VORTEX VISCOSITY IN YBA2CU3O7-DELTA AT LOW-TEMPERATURES

With a bolometric technique, we have measured, in intense fields, the low-temperature surface resistance of both 90- and 60-K YBa2Cu3O7-delta single crystals. In the former, the observed vortex viscosity eta(eff)=5 X 10(-5) N s/M2 exceeds by 2 orders of magnitude the viscosity in conventional superconductors. This corresponds to a normal-state resistivity of 0.4 muOMEGA cm, and a quasiparticle lifetime tau approximately 6 ps. (The corresponding numbers of the 60-K crystal are eta(eff) = 2.5 X 10(-6) N s/m2 and tau = 1.9 ps at 3 K.) These lifetimes place YBa2Cu3O7-delta firmly in the superclean regime where the lifetime broadening of core energy levels is much less than the energy spacing. We discuss how a very large Hall angle in this regime affects the analysis of surface resistance.