  We performed scanning tunneling spectroscopic experiments on hole-doped
NdBa$_2$Cu$_3$O$_{7-\delta}$. The d$I$/d$V$ curves obtained at 4.2 K are
asymmetric with clear peak-dip and hump structures. Energy derivatives of these
curves show peaks at energies beyond the dip features. Highly precise full
potential bandstructure calculations confirm a featureless electronic density
of states in that energy region. Our results indicate that tunneling electrons
couple to a collective mode in the CuO$_2$ plane.
