Self-bound droplets of a dilute magnetic quantum liquid
Self-bound droplets of a dilute magnetic quantum liquid
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DOI：
10.1038/nature20126
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发表时间：
2016-11-10
期刊：
影响因子：
64.8
通讯作者：
Pfau, Tilman
中科院分区：
文献类型：
--
作者：
Schmitt, Matthias;Wenzel, Matthias;Pfau, Tilman
自束缚多体系统是通过引力和排斥力的平衡而形成的，在许多物理场景中都会发生。液滴是自束缚系统的一个例子，它是由粒子间势的不同分量产生的相互吸引力和排斥力的平衡形成的。有人提出(1，2)，当原子数密度比由稠密量子液体形成的氦滴中低10(8)倍时，应该存在超冷原子的自束缚系综。然而，到目前为止，这样的组合一直难以捉摸，因为它们需要的力不是通常的零距离接触相互作用，而是吸引或排斥，但永远不会两者兼而有之。根据最近的发现，不稳定的玻色子偶极气体可以通过排斥多体项来稳定(3)，预测应该存在磁性原子的三维自束缚量子液滴(4，5)。在这里，我们报告了在无陷阱的悬浮场中观察到这样的液滴。我们发现，这种稀磁量子液体需要一个最小的、临界的原子数，低于这个原子数，液体就会蒸发成膨胀的气体，这是单个组分量子压强的结果。因此，在这个临界原子数附近，我们观察到在超冷原子的量子简并区中气体和自束缚液体之间的相互作用驱动的相变。这些液滴是强关联自束缚系统的稀释物，如原子核(6)和氦液滴(7)。
Self-bound many-body systems are formed through a balance of attractive and repulsive forces and occur in many physical scenarios. Liquid droplets are an example of a self-bound system, formed by a balance of the mutual attractive and repulsive forces that derive from different components of the inter-particle potential. It has been suggested(1,2) that self-bound ensembles of ultracold atoms should exist for atom number densities that are 10(8) times lower than in a helium droplet, which is formed from a dense quantum liquid. However, such ensembles have been elusive up to now because they require forces other than the usual zero-range contact interaction, which is either attractive or repulsive but never both. On the basis of the recent finding that an unstable bosonic dipolar gas can be stabilized by a repulsive many-body term(3), it was predicted that three-dimensional self-bound quantum droplets of magnetic atoms should exist(4,5). Here we report the observation of such droplets in a trap-free levitation field. We find that this dilute magnetic quantum liquid requires a minimum, critical number of atoms, below which the liquid evaporates into an expanding gas as a result of the quantum pressure of the individual constituents. Consequently, around this critical atom number we observe an interaction-driven phase transition between a gas and a self-bound liquid in the quantum degenerate regime with ultracold atoms. These droplets are the dilute counterpart of strongly correlated self-bound systems such as atomic nuclei(6) and helium droplets(7).