Evolution of current-voltage characteristics in Bi2Sr2CaCu2O8 intrinsic tunnel Josephson junctions

A series of unique current-voltage characteristics are observed as a function of temperature for a stack of 18 intrinsic tunnel Josephson junctions fabricated on an underdoped Bi2Sr2CaCu2O8 single crystal. The characteristics at low temperatures below T-c clearly show a superconducting energy gap of about 24 meV, which is suppressed significantly by the nonequilibrium superconductivity effect. On the other hand, the characteristics above T-c are nonlinear up to 180 K, which cannot be explained within the framework of conventional tunnel Josephson junctions. This nonlinearity is interpreted to provide evidence for the evolution of a pseudogap, or most likely a spin gap, in this system. The semiconductive temperature dependence of the c-axis resistivity is also explained consistently in terms of this pseudogap.