Critical currents of small Bi2Sr2CaCu2O8+x intrinsic Josephson junction stacks in external magnetic fields

We present a study of the critical currents of small sized intrinsic Josephson junction stacks in external magnetic fields applied parallel to the ${\mathrm{CuO}}_2$ layers. The stacks consisting of 5–12 junctions were realized as mesa structures patterned on top of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ single crystals. Lateral mesa dimensions ranged from $10\times 10$ to $0.8\times 0.8\mu {\mathrm{m}}^2.$ For the largest structures a complex magnetic field dependence of the critical currents was found. With decreasing mesa dimensions the critical currents systematically approached a Fraunhofer-type magnetic field dependence. Measurements agree well with the results of numerical calculations using coupled sine-Gordon equations.