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Friday 14 December 2018, 3:00pm to 4:00pm
Nanobridges as Josephson junctions: Shapiro steps as a probe of overheating
Abstract: Modern nanofabrication techniques has led to relatively easy realisation of sub-50 nm structures. These advantages in fabrication technology have opened the door for creating superconducting nanobridge based Josephson junctions in place of more “traditional” insulator (SIS), or normal barrier (SNS) junctions. The advantage of nanobridges is the ability to create Josephson circuits in a single-lithography step. In the last decade nanobridges have been implemented in nanoSQUIDs in order to create SQUIDs with very small loop sizes for single magnetic particle detection. A disadvantage of using nanobridges is the Joule heating that occurs when the junction enters the normal state. A hot-spot is formed that is suggested to extend deep into the banks and results in a thermal hysteresis in the junction current-voltage characteristics. I will present recent work whereby microwave-induced Shapiro steps are used to infer a reduced critical current in the hysteretic regime, an estimate of the effective local temperature, and ultimately an upper limit on the “length” of the junction. I propose that these results suggest that nanobridge based devices can be used as Josephson junctions, even in the hysteretic regime. The Shapiro step method can be used to further optimise thermal behaviour in these junctions. Finally, I will overview our work related to nanobridges in general – SFQ-based readout of SNSPDs, and nanobridge based voltage standards.
Dr. Connor Shelly
National Physical Laboratory, UK