Long-lived isospin excitations in magic-angle twisted bilayer graphene

Tian Xie, Siyuan Xu, Zhiyu Dong, Zhiyuan Cui, Yunbo Ou, Melike Erdi, Kenji Watanabe, Takashi Taniguchi, Seth A. Tongay, Leonid S. Levitov and Chenhao Jin ()
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Tian Xie: University of California at Santa Barbara
Siyuan Xu: University of California at Santa Barbara
Zhiyu Dong: California Institute of Technology
Zhiyuan Cui: University of California at Santa Barbara
Yunbo Ou: Arizona State University
Melike Erdi: Arizona State University
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Seth A. Tongay: Arizona State University
Leonid S. Levitov: Massachusetts Institute of Technology
Chenhao Jin: University of California at Santa Barbara

Nature, 2024, vol. 633, issue 8028, 77-82

Abstract: Abstract Numerous correlated many-body phases, both conventional and exotic, have been reported in magic-angle twisted bilayer graphene (MATBG)1–24. However, the dynamics associated with these correlated states, crucial for understanding the underlying physics, remain unexplored. Here we combine exciton sensing and optical pump–probe spectroscopy to investigate the dynamics of isospin orders in MATBG with WSe2 substrate across the entire flat band, achieving sub-picosecond resolution. We observe remarkably slow isospin dynamics in a broad filling range around ν = 2 and between ν = −3 and −2, with lifetimes of up to 300 ps that decouple from the much faster cooling of electronic temperature (about 10 ps). This non-thermal behaviour demonstrates the presence of abnormally long-lived modes in the isospin degrees of freedom. This observation, not anticipated by theory, implies the existence of long-range propagating collective modes, strong isospin fluctuations and memory effects and is probably associated with an intervalley coherent or incommensurate Kekulé spiral ground state. We further demonstrate non-equilibrium control of the isospin orders previously found around integer fillings. Specifically, through ultrafast manipulation, it can be transiently shifted away from integer fillings. Our study demonstrates a unique probe of collective excitations in MATBG and paves the way for actively controlling non-equilibrium phenomena in moiré systems.

Date: 2024
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DOI: 10.1038/s41586-024-07880-5

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