Vortex-motion dissipation in high-Tcsuperconductors at microwave frequencies

Measurements of the microwave surface resistance on samples of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O at different frequencies (23 and 48 GHz) indicate a universal behavior of the dissipation due to the vortex motion as a function of temperature, at low magnetic field. A temperature ${\mathit{T}}_0$=(0.97±0.01)${\mathit{T}}_{\mathit{c}}$ separates two regimes such that for T<${\mathit{T}}_0$ the dissipation increases with T while, for ${\mathit{T}}_0$<T<${\mathit{T}}_{\mathit{c}}$ the dissipation decreases with increasing T. The role of fluctuations is expedient for explaining the experimental data near ${\mathit{T}}_{\mathit{c}}$.