Critical Landau Velocity in Helium Nanodroplets
Critical Landau Velocity in Helium Nanodroplets
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DOI：
10.1103/physrevlett.111.153002
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发表时间：
2013-10-08
影响因子：
8.6
通讯作者：
Drabbels, Marcel
中科院分区：
文献类型：
--
作者：
Brauer, Nils B.;Smolarek, Szymon;Drabbels, Marcel
超流氦最著名的特性是当物体以低于所谓的临界朗道速度在液体中运动时所经历的粘性消失。该临界速度通常被认为是宏观性质，因为它与液体中氦原子的集体激发有关。在目前的工作中，我们确定在何种程度上这一概念仍然可以适用于纳米尺度，有限大小的氦系统。为此，嵌入在不同大小的氦纳米液滴中的原子和分子通过光激发被加速出液滴，并且确定喷射粒子的速度分布。测量结果表明，在这些系统中存在一个临界速度，即使是只有一千个氦原子组成的纳米液滴。伴随的理论模拟的基础上的时间依赖性密度泛函描述的氦确认和进一步阐明这一实验发现。
The best-known property of superfluid helium is the vanishing viscosity that objects experience while moving through the liquid with speeds below the so-called critical Landau velocity. This critical velocity is generally considered a macroscopic property as it is related to the collective excitations of the helium atoms in the liquid. In the present work we determine to what extent this concept can still be applied to nanometer-scale, finite size helium systems. To this end, atoms and molecules embedded in helium nanodroplets of various sizes are accelerated out of the droplets by means of optical excitation, and the speed distributions of the ejected particles are determined. The measurements reveal the existence of a critical velocity in these systems, even for nanodroplets consisting of only a thousand helium atoms. Accompanying theoretical simulations based on a time-dependent density functional description of the helium confirm and further elucidate this experimental finding.