THE LOW-TEMPERATURE MICROWAVE SURFACE-RESISTANCE OF 90-K AND 60-K YBA2CU3O7-DELTA IN MAGNETIC-FIELDS

With a bolometric technique, we have measured, in intense fields, the low temperature microwave surface resistance of 90-K and 60-K YBCO sin gle crystals. From the viscocity inferred from the field dependence of surface resistance, the quasiparticle scattering rate 1/tau in the vo rtex core of YBCO is found to be strongly suppressed. The long relaxat ion times of quasiparticle place YBCO in the ''superclean regime'' whe re the lifetime broadening of the core energy levels is much less than the energy spacing.