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中国物理学会期刊

一维超冷原子动量光晶格中的手征对称性破缺拓扑相

CSTR: 32037.14.aps.73.20231566

Topological phase in one-dimensional momentum space lattice of ultracold atoms without chiral symmetry

CSTR: 32037.14.aps.73.20231566
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  • 对称性在理解物质的拓扑态方面具有关键作用. 过去人们认为手征对称性保证了一维晶格的量子化Zak相位及其对应的非平庸拓扑相. 本文展现了在一维手征对称性破缺的情况下, 晶格系统仍具有量子化Zak相位和非平庸拓扑相. 具体而言, 在超冷原子动量晶格系统中有效地模拟了一个链长为26、手征对称性破缺的Zigzag模型, 其中相等的次近邻耦合强度能够在保留空间反演对称性的同时破坏手征对称性. 通过测量原子的时间平均波包位移来获得系统的拓扑不变量, 并得到了其对应的量子化的Zak相位. 此外, 还观测到系统随着最近邻耦合强度比例的变化会从非平庸拓扑相转变为平庸拓扑相. 本文不仅为对称性及拓扑相的相关研究提供了一个完全可控的平台, 还可以通过控制格点间耦合强度和原子间相互作用, 探索例如Tasaki, Aharonov-Bohm caging模型中的平带拓扑以及引入相互作用研究的非线性拓扑现象.

     

    Symmetry plays a crucial role in understanding topological phases in materials. In one-dimensional systems, such as the Su-Schrieffer-Heeger (SSH) model, chiral symmetry is thought to ensure the quantization of the Zak phase and the nontrivial topological phase. However, our work demonstrates that the one-dimensional lattice system with broken chiral symmetry can still possess quantized Zak phase and nontrivial topological phase. Specifically, we use a Bose-Einstein condensate of 87Rb atoms in a momentum space lattice of ultracold atoms to effectively simulate a one-dimensional Zigzag model of 26 sites, which intrinsically breaks the chiral symmetry by additional next-nearest-neighbor coupling. To ensure the existence of the nontrivial topological phase, where the Zak phase can be measured from the time-averaged displacement during the system’s evolution, we need to preserve the inversion symmetry by modulating laser power so that all next-nearest-neighbor coupling strengths are equal. Furthermore, by changing the ratio of nearest-neighbor coupling strengths, we observe a topological phase transition from a nontrivial topological phase to a trivial topological phase at the point where the ratio equals 1. Our work demonstrates that the ultracold atom system provides a controllable platform for studying the symmetrical phase and topological phase, with the potential to explore nonlinear topological phenomena by increasing the interactions among atoms. In addition, our system can be used to investigate other interesting topological phenomena with more complex models, such as critical phenomena at the phase transitions and flat band structures in the extended SSH model with long-range coupling. By controlling the coupling strengths, we can also explore the influence of different symmetries on the topological properties of extended SSH models in the future. Moreover, our platform makes it possible to studythe models with more energy bands, such as the Aharonov-Bohm caging model with a three-level structure, which shows peculiar flat-band properties. This work provides opportunities for various studies in the fields of symmetry, topology, and the interaction of controllable quantum systems.

     

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