TEMPERATURE-DEPENDENT MAGNETIC PENETRATION DEPTH OF CO AND ZN DOPED YBA2CU3O7 OBTAINED FROM THE AC SUSCEPTIBILITY OF MAGNETICALLY ALIGNED POWDERS

We have measured the absolute value and temperature dependence of the magnetic penetration depth lambda(T) of magnetically aligned cobalt an d zinc doped YBa2Cu3O7-delta (YBCO) using a commercial AC susceptomete r. For pure YBCO, our values of lambda(ab)(0) congruent-to 140 nm and lambda(c)(0) congruent-to 1040 nm agree with other published values. W e find that increasing the Co doping increases both the fluctuation ef fects and the anisotropy ratio lambda(c)/lambda(ab), probably due to t he reduction of coupling in the c-direction, while doping with Zn subs tantially increases both lambda(ab) and lambda(c). Our data imply a la rge, highly anisotropic gap whose maximum value is 2DELTA(0) congruent -to 10k(B)T(c). All samples show a T2 term in both lambda(ab)(T) and l ambda(c)(T) at low temperatures (T<0.6T(c)), in apparent disagreement with conventional s-wave BCS pairing. The possible explanations of thi s result are discussed.