Study of Electrons in Superfluid Helium

当一个电子被注入到氦中时，它会形成一个非常小的气泡（直径约38 μ m），氦原子被排除在气泡之外。这种“电子泡”形成了一个独特的量子系统，已经被广泛研究，并得到了相当好的理解。然而，除了正常的电子泡，其他带负电荷的物体也会产生，其起源仍然是个谜。 这些物体被称为外来离子，具有更高的流动性，似乎比正常的电子泡更小，尽管做了大量的工作，这些物体的性质仍然是一个谜。一种可能的解释是，这些奇异离子是液体中的气泡，只包含单个电子的一部分波函数，这一解释仍然存在一定的争议。本项目旨在利用超声、光学和输运测量来探索这些奇异离子的性质和结构。 如果可以确定这些确实是气泡，只有一小部分的电子波函数，这将导致物理学家对量子理论的看法发生根本性的变化。该研究项目非常适合于本科生和研究生的培训，他们将学习广泛的实践技能，包括电子学，真空技术，低温物理学，光学和计算机编程。技术摘要在这项研究中，我们的团队调查了在1 K左右的温度下在超流氦中发现的奇异负电荷离子。 研究的目的是发现这些奇异离子的内部结构。在以前的实验中，我们的小组已经进行了迁移率测量，表明至少有18种不同的离子，每种离子都有不同的特征尺寸。值得注意的是，也存在具有连续尺寸分布的离子。尽管不同的研究小组做了大量的工作，但这些离子的结构仍然未知。有强有力的实验证据表明它们不是杂质，而且似乎不可能将它们与以前在氦气中检测到的原子或分子氦离子联系起来。还有一种可能性是，离子是电子泡，每个电子泡只包含单个电子波函数的一小部分。虽然这是一个激进的想法，但它确实提供了一种理解大量不同离子存在的方法，并使具有连续尺寸分布的离子成为可能。当前项目的目标是更多地了解离子的性质，并以这种方式测试包含一部分波函数的气泡可以作为稳定实体存在的想法。在第一个项目中，将负压施加到液体上，并测量引起所选外来离子爆炸所需的压力大小。然后将负压的大小与理论预测进行比较。在第二个项目中，测量离子的光吸收。这决定了光子的能量，它可以引起离子的光激发，并给出了理论的第二个测试。该研究项目使用了广泛的技术，包括超声波，光学，低温和计算机模拟，并为本科生和研究生提供了良好的培训。

Non-technical AbstractWhen an electron is injected into helium, it forms a very small bubble (diameter ~38 Å) from which helium atoms are excluded. This "electron bubble" forms a unique quantum system which has been extensively studied and is fairly well understood. However, in addition to the normal electron bubbles other negatively charged objects are produced whose origin remains a mystery. These objects, which have been called exotic ions, have higher mobility and appear to be smaller than the normal electron bubbles and, despite much work, the nature of these objects remains a mystery. One possible explanation, which remains somewhat controversial, is that these exotic ions are bubbles in the liquid which contain only a part of the wave function of a single electron. This project is aimed at exploring the nature and structure of these exotic ions using ultrasonic, optical and transport measurements. If it can be established that these are indeed bubbles with only a fraction of the electron wave function it will result in a radical change in how physicists view quantum theory. This research project is ideally suited for the training of undergraduate and graduate students who will learn a broad range of practical skills including electronics, vacuum techniques, low temperature physics, optics and computer programming.Technical AbstractIn this research our team investigates exotic negatively-charged ions which have been discovered in superfluid helium at temperatures around 1 K. The goal of the research is to discover the inner structure of these exotic ions. In previous experiments our group has made mobility measurements which show that there are at least 18 different ions, each with a different characteristic size. Remarkably, there are also ions which have a continuous size distribution. Despite much work by different research groups the structure of these ions remains unknown. There is strong experimental evidence that they are not impurities, and it appears impossible to relate them to atomic or molecular helium ions previously detected in helium gas. One remaining possibility is that the ions are electron bubbles each containing only a fraction of the wave function of a single electron. While this is a radical idea, it does provide a way to understand the existence of a large number of different ions and makes it possible for there to be ions with a continuous distribution of size. The goal of the current project is to learn more about the properties of the ions and in this way to test the idea that bubbles containing a fraction of the wave function can exist as stable entities. In a first project a negative pressure is applied to the liquid and the magnitude of the pressure required to cause a selected exotic ion to explode is measured. The magnitude of the negative pressure is then compared with theoretical predictions. In a second project the optical absorption by the ions is measured. This determines the photon energies which can induce optical excitation of the ions and gives a second test of the theory. The research project uses a wide range of techniques including ultrasonics, optics, low temperature, and computer simulation and provides excellent training for undergraduate and graduate students.