Optimizing interfaces between topological insulators and superconductors: Towards a local detection of Majorana states

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The project aims at the preparation of interfaces between topological insulators (TIs) and s-wave superconductors (SCs) hosting detectable, robust Majorana excitations (MEs) within vortices. This aim faces three major challenges. Firstly, the Dirac point energy ED of the TI has to be rather exactly at the Fermi level EF in order to reduce the density of trivially confined states within the vortex and, thus, to increase the minigap which protects the MEs. In the first project phase, we have established two methods to tune ED to EF, either by adapting the stoichiometry or by growing a p-n junction of variable thickness. Secondly, the ME signal must be disentangled from the Caroli-de Gennes-Matricon states of the SC. Our approach is to remove the SC within the vortices or, more precisely, to pin the vortices within holes of the s-wave SC. This has the additional ad¬vantage that multiple flux quanta can be caught by the holes such that the ME signature disappears in case of an even number of flux quanta. Towards that goal, we have established an ultrahigh-vacuum (UHV) mask aligner, which transfers mask structures to a substrate with 100 nm precision. Thirdly, the ME signatures have to be detected by scanning tunneling spectroscopy (STS). To this end, we have established STS down to 370 mK in UHV and B-field with an energy resolution of 0.1 meV and a voltage noise below 0.02 mV, thus, providing the required properties for the detection of MEs. Besides these achievements, we pursued a project resulting from discussions within the priority programme, namely we confirmed the predicted weak TI properties of Bi14Rh3I9 (first experimental realization of a weak TI) by probing its topologically protected edge states and, meanwhile, identified BiTe as a second weak TI. Within the second funding period, we firstly aim at the deposition of an adequate SC starting with FeSe, which has been shown to exhibit exceptional critical temperatures, if deposited as a monolayer on SrTiO3 (110 K) or BaTiO3 (70 K) or as a multilayer after adequate doping (50 K). However, since it is not clear a priori, if FeSe gets superconducting on BiSbTeSe alloys, we might get back to more conventional superconductors as NbSe2, Nb or Pb. Secondly, we will prepare the hole structure using mask alignment in UHV independently from the optimization of the SC. Finally the resulting structures will be probed in detail by STS varying B-field, temperature, the local chemical potential, the distance between adjacent holes, and, possibly, the materials combination. Note that the mask aligner technique can also be transferred to weak TIs, which should host MEs at the end of a proximity induced edge state.