  We report here the results of scanning tunneling spectroscopic experiments
performed on hole doped NdBa$_2$Cu$_3$O$_{7-\delta}$ single crystals of $T_c$
values of 76 K, 93.5 K and 95.5 K. The energy gaps are observed to be
increasing with decreasing $T_c$ values. The coherence peaks are asymmetric
with the peaks at the filled states being larger than those at the empty ones.
The asymmetry increases with decreasing $T_c$. The observed asymmetry and its
$T_c$ dependence can be explained by considering the Mott insulating nature of
the material at the undoped state.
