Observing the Rosensweig instability of a quantum ferrofluid
Observing the Rosensweig instability of a quantum ferrofluid
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DOI：
10.1038/nature16485
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发表时间：
2016-02-11
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：
Kadau, Holger;Schmitt, Matthias;Pfau, Tilman
由于其成分的磁性，磁流体表现出不同寻常的流体动力学效应。随着磁化强度的增加，经典的铁磁流体经历Rosensweig不稳定性(1)，并产生自组织的有序表面结构(2)或液滴晶体(3)。具有强偶极相互作用的量子磁流体例如玻色-爱因斯坦凝聚体也表现出超流[4]。偶极量子气体领域的动机是寻找打破连续对称性的物质的新相(5，6)。连续对称性的同时破缺，例如超流状态下的相不变性和晶体中的平移对称性，为这些新的物质状态提供了基础。然而，超流中的相互作用诱导结晶还没有被观察到。在这里，我们使用原位成像来直接观察到在镝原子玻色-爱因斯坦凝聚体中从无结构超流体到有序排列液滴的自发转变(7)。通过使用Feshbach共振来控制粒子间的相互作用，我们诱导了有限波长不稳定性(8)，并观察到离散的液滴呈三角形结构，其数量随着原子数量的增加而增加。我们发现这些结构态的寿命惊人地长，并且观察到了滞后行为，这是结晶过程中的典型现象，与Rosensweig不稳定性非常相似。我们的系统既表现出超流性，又表现出自发的平移对称性破缺。虽然我们的观察没有探测到结构态中的超流性，但如果液滴通过弱连接建立了一个公共相，那么我们的系统是一个非常好的候选超固态基态(9-11)。
Ferrofluids exhibit unusual hydrodynamic effects owing to the magnetic nature of their constituents. As magnetization increases, a classical ferrofluid undergoes a Rosensweig instability(1) and creates self-organized, ordered surface structures(2) or droplet crystals(3). Quantum ferrofluids such as Bose-Einstein condensates with strong dipolar interactions also display superfluidity(4). The field of dipolar quantum gases is motivated by the search for new phases of matter that break continuous symmetries(5,6). The simultaneous breaking of continuous symmetries such as the phase invariance in a superfluid state and the translational symmetry in a crystal provides the basis for these new states of matter. However, interaction-induced crystallization in a superfluid has not yet been observed. Here we use in situ imaging to directly observe the spontaneous transition from an unstructured superfluid to an ordered arrangement of droplets in an atomic dysprosium Bose-Einstein condensate(7). By using a Feshbach resonance to control the interparticle interactions, we induce a finite-wavelength instability(8) and observe discrete droplets in a triangular structure, the number of which grows as the number of atoms increases. We find that these structured states are surprisingly long-lived and observe hysteretic behaviour, which is typical for a crystallization process and in close analogy to the Rosensweig instability. Our system exhibits both superfluidity and, as we show here, spontaneous translational symmetry breaking. Although our observations do not probe superfluidity in the structured states, if the droplets establish a common phase via weak links, then our system is a very good candidate for a supersolid ground state(9-11).