  We have magnetically imaged interlayer Josephson vortices emerging from an ac
face of single crystals of the single layer cuprate high-Tc superconductor
(Hg,Cu)Ba2CuO4+d. These images provide a direct measurement of the c-axis
penetration depth of about 10 microns. This length is a factor of 10 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 100 times smaller than is required for
superconductivity.
