ac absorption in the high-Tcsuperconductors: Reinterpretation of the irreversibility line

A simple physical picture for the ac absorption in high-${\mathit{T}}_{\mathit{c}}$ superconductors in the mixed state is proposed. We argue that the transition in the real part and the peak in the imaginary part of the ac susceptibility are due to the skin size effect and occur when the skin penetration depth is of the order of the size of the sample. The frequency of the peak is proportional to the resistivity ρ(H,T), excluding other field- or temperature-dependent factors. Experiments to determine the true irreversibility line associated with the transition between the vortex-glass and the vortex-liquid state are suggested.