Cosmological Experiments in He3 - He4 Mixtures

宇宙相变是拓扑缺陷的一个重要来源。 同样的情况也发生在凝聚态系统中，对这些系统的实验可以帮助确定缺陷形成的机制。 液氦中的超流跃迁与包含拓扑缺陷的宇宙学模型之间的相似性特别强。 因此，近年来，人们开始积极地利用液态He-3和He-4来模拟早期宇宙的相变。之前的实验表明，在He-3和He-4中，缺陷（在这种情况下是涡旋）确实会产生，并且涡旋线密度与基于Zurek提出的机制（现在称为Kibble-Zurek（K-Z）机制）的估计一致。 该机制对涡线密度随a）跃迁发生的速率和B）序参量的相关长度的幅度的标度进行了具体的预测。 在早期的实验中，对前者几乎没有控制，而后者完全由流体决定，也不能明显改变。 Mulder教授的团队将进行一项新的实验，在He-3和He-4混合物中接近三临界点进行操作，将有能力在很大范围内改变两者。 该项目由物理系、材料研究系和MPS多学科活动办公室共同支持。

Cosmological phase transitions can be a copious source of topological defects. The same happens in condensed matter systems, and experiments on these systems can help in determining the mechanism for defect formation. The similarities between the superfluid transitions in liquid helium and cosmological models containing topological defects is especially strong. As a result, in recent years there has been considerable activity to simulate phase transitions of the early universe using liquid He-3 and He-4.Previous experiments in both He-3 and He-4 have shown that defects, in this case vortices, are indeed created, and that the vortex line density is consistent with estimates based on the mechanism proposed by Zurek, now known as the Kibble-Zurek (K-Z) mechanism. This mechanism makes specific predictions about the scaling of the vortex line density with a) the rate at which the transition takes place and b) the amplitude of the correlation length of the order parameter. In earlier experiments there was little control over the former, while the latter, determined solely by the fluid, could not be changed appreciably either. Prof. Mulder's group will carry out a new experiment, operating close to the tricritical point in He-3 and He-4 mixtures, will have the ability to change both over a wide range. This should result in a definitive test of the K-Z mechanism.This project is jointly supported by the Division of Physics, the Division of Materials Research and the MPS Office of Multidisciplinary Activities.