Effects of fluxon dynamics on higher harmonics of ac susceptibility in type-II superconductors

Measurements of fundamental and higher harmonics of ac susceptibility of polycrystalline low-${\mathit{T}}_{\mathit{c}}$ and high-${\mathit{T}}_{\mathit{c}}$ superconductors show a failure of the critical-state model in describing the higher harmonics dependence on temperature at fixed dc and ac magnetic fields. The results of measurements at different frequencies suggest that an effect of the fluxon dynamics could take place. Indeed simple considerations lead to the conclusion that thermally activated flux flow, flux creep, and flux flow (FF) should have an important role in determining the higher harmonics of ac susceptibility. A simplified critical-state model, only including pinning and FF, seems to explain part of the experimental results.