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Host: Prof. Xin Liu
Zoom Meeting link: https://zoom.us/j/94188196165?pwd=LxI4rbLfMp37LGb29BI3ZP5YuQdVwf.1
Meeting ID: 94188196165, Password: 123456
Abstract:
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, we have established a technology to contact a TI nanowire (TINW) from the side using an etching approach. This approach allows us to introduce proximity-coupled superconductivity to both top and bottom surfaces. Tuning the magnetic flux along the TINW, we observed a pronounced SQUID-like oscillation of the critical current [3]. This demonstrates the surface nature of the supercurrent. From a
conceptual point, this setup can be considered as two copies of the Fu-Kane junction, making it an interesting platform for topological superconductivity. In addition, we also found a strong and tunable Josephson diode effect [4].
References:
[1] J. Schluck et al., https://arxiv.org/abs/2406.08265v1
[2] L. Fu and C. Kane, Phys. Rev. Lett. 100, 096407 (2008)
[3] E. Nikodem et al., https://arxiv.org/abs/2412.07993
[4] E. Nikodem et al, Sci. Adv. 11, eadw4898 (2025)
Biography:
2008/4-2011/9 | Bachelor studies in physics at the Heinrich-Heine-University Düsseldorf |
2011/9-2013/6 | Master studies in physics at the Heinrich-Heine-University Düsseldorf |
2016/6-2016/8 | Visiting researcher at the Institute of Solid-State Physics, Tokyo University |
2014/1-2018-7 | PhD student at the Condensed Matter Physics Laboratory at the Heinrich Heine-University Düsseldorf |
Occupation
2013/9-2018/6 | Scientific employee at the Condensed Matter Physics Laboratory, Heinrich-Heine-University Düsseldorf |
2019/2- present | Scientific employee at Institute of Physics II, University of Cologne |
Awards
2009-2013 | Fellow of the German Academic Scholarship Foundation |
2016 | Fellow of the Summer Program of the JSPS |
2022 | ML4Q Independence Grant |