The I_CRₙ Value in Intrinsic Josephson Tunnel Junctions in Bi₂Sr₂CaCu₂O_8+δ (Bi2212) Mesas

The c-axis current-voltage I(V) characteristics have been obtained on a set of mesas of varying height sculpted on Bi ₂ Sr ₂ CaCu ₂ O _{8 + δ} (Bi2212) crystals intercalated with HgB ₂ . The intercalation, along with the small number of junctions in the mesa, N = 6-30, minimizes the degree of self-heating, leading to a consistent Josephson critical current, I _C , among junctions in the mesa. The Bi2212 crystals with a bulk T _C = 74 K are overdoped and display negligible pseudogap effects allowing an accurate measure of the normal state resistance, R _N . These properties make the mesas nearly ideal for the determination of the Josephson I _C R _N product. We find I _C R _N values consistently ~30% of the quasiparticle gap parameter, Δ/e, which was measured independently using a mechanical contact, break junction technique. The latter was necessitated by higher bias heating effects in the mesas which prevented direct measurements of the superconducting gap. These values are among the highest reported and may represent the maximum intrinsic value for I _C R _N . The results indicate that the c-axis transport is a mixture of coherent and incoherent tunneling.