[en] Measurements of the current noise power spectrum of the photoconducting insulators CdS and CdSe irradiated with monochromatic light of different wavelength λ are reported. It is shown that there is an abrupt change of about one order of magnitude in the low frequency power density of the noise when λ crosses the photoconductor gap value λ and the photocurrent and the device conductance are kept constant by varying the light intensity. The effect is explained in terms of an abrupt increase of the carrier recombination rate when the photon energy becomes larger than the energy gap of the photoconductor. Possible applications of these results are briefly discussed

[en] This book contains the proceedings of the two joint conferences: the '8th International Conference on Noise in Physical Systems' and the '4th International Conference on 1/f noise', held in Rome. (Auth.)

[en] Comparative measurements are made on the dissipative effects due to dc current flow and ac magnetization processes on the same YBCO superconducting specimen at 77.3 K. The material is brought into the Abrikosov mixed phase by a dc magnetic field H₀. Different sets of data are reported concerning: a) the dependence of the dc resistivity ρ on H₀ and on the current density J, b) the dependence of H(t) on the frequency and on the value of H₀ of the magnetic power loss generated by a very small (0.34 A/m) sinusoidal magnetic field H(t) superimposed to H₀. By a numerical calculation it is shown that at high frequencies the magnetic power loss is very closely equal to the one expected for eddy current dissipation in a conducting material whose resistivity depends on the current density and has a value rather closely equal to the one measured on the YBCO specimen with a conventional four-terminal technique. A second component of the magnetization power loss, which is closely proportional to the magnetization frequency and corresponds to the hysteretic loss, is also seen. (orig.)

[en] Current noise observed on YBa₂Cu₃O_7-δ ceramic superconductors, brought to a resistive state by the application of a magnetic field, is shown to be very different in amplitude when the same resistance value is obtained by either increasing or decreasing the field. A tentative explanation of this hysteretic effect on current noise in terms of a model of superconducting islands separated by resistive, noise-generating boundaries, whose structure is hysteretically field dependent, is reported. The possibility of using the noise to get information on the internal structure of the ''strong'' and ''weak'' superconducting regions is suggested

[en] We present results of experimental measurements of critical current and voltage drop at constant injected current versus increasing and decreasing magnetic field, for a paralellipipedic YBa₂Cu₃O_7-δ sample. The measurements were carried out at 77 K, cycling the field from 0-76 mT and back. The irreversible dependence on the applied field and the analogy between the two types of experimental curves are discussed. The existence of a biunivocal relationship between critical current and voltage drop, irrespective of the increasing or decreasing character of the applied magnetic field, is pointed out. The modified Ambegaokar-Halperin (AH) model for a Josephson junction turns out to be an appropriate theoretical approach for explaining this behaviour. A unitary treatment mode is proposed that reduces the hysteretical dependences on magnetic field of both critical current and voltage to the irreversible behaviour of the noise rounding parameter characterizing the Josephson junction. The accord between experimental curves and theoretical results supports the idea of representing the superconducting sample as an array of AH-modelled Josephson junctions and also gives the possibility of making predictions for different parameter values characterizing the weak links, such as the normal resistance of a Josephson junction and the critical current density in the absence of a magnetic field and thermal fluctuations. (author)

[en] In this paper we present measurements concerning the current noise produced during the resistive transition in a MgB₂ polycrystalline thin film. The power spectrum of the current noise, observed when the temperature is slowly changed across its critical value, presents a large electrical noise of the 1/fⁿ type (n ≅ 3) over a quite wide range of frequencies. This noise may be considered as generated by the abrupt creation of resistive strips across the specimen constituted by grains which have undergone the resistive transition. A computer model that takes into account fluctations of the grain critical currents and of the number of grain per strips, has been developed to simulate the resistive transition and to evaluate the noise power spectrum. When the temperature is incresed and reaches its critical value, resistive strips are formed according to a percolative process, giving rise to resistance steps which are at the origin of the noise. The theoretical results obtained by this model are in good agreement, concerning both the shape and intensity of the noise power spectrum, with the experimental data directly measured on the specimen