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HELENA Report Summary

Project ID: 617256
Funded under: FP7-IDEAS-ERC
Country: Netherlands

Periodic Report Summary 2 - HELENA (Heavy-Element Nanowires)

We explore the growth of nanowires based on heavy elements, such as InSb and PbTe. Due to their special electronic and thermal properties, these materials have promising applications in topological quantum computation and thermoelectrics. We focus on the growth of high quality InSb wires and new wire architectures, such as networks. In addition, we develop a new class of nanowires based on II-IV-VI elements. During the first phase of the project we have developed an approach to fabricate nanowire networks. The basic platform for this is a (100)-crystal oriented substrate in which trenches have been etched exposing (111)B facets. Using ebeam lithography, catalyst particles have been defined on these sloped facets. When two wires grow from opposing facets they can merge, forming a network. In addition, this geometry has been used to shadow-grow superconductor islands on the wires. By tuning the number of wires standing in front of another wire, the number of superconducting islands can be defined. The superconductor has been grown epitaxially, thereby creating an excellent interface between the semiconductor and the superconductor. This generic approach can be used for many other materials systems and allows for the bottom-up growth of complete quantum circuits.
Transport studies on these hashtags and shadow-grown structures show extremely clean data, phase coherent transport and a hard induced superconducting gap. These are the key ingredients for a Majorana braiding operation. With these wires a quantized zero bias peak (at 2e2/h) has been observed for the first time, which is strong additional evidence for the existence of Majorana fermions. We expect that we can start first Majorana braiding experiments this year in InSb hashtags with superconducting Al islands.

Thermal and electric transport of InSb nanowires are studied to reveal their thermoelectric properties. Due to the small dimensions the phonons are scattered at the nanowire surface and therefore thermal transport is largely suppressed by a factor 100 compared to bulk material. At the same time, these thin wires only host one electronic transport channel and since the wire are very pure and defect free they show ballistic transport (at low temperatures). The Seebeck coefficient has been determined for individual wires. Goal is combine these features to obtain a high thermoelectric efficiency at room temperature.

We have installed a thermal imaging system which we have used to study the thermal transport mechanism through a single nanostructure. Thermal transport has always been considered to be diffusive following Fourier’s law. Recently, hydrothermal transport has been predicted by theory and could form the basis for logical thermal devices, for instance thermal gates and diodes. We have obtained first indications for hydrothermal transport and thermal rectification in our nanostructures. Aim is to further investigate this mechanism and to explore this effect for thermal devices.

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