Quantum Materials based on the Kagome Lattice

Abstract

A central concept in condensed matter physics is the “emergence” of new macroscopic properties from complex microscopic interactions between many constituents. Quantum materials refer to a subset of condensed matter systems whose emergent properties defy description from classical physics or low-level quantum mechanics. Quantum materials exhibit truly diverse and fascinating properties, ranging from high-temperature superconductivity and topologically protected boundary currents to the fractionalization of elementary particles. The exotic properties discovered in quantum materials not only inspire and guide theoretical advancements – for example, the discovery of Kondo effect in impurity spin systems led to the development of renormalization group theory – but also hold great promise for the next generation of quantum technologies – for instance, the discovery of topological insulators opens up the possibility of realizing novel spintronic and magnetoelectronic devices. Designing new quantum materials, understanding their exotic properties, and harnessing them for the development of new quantum technologies represent a central mission of condensed matter physics.

Our strategy for designing new quantum materials and realizing new emergent quantum properties is based on the specific lattice geometry known as the kagome lattice. The insulating kagome lattices have been studied since the 1980s as a platform for novel quantum magnetic states. In the metallic case, the symmetry of the kagome lattice protects rich singularities in its electronic structure, including Dirac fermions at the Brillouin corner K, van Hove singularities at the zone edge M, and the flat band across the entire Brillouin zone. Once properly combined with other perturbations, these electronic singularities of the kagome lattice represent a rich potential to realize diverse emergent quantum phenomena at the intersection of topological and strongly correlated physics. These tantalizing opportunities presented by the kagome lattice have been theoretically recognized and extensively explored since the early 2000s. However, a proper experimental realization of the kagome lattice electronic structure and related quantum phenomena have been elusive for a long time, despite being highly desired.

This dissertation summarizes me and my collaborators’ pioneering contributions in the experimental realization of the kagome lattice physics over the past six years. Together with parallel efforts from several other research groups worldwide, our works have played a central role in initializing, establishing, and advancing a new research field “Quantum materials based on the kagome lattice”. Our achievements are twofolded. First, by employing a suite of synergistic band structure probes, we conducted in-depth investigations of the electronic structure of newly discovered 3d transition metal-based kagome lattice materials and successfully demonstrated the long-sought realization of the kagome lattice electronic structure, namely Dirac fermions, flat bands, and van Hove singularities, near the Fermi level. Second, by combining these electronic singularities with the spin-orbit coupling, magnetism, and strong electronic interactions inherent in the 3d-kagome metals, we realized diverse topological and correlated quantum phenomena on the kagome lattice, thereby confirming numerous theoretical predictions made since the early 2000s. With these radical advancements in the experimental realization of kagome lattice physics, the kagome lattice materials are now established as one of the most versatile and promising platforms for the quantum materials research. It has been a great privilege to contribute to this field and to witness how this field starts, gains interest, spreads globally, and becomes a major pillar of the community during my Ph.D. journey.

I start the thesis with an essential introduction to the kagome lattice physics in Chapter 1. Chapter 2 summarizes a catalogue of kagome lattice materials discovered thus far, along with an overview of the physics studied in them. The subsequent chapters describe the details of our research, namely the realization of the kagome lattice electronic structure and associated quantum phenomena in various 3d kagome lattice materials. These include the realization of massive Dirac fermions and intrinsic anomalous Hall conductivity in a ferromagnetic kagome metal Fe3Sn2 (Chapter 3); the observation of coexisting surface and bulk Dirac fermions as well as flat bands in an antiferromagnetic kagome metal FeSn (Chapter 4); the realization of the topological flat band in a nonmagnetic kagome metal CoSn (Chapter 5); measurements of the quantum geometry associated with the kagome flat band (Chapter 6); observation of the sublattice-decorated van Hove singularity and its connection to charge order in CsV3Sb5 (Chapter 7); and observation of the intricate competition between charge order and superconductivity in the kagome metal series (K,Rb,Cs)V3(Sb,Sn)5 (Chapter 8). I conclude the dissertation by outlining the remaining goals and challenges in this research area and potential extensions to other lattice geometries in Chapter 9. It is my sincere hope that this dissertation will serve as a groundwork, guiding and inspiring future studies of kagome metals and relevant systems.