1,720,989 research outputs found
A vibrational model of F centres in alkali halides
No full textA model of F-centres in alkali halides, which relates absorption energies in F-bands to the dynamic parameters of host lattices, is proposed. According to this model, the electrons trapped in F-centres are treated as classical particles with a proper mass , m standing for the actual electron mass, and for local high frequency and optical dielectric constants, respectively. Hence, by considering the trapped electrons as substitutional impurities of small mass, the dynamics of the lattice is investigated by means of the theory of local modes with the assumption of isotopicity. A simple equation allowing calculation of the local mode frequencies is obtained. Knowledge of the Debye frequency and of transverse and longitudinal mode frequencies at the long wavelength limit is required. With this model, F-band absorption energy is reasonably well accounted for. Copyright Springer-Verlag Berlin/Heidelberg 2003
A DOSIMETRY PROCEDURE BASED ON STORAGE PHOSPHORS WITH SHORT FADING TIME
No full textProblem of retrieving data on exposure to radiation from storage phosphors characterised by short fading time in unpredictably
long exposure times is addressed from a numerical point of view. A simple algorithm is presented for the case of first
kinetic order. Extension to a generic kinetic order is possible provided a suitable characterisation of the fading features is performed.
Simulations are also presented where temperature and reading effects, as well as error measurements, are taken into
account
Effective Linewidth in Raman Spectra of Titanium Dioxide Nanocrystals
No full textRaman spectra of nanocrystals titanium dioxide are discussed and the correlation between the band shape of the
allowed A1g Raman mode and the crystals dimensions is discussed. Data on Raman spectra are reconsidered in the framework
of a modified "hard confinement" model (MHC). The proposed model is based on the idea of using an effective
linewidth parameter, which is a function of the effective dimension of the nanostructure, in spite of the intrinsic Raman
band linewidth.
The comparison with standard hard confinement model reveals better agreement with the experimental results for the
MHC model up to 6 nm. Moreover, the analysis permits to improve the knowledge of the phonon dispersion curve as well
as the intrinsic Raman bulk parameters. An analytical form of the size-dependent peak-position in nanocrystals, useful for
an approximated size estimation, has been explicated. The general structure of the model permits to extended the MHC to
other nano-sized materials
Phonon characterization of nano-crystals by Raman spectroscopy
No full textProposed herein is a method based on the Levenberg–Marquadt technique to retrieve an approximated
average phonon dispersion curve from Raman spectra of nano-sized crystals. To this end we deal with
a modified form of the well-known phonon confinement model in which the intrinsic linewidth C0 is
replaced by an effective linewidth C. An application to experimental results obtained on TiO2 nano-crystals
is presented as a test of the method
A Statistical Model Expounding Weak Bosons as Massless Particles
No full textA statistical model utilizing the Thomas-Fermi approximation is ad-
vanced which, by assuming Z0 and W
§
bosons to be massless particles like photons,
explains the short range of weak forces by means of the screening properties of the
negative-kinetic-energy neutrino distribution which, according to the Dirac equa-
tion, ̄lls the whole universe. In addition, a conjecture is proposed ascribing the
current electron and neutrino masses to an evolutive process which took place while
the universe was cooling
Thermodynamic Basis of the Concept of “Recombination Resistance”
No full textThe concept of “recombination resistance” introduced by Shockley and Read [Phys. Rev. 87, 835 (1952)] is discussed within the framework of the thermodynamics of irreversible processes ruled by the principle of the minimum rate of entropy production (MREP). It is shown that the affinities of recombination processes represent “voltages” in a thermodynamic Ohm-like law where the net rates of recombinations represent the “currents” and the Onsager coefficients of the phenomenological laws represent the inverse of the “resistances.” The quantities thus found allow for the definition of “dissipated power” which is to be related to the rate of entropy production of the recombination processes dealt with. The goal of this paper is thus to give a thermodynamical ground to the recombination resistance concept. But, also stressed is the potentiality of the MREP variational method. To this purpose, the simple case of the ideal (Shockley) diode, where diffusion forces are active as well, is considered
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