Temperature dependence of the magnetic penetration depth in the high-T sub c superconductor Ba sub 2 YCu sub 3 O sub (g- delta): Evidence for conventional S-wave pairing.

Transverse-field Muon Spin Relaxation (mu SR) measurements have been performed on Ba sub 2 YCu sub 3 O sub (9-delta) (delta = 2.1 +- 0.05) above and below its superconducting transition temperature. The data at 6K indicate a magnetic penetration depth lambda = 1365angstroms, with a temperature dependence similar to that of an ordinary s-wave (not d-wave) superconductor. Using the measured value for lambda, one obtains a charge carrier density of 0.6x10 sup 22 cm sup (-3) (= 1/3 carrier/Cu atom).