Excitonic complexes in transition metal dichalcogenides and related materials
25 september 2019
Campus Groenenborger, U0.24 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
Organisatie / co-organisatie:
Department of Physics
Matthias Van der Donck
PhD defence Matthias Van der Donck - Faculty of Science, Department of Physics
In this thesis the properties of excitons, which are bound states of an electron and a hole, and higher order excitonic complexes such as trions and biexcitons are studied in monolayer transition metal dichalcogenides (TMDs) and related materials such as monolayer black phosphorus and TMD heterostructures.
For excitons a comparison is made between the finite element element solutions of the multi- and single-band model, showing that the former lowers the binding energy and leads to a reordering of and breaking of degeneracies between different angular momentum intervalley excitons. For trions and biexcitons the stochastic variational method is employed to numerically solve the single-band model. For all three excitonic complexes it is found that the binding energies, which are calculated for different combinations of TMDs and substrates and which are compared with theoretical and experimental results from the literature, are extremely large.
Next, the presence of a perpendicular magnetic field is considered and shown to increase the binding energy of excitons, trions, and biexcitons in monolayer TMDs. The diamagnetic shifts of these excitonic complexes are found to increase with increasing substrate dielectric constant and by calculating the exciton Landau levels it is demonstrated how the magnetic field alters the degeneracies of the excited states. Furthermore, it is shown that so-called dark excitons exhibit an exceptionally strong valley Zeeman effect in the presence of a tilted magnetic field.
Monolayer materials with anisotropic band masses are considered next and it is shown that in black phosphorus this anisotropy persists in the excitonic complexes while in TiS3 it does not. It is found that applying uniaxial tensile strain increases the exciton binding energy in black phosphorus.
Interlayer excitons in TMD heterostructures are also studied and it is shown that additional polarization effects in these systems can significantly decrease their binding energy, which is calculated for all possible combinations of TMDs. The experimental signature of these interlayer excitons is found to be tunable by means of a perpendicular electric field.
Finally, the possibility of interlayer excitonic superfluidity, which is currently intensively sought after in experiments, in a superlattice of TMD heterostructures is investigated and very high critical temperatures of up to 270 K are obtained.
This thesis contributes new insights to the topic of strongly bound excitonic complexes in two-dimensional materials, including the tunability of their properties and their importance in exotic phenomena such as high-temperature superfluidity.