Strong Pinning in High Temperature Superconductors

Detailed measurements of the critical current density jc of YBa2Cu3O7 films grown by pulsed laser deposition reveal the increase of jc as function of the filmthickness. Both this thickness dependence and the field dependence of the critical current are consistently described using a generalization of the theory of strong pinning of Ovchinnikov and Ivlev [Phys. Rev. B 43, 8024 (1991)]. From the model, we deduce values of the defect density (10^21 m^-3) and the elementary pinning force, which are in good agreement with the generally accepted values for Y2O3-inclusions. In the absence of clear evidence that the critical current is determined by linear defects or modulations of the film thickness, our model provides an alternative explanation for the rather universal field dependence of the critical current density found in YBa2Cu3O7 films deposited by different methods.Comment: 11 pages; 8 Figures; Published Phys. Rev. B 66, 024523 (2002

Tunneling Conductivity of Manganites with Negative Magnetoresistance

Tunneling investigations of ferromagnetic La$\text{}_{0.6}$Sr$\text{}_{0.4}$MnO$\text{}_{3}$ ceramic samples showing negative magnetoresistance effect were carried out in two types of junctions: symmetric "break junctions" and asymmetric ceramics-insulator-metal junctions. The results are consistent with the presence of small ferromagnetic clusters in the insulating barriers of both types of junctions and suggest a two-phase state realized by impurity mechanism of phase separation