Strained HgTe/CdTe topological insulators, toward spintronic applications

Orateur : Soutenance de Thèse de Candice THOMAS (CEA-LETI)

Topological Insulators (TI) are a new class of materials that recently attracts a large interest both theoretically and experimentally thanks to the u nique electronic and spin properties that arise on their interfaces. Indeed, with graphene-like transport properties carried by massless Dirac fermions and a topological protection preventing from backscattering phenomena, TI surfaces are full of promise for the design of future quantum electronics. However, so far, only few material systems fulfill the requirements to make strong TIs. With an inverted band structure, the semi-metal HgTe is one of them assuming the opening of a bandgap by tensile strain. This PhD thesis aims at experimentally demonstrating the topological nature of strained HgTe as well as its eligibility for applications, especially for spintronics. To do so, strong efforts have first been dedicated to the improvement of the growth process by molecular beam epitaxy, leading nowadays to structures with flat interfaces and very high mobility (up to 600,000 cm2/(V.s)). Such a material quality has then enabled quantum Hall effect to be unambiguously demonstrated using low temperature magneto-transport measurements, exhibiting direct evidences of Dirac fermions at the surface of strained HgTe from temperature dependence analysis. Finally, the ability of HgTe surfaces to generate spin-polarized currents has been experimentally verified, paving the way toward the use of this TI system for future spintronics. On this basis, the implementation of strained HgTe into simple p-n junction has been investigated to realize a first spin-based logic element.

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