AC SUSCEPTIBILITY OF A YBCO SINGLE-CRYSTAL IN AN EXTENDED FREQUENCY-RANGE INTERPRETED BY NUMERICAL-SOLUTION OF THE CONTINUITY EQUATION FOR THE MAGNETIC-FIELD

a.c. susceptibility measurements on a YBCO single crystal in an extend ed range of frequency and applied magnetic fields are presented. The d issipation phenomena shown by these measurements are interpreted insid e the framework of the thermally activated flux motion theory develope d for low-temperature superconductors in early '60. For this purpose t he continuity equation for the magnetic field inside the sample is sol ved numerically, avoiding to use a priori the standard approximations of this theory: thermally activated flux creep and thermally assisted flux flow. The whole set of a.c. susceptibility measurements, performe d in a wide range of the parameters, can be fitted with good agreement between the experimental and theoretical data.