Superconducting energy gap in Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223) single crystals

Current-voltage characteristics of $S{-}I{-}S$ tunnel break junctions fabricated from pure undoped Bi2223 single crystals ($T_c=110$ K) have been measured. High quality of the crystals enables to produce the good tunnel junctions with a low or almost zero leakage current and well developed the gap structure in the tunneling spectra. Values of the peak-to-peak energy gap $2\Delta _{p-p}$ in different crystals and tunnel junctions ranged from $80$ to $105$ meV. The tunneling conductance in superconducting state was normalized by that in normal state and compared to a smeared BCS density of states. Simple fit of the data gave average value of the $\Delta =38.5$ meV and reduced gap $2\Delta /kT_c\simeq 8,$ consistent with a very strong coupling mechanism.