determination determination

编译 | 李言
Nature, 3 December 2020，Volume 588 Issue 7836
《自然》2020年12月3日，第588卷，7836期

文章插图
物理学Physics
Determination of the fine-structure constant with an accuracy of 81 parts per trillion
测定精细结构常数，精度为万亿分之81
▲ 作者：Léo Morel, Zhibin Yao, Pierre Cladé & Sa?da Guellati-Khélifa
▲ 链接：
https://www.nature.com/articles/s41586-020-2964-7
▲ 摘要
粒子物理学的标准模型非常成功，因为它（几乎）与所有的实验结果一致。然而，它不能解释暗物质、暗能量以及宇宙中物质和反物质之间的不平衡。
在这里，我们使用物波干涉测量法来测量一个铷原子吸收一个光子的反冲速度，并确定了精细结构常数α?1 = 137.035999206(11)，相对精度为万亿分之81 。
11位数字的准确性导致了电子g因子——标准模型中最精确的预测——极大地减少了不确定性。我们的精细结构常数值与铯反冲测量的最佳结果相差5个标准差以上。
我们的结果修正了对可能的候选暗物质粒子的限制，这些暗物质粒子被提议用来解释8Be原子核激发态的异常衰变，并且为测试电子部门中观测到的介子磁矩异常的差异铺平了道路。
▲ Abstract
The standard model of particle physics is remarkably successful because it is consistent with (almost) all experimental results. However, it fails to explain dark matter, dark energy and the imbalance between matter and antimatter in the Universe. Here we use matter-wave interferometry to measure the recoil velocity of a rubidium atom that absorbs a photon, and determine the fine-structure constant α?1 = 137.035999206(11) with a relative accuracy of 81 parts per trillion. The accuracy of eleven digits in α leads to an electron g factor—the most precise prediction of the standard model—that has a greatly reduced uncertainty. Our value of the fine-structure constant differs by more than 5 standard deviations from the best available result from caesium recoil measurements. Our result modifies the constraints on possible candidate dark-matter particles proposed to explain the anomalous decays of excited states of 8Be nuclei and paves the way for testing the discrepancy observed in the magnetic moment anomaly of the muon in the electron sector.

材料科学Material Science

Electrical switching of magnetic order in an orbital Chern insulator
轨道Chern绝缘子中的超常规磁性切换
▲ 作者：H. Polshyn, J. Zhu, M. A. Kumar, Y. Zhang, F. Yang, C. L. Tschirhart, M. Serlin, K. Watanabe, T. Taniguchi, A. H. MacDonald & A. F. Young
▲ 链接：
https://www.nature.com/articles/s41586-020-2963-8
▲ 摘要
磁性通常是由费米统计和排斥性库仑相互作用共同产生的，这有利于基态与非零电子自旋。因此，只能间接地实现利用电场控制自旋磁性——自旋电子学和多铁学的长期技术目标。
在此，我们实验演示了在一个非普通带拓扑有利于轨道角动量的长程顺序，但自旋被认为是无序的磁系统中的磁态的直接电场控制。在带内每莫尔元胞中填充一和三个电子时，我们发现通过对化学势的场效应控制可以逆转量子反常霍尔效应的符号；此外，这种跃迁具有迟滞性，我们用迟滞性来演示非挥发性电场诱导的磁态反转。
理论分析表明，这种效应是由拓扑边缘状态引起的，它引起磁化符号的变化，从而使有利的磁态发生逆转。
▲ Abstract
Magnetism typically arises from the joint effect of Fermi statistics and repulsive Coulomb interactions, which favours ground states with non-zero electron spin. As a result, controlling spin magnetism with electric fields—a longstanding technological goal in spintronics and multiferroics—can be achieved only indirectly. Here we experimentally demonstrate direct electric-field control of magnetic states in an orbital Chern insulator, a magnetic system in which non-trivial band topology favours long-range order of orbital angular momentum but the spins are thought to remain disordered. At a filling of three electrons per moiré unit cell, we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential; moreover, this transition is hysteretic, which we use to demonstrate non-volatile electric-field-induced reversal of the magnetic state. A theoretical analysis indicates that the effect arises from the topological edge states, which drive a change in sign of the magnetization and thus a reversal in the favoured magnetic state.

Unconventional ferroelectricity in moiré heterostructures
莫尔异质结中的反常铁电性
▲ 作者：Zhiren Zheng, Qiong Ma, Zhen Bi, Sergio de la Barrera, Ming-Hao Liu et al.

determination determination

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