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Area dependence of Josephson critical current density in superconducting Bi2Sr2CaCu2O8+d mesas for terahertz emission
There are numerous application fields of terahertz waves such as airport screening of passengers for weapons, explosives, drugs, secure wireless communications, cancer detection, etc. high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystal has been observed as an intense, coherent, continuous electromagnetic wave source in terahertz frequency region. Bi2212, which is highly anisotropic high-Tc superconductor, is considered as a stack of intrinsic Josephson junctions (IJJs) on atomic scale. In this study, we have fabricated triple mesa structures on a same chip with various mesa areas (300x50, 200x50, 100x50 μm2). Firstly, single crystal of Bi2212 is glued onto a sapphire substrate from its smooth a-b surface by silver epoxy. After deposition of 100 nm Au layer, rectangular mesa structures were fabricated on the surface of an under-doped Bi2212 crystal by using e-beam lithography and Ar-ion etching step by step. On account of the difficulties in making a contact on small area of the mesa, CaF2 insulating layer deposition was performed. After that, a gold stripe with the width of 30 μm was created by lift-off technique on the mesa and CaF2 layer. Finally, three gold probe wires were connected to the two contact paths and mesa by silver epoxy. After the mesa fabrication, the exact dimensions of the mesas were obtained using atomic force microscope. To obtain the electrical characterization, c-axis resistance versus temperature (R-T), and current-voltage behavior (I-V) were measured. From I-V characteristics, critical current of each mesa structure having different dimension was obtained, after that, we have calculated the critical current densities of each mesa structure and then we have studied change in Josephson critical current density of mesas with different dimensions. We can conclude from the I-V measurements of the mesas that the Josephson critical current density is decreasing when the area of mesa is increasing. Furthermore, the backbending voltage points are increasing since heating effects dominate for the large areas of the mesa structures.