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 (/spl chi//sub n/) of the ac magnetic susceptibility in HTS have been investigated by numerical solutions of the non linear diffusion equation for the magnetic flux. Within the framework of the collective pinning model, we show that the transition between different regimes for the flux dynamics (taff, creep, flow) is determined by the field frequency for a fixed dc field, or by the dc field for a fixed frequency. As a consequence, a non universal behavior arises for the temperature dependence of /spl chi//sub n/. In particular, in this approach the frequency dependencies of both amplitude and temperature of the peak of the imaginary part of the first harmonic and of the modulus of the third harmonic are discussed.