MAGNETIC PENETRATION DEPTHS IN Y2BA4CU6+NO14+N-DELTA AND LOCAL-PAIR CONDENSATION

The magnetic penetration depth lambda-(T) has been measured by means of positive-muon spin rotation and relaxation for YBa2Cu4O8, Y2Ba4Cu7O15-sigma and quenched YBa2Cu3O7-sigma samples, altogether spanning a range of critical temperatures (T(c)) from 29 to 90 K. The extrapolated (T --> 0) spin relaxation rate (proportional to lambda(0)-2, and thus to the ratio of density to effective mass n(s)/m* for the superconducting condensate) for YBa2Cu4O8 and Y2Ba4Cu7O15-sigma increases almost linearly with T(c) similar to the monotonic increase of T(c) with decreasing sigma previously determined for Y2Ba4Cu7O15-sigma. Within the framework of models based on Bose condensation of local pairs, the data indicate that the double-chain ribbons play a more active role in the superconductivity than as mere charge reservoirs.