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Heavy-Element Nanowires

Final Report Summary - HELENA (Heavy-Element Nanowires)

At the start of the project, we had just observed the first signatures of Majorana states in InSb nanowires. These Majorana states are interesting to detect because 1) they represent a new class of yet unobserved particles -non-Abelian anyons- and their detection is important to test theoretical models, 2) these particles are expected to be ideal carriers of quantum information. The great challenge in the field of topological quantum computation is to unambiguously demonstrate the existence of Majorana states. For this, the host material system needs to be improved. This quest has opened a new field in material science with the aim to identify better semiconductors and superconductors with a special focus on their interface.

In this ERC CoG project, we have set-up a cluster of Molecular Beam Epitaxy (MBE) chambers to combine different classes of semiconductors with superconductors. An MBE chamber for the low-temperature deposition of superconductors has been developed. This set-up is unique in the world and has already led to improved material quality and the detection of new Majorana properties.

In this system, we developed the next generation material system for the detection and manipulation of Majorana states. In this new system, quantized zero bias peaks have been observed, which is an important additional signature of a Majorana bound state. To improve the electronic properties, we have developed a route to grow ultra clean InSb nanowires, in which the electron mobility has been significantly increased. The interface between the semiconductor nanowire and a superconductor has been improved by in-situ deposition of the superconductor (Al), using a shadow configuration. This is a generic approach, which has been developed in this project and which is now being followed by our and many other groups to investigate different super/semi combinations. A special challenge is to generate an atomically flat semi/super interface, since disorder at the interface will impact the overall device quality. This new approach also enabled the growth of nanowire hashtags, a loop of semiconductor material, which will be used to move around the Majorana states. These out-of-plane hashtag structures are single-crystalline and exhibit good transport properties. With this growth mechanism also other structures, such as nanoflakes can be grown, which allows for more complicated shadow structures. This project has yielded a wealth of information on the nanowire growth mechanism and the epitaxial growth of superconductors on the surface of a semiconductor.

In parallel, we have developed a completely new material system, which should have even better, and more extreme properties. We have established the growth of PbTe wires for the first time in a Molecular Beam Epitaxy (MBE) reactor. This material is expected to have stronger spin-orbit interactions and a higher electron mobility. An important advantage of growth in an MBE reactor is that the superconductor can be grown in-situ enabling a clean interface. In addition, PbTe can be combined with Pb as a superconductor such that interface reactions will not lead to interface roughening. This new material system will be studied in more detail in recently started follow-up projects.

Due to this ERC CoG project, we are (still) at the forefront of topological quantum computation and are able to develop new materials and unexplored material combinations. Due to the successes of the project, we attract top level students and world leading scientists to our university.