  In type-II superconductors, in the regime of very high applied current
density, an abrupt transition is experimentally observed from the mixed state
to a highly dissipative regime, probably the normal state. This provokes a high
voltage jump in the Current-Voltage Characteristics (CVC) of the samples, at a
current density J* larger than the critical current Jc. In this work we study
Y-123 (YBCO) microbridges sputtered over SrTiO3 substrates. We measure their
CVCs for different temperatures close to Tc, and with low applied magnetic
field, from 0 to 1 T. We compare the dependences of Jc and J*, and find that
some of the typical experimental results in this regime could be explained by
only considering the effect of the current self-field. This suggests that the
depairing of the supercarriers could be involved in the abrupt transition. We
also study the influence of thermal effects, by using a simplified model to
describe the thermal evolution of our samples. We find that the uniform
self-heating can produce an abrupt thermal runaway, at high current density,
which could explain the occurrence of J*, as well as its dependence on
temperature and applied magnetic field. These results do not totally exclude
the implication of other more intrinsic mechanisms (like Cooper pairs
depairing, or a change in the regime of the vortex dynamics), but suggest that
the uniform self-heating is more relevant than what was considered up to now.
In a shorter second part of the thesis, the contribution of the author to the
study of the paraconductivity in his laboratory is reflected.
