The combination of topological insulators (TIs) with a conventional superconductor has been pro-posed to create Majorana bound states (MBS), which were the focus of this project. We pursued four main approaches to probe MBS:
For two approaches, one-dimensional wire structures with a few electronic modes and conductance quantization were needed. Conductance steps are usually multiples of e^2/h, but we found anoma-lous steps with fractional e^2/h values in a perpendicular magnetic field. This unexpected result com-plicates the use of quantized conductance to probe MBS. A publication on this topic is in preparation.
The third approach investigated MBS in HgTe wires utilizing the fractional ac Josephson effect, with the normal part of the junction composed of a topological insulator wire. This experiment, which relies on tuning the wires' band structure with an axial magnetic flux, predicted a 4π-periodic current in the topological case. The suppression of odd Shapiro steps under microwave irradiation indicated a 4π-periodic supercurrent, suggesting the presence of MBS. Further analysis showed trivial suppres-sion of Shapiro steps at zero magnetic field but potential MBS-related suppression at higher fields [1].
Further experiments revealed a Fraunhofer-like interference pattern in samples with lower transpar-ency between the superconducting contact and HgTe material. This pattern, occurring despite the magnetic field being parallel to the current direction, was traced to flux-enclosing paths winding around the TI nanowire, highlighting the three-dimensional junction geometry [2].
The fourth approach involved pinning magnetic flux in a hybrid HgTe structure with a supercon-ducting film and periodically arranged holes (antidots), forming a lateral superlattice. MBS were ex-pected to bind to these vortices, affecting the electronic density of states. While density of states measurements have not yet detected the expected MBS signal, the first commensurability (Weiss) oscillations in a gate-defined lateral superlattice in 3D HgTe were observed [3].
Additionally, we investigated the properties of HgTe, which hosts various charge carriers. Surprisingly, the sample exhibited a well-quantized Hall effect in high magnetic fields due to charge redistribution among the electron subsystems, resulting in a single quasi-2D electron system, which shows a clear quantum Hall effect [4]. Another study compares quantum capacitance and conductivity, demon-strating the merging of electronic subsystems more directly. Current distribution measurements showed that in the quantum Hall regime, current flows across the entire sample width rather than just at the edges.
[1] R. Fischer et al., Phys. Rev. Research 4, 013087 (2022)
[2] W. Himmler et al., Phys. Rev. Research 5, 043021 (2023)
[3] A. Koop et al., Phys. Rev. Research 6, 023153 (2024)
[4] J. Ziegler et al., Phys. Rev. Research 2, 033003 (2020)
