Abstract

Using an innovative microwave technique developed in the QM group, precise measurements of the flux-flow resistivity rho_ff(T) and the pinning force alpha_p(T) have been made on high-purity samples of optimally doped YBa2Cu3O6+x (TC = 93.5 K) in the mixed state, at temperatures down to 1 K. Several new effects have been observed, despite the subject being the most intensely studied of cuprate superconductors. The effective resistivity of the vortex cores is found to exhibit the same metalinsulator transition as has been reported in the DC resistivity of lower-TC cuprates, with rho_ff(T) growing logarithmically at low temperatures. The pinning force acting upon the vortices has been found to increase exponentially with temperature below 30 K, consistent with published critical-current measurements but difficult to explain using the popular theory of thermally activated flux creep. At temperatures just below TC, a sharp peak in alpha_p(T) has been seen, whose form is strikingly similar to that of the intrinsic peak effect reported for the low-TC superconductor NbSe2. Finally, the magnetic field dependence of the London penetration depth has been inferred from the apparent field dependences of rho_ff and alpha_p at low temperatures, the results being consistent with published mu-SR measurements.