c-axis transport in naturally grown Bi2Sr2CaCu2O8+δ cross-whisker junctions -: art. no. 094517

We studied the c-axis transport of Bi2Sr2CaCu2O8+delta (Bi2212) cross-whisker junctions formed by annealing naturally formed whisker crosses. These frequently appear during growth when the ab faces of neighboring whiskers come in contact. We obtained Fraunhofer patterns of the cross-junction critical currents in a parallel magnetic field, and found a sharp increase in the quasiparticle tunneling conductance at V-g=50-60 mV, indicating high junction quality. For our weak junctions, the interface critical current density is about 3% of the critical current density across the stack of bulk intrinsic junctions, as is the room-temperature conductivity, and both are independent of the twist angle, in contrast to most of the data reported on artificial cross-whisker junctions [Y. Takano , Phys. Rev. B PRBMDO65, 140513(R) (2002)]. As a minimum, our results provide strong evidence for incoherent tunneling at least at the interface, and for at least a small s-wave order-parameter component in the bulk of Bi2212 for Tless than or equal toT(c). They are also consistent with the bicrystal twist experiments of Li [Phys. Rev. Lett. 83, 4160 (1999)].