Kerim Kiymaç

Field Dependence of Magnetization anddM/dHfor Sm- and Gd-Doped Bi1.7Pb0.3Sr2Ca2 xRExCu3O10+yCompounds

We have performed magnetization measurements as a function of magnetic fields up to 4 kOe on Bi1.7Pb0.3Sr2 Ca2−xRExCu3O10+y (RE: Gd0.01, Sm0.03), at different fixed temperatures T<Tc in the range 15≤T≤75 K. We have observed three important features. Firstly, the M–H curves of both the Sm- and Gd-doped samples are similar and each shows a minimum of M at a certain critical field Hc, above which there appears to be a monotonic decrease in magnitude. Secondly, the magnitude of the initial susceptibility dM/dH, related to the superconducting volume, decreases with increasing temperature. In other words, the decrease of |dM/dH| with T indicates that the superconducting regions reduce in size and the paramagnetic domains probably grow at the expense of superconductivity. The third feature is the fast decrease of hysteresis loops in both the Gd and Sm doped samples with increasing temperature, which implies existence of flux pinning centres. Thus the results show that the Ca2+→Sm3+ and Gd3+ substitution decreases the hole carrier concentration and hence the volume fraction of the superconducting phase.

THE MAGNETIC AND ELECTRICAL PROPERTIES OF RARE-EARTH Sm3+ SUBSTITUTED Bi1.7Pb0.3Sr2Ca2-xSmxCu3O12 SYSTEM

In the present work, the magnetic properties of high-TcBi1.7Pb0.3Sr2Ca2-xSmxCu3O12 system (x = 0.03) have been investigated, for different temperatures below Tc. Substitution of Sm3+ for Ca2+ is found to change the superconducting properties of the system, despite the weak paramagnetic nature of the Sm3+ ion. The dc magnetization measurements performed on the compound reveal that the superconducting regions decrease with increasing temperature and magnetic field. The results are explained on the basis of a possible reduction of hole concentration with trivalent rare-earth ion substitution, and also on the basis of the magnetic nature of the substituted Sm3+ ions.

CRITICAL CURRENT DENSITIES IN Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y (x=0.01, 0.1) SUPERCONDUCTORS PREPARED BY MELT-QUENCHING METHOD AND ANNEALED IN DIFFERENT TIME INTERVALS

We report critical current density (Jc) behaviors of the Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y (x=0.01, 0.1) superconductors prepared by melt-quenching method and annealed in different time intervals at 840°C. The effects of annealing times are found to be very important. For example, for the sample B2, the highest value of Jc is 3.7×104A/cm2 at 9 K. This value is almost 1.5 and 2 times greater than that of the samples B3 and B1, respectively. With increasing annealing time, the 2223 phase increases, thus resulting in a decrease in the insulating phase. For lower annealing times, there are a large number of insulating phases. On the whole, our results indicate that there is an optimum annealing time t to form the 2223 phase, in a time interval of 120<t<192 h. It is clearly seen that Jc decreases smoothly with increasing temperature for all the samples studied. At lower temperatures, especially below 30 K, an extremely rapid decrease of Jc with increasing temperature has been observed. The experimental values also...

A SIMPLE CHAOTIC NEURON MODEL: STOCHASTIC BEHAVIOR OF NEURAL NETWORKS

We have briefly reviewed the occurrence of the post-synaptic potentials between neurons, the relationship between EEG and neuron dynamics, as well as methods of signal analysis. We propose a simple stochastic model representing electrical activity of neuronal systems. The model is constructed using the Monte Carlo simulation technique. The results yielded EEG-like signals with their phase portraits in three-dimensional space. The Lyapunov exponent was positive, indicating chaotic behavior. The correlation of the EEG-like signals was. 92, smaller than those reported by others. It was concluded that this neuron model may provide valuable clues about the dynamic behavior of neural systems.