Interlayer tunneling spectroscopy of Bi₂Sr₂CaCu₂O_(8+δ): a look from inside on the doping phase diagram of high T_c superconductors

A systematic, doping dependent interlayer tunneling spectroscopy of Bi2212 high $T_c$ superconductor is presented. An improved resolution made it possible to simultaneously trace the superconducting gap (SG) and the normal state pseudo-gap (PG) in a close vicinity of $T_c$ and to analyze closing of the PG at $T^*$. The obtained doping phase diagram exhibits a critical doping point for appearance of the PG and a characteristic crossing of the SG and the PG close to the optimal doping. This points towards coexistence of two different and competing order parameters in Bi2212. Experimental data indicate that the SG can form a combined (large) gap with the PG at $T<T_c$ and that the interlayer tunneling becomes progressively incoherent with decreasing doping.