Correlated electron phenomena from electronic flat bands | Riccardo Comin (from MIT, USA)

Abstract:

Electronic flat band systems have gained considerable attention as a platform to realize quantum matter phenomena including the fractional quantum Hall effect, topological phases, unconventional superconductivity, and new forms of magnetism. Electrons in a flat band are characterized by a quenched kinetic energy (bandwidth), a diverging effective mass, and localized wave functions, leading to strong correlation physics in the presence of Coulomb interactions. In recent years, significant efforts have been directed at realizing electronic flat bands in special ‘line graph’ lattices, which under certain conditions exhibit eigenstates that are spatially ‘trapped’ by the total destructive interference of hopping pathways (compact localized states).  The realization of new flat band systems and their tuning via chemical composition, carrier doping, or lattice geometry (dimensionality) is an exciting frontier for engineering quantum phases of matter at potentially high temperatures.

In this talk, I will discuss how flat bands are not just a textbook realization of the line-graph band structure toy model in the noninteracting limit, but rather can support strong correlation physics in two paradigmatic lattices – the kagome and pyrochlore lattice. We investigated the electronic band structure in kagome and pyrochlore metals using angle-resolved photoemission spectroscopy (ARPES). I will first discuss the general phenomenology of flat bands in a few representative kagome and pyrochlore intermetallics [1,2]. Next, I will present evidence for one-dimensional flat bands giving rise to heavy fermion-like behavior in kagome metal Ni3In [3]. Last, I will show recent work elucidating the band structure of ‘f electron-free’ heavy-fermion compound LiV2O4, and the role of flat bands in this system [4].

[1] M. Kang, et al., Topological flat bands in frustrated kagome lattice CoSn. Nature Comm. 11, 4004 (2020).
[2] J. Wakefield, et al., Three-dimensional flat bands in pyrochlore metal CaNi2, Nature 623, 301 (2023).
[3] L. Ye, et al., A flat band-induced correlated kagome metal. Nature Phys. 20, 610 (2020).
[4] D. Oh, et al., in preparation.