Phase interference for probing topological fractional charge in a TI-based Josephson junction array

Fractional charges can be induced by magnetic fluxes at the interface between a topological insulator (TI) and a type-II superconductor due to axion electrodynamics [1]. In a Josephson junction array with a hole in the middle, these electronic states can have phase interference in an applied magnetic field with 4×2π period, in addition to the 2π interference of the Cooper pairs.

In our previously published work [2], we test an experimental configuration for probing the fractional charge and report the observation of phase interference effect in superconducting arrays with a hole in the middle in both Au- and BiSbTeSe₂-based devices. Our numerical simulations based on the resistive shunted capacitive junction model are in good agreement with the experimental results. However, no clear sign of an axion charge-related interference effect was observed.

In the past year, we fabricated Josephson junction array devices on thin films of the topological insulator (Bi_1-xSb_x)₂Te₃, where these films were deposited by molecular beam epitaxy. These arrays again show clear oscillations that match the simulations very well, but in addition show oscillations with a period that hints towards the e/4 charges. More samples are being fabricated to investigate the exact nature in greater detail.

[1] F.S. Nogueira et al., Phys. Rev. Lett. 121, 227001 (2018).

[2] J.M. Brevoord, D.H. Wielens, et al., Nanotechnology 32, 435001 (2021).