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 and zinc doped YBa2Cu3O7-delta (YBCO) using a commercial AC susceptometer. 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. We find that increasing the Co doping increases both the fluctuation effects and the anisotropy ratio lambda(c)/lambda(ab), probably due to the reduction of coupling in the c-direction, while doping with Zn substantially increases both lambda(ab) and lambda(c). Our data imply a large, 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 lambda(c)(T) at low temperatures (T<0.6T(c)), in apparent disagreement with conventional s-wave BCS pairing. The possible explanations of this result are discussed.