# Cologne team sees signs of gap closing in a topological insulator Josephson junction at 50 mK

- Published: 2026-11-05 23:31 UTC
- Section: Science › Physics
- Tags: topological insulator, Josephson junction, Andreev modes, gap closing, spin-momentum locking, Majorana zero modes, topological superconductivity, BiSbTeSe2, Fu-Kane proposal, University of Cologne
- URL: https://realtech.media/news/rt-b111c2fa27b0817421c31a38_cologne_team_sees/

**Physicists at the University of Cologne measured highly transmissive Andreev modes in a 50-nm junction on a topological insulator. The data fit a 2008 prediction but do not prove it.**

![Cologne team sees signs of gap closing in a topological insulator Josephson junction at 50 mK](https://sgzwpqqelzmajkcubgkl.supabase.co/storage/v1/object/public/rt-media/2026/11/00e43b328e61cb1a.jpg)
*Earth.com*

A team at the University of Cologne, with theorists from RWTH Aachen University, has reported highly transmissive Andreev modes in a Josephson junction on the surface of a topological insulator. The results were published in Science Advances. The junction is about 50 nanometers wide and was measured at 50 millikelvin. Within the experimental resolution, the signal could indicate that the energy gap closes and reopens [1].

## Device and measurement

The junction consists of two aluminum strips on a flake of BiSbTeSe2, 10 to 20 nanometers thick. The strips end about 50 nanometers apart and are connected by an aluminum loop, so a small magnetic field sets the superconducting phase difference. Two gold electrodes, separated from the junction by a thin oxide layer, probe the energies of states inside it at different locations. The experiment was done by Jakob Schluck and Ella Nikodem in Yoichi Ando's lab; the simulations were run in Fabian Hassler's group.

At zero field the readout showed an energy gap, with two peaks at 130 microelectronvolts on either side of zero. As the field tuned the phase toward the halfway point, the peaks moved inward and merged into one broad peak at zero. A stronger field reopened the gap, and the pattern repeated every 230 microtesla, which matched the device dimensions [1].

## What the data support and what they do not

Fu and Kane proposed in 2008 that spin-momentum locking in the surface state guarantees perfectly transmitted Andreev modes, which close the gap at a particular phase difference [1]. The observed pattern persisted while the gate voltage was swept from -20 to +20 volts, shifting the surface electrons' energy by at least 40 millielectronvolts. It also appeared in a second, nearly identical device.

In fields of 5 to 6 millitesla, the closing appeared only at particular spots. The two electrodes saw it at different times, in line with theoretical predictions for Josephson vortices in the surface state [1].

The experiment resolves energies down to about 30 microelectronvolts, and a smaller gap would look closed. By the authors' calculation, transmission of 95% or better would produce the observed signal. The authors state that the result fits the prediction but is not proof of it. A stray state in the insulating layer produced a similar signal in one device, but only at a single gate voltage. Disordered junctions can also close a gap through a few open channels, which the authors consider unlikely to persist over such a wide gate sweep.

The predicted transition to a topological superconducting state has not been confirmed. Demonstrating it requires better energy and time resolution, which Schluck named as the next step for the Ando lab.

## Sources

1. [Science Advances](https://doi.org/10.1126/sciadv.aeg0687)
2. [Earth.com](https://www.earth.com/science/quantum-experiment-reveals-signs-of-long-predicted-protected-states)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-b111c2fa27b0817421c31a38_cologne_team_sees/