Numerical study of the frequency and temperature dependence of the ac magnetic susceptibility in presence of a static magnetic field in HTS

The temperature dependence of the harmonics (chi(n)) of the ac magnetic sus ceptibility in HTS have been investigated by numerical solutions of the non linear diffusion equation for the magnetic flux. Within the framework of t he collective pinning model, we show that the transition between different regimes for the flux dynamics (taff, creep, flow) is determined by the fiel d frequency for a fixed de field, or by the de field for a fixed frequency. As a consequence, a non universal behavior arises for the temperature depe ndence of chi(n). In particular, in this approach the frequency dependencie s of both amplitude and temperature of the peak of the imaginary part of th e first harmonic and of the modulus of the third harmonic are discussed. ha rmonic are discussed.