Condensed Matter/Materials & Biological Physics Seminar with Ming Yi on Emergent Phases in Geometrically Frustrated Lattices

Emergent phases often appear when the electronic kinetic energy is small compared to the Coulomb interactions. In 2D, this has been realized in twisted bilayer graphene where folding in momentum space under the moire potential leads to flattened electronic bands. Another approach to realize flat bands in bulk materials is to seek compounds with crystal lattices that exhibit geometric frustration, where the hopping of the electrons is suppressed due to the quantum destructive interference of the electronic wavefunction, resulting in flat bands. Recently, such efforts have found a wide range of exotic phases in the two-dimensional kagome lattice, including magnetic order, time-reversal symmetry breaking charge density wave, nematicity, and superconductivity. In this talk, I will present our recent efforts primarily based on the technique of angle-resolved photoemission spectroscopy in experimentally exploring bulk materials hosting flat bands induced from the geometric frustration of the lattice. In particular, I will present examples of topological flat band systems including the magnetic kagome systems, FeSn and FeGe, where we explore the role of the electronic structure in magnetism and charge density wave orders [1-2]. Then I will extend this geometry frustration design principle to the 3D analogue ”the pyrochlore lattice, where we also observe the role of quantum destructive interference in producing flat bands near the Fermi level in two material systems [3-4].

[1] Teng et al. Nature 609, 490 (2022).
[2] Teng et al. Nat. Phys. 19, 814 (2023).
[3] Huang et al. arXiv: 2304.09066.
[4] Huang et al. arXiv: 2311.01269. (to appear in Nat. Phys.)