The ERC-Project “Engineered Topological Superconductivity in van der Waals Heterostructures” (TopSupra) addresses two aspects of high interest: topological matter and two-dimensional (2D) van der Waals (vdW) materials.
Topological matter is a young research theme with great perspectives. A topological insulator (TI) is a “strange” insulator with an inverted “negative” bandgap. Since the bandgap must be positive for the vacuum, it must cross zero at the surface. A characteristic feature of a TI is, therefore, a conducting surface state.
Alike a TI, a topological superconductor (TSC) is also gapped in the bulk and has a special surface state, which is pinned to zero-energy due to electron-hole symmetry. The in-gap quasiparticles live at zero energy and carry neither charge nor spin. But they have a special “non-Abelian” exchange statistics that can be used for quantum computing. Although first indications of a TSC were reported in 1D systems, TopSupra aimed to search for evidence for a TSC in vdW heterostructures.
This platform builds on graphene, a 2D material that is obtained by exfoliating graphite down to the monolayer (ML). It can be combined with other layered materials by stacking 2D materials on top of each other to realize heterostructures with novel properties. The technology of stacking different 2D layers together has been developed to such a perfection that also arbitrarily twisted heterostructures are studied today.
In TopSupra a large fraction of activity was devoted to searching for the fractional Josephson effect (FJE). The FJE can appear in Josephson junctions (JJs) made from TSCs. While there are multiple claims of evidence for the FJE in contemporary literature based on missing odd Shapiro steps, we could show that missing Shapiro steps can be generated even in the most basics non-topological JJ. Further on, we have conducted an intensive search for the fractional AC radiation. Although we have tested a series of potential candidates for a TSC we have not observed any evidence for the FJE. However, we often find signals at frequencies expected for the FJE due to spurious resonances that form in the environment: Phys. Rev. B 108, 94514 (2023).
In our studies of different vdW materials that are supposedly TIs, we have found one material that provided the largest degree of evidence for “topology”. This is WTe2. While MLs of WTe2 are QSH insulators, the few layers variant is a Weyl semimetal. In TopSupra we discovered strong edge currents, a breaking of inversion symmetry, and a surprisingly robust long distant supercurrent which led us to speculate that few-layer WTe2 may be a higher-order topological insulator (HOTI): “One-dimensional edge transport in few-layer WTe2”, Nano Lett. 20, 4228 (2020) and follow-up papers, J. of Appl. Phys. 129, 113903 (2021), Phys. Rev. Mat. 6, L081201 (2022), and “Current-phase relation of WTe2 Josephson JJs”, Nano Lett. 23, 4654 (2023).
Further on, we reported on the superconducting diode effect: Phys. Rev. Res. 5, 33131 (2023) and on “Charge-4e supercurrent in an InAs-Al superconductor-semiconductor heterostructure”, Comm. Phys. 7, 41 (2024).
Most recently, we were able to realize superconducting devices in magic-angle twisted trilayers of graphene. We measured the kinetic inductance in the intrinsic superconducting phase and found a huge value exceeding the highest known values by up to two orders of magnitude.
The project took quite a different path than initially anticipated. As time passed on, I got more and more skeptical regarding claims of topological superconductivity. I would also like to stress that a conventional shorter-term funding would not have allowed me to research to the same depth.
