The temperature dependence of the London penetration depth $\lambda$ was measured for an untwinned single crystal of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ along the three principal crystallographic directions ($a$, $b$, and $c$). Both in-plane components (${\lambda }_a^{-2}$ and ${\lambda }_b^{-2}$) show an inflection point in their temperature dependence which is absent in the component along the $c$ direction (${\lambda }_c^{-2}$). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of ($s+d$)-wave symmetry whereas it is mainly $s$ wave along the $c$ direction. In conjunction with previous results it is concluded that coupled $s+d$-order parameters are universal and intrinsic to cuprate superconductors.

• Received 26 July 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.237601