分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2022-10-21
摘要： We illustrate the microscopic quantum picture of superfluid $^4$He with the help ofrevealing a hidden property of its many-body levels. We show that, below thetransition point, the low-lying levels of the system form a grouping structure with eachlevel belonging to one specific group only. In a superflow state or a static state, the systemestablishes a group-specific thermal equilibrium with its environment and the levels of aninitially-occupied group shall be thermally distributed. The other initially-unoccupied groupsof levels remain unoccupied, due to the fact that inter-group transitions are prohibited.The macroscopically observable physical quantities of the system, such as superflowvelocity and thermal energy density, are determined statistically by the thermaldistribution of the occupied group(s). We further show that thermalenergy of a superflow has an unusual flow velocity dependence: the largerthe velocity is, the smaller the thermal energy. This velocity dependence isresponsible for several intriguing phenomena of the system, such as themechano-caloric effect and the fountain effect, which demonstrate afundamental coupling between the thermal motion of the system and itshydrodynamic motion. We report an experimental observation of a counter-intuitiveself-heating effect of $^4$He superflows, which confirms that a $^4$He superflowcarries significant thermal energy depending on its velocity.
分类： 物理学 >> 凝聚态:结构、力学和热性能 提交时间： 2016-11-17
摘要： 热力学第二定律要求孤立体系中有序运动模式的能量要逐渐转化成无序运动（热运动）的能量，最终达到热平衡状态（熵的最大化）。超流氦四在不需要任何外力的作用下却能够维持高度有序的流运动的能量，从而让热力学第二定律发挥不了作用，是个十份神秘的现象。朗道凭借他的非凡的物理直觉，得到了超流液氦的一组量子谱态，从而干净利索地说明了超流的流运动是不会衰减下去的。我们近一步说明超流氦在和热力学第二定律的‘对抗’的路上可以走得更远。
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