Some questions of the theory of luminescent centres in crystalline phosphors

The calculation of the properties of admixture centres in the adiabatic approximation consists of two stages: 1) calculation of the adiabatic potentials, and 2) calculation of the different properties of centres on the basis of the known adiabatic potentials and wave functions. This paper is a survey of the theoretical studies carried out on these two problems in the Institute of Physics and Astronomy of the Academy of Sciences of the Estonian SSR. The calculations of adiabatic potentials relate mainly to luminescent centres in the phosphors of alkali halides, the many theoretical problems of which are discussed. The calculations of the spectra are bound up with the use and elaboration of the method of moments, which has permitted very general results to be obtained (the Mössbauer and Spolský effects are also discussed).The paper will be published in full in Czech. J. Phys. A, probably in the May 1963 issue.     

A light induced configurational change of FA centres in Li doped KCl-KBr crystals

Abstract

Spectroscopic properties of F_A centres in Li doped KCl-KBr mixed crystals were studied. At low temperature light induced spectral shifts, for the F_A1 hand towards lower energy and for the F_A2 band towards higher energy, were observed. The shifts are proposed to be due to a configurational change where the electron occupied vacancy finds a new location in relation to the neighbouring chlorine and bromine ions. The recovery to the original configuration, obtained in the F → F_A conversion, is a temperature activated process.     

Stacking faults in crystals and the model of configurational localization

The energy changes of stacking faults in metals and alloys are discussed in connection with the formation of stable electronic configurations of the fault atoms. The results of the discussions can be used to predict the effect of the admixture of one element on the stackingfault energy for another element.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 8, pp. 38–45, August, 1969.     

The use of PDE centres in the local RBF Hermitian method for 3D convective-diffusion problems

In this work an extension is proposed to the Local Hermitian Interpolation (LHI) method; a meshless numerical method based on interpolation with small and heavily overlapping radial basis function (RBF) systems. This extension to the LHI method uses interpolation functions which themselves satisfy the partial differential equation (PDE) to be solved. In this way, a much improved reconstruction of partial derivatives can be obtained, resulting in significantly improved accuracy in many cases.     

Enthalpy relaxation of polymers: comparing the predictive power of two configurational entropy models extending the AGV approach

The Tool-Narayanaswamy-Moynihan (TNM) phenomenological model is widely accepted in order to describe the structural relaxation of glasses. However several quantitative discrepancies can be found in the literature that cannot be entirely ascribed to the experimental errors. In this work we compare the predictive power of two recently proposed configurational entropy approaches extending the TNM formalism. Both of them change the treatment of non linearity by adding a free parameter. We use Differential Scanning Calorimetry (DSC) experiments in order to test the models in two different polymers. One of them is a commercial PMMA sample, the other is a side chain liquid crystal azo-benzene polymer properly synthesized for optical nanorecording purposes. Different results were found for the two systems. In the PMMA sample only one of the new models was able to improve the agreement between DSC experiments and theory with respect to the TNM model, whereas in the second polymer both the approaches were able to describe the experiments better than TNM model.Received: 25 February 2004, Published online: 21 October 2004PACS: 64.70.Pf Glass transitions - 61.43.Fs Glasses - 61.41. + e Polymers, elastomers, and plastics     

Cooling-rate dependence of the residual energy of a one-dimensional configurational glass model

For a one-dimensional configurational glass model we have performed molecular dynamical calculations. The Newtonian equation of motion was solved numerically including a damping term. The residual energye _res() as a function of the damping constant , exhibits a power law behavioure _res(),0.061; in an intermediate range of . This behaviour can be explained as the freezing of a certain type of two level systems with an excitation energy and a barrier heightB. The exponent is approximately equal to /B. This relationship is justified analytically.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Different Er centres in Si and their use for electroluminescent devices

At low temperatures, Er in Si produces a big variety of spectra in the 1.5 μm region which can be identified by high-resolution spectroscopy as being due to either interstitial Er or different complexes of Er with oxygen, intrinsic defects and other light impurities. Although the luminescence yield can be improved by codoping with light elements (C, N, O, F, etc.) all of these centres show strong thermal quenching of the luminescence above 150–200 K. There is, however, one type of rather broad spectrum in heavily Er- and O- doped Si, which is seen up to temperatures of 400 K and above. This spectrum can be excited in Si by hot electrons generated in a reverse biased diode. The same spectrum appears also in other Si related materials like porous Si and in silica with the same temperature dependence. In these materials, excitation spectroscopy is possible and it shows also close agreement of the excitation spectra. From these findings we infer that Er is incorporated in another surrounding and we propose Si–Er–O nano-precipitates since the spectra of other candidates, like Er₂O₃, are clearly different. We review recent work on the excitation and quenching mechanisms and we discuss consequences for technology.     

Comment on the model of the Mg centres in NaCl crystals

The structure models of the Mg₂ centre (substitutional Mg⁺ ion associated with a vacancy pair) and the Mg₄ centre (Mg⁰ atom situated in a vacancy pair) produced in NaClMg crystal by X-irradiation and subsequent F- and thermal bleaching are discussed. The given models explain without any contradiction all the experimental observations.     

Measuring the configurational heat capacity of liquids

A high electric field impedance experiment on supercooled molecular liquids is employed to transfer energy to the slow modes by absorption from the field and detect the increase of their "configurational temperature", T(cfg), via the change of the relaxation times. This allows us to determine the configurational heat capacity, which accounts for most of the excess heat capacity for stronger liquids, but for only half of the heat capacity step in the case of more fragile systems. It is also observed that T(cfg) gradually approaches the phonon temperature on the structural relaxation time scale.     

Simulation of spatial correlations of impurity ions in solids using configurational entropy and the hard-sphere model

Based on the hard-sphere model, the spatial correlations are considered in a system of impurities with variable valency. In a zeroth approximation, the configurational entropy of the spatially correlated system of impurity ions is identified with the configurational entropy of a system of hard spheres. The electron mobility limited by scattering on the correlated system of impurity ions at finite temperatures is found. The theory developed explains experimentally observed anomalies of the carrier mobility in an iron-doped HgSe gapless semiconductor.     

The predictive performance of a path-dependent exotic-option credit risk model in the emerging market

Most empirical research of the path-dependent, exotic-option credit risk model focuses on developed markets. Taking Taiwan as an example, this study investigates the bankruptcy prediction performance of the path-dependent, barrier option model in the emerging market. We adopt Duan’s (1994) [11], (2000) [12] transformed-data maximum likelihood estimation (MLE) method to directly estimate the unobserved model parameters, and compare the predictive ability of the barrier option model to the commonly adopted credit risk model, Merton’s model. Our empirical findings show that the barrier option model is more powerful than Merton’s model in predicting bankruptcy in the emerging market. Moreover, we find that the barrier option model predicts bankruptcy much better for highly-leveraged firms. Finally, our findings indicate that the prediction accuracy of the credit risk model can be improved by higher asset liquidity and greater financial transparency.     

Visual performance improvement analytics of predictive model for unbalanced panel data

Journal of Visualization - An unbalanced panel is a dataset in which at least one subject is not observed some times. Moreover, each subject is recorded with irregular periods and intervals....