Transition-metal dichalcogenides in and out of equilibrium

The growing demand for electronic devices with high performance and low power consumption is a major driving force in the search for new materials. In particular, the continuous miniaturisation of silicon-based electronics and the concomitant need to scale transistors to ever smaller dimensions poses a formidable challenge for the semiconductor industry. In this context, the discovery of graphene with its remarkable properties has raised large interest in two-dimensional (2D) materials. However, due to the lack of a band gap, the use of graphene in transistors for digital electronics is limited. An alternative are transition-metal dichalcogenides (TMDs), which consist of a transition- metal such as tantalum or molybdenum and a chalcogen atom like sulfur or selenium. Monolayers of semiconducting TMDs such as MoS2 are promising candidates for ultrathin field-effect transistors. With their band gaps in the range of visible light, they further represent promising materials for optoelectronic applications such as optical switches, photodetectors, light emitters and thin-film solar cells. Due to their high in-plane bonding, TMDs are mechanically strong and flexible and may be an ideal choice for future flexible electronics, e.g. flexible photovoltaics, bendable displays for mobile phones and wearable electronics for healthcare applications. Besides being very promising materials for technological applications, TMDs are interesting from a fundamental physical point of view. This regards in particular strongly correlated TMDs such as TaSe2 and NbSe2, in which the Coulomb interaction among the electrons plays an important role. Due to the interaction and collective behaviour of the electrons, these materials can host a large variety of phases such as correlated insulating, charge-density-wave, superconducting and spin- liquid phases. In particular, new phases can emerge in monolayers, which are absent in the corresponding 3D materials. An investigation of these so-called quantum phases requires elaborate theoretical and computational techniques such as time-dependent density functional theory (TDDFT), dynamical mean-field theory (DMFT) or the ab-initio dynamical vertex approximation (ADGA). In this project, I will further develop and benefit from synergies between these different methods to cover a wide range of phenomena at different spatial and time scales. My investigations will further extend to the non-equilibrium electronic properties of TMDs, which become relevant e.g. when one shines an intense laser pulse onto a material. By investigating how the electrons in a material react to such an intense light pulse, we can learn more about the material itself and how it reacts to external stimuli, which is important for developing efficient and fast-switching transistors.