Observation of first and second sound in a BKT superfluid
Observation of first and second sound in a BKT superfluid
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DOI：
10.1038/s41586-021-03537-9
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发表时间：
2021-06-10
期刊：
影响因子：
64.8
通讯作者：
Hadzibabic, Zoran
中科院分区：
文献类型：
--
作者：
Christodoulou, Panagiotis;Galka, Maciej;Hadzibabic, Zoran
在二维超流体中实验观测到了第一和第二声，声速随温度的变化揭示了无限阶Berezinskiii-Kosterlitz-Berteless转变时超流体密度的预测跳跃。自从在液氦II中观察到无摩擦流动以来，各种形式的超流体一直受到人们的关注1，2。在三维空间中，它在概念上与临界温度下长程有序的出现有关。正如双流体模型（3，4）所预测的，在液氦（5）和超冷原子气体（6，7）中观察到的，超流性的标志之一是存在两种声音激发-第一和第二声音。在二维系统中，热涨落排除了长程有序性（8，9）;然而，超流性仍然通过无限阶Berezinskiii-Kosterlitz-无相变（BKT）在非零临界温度下出现（10，11），这与超流密度的普遍跳跃（12）有关，而流体的热力学性质没有任何间断。BKT超流体也被预测支持两种声音，但到目前为止，这还没有被实验观察到。在这里，我们观察到均匀二维原子玻色气体中的第一和第二声音，并使用两个温度依赖的声速来确定气体的超流密度（13-16）。我们的结果与BKT理论的预言一致，包括在临界温度下超流密度普遍跳跃的预言。
First and second sound are experimentally observed in a two-dimensional superfluid, and the temperature-dependent sound speeds reveal the predicted jump in the superfluid density at the infinite-order Berezinskii-Kosterlitz-Thouless transition.Superfluidity in its various forms has been of interest since the observation of frictionless flow in liquid helium II1,2. In three spatial dimensions it is conceptually associated with the emergence of long-range order at a critical temperature. One of the hallmarks of superfluidity, as predicted by the two-fluid model(3,4) and observed in both liquid helium(5) and in ultracold atomic gases(6,7), is the existence of two kinds of sound excitation-the first and second sound. In two-dimensional systems, thermal fluctuations preclude long-range order(8,9); however, superfluidity nevertheless emerges at a non-zero critical temperature through the infinite-order Berezinskii-Kosterlitz-Thouless (BKT) transition(10,11), which is associated with a universal jump(12) in the superfluid density without any discontinuities in the thermodynamic properties of the fluid. BKT superfluids are also predicted to support two sounds, but so far this has not been observed experimentally. Here we observe first and second sound in a homogeneous two-dimensional atomic Bose gas, and use the two temperature-dependent sound speeds to determine the superfluid density of the gas(13-16). Our results agree with the predictions of BKT theory, including the prediction of a universal jump in the superfluid density at the critical temperature.