A.N. Anagnostopoulos

Electrical and optical properties of as-grown TlInS2, TlGaSe2 and TlGaS2 single crystals

Abstract Single crystals of the layered compounds TlInS 2 , TlGaS 2 and TlGaSe 2 were grown by direct synthesis of their constituents. Their electrical conductivity was studied as a function of the temperature, perpendicularly and parallel to the layer planes and it proved to be highly anisotropic. Furthermore, a differential evaluation of measured lno vs. 10 3 /T curves reveals the existence of an acceptor level at 0.237 eV, 0.370 eV and 0.207 eV above the valence band maximum of each compound, respectively. Optical absorption measurements were performed in the temperature range 15 to 300 K and the corresponding absorption coefficients were studied as a function of the temperature.

On the non-linear properties of TlInX2 (X = S, Se, Te) ternary compounds

Abstract Optical and electrical properties of the TlInX 2 (X = S, Se, Te) were studied. The selenides and tellurides are of chain-like structure while the sulfides are layered. The measurements ( I-U characteristics and optical absorption) seem to have a direct connection with the main structural features. The I-U -curves of the first two compounds are S-type with characteristic oscillations in the negative differential resistance (NDR) region while the sulfides show an Ohmic behavior. The optical absorption measurements in the temperature range 15–300 K for Se- and Te-compounds show a positive temperature coefficient of the gap while for the S-compound it is negative. Possible explanations are mentioned.

B-spline method for solving Bratu's problem

In this paper, we propose a B-spline method for solving the one-dimensional Bratus problem. The numerical approximations to the exact solution are computed and then compared with other existing methods. The effectiveness and accuracy of the B-spline method is verified for different values of the parameter, below its critical value, where two solutions occur.

Optical and photoelectric properties of TlGaSe2 layered crystals

Abstract Absorption spectra of thin layers of TlGaSe2 crystals are used to study the energy gap and the interband transitions of the compound in the energy region 1.5–3.8 eV and in the temperature range 12–290K. A peak of excitonic origin appears at 2.39 eV at 12 K, 0.3 eV above the fundamental direct gap. It is followed by two sharp structures at 2.49 eV and 2.56 eV (12K). A broad structure appears at 2.64 eV. A room temperature reflection spectrum is also presented. The temperature dependence of the critical energies of all the observed structures is given. The dependence of the broadening parameter on temperature and the binding energy of the main exciton peak are also presented. Photoconductivity measurements at 290K and 90K resolve a number of structures that can be identified with several optical transitions. A peak associated with an acceptor level at 0.43 eV above the valence band maximum is also registered.

Chaotic behaviour of a fourth-order autonomous electric circuit

Abstract The chaotic dynamics of a fourth-order autonomous nonlinear electric circuit was studied by measuring its response in the form of chaotic time series. The circuit consists of two active elements, one linear negative conductance and one nonlinear resistor exhibiting a symmetrical piecewise-linear v – i characteristic and two capacitances C 1 and C 2 , which serve as the control parameters of the system. From the experimental time series the minimum embedding dimensions m min =4, correlation dimensions ν , with positive noninteger values, and Kolmogorov entropies, tending to constant positive values, were determined by numerical evaluation for the examined states of the system.

Optical and photoelectrical properties of the TlGaS2 ternary compound

Abstract Absorption and photoconductivity spectra of thin TlGaS 2 layers are used to study the fundamental and other transitions of TlGaS 2 in the energy region 1.0–3.0 eV and in the temperature range 10–300 K. One of the peaks resolved by photoconductivity measurements corresponds to the excitonic peak registered in the absorption spectrum of this compound. The temperature dependence of the critical energy and of the broadening parameter of this excitonic peak are presented. Most of the rest photoconductivity peaks are related to transitions from localized levels to the conduction band.

Optical investigation of SnS2 single crystals

The optical properties of the layered compound SnS2 have been studied. Reflectivity spectra were measured at room temperature, in the energy region 1.8–5.6 eV. Optical transmission measurements were carried out in the temperature range 13–300 K, in order to evaluate the energy gaps and their temperature dependences. The spectra of the optical constantsn, κ, ɛ1 and ɛ2 versus photon energyhν have also been presented.

Separation of chaotic signal sum into components in the presence of noise

The problem of separation of an observed sum of chaotic signals into the individual components is considered in the presence of noise. A noise threshold is found above which high-quality separation is impossible. This effect is shown to be associated with the information content of chaotic signals and a theoretical estimate is given for the threshold. A method for signal separation is proposed, which uses iteration of the chaotic source equations in reverse time. The method allows us to approach the theoretical limit threshold.

THE INTERMITTENCY ROUTE TO CHAOS OF AN ELECTRONIC DIGITAL OSCILLATOR

An electronic oscillator producing digital signals, formed by a second-order, nonlinear, nonautonomous, closed loop topology that is based on two integrators and nonlinear elements (a comparator and a XOR gate), was experimentally investigated. For a certain frequency and amplitude range of the periodic driving signal, the intermittency route to chaos was observed and studied.

Optical properties of CuAlX2 (X=Se, Te) thin films obtained by annealing of copper, aluminum and chalcogen layers sequentially deposited

Abstract Thin layers of copper, aluminum and chalcogen sequentially deposited by evaporation are annealed to synthesize CuAlX2 (X=Se, Te) films. The films crystallized in the chalcopyrite structure. For CuAlSe2, three characteristic energy gaps of 2.66(6), 2.78(2), and 2.91(5) eV were obtained from an analysis of the optical transmission spectra in the wavelength range 350–800 nm. The energies of the spin-orbit and the crystal field were found to be, respectively, 162 meV and −143 meV. Reflectivity measurements in the far infrared yield four mode frequencies for CuAlSe2 and CuAlTe2.