Andreev bound state spectroscopy of phase-biased topological insulator Josephson junction

Topological superconductivity and the associated Majorana physics remain a hot and controversial topic in condensed matter physics. In the Ando group at Cologne, we focus on bulk-insulating topological insulators (TIs) in combination with conventional superconductors to realize devices exhibiting signs of topological superconductivity in transport experiments.

Recently we demonstrated for the first time the ability to probe the bound state spectrum of a TI Josephson junction with a tunnel contact deposited directly on it [1] and showed that the spectrum can be tuned via the superconducting phase difference. For this platform, a topological phase transition is expected, whenever the phase difference reaches an odd multiple of π [2]. Indeed, we observe a spectrum consistent with a periodic gap-closing and reopening. Furthermore, in line with expectations, the gap-closing is found to be robust with respect to tuning the chemical potential. For the case of a finite magnetic field in the junction, the formation of Josephson vortices is expected, which host Majorana zero modes at their cores. While we do observe a spatial dependence of the gap-closing consistent with vortex formation, our energy resolution is so far insufficient to resolve individual states. Possible ways to overcome this limitation will be discussed. We also discuss alternative possibilities to cause the gap-closing phenomenology.

In addition, I will briefly introduce other experiments performed in Cologne to address topological superconductivity to be realized in proximitized TIs.

[1] J. Schluck et al., https://arxiv.org/abs/2406.08265v1

[2] L. Fu and H. Kane, Phys. Rev. Lett. 100, 096407 (2008)