SFB 1170


Cooper pair injection into edge states of two-dimensional topological insulators


The prospect of implementing concepts of topological quantum computation is the powerful driving force behind the search for physical systems that host topological quantum states. These can arise from an interplay between superconductivity and band structure topology. One promising platform for topological superconductivity are hybrid structures of two-dimensional (2D) and three-dimensional (3D) mercury telluride topological insulators (HgTe TIs) that are coupled to conventional s-wave su- perconductors. Previously, we had demonstrated proximity-induced superconductivity in 3D TI HgTe Josephson junctions. In the first funding period, the project focused on developing the necessary mi- crofabrication techniques for inducing superconductivity in the quantum spin Hall (QSH) edge states of 2D TI HgTe. The technological improvements enabled our recent experiments on the 4π-periodic Josephson effect in 2D and 3D HgTe TI Josephson junctions that provide some of the most com- pelling evidence for the existence of Majorana bound states available to-date.

In the second funding period, we want to advance our research in the direction of localization and controlled interactions of topological excitations. Initially, we will focus on the microscopic mecha- nisms that enable the observation of the fractional Josephson effect with and without explicitly break- ing time-reversal-symmetry (TRS). The experiments will address dissipation, interaction effects that couple Andreev bound states, and the possibility of generating a spontaneous magnetization in an inhomogeneous topological superconductor. On the technical side, we will explore new, noninvasive techniques for incorporating ferromagnetic elements in TI nanostructures, with the objective of local- izing Majorana bound states. A second focus topic will be tunable interactions of QSH edge channels in quantum point contacts (QPC) with superconducting electrodes. We will fabricate Josephson junc- tions with an embedded QPCs as sensitive detector of interedge scattering by generating a π-phase shift in the current-phase relation of the device. A similar setup will be used to study odd-frequency correlations and Majorana Kramers pairs in nonlocal conductance measurements. The program will be complemented by an investigation of the Josephson effect in CdTe-HgTe core-shell nanowires—a new, quasi-one dimensional topological material of technological relevance.