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Four experiments in Topological Superconductivity.

Periodic Reporting for period 2 - 4-TOPS (Four experiments in Topological Superconductivity.)

Reporting period: 2018-12-01 to 2020-05-31

The ERC-AG 4-TOPS project seeks to advance our understanding of induced superconductivity in topological materials. Inducing superconductivity into the surface or edge states of topological insulators via the proximity effect is the most promising route to studying topological superconductivity at present. We aim to lay the groundwork for future applications such as topological qubits for quantum computation in topological materials.
This report summarizes the research activities in the period from 01/06/2017 to 30/11/2019.
Our first research direction concerns the investigation of the Josephson effect in topological insulators (TIs). For this purpose we employ interferometric and spectroscopic methods. We succeeded in mapping out the current-phase relation (CPR) of HgTe 2D TI Josephson devices which were placed in asymmetric DC SQUID interferometer configuration. The modulation of the Josephson supercurrent with an applied magnetic field traces the CPR of the test junction. We obtained negatively-skewed CPRs comparable to previous results on HgTe 3D TI Josephson devices. To carry out spectroscopic measurements on the Andreev bound states that carry the supercurrent in the Josephson junction, we need to couple the device to a co-planar microwave resonator structure. We tested various microfabrication methods and concluded that standard etching methods damage the surface of the TI material resulting in low resonator quality factors. The problem was overcome fabricating the resonator on a separate substrate and coupling the RF SQUID with the embedded test junction inductively to it in “flip-chip” configuration.

Tunneling spectroscopy can be used to obtain information about the spatial localization of Majoranas, i.e. topological quasiparticles that exist close to the interfaces between superconductors and topological insulators. Our second activity concerned the development of a suitable tunneling barrier to conduct tunnel spectroscopy measurements. We successfully demonstrated quantized conductance in a Quantum Point Contact (QPC) of a HgTe quantum well. By creating QPCs with narrow constrictions in the TI, we realized tunnel contacts with electrostatically tunable barrier height, suitable for tunnel spectroscopy measurements. In parallel, we developed processes to exfoliate and place thin layers of hexagonal boron nitride on HgTe devices which can be used as tunnel barrier or gate dielectric.

A third area of research comprises the exploration of induced superconductivity in novel topological materials. We placed superconducting electrodes on vanadium-doped (Bi1-xSbx)2Te3 , a material that exhibits the Quantum Anomalous Hall effect, and tested for the Josephson effect and for chiral Majoranas. Compressively strained bulk HgTe is a Dirac/Weyl semimetal. In this material we obtained a robust Josephson effect. Supercurrent diffraction patterns exhibit different character for n- and p-carrier regimes. When gating the device electrostatically with a top gate, they evolve distinctly with applied (perpendicular) in-plane magnetic field, suggesting a change in the modulation of the Cooper pair wavefunction.
On-going spectroscopic measurements are expected to clarify the topological character of the Andreev bound states in TI Josephson junctions. The realization of QPC devices in 2D TIs is a major technological feat that will enable us not only to probe localized Majoranas but also to manipulate these quantum states by tuning the interactions between them electrostatically with a gate. This constitutes an important step towards the implementation of qubit devices. Follow-up measurements on QAHE and Dirac/Weyl materials are directed at probing the induced pairing symmetry. We will further explore the role of the chiral anomaly and the contribution of Fermi arcs to supercurrent transport in the Dirac/Weyl semimetal.
Quantum point contact (QPC) in a HgTe quantum well.