Expérimental
Topological Insulators (TIs) hold great promise for making novel electronic devices, thanks to the existence at their boundaries of topologically protected conduction channels. Unfortunately, the expected topological protection has turned out to be less robust than anticipated, notably due to the existence of conduction in the bulk. This complicates the fundamental study of the edge states, and motivates the search for different TIs with a reduced contribution of the non-topological bulk states. Among newly discovered TIs, Bi4Br4 appears to be a very promising material, with a large bulk gap, and experimental indications of a Second Order Topological Insulator (SOTI) character. SOTIs are topological insulators with (d-2)-dimensional topological states, d being the dimension of the bulk. In the case of Bi4Br4, current should be carried without dissipation at the hinges of the crystal by helical states, which are counter-propagating ballistic states with a spin orientation locked to that of the propagation direction. We propose during this internship to explore these hinge states usingquantum transport at low temperature. The superconducting proximity effect and quantum interferences induced by a magnetic field will be used to evidence these hinge states, determine the spatial distribution of conduction paths, their spatial transverse extension and test their ballisticity.