Bose Einstein Condensate of Spontaneously formed Triplet Excitons in Yin-Yang Flat Bands
ORAL · Invited
Abstract
Excitonic Bose-Einstein condensation (BEC) is a fascinating phenomenon manifesting macroscopic coherence of excitons, yet to be conclusively demonstrated either theoretically or experimentally. In this work, we show that flat valence and conduction bands (so called yin-yang flat bands (FBs)) of diatomic Kagome lattice, as exemplified in a superatomic graphene lattice, conspire to provide an ideal platform for realizing a triplet excitonic insulator state. Based on density-functional theory calculations combined with many-body GW and Bethe-Salpeter equation, we show the binding energy of a single triplet exciton can exceed the band gap, indicative of spontaneous exciton formation. Moreover, by developing an efficient exact diagonalization method for solving and analyzing many-exciton wavefunctions, we go beyond the state-of-the-art computational studies which are largely limited to single exciton calculations. This has enabled us to show directly spontaneous BEC of triplet excitons in an extended Hubbard model. We elucidate the critical role of FBs in promoting quantum coherence, evidenced by an off-diagonal long-range order in many-exciton states. This work significantly enriches FB and excitonic physics leading to the material realization of spinor BEC and spin superfluidity.
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Publication:[1] Sethi, G., Cuma, M., & Liu, F. (2022). Excitonic Bose-Einstein Condensate in Flat Conduction and Valence Bands of Opposite Chirality. Under Review. [2] Zhou, Y.*, Sethi, G.*, Liu, H., Wang, Z., & Liu, F. (2022). Excited quantum anomalous and spin Hall effect: dissociation of flat-bands-enabled excitonic insulator state. Nanotechnology, 33(41), 415001. [3] Liu, H.*, Sethi, G.*, Sheng, D. N., Zhou, Y., Sun, J. T., Meng, S., & Liu, F. (2022). High-temperature fractional quantum Hall state in the Floquet kagome flat band. Physical Review B, 105(16), L161108. [4] Sethi, G., Zhou, Y., Zhu, L., Yang, L., & Liu, F. (2021). Flat-band-enabled triplet excitonic insulator in a diatomic kagome lattice. Physical Review Letters, 126(19), 196403. * Authors contributed equally to the work
