[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] A detailed understanding of the physical determinants of the ablation rate in multiple nanosecond laser pulses regime is of key importance for technological applications such as patterning and pulsed-laser deposition. Here, theoretical modeling is employed to investigate the ablation of thick metallic plates by intense, multiple nanosecond laser pulses. A new photo-thermal model is proposed, in which the complex phenomena associated to the ablation process are accounted for as supplementary terms of the classical heat equation. The pulsed laser ablation in the nanosecond regime is considered as a competition between thermal vapourization and melt ejection under the action of the plasma recoil pressure. Computer simulations using the photo-thermal model presented here and the comparison of the theoretical results with experiment indicate two different mechanisms that contribute to the decrease of the ablation efficiency. First, during the ablation process the vapour/plasma plume expanding above the irradiated target attenuates the laser beam that reaches the sample, leading to a marked decrease of the ablation efficiency. Additional attenuation of the laser beam incident on the sample is produced due to the heating of the plasma by the absorption of the laser beam into the plasma plume. The second mechanism by which the ablation efficiency decreases consists of the reduction of the incident laser intensity with the lateral area, and of the melt ejection velocity with the depth of the hole