FLUCTUATIONAL PHENOMENOLOGICAL MODEL FOR THE MAGNETODISSIPATION IN HIGH-T-C SUPERCONDUCTORS

We develop a phenomenological model for the magnetoresistivity in high -T-c superconductors that includes the contribution of the fluctuation excess conductivity and the effects of the phase slip due to thermal motion of vortices above the irreversibility line over local depressio ns of the order parameter. The fluctuation conductivity in the proximi ty of the mean-field transition is inserted into the final expression for the resistivity through a scaling function, obtained theoretically by Ullah and Dorsey. The behavior of the system of vortices is taken into account assuming that below the irreversibility line the solid ph ase is a glass phase. Crossing the irreversibility line, the vortex sy stem becomes a viscous fluid and, finally, a liquid. It is possible to fully describe the resistivity by recalling some of the main concepts of the conventional glass transitions. We obtain a compact expression for the resistivity that we compare to previously reported experiment al data in twinned and untwinned Y-Ba-Cu-O single crystals. With very few parameters we can fit extremely well the resistive transitions in the full temperature and field range. Also, the transitions in very pu re, untwinned crystals can be entirely fitted, including the ''kink' a t the so-called melting transition. Moreover, the resistivity is shown to be heavily influenced by fluctuations.