  A subharmonic structure in Josephson junctions appears due to Andreev
reflections within the junction. Here we report on experimental observation of
a subharmonic half-gap singularity in interlayer tunneling characteristics of a
layered high temperature superconductor Bi2Sr2CaCu2O8+d. The singularity is
most pronounced in optimally doped crystals and vanishes with decreasing
doping. It indicates existence of non-vanishing electronic density of states
and certain metallic properties in the intermediate BiO layers, which grows
stronger with increasing doping. This provides an additional coherent
interlayer transport channel and can explain a gradual transition from an
incoherent quasi-two-dimensional $c$-axis transport in underdoped to a coherent
metallic transport in overdoped cuprates. Furthermore, due to a very small
sub-gap current, the singularity allows unambiguous extraction of the
superconducting gap, without distortion by self-heating.
