Master's defense: Luca Buiarelli

Structural properties of kagome-layered crystals

This thesis investigates the structural properties of materials with kagome-layered crystal structure, focusing on the new kagome metals AV3Sb5 and the shandites M3A2Ch2. The former have only recently been synthesized and found to have a charge ordering phase that doubles its unit cell, called a charge density wave (CDW). The latter have been known for a long time and have also recently resurfaced because of some interesting transport properties, possibly connected to topological effects, but none of them is known to have a similar CDW phase. The thesis begins by providing an overview of the materials and their crystal structures, highlighting the unique electronic features of the kagome lattice. It then extends the existing phenomenological Landau theory for CDW-like structural transitions in AV3Sb5 to the shandite crystal structure, revealing that the symmetries of M3A2Ch2 could potentially allow for a CDW phase. Density Functional Theory (DFT) is then introduced as a computational tool to study the materials of interest. The simulations on CsV3Sb5 confirm the presence of an unstable phonon mode, signifying a structural instability, but are not able to identify electronic instabilities. Regarding the shandites, we focus on Pd3Sn2Se2, where we examine its similarities and differences compared to the kagome metals. Through simulations, we demonstrate that there are no indications of any structural or electronic instability in Pd3Sn2Se2 and other shandite compounds. Throughout the study, further investigations are proposed, such as studying the role of apical ions, exploring different shandite compounds, searching for shandites exhibiting CDW phases, and delving deeper into the interplay between phonons and electronic effects in the CDW formation. In summary, this thesis contributes to our understanding of the structural properties of kagome-layered compounds and their interplay with the electronic properties, laying the groundwork for future research and applications in the shandite materials.