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SPAJORANA Report Summary

Project ID: 335497
Funded under: FP7-IDEAS-ERC
Country: Austria

Mid-Term Report Summary - SPAJORANA (Towards spin qubits and Majorana fermions in Germanium self-assembled hut-wires)

The interest in electrons confined in group IV materials for spin qubits has been continuously increasing over the past few years. Silicon has not only the advantage of being the most important element in semiconductor industry, it can be also isotopically purified eliminating the problem of decoherence from hyperfine interactions. One limitation of electrons confined in Si is it’s low spin orbit coupling making the electrical spin manipulation difficult. Holes confined in Germanium (Ge) have a rather strong spin-orbit coupling and together with the rather weak hyperfine interaction, make Ge quantum dots a promising platform for the realization of high fidelity spin qubits. In 2002 the first Ge/Si core shell nanowires (NWs) were grown by chemical vapour deposition and soon after, QDs were investigated in such structures. Promising spin relaxation and dephasing times have been reported.
In this project Ge hut wires (HWs) are studied; these nanostructures are grown by means of the Stranski-Krastanov growth mode. They were reported for the first time in 2012 when it was shown that they can be monolithically grown on Silicon. Such HWs have a triangular cross section with a height of about 2 nm and are fully strained. These structural properties should lead to a very large heavy hole (HH) – light hole (LH) splitting, minimizing the mixing and as a consequence the non Ising type coupling to the nuclear spins. Despite these interesting structural properties, not much is known about the electronic properties.
During the first half of the ERC Project Spajorana the first magnetotransport measurements on this so far unexplored type of nanostructure were performed. The low temperature electronic transport measurements revealed a very strong anisotropy between the in plane and out of plane g-factors. This very strong anisotropy suggest that the confined carriers are of HH character. Numerical simulations performed in the group of Prof. Loss (University of Basel) have verified that indeed the confined states have a less than 1% LH character. The experimental data and theory support suggest that confined holes in Ge hut wires are promising for the realization of spin qubits.
In a next step, the PI’s group realized charge sensors. Two HWs were used for this purpose. In the charge sensor HW the current switches between the on and off state depending on the number of confined carriers in a close by HW. Such a charge sensor is important in order to be able to measure the spin relaxation time.
Finally, in order to be able to create a singlet triplet spin qubit and measure its coherence time, double quantum dot structure devices have been realized. This was done with the help of 50nm wide top gates. 4K measurements have verified the realization of tunnel coupled quantum dot devices.

Reported by

Institute of Science and Technology Austria
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