Investigating "kagome-derived" phenomena in the RT6X6 family

Abstract

Materials containing kagome lattices have gained substantial attention in part since their band structures are predicted to simultaneously host Dirac points, van Hove singularities, and flat bands. In this thesis I investigate the consequences (or lack thereof) of the first two of these features on a couple of materials in the RT6X6 kagome family. First, I will discuss the ferrimagnet TbMn6Sn6 which displays a large intrinsic anomalous Hall effect (AHE) previously attributed to gapped Dirac points in the band structure. Through tuning this material with Cr doping, we reveal key issues with this interpretation, finding instead that the large intrinsic AHE can be attributed to multiple Berry curvature “hot spots” that exist throughout the Brillouin zone. Coincidentally, this doping series leads to magnetic compensation that is highlighted by record-breaking coercive fields, offering a system with a large intrinsic AHE coupled with nearly vanishing magnetization. After this, I will focus for the rest of the thesis on my work on the charge density wave (CDW) system ScV6Sn6. The first result comes from careful measurements of the magnetotransport of this system; these data and accompanying analysis provide strong evidence for pseudogap behavior in the “normal” state of this compound in which short-range CDW fluctuations open up a partial electronic gap. Then I will discuss my work carefully optimizing the crystal growth of ScV6Sn6 which allowed me to obtain crystals with a far better quality than those reported in literature. Magnetotransport measurements of these crystals reveal a lack of evidence of any AHE (which was originally reported) and a complicated series of apparent phase transitions near the nominal TCDW . Lastly, I will present recent angle-dependent magnetoresistance measurements these high quality samples of ScV6Sn6. While electrical transport on RT6X6 materials often is dominated by small, light 3D Dirac like pockets, our high-quality single crystals allow us to detect the transport contribution of a large quasi-2D vHs pocket near the Fermi energy at low temperatures. These results indicate the transport properties of this nominally 3D stannide maintains some connection to the 2D kagome layers it contains.