Propriétés topologiques de nanohexagones de bismuth // Probing the topological properties of bismuth nanohexagons

Unidimensional conduction is extremely rare, because even a small amount of disorder tends to localise the electron wavefunction at low temperature. The recently discovered Quantum Spin Hall state, in which current is carried by “helical” 1D electrons, i.e. electrons whose spin direction is locked to their propagation direction, thereby hindering backscattering if no spin-flip mechanism is present, may be a remarkable exception to this rule.
The Quantum Spin Hall state is realized in topological materials such as specific graphene assemblies, 2D topological insulators, as well as the newly discovered 3D Second Order Topological Insulators (SOTI), which are three dimensional materials with unidimensional helical states at some of their edges. Our group discovered that bismuth was such a 3D SOTI, using experiments in which superconductivity is induced in perfectly crystalline bismuth nanowires [1-4]. Those nanowires however vary in crystalline orientation, and are rather large (200 nm in diameter and several microns in length). This leads to many non-topological surface and bulk states, that coexist with the topological state. This can hinder a deeper understanding of the topological properties of bismuth, and motivates the search for other SOTI materials, as well as more controlled forms of bismuth.
The PhD project consists in detecting signatures of helical edge states in thin (8 nm) bismuth nanohexagons, synthesized by collaborators (ICMol Valencia, Spain [5]). To this end, several experiments will be conducted, from low temperature transport measurements with different electrodes configurations and materials (non-superconducting or superconducting), to high sensitivity detection of orbital moments due to currents circulating around the edges of the hexagons (see figure).
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Unidimensional conduction is extremely rare, because even a small amount of disorder tends to localise the electron wavefunction at low temperature. The recently discovered Quantum Spin Hall state, in which current is carried by “helical” 1D electrons, i.e. electrons whose spin direction is locked to their propagation direction, thereby hindering backscattering if no spin-flip mechanism is present, may be a remarkable exception to this rule.
The Quantum Spin Hall state is realized in topological materials such as specific graphene assemblies, 2D topological insulators, as well as the newly discovered 3D Second Order Topological Insulators (SOTI), which are three dimensional materials with unidimensional helical states at some of their edges. Our group discovered that bismuth was such a 3D SOTI, using experiments in which superconductivity is induced in perfectly crystalline bismuth nanowires [1-4]. Those nanowires however vary in crystalline orientation, and are rather large (200 nm in diameter and several microns in length). This leads to many non-topological surface and bulk states, that coexist with the topological state. This can hinder a deeper understanding of the topological properties of bismuth, and motivates the search for other SOTI materials, as well as more controlled forms of bismuth.
The PhD project consists in detecting signatures of helical edge states in thin (8 nm) bismuth nanohexagons, synthesized by collaborators (ICMol Valencia, Spain [5]). To this end, several experiments will be conducted, from low temperature transport measurements with different electrodes configurations and materials (non-superconducting or superconducting), to high sensitivity detection of orbital moments due to currents circulating around the edges of the hexagons (see figure).
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Début de la thèse : 01/10/2025
WEB : https://equipes2.lps.u-psud.fr/meso/

Europe - ERC (European Research Council)

Université Paris-Saclay GS Physique

564 Physique en Ile de France

Formation souhaitée en matière condensée ou physique quantique
Masters Degree with Quantum physics and/or Condensed Matter Physics