Tunneling characteristics of Bi-Sr-Ca-Cu-O intrinsic Josephson junctions

Tunneling-type current-voltage (I - V) characteristics with a clear energy gap structure have been observed for very thin intrinsic Josephson junction stacks fabricated on the surface of underdoped Bi2Sr2CaCu2O8 single crystals with the CuO2 bilayer numbers of 27, 18, and 9, It is found that the nonequilibrium heating effect is significant in this layered material, and that an increase in the number of CuO2 bilayers or a decrease in the c-axis resistivity leads to significant gap suppression and negative resistance. The tunneling properties are characterized by the three points: (I) the energy gap suppression by the nonequilibrium superconductivity effect, (2) a large subgap conductance, and (3) an extraordinarily small maximum Josephson current compared with the BCS value. Numerical calculation shows that the observed I - V curves are consistent with the two-dimensional d-wave order parameter with the gap suppression due to the nonequilibrium effect.