Seminars Archive

Electronic structure and electron dynamics in novel two-dimensional materials

Abstract
Changing the dimensionality of a material results in significant modifications of its electronic properties. This is even the case if the parent material already has a layered structure with little interaction between the layers, as in the case of graphene, bilayer graphene and single-layer transition metal dichalcogenides. Here we explore the properties of novel two-dimensional materials such as single layers of MoS2, WS2 and TaS2 by scanning tunnelling microscopy angle-resolved photoemission spectroscopy (ARPES). The layers are grown epitaxially on Au(111), Ag(111) and graphene. For the semiconducting materials (MoS2, WS2), strong band gap renormalizations are observed due to the interaction with the substrate. For the metallic layers (TaS2), we can study the effect of low dimensionality on electronic instabilities such as charge density waves and superconductivity. While the static electronic properties of novel two-dimensional materials can be studied by standard ARPES, investigations of the ultrafast carrier dynamics require both time- and angular resolution and thus time-resolved (TR)-ARPES. There is, moreover, the technical requirement of high photon energies since the interesting part of the aforementioned materials' electronic structure (i.e. the (gapped) Dirac cone) is placed at the two-dimensional Brillouin zone boundary. Recently, it has become possible to probe states at such high k by TR-ARPES, thanks to the arrival of ultrafast high harmonic laser sources. Using this technique, we can also address the electron dynamics in graphene and single layer MoS2.