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Cooperation between Superconductivity and Magnetism in Mesoscopic systems: towards Majorana states

Periodic Reporting for period 1 - SuperMag (Cooperation between Superconductivity and Magnetism in Mesoscopic systems: towards Majorana states)

Reporting period: 2015-07-01 to 2017-06-30

A major challenge in contemporary physics is to understand and control unconventional states of matter enabling future low-power superconducting digital electronics, and potentially leading to scalable fault-tolerant “topological” quantum computing hardware whose capabilities far exceed those of classical systems. Recent proposals and experiments showed that these exotic quantum states can be engineered at the nanoscale from a precise cooperation between superconductivity and magnetism.

In this project I studied this cooperation in hybrid devices consisting of nanometer-size semiconductors and/or metals where superconductivity and magnetism can coexist and be finely controlled. I developed a series of milestone experiments to independently target different aspects of this cooperation ad relate it to size, geometry and materials composition.
The electrical characterization, at ultra-low temperatures, of my devices allowed me to clarify many aspect of this cooperation. Moreover the experimental observations supported by the my theoretical models contributed to the understating and control of the unconventional quantum state achieved in the electrons of my nanoscopic devices, towards the highly sought-after and ambitious target of demonstrating fault-tolerant quantum computing in the solid-state.
"The work performed during the project can be summarized in the following tasks:

1-Characterization of the electrical conductance of multi-terminal metallic ""Josephson"" junctions (namely: the omaga-SQUIPT).
I fabricated a nanoscopic T-shape island of copper embedded in two superconducting loops of aluminium (see Fig.1). By measuring, at ultra-low temperature, electrical conductance of the copper island in contact to a metallic aluminium-manganese probe I was able to demonstrate the fine control and exotic quantum states generated in the conducting island when an external magnetic field is applied. Moreover, I developed a theoretical model able to demonstrate the unusual properties of these quantum states. Finally, I was able to extend the operating temperatures of the device from 1 Kelvin up to 3 Kelvin by adding a vanadium layer on top of the aluminium loop.

2-Study of the anomalous superconducting current in indium arsenide nanowires.
I realized a nanoscopic “Josephson” junction with an indium arsenide nanowire contacted by two aluminium leads. By measuring the amplitude of the electrical superconducting current as a function of the external magnetic field I observed clear signatures of the unconventional quantum states enabling fault-tolerant quantum computing scheme.

3-Development of a molecular doping in different materials
I extended, in collaboration with Twente University, the molecular doping technique to cover a larger spectrum of materials. Specifically we found a recipe based on a different chemical compositions of the molecule able to efficiently cover the oxide surface of conventional silicon wafers. The thin copper films evaporated on top of these active surfaces showed at low temperature characteristic feature in the electrical conductance demonstrating the magnetic doping induced by the molecular layer.

4-Understanding the magnetic properties of europium-sulphide/aluminium bilayers
I measured the electrical conductance of thin films made of europium-sulphide/aluminium bilayers in electrical contact with an aluminium probe. The evolution of the electrical conductance as a function of an external magnetic field allowed me to understand the dynamics of the ferromagnetism of this unconventional bilayer. I then developed a theoretical model able to describe the electrical properties of the bilayer and deeply understand the interplay between the magnetism of the europium-sulphide and the superconductivity of the Aluminium layer.

All these results have been disseminated in the major international conference dedicated to superconductivity magnetism and unconventional state of matter, as well as public events organized by my institute. They represents an important step towards the understanding of robust quantum states of matter for the realization of fault-tolerant quantum computers. Moreover, in the short term, the patent I submitted during this project could be immediately exploited in the implementation of modern supercomputers.
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In this project different progresses have been achieved as demonstrated by the consistent number of high-impact scientific publications, oral presentations to international conferences and manuscripts in phase of evaluation in which the MSCA action have been always acknowledged. In particular starting form the tasks described before different progresses beyond the state of the art have been achieved:

1-The experiment on tree-terminal nanoscopic metallic islands (“omega-SQUIPT”) demonstrated for the first time a new class (topology) of quantum electronic states.
These results have been awarded with two important publications and have been successfully disseminated with seminars, workshops, and invited talks to a broad scientific community including PhDs students, condensed matter physicists, metrologiests, and quantum engineers employed in companies of quantum technology. The new physics discovered in these systems is now source of inspiration of new fundamental paradigms of quantum mechanics and can be exploited in the future to achieve fault-tolerant computational scheme.

2-The enhancement of the superconducting current observed in indium arsenide nanowires provided a new important piece of the big puzzle of demonstrating the unusual quantum states in the solid state for fault-tolerant quantum computation.
This result have been rewarded with a high impact publication and disseminated to conferences scientifically dedicated to this topic. Moreover due to the wide international interest in this new physics I have been invited to explain this fascinating topic to a public audience of specific themed movie shows on the story of Ettore Majorana as well as national news papers.

3-The molecular doping technique applied to conventional silicon wafer demonstrated the flexibility of this technique enabling the magnetic doping to different materials ranging from metallic thin films to two-dimensional semiconductors.

4-The measurements performed at ultra-low temperatures on europium-sulphide/aluminium bilayers allowed me to quantified the electronic states generated by the interplay between the magnetism of the europium sulphide and the superconductivity of the aluminium. In this interplay the important role of the magnetic structure (domains) of the europium-sulphide have been addressed for the first time and is now the argument of a manuscript in press to an international journal of material science. Furthermore I divulgated these results in an international conference dedicated to superconductivity and magnetism.
Starting from this first device a more complex structure, based on the same materials, have been recently fabricated and characterized. The promising results obtained are now in preparation to a high impact publication as well as in a patent exploiting the strong magnetism of the europium sulphide for a new concept of superconducting electronics. This patent could have a strong socio-economic impact by solving the requirement of low-dissipation now limiting the performances of state-of-the-art classical computers.
Fig.1