Magnetic Penetration Depth in YBa 2 Cu 3 O 9-δ : Evidence for Strong Electron-Phonon Coupling?

Published experimental data on the temperature dependence of the magnetic penetration depth λ( T ) in YBa 2 Cu 3 O 9-δ by means of muon-spin-relaxation (μSR) measurements are analysed in terms of conventional ( s -wave) weak- and strong-coupling theory. The data are consistent with very strong electron-phonon coupling. It is emphasized that the often used empirical formula [λ(0)/λ( T )] 2 = [1 - ( T/T c ) 4 ] does not represent the weak-coupling magnetic penetration depth.