Nathan Satchell, Texas State University
When
Where
Title: Hiding the Magnet: Building Better π Josephson Junctions
Abstract: A Josephson junction, two superconductors separated by a thin barrier, carries a supercurrent set by the quantum-mechanical phase difference across it. With a ferromagnetic barrier of the right thickness, the junction's preferred phase difference shifts from zero to π, a built-in half-turn. Such π junctions could serve as phase shifters in superconducting classical and quantum computers, making circuits smaller and simpler. Making good ones is a materials challenge: supercurrent dies away quickly inside a ferromagnet, and its magnetization adds unwanted magnetic flux [1].
I will describe two solutions. The first, the weak ferromagnet PdNi, passes a large supercurrent in the π state and hides its magnetization from the junction by pointing it along the current [2]. The second uses altermagnets, a newly recognized class of magnets with spin-split bands but no net magnetization. Our first junctions with the candidate Mn₅Si₃ pass a supercurrent but do not yet show a π state [3].
Finally, I will introduce point-contact Andreev reflection, a technique from the superconducting toolbox once widely used to measure spin polarization in ferromagnets. I will show how it can become important for altermagnets: in films of another candidate, RuO₂, we measure a finite spin polarization of the current [4].
[1] N. O. Birge and N. Satchell, APL Mater. 12, 041105 (2024). [2] A. Sapkota et al., Appl. Phys. Lett. 128, 102601 (2026). [3] A. Sapkota et al., arXiv:2608.14842 (2026). [4] A. J. Howzen et al., arXiv:2601.19052 (2026).
Bio: Nathan Satchell joined Texas State University in 2023 as an Assistant Professor in the Department of Physics. His current research is funded by the National Science Foundation. Previously, he was a Postdoctoral Fellow at the University of Leeds (UK), Michigan State University, and the ISIS Neutron and Muon Source (UK). He held a Marie Skłodowska-Curie Global Fellowship (2017–2020) and received UK government funding (2021–2023). He obtained his PhD (2016), MPhys and BSc (2012) from the University of Leeds.
3:00 PM in PAS 201 / YouTube
Refreshments in PAS 236, 2:30PM