Strong temperature dependence of the c-axis gap parameter of Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions

Abstract

Stacked series arrays of intrinsic Josephson tunnel junctions have been fabricated on the surfaces of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ single crystals using photolithography and Ar-ion milling together with in situ monitoring of the resulting current-voltage ($I-V$) characteristics. The number of junctions in the stack (along the $c$ axis) is varied from 1-5 to ∼200 in a controlled way. Both Josephson coupling and multiple branches with well developed gap features are seen in the $I-V$ dependences of the arrays at $T\lesssim 90$ K. The gap parameter ${\Delta }_c$ is 10-13 meV at 4.2 K, which is approximately half the value reported in the literature. The temperature dependence of ${\Delta }_c$ deviates strongly from the BCS one. Proximity induced superconductivity of the Bi-O layers may possibly explain both the reduced gap parameter and its temperature dependence.