Recently discovered two-dimensional (2D)-honeycomb semiconductor materials have two inequivalent, degenerate valleys in their electronic band structure. This leads to a new “valley” degree of freedom known as pseudospin that, similar to real spin, has been proposed as an extra information carrier for new classes of electronic and optoelectronic devices. Single-layer transition-metal dichalcogenides (TMDs) are an important type of 2D material, in which, due to the strong 2D confinement and a reduced dielectric screening of the Coulomb interactions, electron-hole (e-h) correlations are extremely strong. This results in the creation of e-h pairs (excitons) that are so strongly bound that excitonic effects completely dominate the optical properties of TMDs even up to room temperature. The pronounced excitonic effects in single-layer TMDs, therefore, provide a unique opportunity to investigate strong light-matter interactions associated with valley effects exhibiting exotic behaviour. However, the key fundamental question regarding the exact excitonic band structure, the valley-exciton energy-momentum (dispersion) relationship, and the corresponding excitonic transport properties, remains open. Several theories are proposed concerning the exciton energy-momentum (dispersion) relationship. Two main scenarios can be distinguished: 1) the exciton dispersion is parabolic around K = 0, but split into two branches, each having a different curvature (mass) resulting from the difference in the effective masses of electrons and holes. 2) The exciton dispersion is linear around K = 0, characteristic for massless Dirac particles.
The main purpose of this research project was to provide a fundamental understanding of valley-exciton dispersion and, resulting transport, combining two main research objectives: (R1) to determine the exciton dispersion and the corresponding transport using momentum- and real-space imaging, and (R2) to achieve an external control of the exciton dispersion via applied magnetic fields.
