Interlayer tunneling spectroscopy and doping-dependent energy-gap structure of the trilayer superconductor Bi₂Sr₂Ca₂Cu₃O_10+δ

The superconducting gap, the pseudogap, and their doping and temperature dependences have been measured by the short-pulse interlayer tunneling spectroscopy for the CuO₂ triple-layer high-T_c superconducting Bi₂Sr₂Ca₂Cu₃O_10+δ system. It is found for a nearly optimally doped sample that the superconducting gap magnitude is ≈80 meV and the pseudogap is ≈120 meV, the values of which are slightly larger than those for CuO₂ double-layer system. Both gap magnitudes show a clear tendency to decrease with increasing doping. In an underdoped sample, a clear dip-and-hump structure is observed, which declines with increasing doping and tends to diminish in overdoped samples. The relationship between unchanged T_c and decreasing superconducting gap in the overdoped region is discussed in terms of the proximity effect applied to the inequivalent doping model. We also discuss the dip-and-hump structure in comparison with other spectroscopic results. Finally, we argue an important implication of the increasing maximum Josephson current and the decreasing superconducting gap magnitude, both with increasing doping.