PhD defence by Mathias Rosdahl Brems

Title: kp Theory for Two-Dimensional Materials and Topological Insulators Electronic Structures of Boron Arsenide and Bismuth Selenide


Principal supervisor: Professor Morten Willatzen, Fotonics Engineering, DTU
Co-supervisor: Professor Jesper Mørk, Fotonics Engineering, DTU

Evaluation Board
Associate Professor Sanshui Xiao, Fotonics Engineering, DTU, Denmark
Professor Eoin O´Reilly, Tyndall National Institute, Cork, Ireland
Professor Thomas Gram Pedersen, Ålborg University, Denmark

Master of the Ceremony
Associate Professor Martijn Wubs, Fotonics Engineering, DTU

For many applications in electronics, optics and nanotechnology understanding the properties of electrons inside the materials plays an important role. Electrons behave according to the laws of quantum mechanics and due to their large number and interactions the problem becomes highly complex. Modern computational methods such as density functional theory allows us to calculate electronic properties from first principles. But in many cases simpler effective models can describe the most important features. In this thesis, we study effective electronic structure models derived from the k.p method.

Two dimensional hexagonal boron arsenide was predicted to be stable and show promising semiconducting electronic properties. We combine density functional theory calculations and the k.p method to develop effective models describing the electronic structure. In particular, we explore possibilities to tune the electronic properties by application of strain, electric and magnetic fields, and show the possibility of going from a semiconducting to a metallic state for high strains and/or electric fields.

Bismuth selenide is a prime example of a topological insulator, an exotic state of matter characterized by their topological surface states. These states have been shown to be well described by a simple model, which we in this thesis extend to include external fields. We consider in detail the influence of strain on topological surface states, showing that the band gap of a thin film is quite sensitive to strain. The optical absorption spectrum for a thin film is calculated within the derived model, and possibilities for optimizing the optical properties by application of strain is showcased.

The electronic structure models, derived in this thesis, show good agreement with first principle calculations, but are applicable for more complex problems such as device simulations where first principle calculations become too computationally demanding.


Fri 10 Jan 20
14:00 - 17:00


DTU Fotonik


Lyngby Campus
Building 308, auditorium 11