Magnetic penetration depth of oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals (${\mathit{T}}_{\mathit{c}}$=66 K)

For single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with ${\mathit{T}}_{\mathit{c}}$=66 K, we measured the magnetic penetration depth ${\lambda }_{\mathit{a}\mathit{b}}$(T), with the applied magnetic field oriented perpendicular to the ${\mathrm{CuO}}_2$ planes, using high-field magnetization isotherms M(H,T). The zero-temperature value ${\lambda }_{\mathit{a}\mathit{b}}$(0) is about 2600 Å, a value almost twice that of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with ${\mathit{T}}_{\mathit{c}}$=90 K, but similar to that of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$. The temperature dependence of ${\lambda }_{\mathit{a}\mathit{b}}$(T) reveals some deviation from the theoretical BCS weak-coupling clean limit, and from the two-fluid model. These observations are discussed and compared with magnetic-penetration-depth data for other high-${\mathit{T}}_{\mathit{c}}$ superconductors.