During the two years of the project, Objective 1 and Objective 2 have been accomplished. Objective 3 has been explored, but conclusive and publishable results have not been achieved.
Objective 1): we explored the possibility to induce superconductivity in high mobility InAs heterostructures via deposition of superconducting electrodes. This resulted in low superconductor/semiconductor interface transparency and resulting low probability of Andreev reflection. Technical issues related to the fabrication of superconducting contacts on the buried InAs quantum well delayed the project by six months, approximately.
We therefore developed a new material platform consisting of an InAs quantum well grown very close to the surface of the wafer and capped by a thin layer of superconducting Al. The Al layer is grown in-situ, meaning in the same deposition chamber used for the semiconductor and results in a pristine interface with epitaxial atomic match. The main drawback with respect to the initial idea is the low electron mobility of two-dimensional electron gases grown close to the wafer surface. Hybrid devices in the InAs/Al heterostructures demonstrated large supercurrents, large and hard induced superconducting energy gaps and optimal gate tunability. Further research demonstrated the possibility to deposit a second superconductor on top of the Al layer with conventional techniques. In this way an even larger superconducting gap is induced in the density of states of the semiconductor.
This work was published in the following peer reviewed scientific articles:
[M. Kjaergaard et al. Phys. Rev Applied 7, 034029 (2017)], [H. Suominen et al. Phys. Rev. B 95, 035307 (2017)], [A. Drachmann, Nano Lett. 17, 1200 (2017)], [M. Kjaergaard et al. Nat. Commun. 7, 12841 (2016)], [J. Shabani et al. Phys. Rev. B 93 155402 (2016)]
Objective 2): We realized superconductor/semiconductor nanowires in a two-dimensional platform by lithographic techniques and gating instead of using bottom up self-assembled nanowires. These devices allow for a much more flexible implementation of complex designs for realizing large networks of Majorana modes, compared to nanowires.
Performing tunneling spectroscopy experiments in nanowire at high magnetic fields, we observe Andreev bound states coalescing to form zero energy states compatible with Majorana zero modes. The initial discovery was published in an article, currently in press in Physical Review Letters: [H. Suominen et al. arXiv:1703.03699].
A subsequent work demonstrated extensive characterization of the zero bias peaks, showing consistencies with a theory for Majorana zero modes. [F. Nichele et al. Phys. Rev. Lett. 119, 136803 (2017)].
Objective 3): Initial steps towards the realization of a large network of Majorana wires were taken. In particular, we studied the properties of a large array of superconducting Al islands patterned on a planar InAs/Al heterostructure, with the coupling between islands controlled by a gate voltage. The great tunability offered by our material allows to tune the resistivity of the sample in a range larger than eight orders of magnitude. In this regime we observe superconducting, insulating and anomalous metallic regimes. The anomalous metallic regime is attracting considerable attention lately, as its nature is not understood. This result will be published in a paper currently in preparation.
