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

• 成果类型: Article
• 署名作者: Xie, Tian; Xu, Siyuan; Dong, Zhiyu; Cui, Zhiyuan; Ou, Yunbo; Erdi, Melike; Watanabe, Kenji; Taniguchi, Takashi; Tongay, Seth A.; Levitov, Leonid S.; Jin, Chenhao
• 署名单位: University of California System; University of California Santa Barbara; California Institute of Technology; California Institute of Technology; Arizona State University; Arizona State University-Tempe; National Institute for Materials Science; National Institute for Materials Science; Massachusetts Institute of Technology (MIT)
• 刊物名称: Nature
• ISSN/ISSBN: 0028-4131
• DOI: 10.1038/s41586-024-07880-5
• 发表日期: 2024-09-05
• 页码: 77-+
• 关键词: unconventional superconductivity; spin; transitions; cascade; valley

摘要

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 nu=2 and between nu=-3 and -2, with lifetimes of up to 300ps 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 Kekule 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 moire systems.