Novel Josephson junctions

Novel high-Tc step-edge Josephson junctions for biomagnetometers

Ar+ ion beam etching and negative photoresist nLOF2020 were used for the preparation of 45o steps on (001) MgO substrates. The step was covered by a double buffer layer: a bottom 10-nm-thick non-superconducting YBa2Cu3O7-x (YBCO) film as a “seed” layer that was covered by a 30-nm-thick SrTiO3 film as a “blocking” layer. High-resolution transmission electron microscopy images show two 45o [100]-tilted grain boundaries in the top YBCO layer: one at the top corner of the step and other at the lower corner of the step. The top YBCO layer has a thickness of 200 nm, a superconducting transition temperature Tc of 93 K and a critical current density of 5 MA/cm2 at 77 K. The 2-µm-wide Josephson junctions have normal state resistance Rn=20 ohm and IcRn product 800 µV at 77 K. Magnetic field resolution of high-Tc SQUID biomagnetometers 2 fT/rt(Hz) at 77 K was achieved.

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High-Tc step-edge Josephson junction on MgO substrate.

  1. M. I. Faley, Patent US 9666783 B2 granted 30.05.2017.
  2. M. I. Faley et al., Superconductor Science and Technology 30, 083001 (2017). https://doi.org/10.1088/1361-6668/aa73ad
  3. M. I. Faley et al., IEEE Transactions on Applied Superconductivity 28 (4) 1600505 (2018).

Josephson junctions between d-wave and s-wave superconductors

Josephson junctions between d-wave superconductor YBa2Cu3O7-x (YBCO) and s-wave superconductor Nb were realized. The normal conducting Au barrier between Nb and YBCO served also as a diffusion barrier for oxygen ions. In π-loops based on such junctions the positive [100] and negative [010] lobes of thin film of YBCO are connected by Nb film. Exactly half quantum of magnetic flux is appeared due to the spontaneously induced persistent currents in such pi-loop. Such π-loops are intended for implementation in self-biased superconducting flux qubits that have potentially longer coherence time because they are naturally decoupled from perturbations produced by the environment: their double-well energy potential is symmetrized by integration in the qubit a pi-junction instead of using a conventional inductance with an external current sources.

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SEM image of YBCO-Au-Nb Josephson edge junction produced at PGI-5 FZJ.
  1. Novel JosephsonH. Hilgenkamp et al., Nature 422, 50, (2003).
  2. M. I. Faley et al., ASC'2018 (2018).

Nanofabrication and Quantum Sensing

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Prof. Dr. Michael Faley

Phone: +49 2461 61-4366
Fax: +49 2461 61-6444
E-Mail: m.faley@fz-juelich.de

Last Modified: 15.06.2022