Topological IV-VI semiconductor heterostructures

Topological insulators are a novel class of materials that are electrically insulating in the bulk but conducting at the surface. This surface conduction is similar to the relativistic motion of light in vacuum and thus, exhibits peculiar properties compared to conventional conductors. Topological crystalline insulators are a subclass of these materials where the relativistic character of the surface electrons is governed by the symmetry of the crystal structure. The objective of the joint research project of the Johannes Kepler University in Linz and the Ecole Normale Superiéure in Paris is to fabricate low dimensional topological crystalline insulator thin film heterostructures and to investigate the properties of the surface channel. Particular focus will be to develop control the topology character by external means to lay the grounds for practical device applications. This control will be achieved based on the fact that the relativistic surface conduction can be manipulated by the chemical composition, mechanical strain, pressure, electric fields and magnetism. One can thus obtain functional devices such as transistors based on nanostructures of such materials that make use of the relativistic conduction properties. The international project brings together the expertise in fabrication of thin film heterostructures in Linz and the study of their properties using optical and electrical measurements at high magnetic fields and low temperatures in Paris. The goal is to solve the technological challenges and provide a novel platform for device applications. In particular, we will develop approaches to fabricate nanolayers and heterostructures with optimized properties using layer-by-layer growth with atomic precision. The novel physics of the obtained structures will be studied in detail by optical and electrical techniques that enable us to extract the velocity, mass and energy of surface electrons and study the evolution of their relativistic character as a function of band topology. This is essential to understand how this state of matter comes to exist and thus to understand the fundamental physics behind. In the second part, we will develop schemes to alter the crystalline symmetry of the material by introducing lattice deformations and we will explore how magnetism impacts surface conduction and suppresses its relativistic character. Overall, the project will establish that certain materials and material combinations provide an on-demand relativistic physics platform where novel exotic effects can be realized.

Topological insulators are a novel class of materials that are electrically insulating in the bulk but conducting at the surface. The surface conduction is mediated by relativistic Dirac, Weyl and Majorana quasiparticles that exhibit a peculiar linear energy-momentum relation similar to the relativistic motion of light in vacuum giving rise to unique properties compared to conventional conductors. In this international joint research project of the Johannes Kepler University in Linz and the Ecole Normale Superiéure in Paris the focus was on topological "crystalline" insulators that are a particularly interesting subclass of these materials because the relativistic character of their surface electrons is highly sensitive against internal and external perturbations. Accordingly, the project developed novel ternary and quaternary materials to create a powerful toolbox for engineering of topological quantum structures suited for device applications. In particular, the project demonstrated effective tuning of band topology, control of the electron wave functions, coexistence of Dirac and Weyl fermions and lifting of the electron degeneracies using electrical transport measurements and spectroscopies at low temperatures and high magnetic fields, as well as by angle resolved photoemission experiments performed under ultra-high vacuum conditions at large scale European research facilities such as synchrotrons. Particular focus was on the realization of multilayer structures with atomic scale precision, as well as on the control of the electronic properties by ferroelectric phase transitions. The results opened up a new realm for novel physics and exotic phases of matter that display a high potential for future data storage and information processing applications.