
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1717-1722 
    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
    Source: Springer Online Journal Archives 1860-2000
    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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  • 18
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1743-1746 
    ISSN: 0392-6737
    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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 19
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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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 20
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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
    Source: Springer Online Journal Archives 1860-2000
    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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  • 21
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1801-1805 
    ISSN: 0392-6737
    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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 22
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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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 23
    ISSN: 0392-6737
    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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 24
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 725-731 
    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
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 25
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 653-660 
    ISSN: 0392-6737
    Keywords: Chaos ; Theory and models of chaotic systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 26
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 685-697 
    ISSN: 0392-6737
    Keywords: Active transport processes ; Fluctuation phenomena, random processes, and Brownian motion ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 27
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 709-723 
    ISSN: 0392-6737
    Keywords: Nonlinear theories (including bifurcation theory) ; Fluctuation phenomena, random processes, and Brownian motion ; Theory and models of chaotic systems ; Computational modeling: simulation ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 28
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 855-861 
    ISSN: 0392-6737
    Keywords: Chaos ; Theory and models of chaotic systems ; Instability and shear flows ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    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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  • 29
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 883-891 
    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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    Topics: Physics
    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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    Topics: Physics
    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 
    ISSN: 0392-6737
    Keywords: Optoelectronic devices ; Conference proceedings
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    Topics: Physics
    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 
    ISSN: 0392-6737
    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 
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    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 
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    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 
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    Keywords: Mösbauer effect ; other γ-ray spectroscopy ; Fe and its alloys ; Conference proceedings
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    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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    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 17 (1995), S. 1265-1276 
    ISSN: 0392-6737
    Keywords: Polaritons (including photon-phonon and photon-magnon interactions) ; Optical properties of thin films, surfaces, and layer structures (superlattices, heterojunctions, and multilayers) ; Conference proceedings
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    Notes: Summary The transport of radiative and electronic excitations in coherently driven semiconductor quantum wells is analysed. On ultrashort time scales the spatial dynamics is determined by light propagation only, whereas for longer times the propagation of light and electronic excitations may be coupled. The theory is applied to the propagation of electronic excitations in the plane of a single quantum well and to polariton propagation in quantum well wave guides.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1323-1332 
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    Keywords: 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 present an extensive set of absorption and PL measurements on semiconductor microcavities in the strong-coupling regime. We observe strong coupling from 110 K to room temperature. We look at the behaviour of the photoluminescence for different exciton-photon detuning. We discuss this data using two different pictures, the first is a non-interacting (uncoupled) picture of an exciton in a cavity filter. The second model is that of a cavity polariton and this explains the observed dynamics in the strong-coupling regime. We show how the opposing needs of strong coupling and high photoluminescence extraction efficiency can be reconciled.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1343-1347 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Polarons and electron-phonon interactions ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; III-V compounds and systems ; Conference proceedings
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    Notes: Summary We have calculated the binding energies of excitons in quantum well structures based on ionic semiconductors by including the electron-hole interactions with the longitudinal-optical-phonon field. We have taken into account these interactions by using different effective interaction potentials between the electron and the hole as derived by Haken, Aldrich and Bajaj, and Pollman and Buttner. We have calculated the binding energies of excitons in several ionic quantum well structures as functions of well width using these effective potentials following a variational approach. We find that the values of the exciton binding energies calculated using these potentials are always larger than those, obtained using a Coloumb potential screened by static dielectric constant.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1355-1358 
    ISSN: 0392-6737
    Keywords: Time-resolved optical phenomena (including electron-hole drops) ; Optical properties of thin films surfaces, and layer structures (superlattices, heterojunctions and multilayers) ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Conference proceedings
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    Notes: Summary We show that a detailed understanding of the influence of the exciton-exciton interaction on the non-linear excitonic response in a GaAs/AlGaAs superlattice can be obtained by changing the external applied field. In time-resolved four-wave mixing this implies a change from a third-order polarization signal dominated by phase space occupation to a Coulomb interaction-induced signal when the bias is increased at high excitation densities. The experimental data are compared with theoretical results based on the numerical solution of the semiconductor Bloch equations.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1371-1375 
    ISSN: 0392-6737
    Keywords: III–V semiconductors ; III–V compounds and systems ; 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 A photoluminescence study of self-ordered InAs quantum dots grown by molecular beam epitaxy on a GaAs substrate is reported. Short pulses and high excitations have been used in order to observe emission from higher states of the quantum dots. The energy spacing between adjacent transitions seems to be of the order of 40–50 meV for dot diameters around 20 nm. The photoluminescence decay time from the ground state is of the order of 650–700 ps and decreases down to roughly 100 ps for the highest confined states. A cascade-like mechanism for the carrier relaxation in these structures is strongly suggested by the time-resolved data.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1453-1458 
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    Keywords: Electron states in low-dimensional structures (including quantum wells superlattices, layer structures, and intercalation compounds) ; Other semiconductor-to-semiconductor contacts,p-n junctions, and heterojunctions ; Conference proceedings
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    Notes: Summary We consider the intrawell and interwell relaxation of circularly polarized excitons in an asymmetric double-quantum-well structure. For the intrawell case (photocreated light excitons coupled to the detected heavy excitons by an optical-phonon emission) the polarization changes sign after a fast relaxation. For the interwell case (transfer of polarization from the ground heavy exciton of the narrow well towards the ground heavy exciton of the wide well) we observe that the initial polarization is recovered after energy relaxation only if a fast-relaxation path is available which involves only intermediate heavy exciton states.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1493-1498 
    ISSN: 0392-6737
    Keywords: Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; III-V semiconductors ; Conference proceedings
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    Notes: Summary Optical properties of GaAs/Al x Ga1−x As shallow quantum wells have been studied in the 2–200K range, both by c.w. and time-resolved experiments. The results agree with a variational calculation of the excitonic transitions. The LO-phonon-assisted carrier relaxation and a temperature-activated non-radiative channel are evidence.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1509-1512 
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    Keywords: Excitons and related phenomena (including electron-hole drops) ; III-V Compounds and systems ; Conference proceedings
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    Notes: Summary The reflectance spectra of GaN/6H-SiC, films and the absorption spectra of GaN/Al2O3 films were studied and several fundumental parameters of GaN-energy positions of exciton, resonances ω0, damping constants⩾s, longitudinal-transverse splittings ωLT and the oscillator, strengthsf of the exciton states were determined. A blue shift and broadening of the luminescence band were observed in the luminescence spectra of GaN quantum dots. The mean dot radius in the sample was estimated to be about 4.5nm.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1531-1536 
    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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    Notes: Summary The localization dynamics of excitons within growth islands of GaAs/Al x Ga1−x As single quantum wells (SQW) have been investigated by time-resolved photoluminescence spectroscopy (TRPL). Several samples with different substrate misorientation and growth interruption times are compared with regard to the photoluminescence (PL) emission dynamics. For monolayer (ML) islands larger than the exciton radius,i.e. long growth interruption, the time evolution of the different PL peaks reflects the transfer of free excitons between growth islands as well as the localization within a single growth island. The samples with shorter growth interruption reveal a much less pronounced PL splitting due to ML islands. The dynamics appears to be mainly determined by exciton localization to thicker ML islands. The density of localization centres is larger for the sample grown on misoriented substrates.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1555-1559 
    ISSN: 0392-6737
    Keywords: Nonlinear optics ; Optical properties of thin films, surfaces, and thin layer structures (including superlattices, heterostructures, and intercalation compounds) ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Excitons and related phenomena (including electron-hole drops) ; Conference proceedings
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    Notes: Summary In a system comprising an organic layer coupled to a semiconductor layer, if the energy of the Frenkel exciton in the first is close to that of the Wannier exciton in the second, the lowest electronic excitations can be described as hybrid resonances. These new excitons give rise to very pronounced optical non-linearities because they have at the same time large oscillator strengths, typical of Frenkel excitons, and large radii, typical of Wannier excitons. The results of a microscopic calculation of the optical properties of such a system are presented and discussed: compared with the usual semiconductor quantum wells, enhancements of two orders of magnitude both in the size of the susceptibility (linear part) and in its relative change with increasing excitation density (Kerr non-linearity) are predicted.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1579-1583 
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    Keywords: Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Other semiconductor-to-semiconductor contacts,p-n junctions, and heterojunctions ; Conference proceedings
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    Notes: Summary We report on the calculation of electron and hole states in InAs/GaAs quantum dots. The latters are modelled by cones floating on a wetting layer. The calculations include the effect of an electric field applied parallel to the cone axis as well as the evaluation of the electron-hole correlation and exchange (short-range part) energies.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1383-1387 
    ISSN: 0392-6737
    Keywords: III–V semiconductors ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Excitons and related phenomena (including electron-hole drops) ; III–V compounds and systems ; Conference proceedings
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    Notes: Summary We report an experimental study on a large set of InGaAs/GaAs quantum well structures by means of continuous-wave photoluminescence and photoluminescence excitation. The luminescence spectrum at low temperature systematically exhibits a doublet structure whose lineshape is very sensitive to the excitation energy. Accordingly, the excitation spectra detected by monitoring the emission at the two different luminescence peaks have very different profiles, with peaks and/or dips which are not directly related to absorption resonances. As a matter of fact, the anomalies disappear when increasing the temperature or when using an optical bias with energy above the GaAs energy gap. In these cases, the excitation profiles recover the behaviour typical of 2D systems.
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  • 57
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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 report here on the study of the optical properties of ZnCdSe−ZnSe quantum structures elaborated by metalorganic chemical vapour deposition (MOCVD). The band structure is experimentally investigated by means of photoluminescence and photoreflectance. We have computed the exciton binding energies for heavy- and light-hole excitons in the context of a self-consistent two-parameter trial function. As a complement, we study the temperature dependence of the photoluminescence intensity under both direct and indirect photoexcitation in graded-index separate confinement heterostructures based on these materials.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1595-1599 
    ISSN: 0392-6737
    Keywords: Optical properties of thin films, surfaces, and thin layer structures (including superlattices, heterostructures, and intercalation compounds) ; III-V semiconductors ; Surface and interface electron states ; Conference proceedings
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    Notes: Summary The optical nonlinearities produced by screening of the piezoelectric field byphotogenerated carriers in a [111]-grown (In, Ga)As/(Al, Ga)As single strained-layer quantum well structure are calculated and compared with those in a [001]-grown structure. Nonlinear absorption and refractive index spectra show that screening of the strain-induced electric field by photo-carriers that have escaped from the [111]-grown quantum well produces optical nonlinearities comparable to those in the more conventional [001]-grown quantum well.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1607-1612 
    ISSN: 0392-6737
    Keywords: Optical properties of thin films, surfaces, and thin layer structures (including superlattices, heterostructures, and intercalation compounds) ; Nonlocalized single-particle electronic states ; Collective effects ; Conference proceedings
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    Topics: Physics
    Notes: Summary High magnetic and electric fields are used to investigate the excitonic properties of strain-balanced InGaAs coupled quantum wells, providing details of their band structure. The bands are strongly non-parabolic and the heavy in-plane hole mass deduced for a lattice-matched structure is shown to be reduced by either tensile or compressive strain. The different band-offsets on changing from InGaAs to AlInAs barriers modify the interwell coupling and result in the observation of Stark ladder transitions and conventional Stark effect behaviour, respectively.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1699-1703 
    ISSN: 0392-6737
    Keywords: Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Localized single-particle electronic states (including impurities) ; III–V compounds and systems ; Photoluminescence ; Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Conference proceedings
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    Notes: Summary We have performed time-integrated and time-resolved photoluminescence experiments on high-quality stressor-induced InGaAs/GaAs quantum dots. The optical spectra of these dot structures exhibit large quantization effects together with remarkably low inhomogeneous broadenings of the respective excitonic transitions. Thus a detailed investigation of the relaxation and recombination dynamics within the distinct electronic dot states becomes feasible. We find that the initial relaxation of optically generated carriers down to the lowest dot states is very efficient. This fast thermalization is ascribed to Coulomb scattering between carriers confined in dot states and carriers located in the higher-energetic quantum well states.
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  • 61
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    Keywords: Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; III–V and II–VI inorganics ; Conference proceedings
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    Notes: Summary Photoluminescence excitation experiments have been performed with a non-intentionally doped GaInAs/AlGaInAs superlattice lattice-matched to an InP substrate. The superlattice is the intrinsic part of a p-i-n diode. We have observed the 1s and 2s heavy-hole exciton states and the Wannier-Stark ladder associated to the light-hole exciton. Three other lines in our spectrum are related to the Wannier-Stark ladder associated to the first excited state of the heavy hole. These transitions are weakly allowed from the built-in voltage existing in the superlattice. Calculations based on Airy functions quantitatively support our interpretation.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1787-1790 
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    Keywords: Excitons and related phenomena (including electron-hole drops) ; II–VI compounds and other chalcogenides ; II–VI semiconductors ; Conference proceedings
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    Notes: Summary We report on the observation of the doubly resonant secondary emission involving 2s and 1s exciton states. The resonance conditions have been realized in CdTe/Cd1-xMnx Te quantum well structures by magnetic-field tuning of the 2s-1s energy spacing to the LO-phonon energy. Theoretical analysis shows the doubly resonant process to be indirect, with hot 1s excitons acting as intermediate states. It is concluded that the resonance width of 2.6 meV gives the 2s-exciton homogeneous linewidth governed by elastic scattering into the hot 1s exciton states.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1797-1799 
    ISSN: 0392-6737
    Keywords: Excitions and related phenomena (including electron-hole drops) ; Electron states in low-dimensional structures (including quantum wells) ; superlattices ; layer structures ; and intercalation compounds ; Optical properties of thin films ; surfaces ; and layer structures ; superlattices ; heterojunctions ; and multilayers ; Conference proceedings
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    Notes: Summary We demonstrate that a hybridization of Frenkel and Wannier-Mott excitons in parallel neighbouring organic and semiconductor quantum wires may occur due to the resonance dipole-dipole interaction between the wires. The hybridization might be especially efficient for excitons with small wave vectors along the wires when the transition dipole moments of excitons are oriented perpendicular to the wires. The hybrid wire excitons may possess both a large radius and a relatively large oscillator strength which could be important in non-linear optics.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 803-810 
    ISSN: 0392-6737
    Keywords: Nonlinear optics ; Fluctuation phenomena, random processes, and Brownian motion ; Conference proceedings
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    Notes: Summary We have investigated, experimentally and theoretically, the occurrence of stochastic resonance (SR) in a laser with saturable absorber: a non-linear optical system presenting optical bistability and self-pulsed regimes for the laser output intensity. In the optical-bistability regime we have obtained an excellent demonstration of SR by studying Fourier spectra and residence time distributions; operating just below the Hopf bifurcation that originates pulsed periodic regime we have obtained experimental evidence of the SR phenomenon. For operation close to the Hopf bifurcation, the theoretical analysis (performed through numerical simulations) has demonstrated that simultaneous presence of noise and periodic forcing generates excursions far from equilibrium; from the locking between these excursions and periodic forcing the SR phenomenon may arise.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 819-825 
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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) ; Fluctuation phenomena, random processes, and Brownian motion ; Conference proceedings
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    Notes: Summary The phenomenon of stochastic resonance is predicted in a monostable system displaying transient bimodality during the evolution. The signal-to-noise ratio as a function of the noise intensity calculated within the rate equation theory with many simplifying assumptions exhibits a promising peak. The numerical simulations are also reported.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 827-834 
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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 We consider the transmission of a periodic signal by noisy threshold devices. A general expression for the input-output characteristic is developed and applied to two particular threshold devices. It is shown that the amplitude of the signal output shows in the subthreshold regime a maximum as a function of the noise strength—the fingerprint of stochastic resonance.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 835-846 
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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) ; Fluctuation phenomena, random processes, and Brownian motion ; Probability theory, stochastic processes, and statistics ; Conference proceedings
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    Notes: Summary The Fitzhugh-Nagumo equations (FHN) provide a simple description of the dynamics of a large class of neurons. We characterize synchronization and stochastic resonance in this model using two complementary points of view: the signal-to-noise ratio (SNR), and the absolute as well as normalized peak heights of the interspike interval histograms (ISIHs). At low stimulus frequencies, multiple firings can occur during one period, while at high frequencies, the refractoriness precludes firing at every cycle. The behaviors of the SNR and ISIHs are thus investigated at low, medium and high frequencies to illustrate special synchronization properties of the FHN system. In particular, the behavior of the SNRvs. noise is found to be similar for forcing amplitudes just below and above that at which a 2:1 deterministic phase-locked firing solution becomes stable. Our results rely on an accurate method of estimation of the power spectrum of the point process formed by the firing times. A theoretical analysis for the shape of the simulated power spectra is also presented.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 699-707 
    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 The purpose of this study was to investigate the sex particularities in human cardiovascular responses to different external influences using the traditional physiological approach and non-traditional methods of dynamical systems theory. Measurements were taken in 21 healthy women and 15 men exposed to noise stress, passive coping, and in 35 women and 25 men exposed to double mental arithmetic stress, active coping. Results showed that, firstly, cardiovascular reactivity in men and women was determined by the type of stress. Thus, in a majority of humans heart rate did not change during noise but increased during mental stress, blood pressure decreased during noise but increased during mental stress. Secondly, results demonstrated the significant sex differences in the basal and stress cardiovascular activity. So, the women showing the greater basal and stress heart rate displayed the hypotensive responses to passive coping more often and the hypertensive responses to active coping less often than men. Thirdly, the changes in normalized entropy of the electrocardiogram signal, reflecting the changes in heart physiological variability, also depended on the sex of subjects and the nature of stress. During noise normalized entropy increased in a majority of women but decreased in a majority of men. Mental stress caused both decreases and increases in normalized entropy in men and women. But the increases in normalized entropy during stress and especially during recovery were greater in women than in men. Normalized entropy was demonstrated to be a more sensitive marker of sex and individual differences in cardiovascular responses to stress than heart rate and blood pressure. Results suggest that the lower blood pressure reactions and the greater increases in normalized entropy,i.e. in physiological heart variability, may partly result in higher cardiovascular stress resistance in women relative to men.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 765-773 
    ISSN: 0392-6737
    Keywords: Muscle contraction, nerve conduction, synaptic transition memorization and other neurophysiological processes (excluding perception processes and speech) ; Biophysical instrumentation and techniques ; Conference proceedings
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    Notes: Summary Two procedures aimed at a synthetic, quantitative characterization of the integrative activity in a small system of nerve cells are proposed, and applications to the buccal ganglion of the molluskAplysia are described. Both methods are based on cross-correlation evaluation. The first method processes intracellular signals simultaneously recorded from pairs of neurones, and reveals and measures the occurrence of shared synaptic input. The second method processes extracellular multiunit signals recorded by two cuff electrodes placed around one nerve, and evaluates the impulse traffic, subdivided according to direction and speed of propagation. The outcome of both methods may be used to represent the state of the system, and may provide a basis to further studies on its dynamics.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1149-1157 
    ISSN: 0392-6737
    Keywords: Optoelectronic devices ; Other solid inorganic materials ; Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Interfaces ; heterostructures ; nanostructures ; Conference proceedings
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    Topics: Physics
    Notes: Summary We report the observation of strong red- and blue-light emission in free-standing porous-silicon samples prepared fromn + substrates at different anodization current densities. The surface morphology of the free-standing samples has been analyzed by means of atomic-force microscopy. Upon excitation with nanosecond pulses at room temperature, both blue and red luminescence bands appear, peaked around 3.2 and 2.0 eV, respectively. An extensive study of the time-resolved behavior of the photoluminescence signal reveals different dynamical features for the two spectral regions. The observed long decay time (several μs) of the red band reflects the predominant effect of non-radiative processes and is consistent with models based on excition diffusion through the interconnected silicon nanocrystals (quantum dots) skeleton. The relatively fast blue-band decay time (≈400 ns) is shown to be related to non-radiative recombination through trap states at the silicon nanocrystal surface.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1179-1186 
    ISSN: 0392-6737
    Keywords: Methods of deposition of films and coatings ; film growth and epitaxy ; Electrochemistry and electrophoresis ; Surface and interface chemistry ; Infrared and Raman spectra and scattering ; Photoluminescence ; Conference proceedings
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    Notes: Summary Porous-silicon layers were prepared by anodic oxidation of mono- and multi-crystalline Si substrates,n + andp-doped, in aqueous HF solutions containing surfactants. The resulting samples show a bright red-orange photoluminescence: PL spectra and uniformity are analysed as a function of surfactants kind and concentration. Moreover, the results of Raman spectroscopy are discussed in terms of current theories.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1225-1232 
    ISSN: 0392-6737
    Keywords: Methods of deposition of films and coatings ; film growth and epitaxy ; Surface treatments ; Optical constants: refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity ; Photoelectric conversion: solar cells and arrays ; Conference proceedings
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    Notes: Summary Porous silicon was formed on multicrystalline Si substrates by stain etching in aqueous HF/HNO3 solutions. In this work optical and electrical properties of the resulting films are discussed as a function of process parameters. Porous-Si films have been shown to be able to reduce surface reflectance to 3% in 350–700 nm wavelength range and their application in anti-reflection coating of photovoltaic solar cells has been demonstrated, obtaining (10×10) cm2 multicrystalline Si solar cells with efficiency approaching 12% under standard AM 1.5 simulated sunlight.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1233-1239 
    ISSN: 0392-6737
    Keywords: Optoelectronic devices ; Conference proceedings
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    Notes: Summary A first structural investigation, carried out by transmission electron microscopy on porous-silicon samples fromp +, 〈111〉-oriented substrates is presented. The samples, which show intense visible room temperature luminescence, are composed by an interconnected network of crystalline nanostructures. Evidences of the pores propagation along the 〈100〉 directions are provided. The optical and morphological characteristics of the investigated samples are found to be much similar to those of samples coming fromp-type, non-degenerate, 〈100〉-oriented substrates rather than those obtained from 〈100〉 substrates with comparable resistivity. This striking effect is explained by invoking different etch-limiting mechanisms during pore formation. Their relative weights are proposed to depend on the crystallographic orientation of the silicon specimen subjected to etching.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 263-268 
    ISSN: 0392-6737
    Keywords: Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) ; Critical-point effects, specific heats short-range order ; Antiferromagnetics ; Conference proceedings
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    Topics: Physics
    Notes: Summary Mössbauer measurements have been made on two kinds of spin-glass (SG) systems: one is the magnetically non-diluted mixed compound Fe0.5Mn0.5TiO3 with the SG-freezing temperatureT SG=21.5K, and the other is the diluted one Fe0.2Mg0.8TiO3 withT SG=6K. We have shown that the temperature variation of the Mössbauer spectrum of Fe0.2Mg0.8TiO3 above and around itsT SG is essentially different from that of Fe0.5Mn0.5TiO3: the former is typical of a cluster-glass and the latter of an ordinary spin-glass. The present work has clearly demonstrated that the Mössbauer spectroscopy is the most useful and unique technique to distinguish a cluster-glass from an ordinary spin-glass.
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    Keywords: Mössbauer effect ; other γ-ray spectroscopy ; Conference proceedings
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    Notes: Summary Sediment cores with different sub-bottom depths (I: 45 cm and II: 700 cm) from the Peru Basin have been investigated. From the depth profile of the relative amount of Fe(II) a redox zone is obtained which correlates with the organic carbon flux into the sediment (core I). Mössbauer parameters suggest that the iron in the sediments is mainly contained in clay minerals and to varying extent also in goethite.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 371-374 
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    Keywords: X-ray ; Mössbauer and other γ-ray spectroscopic analysis methods ; Molecular biophysics ; Conference proceedings
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    Notes: Summary The Rayleigh scattering of Mössbauer radiation has been measured on highly oriented fibres of Na-hyaluronate at different hydration levels. The elastic-and inelastic-scattering intensities, measured as a function of the scattering vectorQ, have provided information on the dynamic structuring of the water molecules to the polysaccharidic chains.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 385-388 
    ISSN: 0392-6737
    Keywords: Mössbauer effect ; other ψ-ray spectroscopy ; Strong field excitation of optical transitions in quantum systems multiphonon processes ; dynamic Stark shift ; Effects of internal magnetic fields ; Conference proceedings
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    Notes: Summary We have studied the time dependence of Mössbauer absorption in a magnetically soft sample of Fe18Ni82 permalloy under the influence of a radio-frequency (r.f.) magnetic field. Absorption was measured as a function of the r.f. field phase at specific gamma-ray energies. We have shown that measuring the time dependence of absorption makes it possible to differentiate between purely magnetic and magnetoacoustic modulation mechanisms even in cases where clear distinction is not visible in conventional energy domain spectra.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 145-161 
    ISSN: 0392-6737
    Keywords: Metal-insulator transitions ; High-pressure and shock-wave effects in solids ; Mössbauer effect ; other γ-ray spectroscopy ; Transition-metal compounds ; Conference proceedings
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    Notes: Summary Most previous studies of magnetism in various compounds under extreme conditions have been conducted over a wide pressure range at room temperature or over a wide range of cryogenic temperatures at pressures below 20 GPa (200 kbar). We present some of the most recent studies of magnetism over an extended range of temperatures and pressures far beyond 20 GPa,i.e. in regions of pressure-temperature (P-T) space where magnetism has been largely unexplored. Recent techniques have permitted investigations of magnetism in selected 3d transition metal compounds in regions ofP-T where physical properties may be drastically modified; related effects have often been seen in selected doping studies at ambient pressures. We present57Fe and129I Mössbauer isotope studies covering the range 300–4 K to sub-megabar pressures in compounds such as Sr2FeO4, LaFeO3 and FeI2, representative of a broad class of 3d transition metal compounds. At ambient pressure the electronic structure of the transition metal atom in these antiferromagnetic insulators extends from 3d 4 to 3d 6 and has a distinct influence on the pressure evolution of their magnetic properties. Mössbauer studies of these compounds are considered in conjunction with available structural and electrical transport data at pressure.
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    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 299-304 
    ISSN: 0392-6737
    Keywords: Layer structures: multilayers, and superlattices (growth structure, and nonelectronic properties) ; Magnetism in interface structures (including layer and superlattice structures) ; Mössbauer effect ; other γ-ray spectroscopy ; Conference proceedings
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    Notes: Summary The synthesis of a new BCT Fe phase was performed in Fe/Ir(100) superlattices grown by MBE. Magnetic properties of57Fe/Ir(100) superlattices with 4 ml Fe and variable Ir thickness (2–30 Å) are investigated by57Fe conversion electron Mössbauer spectroscopy in the 4.2–300 K temperature range. Two spectral components are evidence, related, respectively, to Fe atoms involved in the central part of the iron layers and at the interface between iron and iridium layers. The appearance of a high magnetic hyperfine splitting in the iron BCT structure above a volume threshold of 12 Å3 is evidenced. Marked differences are observed between the mean magnetic properties and the local ones suggesting strong relaxation effects.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1277-1284 
    ISSN: 0392-6737
    Keywords: II–IV semiconductors ; 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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    Notes: Summary Steady-state and picosecond photoluminescence experiments on CdTe/CdZnTe and CdTe/CdMnTe asymmetric double quantum wells are presented. An external magnetic field is used to tune the coupling between the two QWs by allowing resonances to occur between excitonic states. Very efficient tunnelling is found when an exciton from the high-energy QW can transfer either with the emission of an LO phonon or via the resonance with the 2s state of the low-energy QW.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1305-1313 
    ISSN: 0392-6737
    Keywords: Molecular and atomic beam epitaxy ; Localized single-particle electronic states (excluding impurities) ; Electron states in low-dimensional structures (including quantum wells, superlattices layer structures, and intercalation compounds) ; III–V compounds and systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
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    Notes: Summary We report on the intrinsic radiative lifetime of excitons in InP quantum dots embedded in an In0.48Ga0.52 P matrix. This lifetime is about 400 ps as measured by time-resolved photoluminescence with resonant excitation of the ground states. It is independent of excitation density and temperature demonstrating the quantum dot nature of the InP islands and the strong localization of the excitons. Similar experiments on excitons localized by well width fluctuations in a GaAs quantum well reveal distinct differences: weakly localized excitons disappear with increasing density or temperature.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1339-1342 
    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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    Notes: Summary Fast initial decays of both the luminescence intensity and the circular luminescence polarization at high exciton densities, under resonant excitation, are reported for the first time. These fast decays are simultaneously initiated by the increase of the ellipticity of the photogenerating picosecond laser beam. The theory of the mechanism illustrates the driving role of the interexciton exchange interaction.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1367-1370 
    ISSN: 0392-6737
    Keywords: Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Optical properties of thin films surfaces and layer structures (superlattices, heterojunctions and multilayers) ; Conference proceedings
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    Notes: Summary Our interest is centred on the very thin layers consisting of only one or a few monolayers of InAs. The optical transition energies, measured by photoluminescence spectroscopy, are compared with theoretical calculations obtained in envelope function approximation and through an empirical tight-binding method. This comparison yields values for the not well-known valence band offset at the InAs/InP interface, and the luminescence lines observed at different energies could be assigned to layers between one and 13 monolayers thick.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1389-1393 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Surface and interface electron states ; Conference proceedings
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    Notes: Summary We calculated the energy and the wave function of the exciton for i) a V-shaped quantum wire, ii) a T-shaped quantum wire and iii) a quantum wire resulting from strain-induced lateral confinement. The full Luttinger and Bir-Pikus Hamiltonian are taken into account in the three calculations. We used different parameters inside the well and at the barrier. The basis states |3/2m〉 are defined with respect to [110], the direction of the wire in the three kinds of structures, instead of the usual [001] direction. The hole dispersion curve allows one to define an effective mass which is to be introduced in the variational calculation of the exciton binding energy. We show that the hole is confined in a wire only in case i).
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  • 85
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1413-1416 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Surface and interface electron states ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary An opening (antidot) in two-dimensional (2D) electron gas placed in a strong perpendicular magnetic field makes it possible the formation of ring-shaped excitons stemming from the skipping-states of electrons and holes. The binding energy of the ground state of such excitons is logarithmically larger than that of the «bulk» 2D excitons. The possibility of tunnelling electron to hole (or vice versa) around the antidot results in a shift for each of the excitons levels that oscillates with the magnetic field.
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  • 86
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1359-1366 
    ISSN: 0392-6737
    Keywords: Optical transient phenomena (including quantum beats, dephasings and revivals, photon echoes, free induction decay, and optical mutation) ; Excitons and related phenomena (including electron-hole drops) ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Weak localization effects (e.g., quantized states) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The optical properties of multiple quantum wells in the transition from isolated wells to a superlattice are investigated theoretically and experimentally. For superlattices with a miniband width similar to the binding energy of the 2s exciton of the isolated quantum well we find an exciton state energetically between the 1s exciton state and the onset of the miniband absorption. We find this state to be an interwell exciton, with the electron confined to one well and the hole to the neighboring well. The interwell exciton resembles the first Wannier-Stark localized exciton of a biased superlattice. However, the localization in the present case is mediated by the Coulomb interaction of the electron and hole. The state has considerable oscillator strength and is observed experimentally in linear and nonlinear experiments.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1395-1400 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Tunneling ; Other semiconductor-to-semiconductor contacts,p-n junctions, and heterojunctions ; Electroluminescence ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The electroluminescence of p-i-n GaAs double-barrier resonant tunnelling diodes has been investigated for various concentrations of free carriers (either holes or electrons) in the quantum well (QW). For these structures it is possible to change the relative electron or hole density quasi-continuously with applied bias-voltage. New low-temperature excitonic recombinations are identified in the electroluminescence spectra. Continuous variation of the carrier density in the quantum well from a hole-rich into an electron-rich environment leads to the observation of positively charged excitons (X 2 +), neutral excitons (X), negatively charged excitons (X −) and finally an unbound electron-hole recombination. An increased temperature causes the dissociation of the charged excitons in favour of the neutral heavy-hole free excitonX. In high magnetic fields the unbound electron-hole recombination is transferred into an excitonic recombination.
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  • 88
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1417-1422 
    ISSN: 0392-6737
    Keywords: Polaritons (including photon-phonon and photon-magnon interactions) ; Excitons and related phenomena (including electron-hole drops) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The excitonic energies are computed in superlattices from the poles in the imaginary part of the susceptibility, using an appropriate Green's function expression for the coherent amplitude function of the electron-hole pair. Solving the Maxwell equations, the polariton states are obtained and the optical properties are computed. The procedure includes the effects of coherence between the carriers and the electromagnetic field, and it does not require any type of additional boundary conditions. Detailed calculations in GaAs/Ga1−x Al x As superlattices show polaritonic effects in the optical properties, in particular a peculiar fine-structure lineshape of then=1 andn=2 hh and lh excitons is obtained. A comparison with similar calculations in bulk GaAs clearly shows the roles of quantum confinement and of anisotropy in the superlattice.
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  • 89
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1441-1446 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Polaritons (including photon-phonon and photon-magnon interactions) ; Optical properties of thin films, surfaces, and layer structures (superlattices, heterojunctions, and multilayers) ; II–VI compounds and other chalcogenides ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Experimental studies of high-resolution reflectivity and a temperature dependence of the photoluminescence for periodic MQWs of CdTe/CdZnTe with periods equal to a one-half and one-forth of the light wavelength are presented. Energies and radiative lifetimes of the polariton-like excitations in the relevant structures are computed, and linked to experimental observations.
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  • 90
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1465-1471 
    ISSN: 0392-6737
    Keywords: Surface and interface electron states ; Excitons and related phenomena (including electron-hole drops) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The localization of the wave function on the scale length of a single monolayer has been studied by magnetophotoluminescence in GaSb/AlGaSb quantum wells. The studied range of well width includes the direct-indirect transition involvingL-point conduction states and Γ-point valence states induced by quantum size effects. Separate carrier localization dominates at higher field values (B〉2T), whereas the excitonic effects are important only in the low field range. Variational calculations of the excitonic transverse extension provide a quantitative description of the experimental data. The dependence of the effective reduced mass on the well width has been obtained experimentally by magnetoluminescence that is highly sensitive to the modifications of the wave function, even on the scale of a single monolayer.
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  • 91
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1505-1508 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; III-V and II-VI inorganics ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A model for calculating exciton binding energies in quantum wells (QWs) is presented, which can be applied to situations in which one of the two band discontinuities is large, while the second one can be arbitrary (positive, negative, or zero). The model is applied to ZnSe/ZnSSe and InGaAs/GaAs QWs., The crossover between strong-and weak-confinement, regimes for excitons in narrow QWs is studied within a simplified model with two one-parameter, variational wave functions.
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  • 92
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1527-1530 
    ISSN: 0392-6737
    Keywords: Exchange and superexchange interactions ; Magnetic semiconductors ; II–VI semiconductors ; Conference proceedings
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    Topics: Physics
    Notes: Summary Interaction of two magnetic polarons formed either by single carriers or by excitons is considered in bulk crystal, quantum well and quantum wire. It is shown that the magnetic bipolaron can be formed if the distance between two polarons is less than some critical value. This critical radius is the larger the lower the dimensionality of the system, the lower the temperature and the larger the single-polaron radius. The critical distance for the neutral-magnetic-bipolaron formation is much longer than for the charged bipolaron.
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  • 93
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1549-1553 
    ISSN: 0392-6737
    Keywords: Studies of specific magnetic materials ; Excitons and related phenomena (including electron-hole drops) ; Conference proceedings
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    Topics: Physics
    Notes: Summary The influence of the electron spin splitting on the stability of quasi-two-dimensional donor bound excitons (D 0 X) has been investigated in the Cd0.97Mn0.03Te/Cd0.7Mg0.3Te semi-magnetic semiconductor quantum well (QW) structure. TheD 0 X binding energy has been measured directly from emission spectra recorded at magnetic field parallel to the QW plane. Complexes have been shown to dissociate into a neutral donor and a free exciton with a spin flip of the electron at high magnetic fields when the electron spin splitting exceeds the binding energy.
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  • 94
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1561-1565 
    ISSN: 0392-6737
    Keywords: III–V semiconductors ; III–V compounds and systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We report on a theoretical calculation of the optical anisotropies in (113)-grown corrugated GaAs/AlAs superlattices using a 6-bankk·p valence band Hamiltonian. By comparison of the calculation with the experimental polarization of the superlattice luminescence we can estimate the corrugation height to be at most 0.3 nm.
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  • 95
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1629-1633 
    ISSN: 0392-6737
    Keywords: III–V semiconductors ; Excitons and related phenomena (including electron-hole drops) ; III–V compounds and systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We report on the biexciton formation of the pseudodirect exciton consisting of the AlAs-X z electron and the GaAs-Γ heavy hole in (GaAs) m /(AlAs) m type-II superlattices withm=10, 12, and 13 monolayers. The photoluminescence lineshape of the pseudodirect exciton exhibits a doublet feature having an energy separation of ∼3 meV at the excitation power of the order of mW/cm2 in all the samples, and the low-energy band grows superlinearly. The transient profiles of the doublet photoluminescence band early indicate the biexciton formation: the delay of the rise of the biexciton photoluminescence on the low-energy side and its shorter decay time in comparison with the exciton photoluminescence.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1635-1639 
    ISSN: 0392-6737
    Keywords: II–VI compounds and other chalcogenides ; Nonlinear optics ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We report two-photon absorption photoluminescence excitation (TPA-PLE) measurements in ZnSe/ZnSSe strained-layer superlattices (SLS). Polarization-dependent selection rules are observed in TPA spectra depending on the direction of the exciting field with respect to the growth axis of the SLS. Experimental exciton transition energies have found to be in good agreement with theoretical predictions considering strain and confinement effects. Furthermore by means of comparative non-linear technique the TPA coefficient has been measured at different energies and compared with theoretical predictions including quasi-bidimensional excitonic effects. A satisfactory agreement has been shown for the overall spectral behaviour and in some cases also for the TPA coefficient values.
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  • 97
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1487-1492 
    ISSN: 0392-6737
    Keywords: III-V semiconductors ; Electron states in low-dimensional structures (including, quantum wells, superlattices, layer structures, and intercalation compounds) ; Excitons and related phenomena (including electron-hole drops) ; Brillouin and Rayleigh scattering ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A detailed study of the relative role played by localized and/or propagating intermediate excitonic states in, resonant Rayleigh scattering (RRS) is presented for a large set of GaAs quantum well (QW) and bulk structures. We show that the two kinds of states contribute to RRS through different mechanisms. We concluded that RRS occurs via localized states in QW heterostructures, very likely due to localization by the interface roughness, while bulk, crystals turn out to be better candidates for RRS via propagating states.
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  • 98
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    Keywords: Surface and interface electron states ; Excitons and related phenomena (including electron-hole drops) ; Conference proceedings
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    Topics: Physics
    Notes: Summary Quantum wire heterostructures, such as V- and T-shaped wires, are very promising candidates for low-threshold lasing. A crucial issue is the excitonicvs. free-carrier nature of the radiative recombination. Here, we report on magnetophotoluminescence studies of GaAs and InGaAs V-shaped wires that allow to discriminate different regimes of radiative recombination.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1601-1605 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Polaritons (including photon-phonon and photon-magnon interactions) ; Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Low-temperature time-resolved luminescence experiments with picosecond resolution have been performed on excitons in a microcavity containing InGaAs quantum wells. The decay curves are very dependent on the energy difference between the cavity mode and the exciton. They agree qualitatively with a model which takes into account a reservoir at the exciton energy and two splitted radiant states.
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    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1613-1617 
    ISSN: 0392-6737
    Keywords: Excitons and related phenomena (including electron-hole drops) ; Optical properties of thin films surfaces and layer structures (superlattices, heterojunctions and multilayers) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We present an analysis of the inter- and intrasubband relaxation dynamics of excitons due to Fröhlich coupling in an unbiased asymmetric double quantum well made from CdTe/CdMnTe. We consider the relaxation processes that take place in the time interval between the excitation of the lowest exciton localized in the narrow well and the recombination in the wide well, by means of Fermi's golden rule. As the excitation falls in the range of a Fano resonance, we have to employ an extended set of rate equations to incorporate the effect of the continuum of initially populated states. Thus, we obtain the transient luminescence, which can directly be compared with time-resolved luminescence experiments.
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