介观物质中的量子简并：Casimir效应与Bose-Einstein凝聚
Quantum degeneracy in mesoscopic matter: Casimir effect and Bose-Einstein condensation
AI总结：
本研究测量了碳纳米管上固态氦中零点声子诱导的声学Casimir压力，发现介观尺度下热力学量具尺寸依赖性并可能出现四相点，且实验发现了50多年前预言的零点空位BEC。
摘要：
基态声子压力类似于由零点光子产生的著名真空Casimir压力。然而，由于典型声速比光速小100 000倍，声学Casimir力比电磁Casimir力弱许多个数量级。由于其微弱性，零点声学Casimir压力从未被观测到，尽管已有关于人为引入的噪声在狭窄孔径上产生的压力的报道。然而，Casimir压力的大小随样品尺寸$L$的减小以$1/L^3$的规律增加，并在亚微米尺度上达到皮牛顿量级。我们演示并测量了吸附在碳纳米管上的固态氦中由零点声子诱导的声学Casimir压力。我们还观察到随温度或长度降低，类Casimir的将热声子“挤出”的现象。我们还表明，所有热力学量均具有尺寸依赖性，因此在介观范围$L\lesssim\hbar{c}/(k_BT)$内，相图上可能出现四个不同相共存的四相点。由于固态氦样品极小，空位的Bose-Einstein凝聚（BEC）温度相对较高，为$10-100$ mK。这使我们得以在实验上发现零点空位系统中的BEC，这一现象在50多年前就被预言。
英文摘要：
The ground-state phonon pressure is an analogue to the famous Casimir pressure of vacuum produced by zero-point photons. The acoustic Casimir forces are, however, many orders of magnitude weaker than the electromagnetic Casimir forces, as the typical speed of sound is 100 000 times smaller than the speed of light. Because of its weakness, zero-point acoustic Casimir pressure was never observed, although the pressure of artificially introduced sound noise on a narrow aperture has been reported. However, the magnitude of Casimir pressure increases as $1/L^3$ with the decrease of the sample size $L$, and reaches picoNewtons in the sub-micron scales. We demonstrate and measure the acoustic Casimir pressure induced by zero-point phonons in solid helium adsorbed on a carbon nanotube. We have also observed Casimir-like "pushing out" thermal phonons with the decreasing temperature or the length. We also show that all thermodynamic quantities are size-dependent, and therefore in the mesoscopic range $L\lesssim\hbar{c}/(k_BT)$ quadruple points are possible on the phase diagram where four different phases coexist. Due to the smallness of solid helium sample, temperature of Bose-Einstein condensation (BEC) of vacancies is relatively high, $10-100$ mK. This allowed us to experimentally discover the BEC in a system of zero-point vacancies, predicted more than 50 years ago.
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