T_c Dependence of Energy Gap and Asymmetry of Coherence Peaks in NdBa₂Cu₃O_(7-δ)

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.