TEMPERATURE-DEPENDENCE OF SURFACE IMPEDANCE Z(T,OMEGA) AND MEAN FREE PATHS I(T) OF YBCO-SUPERCONDUCTORS

The surface impedance Z(T,omega) at 10 and 145 GHz and between 4 and 3 00 K is obtained experimentally. Z(T-a less than or equal to T less th an or equal to T-c) is quantitatively fitted by the BCS theory with a mean free path I(T) increasing.rapidely below T-c. This I-increase in the frame of the BCS-theory is limited at T-a by 'inelastic surface sc attering' at weak or strong links, e.g., by twin boundaries in distanc es a(TW) which dominates scattering for a(TW) less than or equal to 2l (T-a). Below T-a the enforced energy transfer from YBCO-crystallites t o weak links may enhance Z(T<T-a) until at T approximate to T the 'we ak link surface impedance' dominates Z(res)(T < T).