Thesis Defense: Clara Nyby, Lindenberg & Heinz Groups
"Ultrafast structural dynamics in layered transition metal dichalcogenides"
This work focuses on visualizing and manipulating atomic-scale structural changes in transition metal dichalcogenide (TMD) materials, with the goals of understanding fundamental light-matter interactions in these materials and modulating material properties on an ultrafast time scale.
Tungsten ditelluride is a layered TMD that crystallizes in a distorted hexagonal net with an orthorhombic unit cell (Td phase). The lack of inversion symmetry in this phase leads to a predicted new topological semimetal hosting the so-called type-II Weyl points. Here, we use a single THz pulse to trigger a structural phase transition to a centrosymmetric phase, and probe the switching using an ultrafast electron diffraction technique. These findings serve as the first direct evidence of a THz field induced structural transition in a two-dimensional material, and offer a new promising way to optically control the topological properties of solids.
Additionally, we present a new non-destructive method for probing heat transport in nanoscale crystalline materials using time-resolved x-ray measurements of photo-induced strain. This technique directly probes time-dependent temperature changes in the crystal and the subsequent relaxation across buried interfaces by measuring changes in the c-axis lattice spacing after optical excitation. This work is motivated by the need to understand the fundamentals of nanoscale heat propagation, particularly at buried interfaces in functional devices.