Spin transport in a tunable Heisenberg model realized with ultracold atoms
Spin transport in a tunable Heisenberg model realized with ultracold atoms
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DOI：
10.1038/s41586-020-3033-y
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发表时间：
2020-12-17
期刊：
影响因子：
64.8
通讯作者：
Ketterle, Wolfgang
中科院分区：
文献类型：
--
作者：
Jepsen, Paul Niklas;Amato-Grill, Jesse;Ketterle, Wolfgang
相互作用自旋的简单模型在物理学中具有重要作用。它们捕获了许多磁性材料的特性，但也扩展到其他系统，例如晶格中的玻色子和费米子、规范理论、高温超导体、量子自旋液体以及具有奇异粒子（例如任意子和马约拉纳费米子）的系统(1,2)。为了研究和比较这些模型，需要一个多功能平台。实现这样的系统一直是超冷原子领域的长期目标。到目前为止，自旋输运仅在具有各向同性自旋-自旋相互作用的系统中进行了研究(3-12)。在这里，我们实现了描述晶格上自旋的海森堡模型，具有完全可调的最近邻自旋-自旋耦合的各向异性（称为 XXZ 模型）。在这个模型中，我们研究了印记自旋螺旋图案的量子淬灭后远离平衡的自旋输运。当自旋仅沿三个可能方向中的两个耦合时（XX 模型），我们发现自旋动力学的弹道行为，而对于各向同性相互作用（XXX 模型），我们发现扩散行为。更一般地，对于正各向异性，动力学范围从异常超扩散到亚扩散，而对于负各向异性，我们观察到时域中从弹道传输到扩散传输的交叉。这种行为与线性响应机制的预期相反，并在理解远离平衡的量子多体动力学方面提出了新的问题。
Simple models of interacting spins have an important role in physics. They capture the properties of many magnetic materials, but also extend to other systems, such as bosons and fermions in a lattice, gauge theories, high-temperature superconductors, quantum spin liquids, and systems with exotic particles such as anyons and Majorana fermions(1,2). To study and compare these models, a versatile platform is needed. Realizing such systems has been a long-standing goal in the field of ultracold atoms. So far, spin transport has only been studied in systems with isotropic spin-spin interactions(3-12). Here we realize the Heisenberg model describing spins on a lattice, with fully adjustable anisotropy of the nearest-neighbour spin-spin couplings (called the XXZ model). In this model we study spin transport far from equilibrium after quantum quenches from imprinted spin-helix patterns. When spins are coupled only along two of three possible orientations (the XX model), we find ballistic behaviour of spin dynamics, whereas for isotropic interactions (the XXX model), we find diffusive behaviour. More generally, for positive anisotropies, the dynamics ranges from anomalous superdiffusion to subdiffusion, whereas for negative anisotropies, we observe a crossover in the time domain from ballistic to diffusive transport. This behaviour is in contrast with expectations from the linear-response regime and raises new questions in understanding quantum many-body dynamics far away from equilibrium.