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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-δ} with δ= 0.0, 0.3 and 0.43. The values of the in-plane λ_{ab}(0) and out-of-plane λ_{c}(0) penetration depths, the low temperature linear term in λ_{ab}(T), and the ratio λ_{c}(0) /λ_{ab}(T) were found to increase with increasing $δ$. The systematic changes of the linear term in λ_{ab}(T) with T_c found here and in recent work on HgBa_2Ca_{n-1} Cu_nO_{2n+2+δ} (n = 1 and 3) are discussed.