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 Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/ single crystals with the CuO/sub 2/ 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 CuO/sub 2/ 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: (1) 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.