Vortex viscosity in YBa2Cu3O7-δ 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-δ single crystals. In the former, the observed vortex viscosity ηeff=5×10-5 N s/m2 exceeds by 2 orders of magnitude the viscosity in conventional superconductors. This corresponds to a normal-state resistivity of 0.4 μΩ cm, and a quasiparticle lifetime τ∼6 ps. (The corresponding numbers for the 60-K crystal are ηeff=2.5×10-6 N s/m2 and τ=1.9 ps at 3 K.) These lifetimes place YBa2Cu3O7-δ 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.