MUON-SPIN-ROTATION MEASUREMENTS OF THE PENETRATION DEPTH IN THE YBA2CU4O8 FAMILY OF SUPERCONDUCTORS

We report muon-spin-rotation (mu SR) measurements of the in-plane magn etic penetration depth (lambda(ab)) in polycrystalline Ca- and La-dope d YBa2Cu4O8 superconductors with T-c ranging from 72 to 88 K. In all s amples the temperature dependence of the mu SR depolarization rate sig ma(T)proportional to 1/lambda(ab)(2)(T) is linear below similar to 25 K. This is similar to what was observed in recent mu SR and surface im pedance experiments on YBa2Cu3O7-delta single crystals. The low-temper ature behavior of sigma(T)/sigma(0) = lambda(ab)(2)(0)/lambda(ab)(2)(T ) shows practically no dependence on doping when plotted versus t = T/ T-c. This observation puts important constraints on models proposed to explain the linear temperature dependence of lambda(ab) in cuprate su perconductors at low temperatures.