SemiTopo

When strained is applied to Mercury Telluride, a semiconducting gap is opened. When the chemical potential is placed within this gap by an external gate, the electrical conduction takes place at the material surface by charge Dirac carriers (with zero effective mass): this is an excellent (the best) topological insulator with very little residual bulk conduction. In this project, this material will be developed in order to control the energy and properties of the surface states (the Dirac carriers) in order to reach the best electronic transport properties (mobility, interface rugosity etc.). The band structure and the energies and the dispersion of the surface states will be studied by ARPES photoemission at the SOLEIL synchrotron and compared to the theoretical models appropriate to this system. The magneto-transport properties will be studied, with a particular interest on the quantum Hall phases which appear at high magnetic field. Specific experiment will be carried out to understand and characterize the bilayer quantum Hall states, we have recently discovered. At very high magnetic field, we will study an insulating phase which looks like a Hall insulator state (for which the longitudinal resistivity is infinite while the transverse Hall resistivity remains finite). Hg1-xCdxTe is a non centro-symmetric material which gap changes sign and in magnitude by varying x. Between the insulating (CdTe) and topological (HgTe) phases, a semi-metallic phase has been predicted to exist on symmetry arguments which nature is presently unknown. Its theoretical analysis should specify under which circumstances it appears and what experiment will reveal its presence. At the device level, we will design and fabricate hybrid structures where the Dirac carriers are in proximity of a superconductor. These structures will be experimentally studied, and the spectroscopy of the Andreev state measured, with the detection of Majorana fermions which have been predicted in such device as an ultimate objective. Finally we will fabricate simple structures capable of selecting and manipulating the spins of the Dirac carriers in the 2D spin-Hall limit (quantum wells) as well as in the topological insulator (3D) limit.