Figure 1. Temperature and doping dependence of intrinsic tunneling spectra of Bi₂Sr₂CaCu₂O_8+δ superconductors.

(a–d) doping strength increasing from underdoped (UD) to overdoped (OD) with T_c = 71, 80, 89 and 79 K, respectively. The spectra are presented with V corresponding to the voltage per junction and in each case are normalized to the respective ones at the pseudogap opening temperature T* = 310, 280, 230 and 140 K. Inset displays a crystal unit cell, showing that electron tunneling occurs between CuO₂ double layers, which form planar-type superconductor-insulator-superconductor (SIS) tunnel junctions.