Kim E, Chan M H W
Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Nature. 2004 Jan 15;427(6971):225-7. doi: 10.1038/nature02220.
When liquid (4)He is cooled below 2.176 K, it undergoes a phase transition-Bose-Einstein condensation-and becomes a superfluid with zero viscosity. Once in such a state, it can flow without dissipation even through pores of atomic dimensions. Although it is intuitive to associate superflow only with the liquid phase, it has been proposed theoretically that superflow can also occur in the solid phase of (4)He. Owing to quantum mechanical fluctuations, delocalized vacancies and defects are expected to be present in crystalline solid (4)He, even in the limit of zero temperature. These zero-point vacancies can in principle allow the appearance of superfluidity in the solid. However, in spite of many attempts, such a 'supersolid' phase has yet to be observed in bulk solid (4)He. Here we report torsional oscillator measurements on solid helium confined in a porous medium, a configuration that is likely to be more heavily populated with vacancies than bulk helium. We find an abrupt drop in the rotational inertia of the confined solid below a certain critical temperature. The most likely interpretation of the inertia drop is entry into the supersolid phase. If confirmed, our results show that all three states of matter-gas, liquid and solid-can undergo Bose-Einstein condensation.
当液态氦（4He）被冷却至2.176K以下时，它会经历一种相变——玻色-爱因斯坦凝聚，并变成一种零粘度的超流体。一旦处于这种状态，它甚至可以通过原子尺寸的孔隙无耗散地流动。虽然人们直观地认为超流仅与液相有关，但从理论上提出超流也可能发生在氦（4He）的固相。由于量子力学涨落，即使在零温度极限下，预计在晶体固态氦（4He）中也会存在离域空位和缺陷。这些零点空位原则上可以使固体中出现超流性。然而，尽管经过多次尝试，在块状固体氦（4He）中尚未观察到这样的“超固相”。在此我们报告了对限制在多孔介质中的固体氦进行的扭摆测量，这种构型可能比块状氦含有更多的空位。我们发现在低于某个临界温度时，被限制固体的转动惯量突然下降。对转动惯量下降最可能的解释是进入了超固相。如果得到证实，我们的结果表明物质的所有三种状态——气体、液体和固体——都可以经历玻色-爱因斯坦凝聚。
