《自然》（20240905出版）一周论文导读
编译|未玖
Nature, 5 September 2024, Volume 633, Issue 8028
《自然》2024年9月5日，第633卷，8028期
天文学Astronomy
Gravitational instability in a planet-forming disk
行星形成盘的引力不稳定性
▲ 作者：Jessica Speedie, Ruobing Dong, Cassandra Hall, Cristiano Longarini, Benedetta Veronesi, Teresa Paneque-Carre?o, et al.
▲链接：
https://www.nature.com/articles/s41586-024-07877-0
▲摘要：
关于行星在星周盘中形成的正则理论提出，行星最初是由小得多的种子长成的。长期考虑的替代理论提出，巨大的原行星可由引力不稳定引起的巨大旋臂的坍塌碎片直接形成——如果星周盘引力不稳定的话。
要实现这一点，星周盘的质量必须比中心恒星大得多：星周盘与恒星的质量比为1:10被广泛认为是触发引力不稳定性、激发大量非开普勒动力学、产生明显旋臂的粗略阈值。虽然估计星周盘质量历来颇具挑战性，但气体的运动可通过其对盘速度结构的影响揭示引力不稳定性的存在。
研究组利用阿塔卡马大型毫米波/亚毫米波阵列（ALMA）对13CO和C18O谱线发射的深入观测，提出了AB Aurigae星周盘引力不稳定的运动学证据。观测到的运动信号与模拟和分析建模的预测非常相似。通过定量比较，研究组推断出在天空中1″到5″范围内的星周盘质量高达恒星质量的三分之一。
▲ Abstract：
The canonical theory for planet formation in circumstellar disks proposes that planets are grown from initially much smaller seeds. The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms induced by gravitational instability—if the disk is gravitationally unstable. For this to be possible, the disk must be massive compared with the central star: a disk-to-star mass ratio of 1:10 is widely held as the rough threshold for triggering gravitational instability, inciting substantial non-Keplerian dynamics and generating prominent spiral arms. Although estimating disk masses has historically been challenging, the motion of the gas can reveal the presence of gravitational instability through its effect on the disk-velocity structure. Here we present kinematic evidence of gravitational instability in the disk around AB Aurigae, using deep observations of 13CO and C18O line emission with the Atacama Large Millimeter/submillimeter Array (ALMA). The observed kinematic signals strongly resemble predictions from simulations and analytic modelling. From quantitative comparisons, we infer a disk mass of up to a third of the stellar mass enclosed within 1″ to 5″ on the sky.
物理学Physics
Long-lived isospin excitations in magic-angle twisted bilayer graphene
魔角扭曲双层石墨烯的长寿命同位旋激发
▲ 作者：Tian Xie, Siyuan Xu, Zhiyu Dong, Zhiyuan Cui, Yunbo Ou, Melike Erdi, et al.
▲链接：
https://www.nature.com/articles/s41586-024-07880-5
▲摘要：
人们已报道了魔角扭曲双层石墨烯（MATBG）诸多相关的多体相，包括传统相和奇异相。与这些相关态有关的动力学对于理解潜在的物理学至关重要，但尚未得到探索。
研究组结合激子传感和光泵浦-探针光谱来研究具有WSe2基底的MATBG在整个平带上的同位旋序动力学，达到了亚皮秒分辨率。结果观察到，在ν=2附近和ν=-3和-2之间的宽填充范围内，同位旋动力学非常缓慢，寿命高达300 ps，与电子温度更快的冷却（约10 ps）解耦。这种非热行为表明在同位旋自由度中存在异常的长寿命模式。
这一出乎理论预测的观察结果，表明了长程传播集体模式、强同位旋涨落和记忆效应的存在，并可能与谷间相干或不匹配的凯库勒螺旋基态有关。研究组进一步证明了先前在整数填充周围发现的同位旋序的非平衡控制。具体来说，通过超快操作，可暂时偏离整数填充。该研究展示了MATBG中集体激发的独特探索，并为主动控制莫尔系统中的非平衡现象铺平了道路。
▲ Abstract：
Numerous correlated many-body phases, both conventional and exotic, have been reported in magic-angle twisted bilayer graphene (MATBG). 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.
材料科学Materials Science
Photoredox phase engineering of transition metal dichalcogenides
过渡金属二硫族化物的光氧化还原相工程
▲ 作者：Juhwan Lim, Jung-In Lee, Ye Wang, Nicolas Gauriot, Ebin Sebastian, Manish Chhowalla, et al.
▲链接：
https://www.nature.com/articles/s41586-024-07872-5
▲摘要：
