Microscopic picture of superfluid helium-4

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.