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Unveiling topological protection of edge states via current fluctuations

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General information

Reference : UMR8502-HELBOU-003
Workplace : ORSAY
Date of publication : Thursday, July 23, 2020
Scientific Responsible name : Meydi Ferrier
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2020
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

When a nanowire is connected to two superconductors (S), a supercurrent can flow, carried by Andreev bound states made of correlated electron- and hole-like quasiparticles. In the presence of spin-orbit interaction, the supercurrent can be carried by topologically protected edge states insensitive to disorder, which are a manifestation of a non-trivial topological phase, a new state of matter recently predicted but still elusive to experiments.
In the mesoscopic physics group, we have demonstrated experimentally that 1D edge states exist in Bismuth nanowires, which possess a strong spin-orbit interaction. Thus, we propose to detect the topological nature of those states via the supercurrent fluctuations. Indeed, we have shown theoretically that protected edge states should be responsible for strong fluctuations of the supercurrent which disappear for a non-topological state.
The internship work aims to develop a new very sensitive noise measurement set-up on a dilution refrigerator. This will allow us to measure such fluctuations in an existing sample. In addition, new samples will be fabricated to investigate different limits, from the short to the long junction compared to the superconducting length. Finally, we want to investigate the effect of the orientation of the magnetic field, which also contains clear signatures of the topological protection.

Work Context

When a nanowire is connected to two superconductors (S), a supercurrent can flow, carried by Andreev bound states made of correlated electron- and hole-like quasiparticles. In the presence of spin-orbit interaction, the supercurrent can be carried by topologically protected edge states insensitive to disorder, which are a manifestation of a non-trivial topological phase, a new state of matter recently predicted but still elusive to experiments.
In the mesoscopic physics group, we have demonstrated experimentally that 1D edge states exist in Bismuth nanowires, which possess a strong spin-orbit interaction. Thus, we propose to detect the topological nature of those states via the supercurrent fluctuations. Indeed, we have shown theoretically that protected edge states should be responsible for strong fluctuations of the supercurrent which disappear for a non-topological state.
The internship work aims to develop a new very sensitive noise measurement set-up on a dilution refrigerator. This will allow us to measure such fluctuations in an existing sample. In addition, new samples will be fabricated to investigate different limits, from the short to the long junction compared to the superconducting length. Finally, we want to investigate the effect of the orientation of the magnetic field, which also contains clear signatures of the topological protection.

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