a.c. 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 extended range of frequency and applied magnetic fields are presented. The dissipation phenomena shown by these measurements are interpreted inside the framework of the thermally activated flux motion theory developed for low-temperature superconductors in early '60. For this purpose the continuity equation for the magnetic field inside the sample is solved 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, performed in a wide range of the parameters, can be fitted with good agreement between the experimental and theoretical data. [ABSTRACT FROM AUTHOR]