Construction and manipulation of topological insulators from III-V heterostructures

• Applicant: Professor Dr. Sven Höfling
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 530143959

Topological insulators (TIs) are new state of matter, characterized by an insulating bulk but gapless and counter-propagating surface or edge states. Among the vast amount of TI materials, composite quantum wells (QWs) heterostructures based on III-V materials, i.e., InAs/GaSb, provide unprecedented device functionality in combination with the advanced epitaxial and device fabrication routines of III-V semiconductors. These composite QW heterostructures are particularly attractive two-dimensional (2D) TIs thanks to their rich phase diagram accessible by band and symmetry engineering and by the application of external electric fields, allowing the modification of their properties with unique flexibility. However, despite many investigations, a fully convincing demonstration of helical edge states in the topological insulating phase in this material system is still elusive. This project therefore proposes first to overcome the main obstacles to the observation of the so-called quantum spin Hall effect (QSHE) in these materials, such as the small energy gap inherent in these asymmetric structures. Moreover, since the discovery of 2D TIs, a large variety of three-dimensional (3D) TIs and semi metallic (SM) states have also been identified in different condensed matter systems. However, the materials in which these topological states have been observed do not allow any continuous tunability of their band structure or topological properties. Therefore, a flexible platform with many adjustable parameters is still very much in demand, both to better probe their physical properties and to consider practical applications. By controlling crystal symmetries and multiple band inversions in InAs/GaSb-based superlattices, it becomes possible to create all known topological quantum states such as 3D TIs, Dirac and Weyl SMs, but also so-called higher order TIs (HOTIs) with a phase diagram of unprecedented richness. Unlike the "lower order" case, edge states of HOTIs are at least two dimensions smaller than that of the system. The project therefore aims on the one hand to create a flexible platform for the study of 2D and 3D topological states based on QWs and III-V superlattices and on the other hand to evidence and control the QSHE in structures of optimized growth and technological processes. The first objective is the realization of a technological breakthrough: the control of dual gated high-performance field effect devices, allowing the observation of the trivial-to-topological phase transition by external voltage rather than structural change. The second objective is to observe quantized conduction of edge states in three-layer QW structures. The third objective is to validate various topological states existing in 3D InAs/Ga(In)Sb superlattices, such as 3D TIs, Dirac and Weyl SMs, and 3D HOTIs.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Dr. Benoit Jouault