Helical States in strained ultrathin Ge nanowires

Ge has emerged as a promising material system for the realization of spin qubits, due to the absence of hyperfine interaction in isotopically purified samples. Very recently a theoretical proposal has suggested that core/shell Ge/Si nanowires constitute also an interesting platform for the realization of helical states in a semiconductor. In order to realize helical states in Ge two conditions were predicted to be necessary: a) it needs to be strained and b) the one- dimensional conductor needs to be very thin. Here we aim at investigating unique nanostructures, which have been developed in the last month in the applicants group: coherently strained ultra-thin Ge nanowires grown monolithically on flat silicon surfaces, also referred as hut wires (HW). In particular we want to assess whether the investigated material has potential in the field of Majorana Physics. By combining experiments and theory we aim at achieving a very good understanding of the system. Specifically we are going to study the influence of strain on the parameters that are relevant for Majorana fermion physics. These parameters are the Landé g-tensor, the spin-orbital splitting of the valence band (due to structural inversion asymmetry), and the proximity-induced superconducting gap in theW. The final experimental goal of this project is to be able, under the guidance of the theory group, to detect signatures of Majorana bound states in a Ge nanowire coupled to a s-wave superconductor.

It is impossible to picture modern life without thinking of the vast amount of microelectronic applications that surround us. Such development has only been made possible with the emergence of semiconductor technology. The semiconductor industry was and will be a fundamental building block of the world economy. According to the Semiconductor Industry Association, it accounts for a world market of approximately US$ 305 billion in 2013, and an estimated US$ 355 billion in 2016. However the European share in 2013 was just 9% with a downward trend in the past 10 years. It is therefore important to introduce new concepts in the operation of devices. This project has been dealing with the development of such novel concepts in nanoelectronics. In particular, it aimed at enhancing the knowledge related to the spin, a quantum mechanical property of most elementary particles. By using the spin of charge carriers in a semiconductor one might be able in the future to create qubits, the quantum analog of the classical bits. In this project we focused our efforts on studying the fundamental properties of the spin of charges confined in very thin Germanium nanostructures. Our research revealed that the ultrathin nanostructures we studied are promising candidates for realizing such spin qubits. In particular low temperature electronic transport measurements revealed that spins react differently to an applied magnetic field depending on its direction. Such a property is important in order to reduce the detrimental effect the magnetic field of the nuclei can have on the spin properties. In addition, during the duration of the project, we have developed a complex process to integrate novel devices on piezoelectric substrates. Such an approach might allow to tailor in the future the fundamental electronic properties of nanoscale devices, which so far are mainly determined during the growth of the materials.