Systematic behaviour of the in-plane penetration depth in d-wave cuprates

We report the temperature T and oxygen concentration dependences of the penetration depth of grain-aligned YBa_2Cu_3O_{7-\delta} with \delta= 0.0, 0.3 and 0.43. The values of the in-plane \lambda_{ab}(0) and out-of-plane \lambda_{c}(0) penetration depths, the low temperature linear term in \lambda_{ab}(T), and the ratio \lambda_{c}(0) /\lambda_{ab}(T) were found to increase with increasing $\delta$. The systematic changes of the linear term in \lambda_{ab}(T) with T_c found here and in recent work on HgBa_2Ca_{n-1} Cu_nO_{2n+2+\delta} (n = 1 and 3) are discussed.