Fabrication of MgB₂ nanobridge dc SQUIDs by focused ion beam

다국어 초록 (Multilingual Abstract)

We have studied fabrication of MgB<SUB>2</SUB> intragrain nanobridge dc SQUIDs by a focused ion beam (FIB) patterning technique. Not only the nanobridges as Josephson elements but the SQUID loop was patterned by FIB. The beam voltage was 30kV and the beam current was 0.9nA for the SQUID loop and 1.5pA for the nanobridges. Each bridge had a nominal width and length of about 100nm and a thickness of 650nm. The SQUID loop had a 3μmx3μm hole with a 2μm average linewidth. The zero-field superconducting transition temperature (T<SUB>c</SUB>) of the SQUID was 37K. Current-voltage (I-V) characteristics of the SQUID showed large excess currents at all temperatures with a small portion of a resistively-shunted-junction (RSJ) component which increases as temperature approaches T<SUB>c</SUB>. At low temperatures, the I-V curves exhibit a large heating effect with a second transition step, which is believed to be due to the transition of a grain boundary near the nanobridge. The SQUID showed well-behaving modulation properties at all temperatures with a modulation depth of more than 30μV at 33.5K and 110μV at 15K. These results together with our previous results on the intergrain nanobridge dc SQUID suggest that fabrication of dc SQUIDs based on FIB-patterned MgB<SUB>2</SUB> nanobridges is highly tolerant of fabrication conditions.