c-Axis Penetration Depth of Hg-1201 Single Crystals

We have magnetically imaged interlayer Josephson vortices emerging from an $\mathrm{ac}$ face of single crystals of the single layer cuprate high- $T_c$ superconductor $(\mathrm{Hg},\mathrm{Cu}){\mathrm{Ba}}_2\mathrm{CuO}{}_{4+\delta }$. These images provide a direct measurement of the $c$-axis penetration depth, ${\lambda }_c\sim 8\mu \mathrm{m}$. This length is a factor of 8 longer than predicted by the interlayer tunneling model for the mechanism of superconductivity in layered compounds, indicating that the condensation energy available through this mechanism is 50 times smaller than is required for superconductivity.