Measurements of the absolute value of the penetration depth in high-T_c superconductors using a low-T_c superconductive coating

A method is presented to measure the absolute value of the London penetration depth, λ(T = O), from the frequency shift of a resonator. The technique involves coating a high-T_c superconductor with film of low-T_c material of known thickness and penetration depth. The method is applied to obtain λ(YBa₂Cu₃O_{7 -δ}) ≈ 1460 ± 150 Å, λ(Bi₂Sr₂CaCu₂O_{8 + δ}) ≈ 2690 ± 150 Å and λ(Pr_1.85Ce_0.15CuO_{4 - δ}) ≈ 2790 ± 150 Å. λ(YBa₂Cu₃O_{7 - δ}) from this method is very close to that obtained by several other techniques. For both Bi₂Sr₂CaCu₂O_{8 + δ} and Pr_1.85Ce_0.15CuO_{4 - δ} the values exceed those obtained by other methods.