Strong correlations of emergent flat bands through compact molecular orbitals

Flat band materials without localized 4f atomic orbitals represent new types of platforms for realizing strongly correlated electronic states. In addition to moiré super-lattices, bulk quantum materials are emerging as exciting systems for active flat bands, especially when they contain geometrically frustrated lattices, such as kagome and pyrochlore lattices. These flat bands typically carry non-trivial topology, making the systems ideal platforms to explore the interplay between band topology and strong electron interaction. In this talk, I will present our recent studies on flat band materials without localized atomic orbitals, and demonstrate how compact molecular orbitals can be introduced [1, 2] in geometric frustrated lattices and how they can be used to characterize the dominating correlation effects and derive and design exotic strongly correlated quantum phases. Going beyond generality, we give a materials-specific construction of the compact molecular orbitals [3] in the non-Fermi liquid kagome metal Ni3In, which set the stage for its probing by STM measurements [4]. Another realization of the compact molecular orbitals in the kagome metal CsCr3Sb5 provides the means to overcome the challenge of having too many overlapping bands and demonstrate the correlation-driven flat bands at the Fermi energy [5], as observed experimentally [6]. I will also discuss similar mechanisms for compact molecular orbitals that provide the understanding of the Kondo effects observed in heterobilayer transition metal dichalcogenides [7,8] and twisted WSe2 [9].

References:

[1] L Chen, F. Xie, et al, Nat. Comm. 15 (1), 5242 (2024); H. Hu et al, Sci. Adv. 9, eadg0028 (2023).

[2] L Chen, F. Xie, et al, arXiv: 2307.09431

[3] M. Mahankali, F. Xie, et al., arXiv: 2503.09706

[4] J. C. Souza, …, F. Xie, et al., arXiv: 2503.09704

[5] F. Xie et al, Phys. Rev. Research 7, L022061 (2025)

[6] Z. Wang*, Y. Guo*, H-Y. Huang*, Fang Xie*, et al., Nat. Comm 16, 7573 (2025)

[7] F Xie, et al, Phys. Rev. Research 6 (1), 013219 (2024)

[8] F. Xie, et al, Phys. Rev. Research 7, 033093 (2025)

[9] F. Xie et al, Phys. Rev. Lett. 134, 136503 (2025).