近日，北京大学的张熙博及其研究团队取得一项新进展。经过不懈努力，他们实现超越旋转波近似的拓扑自旋轨道耦合费米子。相关研究成果已于2024年1月19日在国际知名学术期刊《科学通报》上发表。

该研究团队提出并实验实现了一种新的方案，用于实现一对超越旋转波近似（RWA）的二维（2D）超冷费米子的自旋-轨道（SO）耦合。这项工作不仅实现了第一个超越RWA的反常Floquet拓扑费米气体，而且显著提高了2D - SO耦合费米气体的寿命。基于泵浦-探针猝灭测量，研究人员观察到两组SO耦合之间存在确定的相位关系，这是他们的超越RWA方案的一个特征。

此外，通过测量双环带反转表面，他们观察到了有趣的带拓扑现象，这与高Chern数下的Floquet拓扑费米气体在定量上是一致的。这项研究为探索基于长寿命粒子耦合的超冷费米子的奇异粒子物理和反常拓扑态开辟了一条新的途径。

据悉，自旋轨道耦合超冷气体是近年来研究的热点领域，其实现通常基于旋转波近似（RWA）。旋转波近似通过忽略反向旋转项，描述了双能级系统中的单一近共振自旋-轨道耦合。

附：英文原文

Title: Topological spin-orbit-coupled fermions beyond rotating wave approximation

Author: Han Zhang, Wen-Wei Wang, Chang Qiao, Long Zhang, Ming-Cheng Liang,Rui Wu, Xu-Jie Wang, Xiong-Jun Liu, Xibo Zhang

Issue&Volume: 2024/01/19

Abstract: The realization of spin-orbit-coupled ultracold gases has driven a wide range of researches and is typically based on the rotating wave approximation (RWA). By neglecting the counter-rotating terms, RWA characterizes a single near-resonant spin-orbit (SO) coupling in a two-level system. Here, we propose and experimentally realize a new scheme for achieving a pair of two-dimensional (2D) SO couplings for ultracold fermions beyond RWA. This work not only realizes the first anomalous Floquet topological Fermi gas beyond RWA, but also significantly improves the lifetime of the 2D-SO-coupled Fermi gas. Based on pump-probe quench measurements, we observe a deterministic phase relation between two sets of SO couplings, which is characteristic of our beyond-RWA scheme and enables the two SO couplings to be simultaneously tuned to the optimum 2D configurations. We observe intriguing band topology by measuring two-ring band-inversion surfaces, quantitatively consistent with a Floquet topological Fermi gas in the regime of high Chern numbers. Our study can open an avenue to explore exotic SO physics and anomalous topological states based on long-lived SO-coupled ultracold fermions.

DOI: 10.1016/j.scib.2024.01.018

Source: https://www.sciencedirect.com/science/article/pii/S2095927324000367