Multiterminal Josephson Junctions: Quartets, Sextets, Non-Hermiticity and Topology

In multiterminal Josephson junctions (MTJJs), the Andreev bound state (ABS) energies depend on multiple phase differences, enabling band structure engineering with external flux control. This tuneability offers unique controllability of the momentum-like phases, granting insight into unique transport processes as well as phase transitions. In three-terminal devices so-called quartet processes have been predicted to appear under voltage biasing [1,2]. Additionally, MTJJs are predicted to host non-trivial topological phases and associated Weyl nodes in the synthetic Brioullin zone spanned by the superconducting phases [3].

In a recent work [4], the hybridisation of ABSs was observed via spectroscopy with complete phase control. We managed to extract the ABS energy out of this experimental data to detect quartet contributions via a tomography scheme [5]. Additionally, we predict that MTJJs can host non-trivial non-hermitian topology, leading to spectral topology in the form of point gaps and Weyl disks [6]. However, the specific role of Quartets (and similar processes, e.g., Sextets) in the transport, and whether they can provide useful information for future studies concerning the topological properties of MTJJs, is not yet clear. Additionally, the ultimate reason behind the existence of Weyl nodes in certain MTJJs is still a mystery. We aim to clarify these questions, by combining several of the above-mentioned aspects to grant a fresh perspective on MTJJs [7].

[1] J. C. Cuevas and H. Pothier, Phys. Rev. B 75, 174513 (2007)

[2] A. Freyn et al., Phys. Rev. Lett. 106, 257005 (2011)

[3] R.-P. Riwar et al., Nat. Commun. 7, 1 (2016)

[4] M. Coraiola et al., Nat. Commun. 14, 6784 (2023)

[5] D. C. Ohnmacht et al., Phys. Rev. B 109, L241407 (2024)

[6] D. C. Ohnmacht et al., arXiv:2408.01289 (2024)

[7] D. C. Ohnmacht et al. (in preparation)