Josephson nano-junctions fabricated with a focused He-ion beam

• Applicant: Dr. Edward Goldobin
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 424544156

The recently developed and commercially available He ion microscope (HIM) provides a highly focused He ion beam (He-FIB) with spot size ~0.5 nm. This enables not only ultra-high-resolution imaging, but also offers exciting perspectives for the controlled modification of thin films on the nm scale. Within this project, we will explore the potential of controlled nanoscale modification of thin films made of cuprate superconductors by using a He-FIB. The choice of cuprate superconductors is motivated by their peculiar properties: (i) the ultra-short coherence length allows one to change their properties on the atomic scale, (ii) the d-wave symmetry of the superconducting order parameter enables the realization of unconventional Josephson junctions (JJs), e.g. geometric pi JJs, and (iii) upon variation of doping one can drive these materials from the superconducting to the insulating state.The project mainly focuses on the well-studied cuprate superconductor YBCO, where doping (oxygen content) can be modified locally by He-FIB irradiation. By “drawing” a line using a He-FIB of proper dose across the YBCO film one can create a Josephson barrier. Such JJs can be freely distributed on a chip and can have different orientations. The critical current densities can controllably vary on a chip by adjusting the He-FIB dose. Using He-FIB irradiation with high dose one can “write” highly resistive walls and areas. This will be used to realize ultra-narrow constriction JJs, superconducting quantum interference devices (SQUIDs) and more advanced Josephson devices. Adjustable irradiation dose and ultra-high spatial resolution shall be exploited to create novel superconducting nano-scale electronic devices and circuits that were not feasible so far.The project is divided into three major parts: optimization of the He-FIB technology for nanoscale modification of cuprate superconductors; studies on basic properties of He-FIB induced JJs and SQUIDs and advanced devices. Altogether, we intend to examine the fundamental properties of the JJs made by a He-FIB in cuprate superconductors, understand the limitations of this new technology and explore the perspectives for the realization of more complex and advanced JJ devices and circuits.

• DFG Programme: Research Grants
• International Connection: Sweden, USA
• Cooperation Partners: Professor Dr. Thilo Bauch; Professor Dr. Shane Cybart