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~38？)，从中排除氦原子。这个“电子气泡”形成了一个独特的量子系统，它已经被广泛地研究并被相当好地理解。然而，除了正常的电子气泡外，还会产生其他带负电荷的物体，它们的起源仍然是一个谜。这些被称为奇异离子的物体具有更高的迁移率，看起来比正常的电子气泡更小，尽管做了很多工作，这些物体的性质仍然是一个谜。一种可能的解释是，这些奇怪的离子是液体中的气泡，只包含单电子波函数的一部分，这一解释仍然存在一些争议。该项目旨在通过超声波、光学和输运测量来探索这些奇异离子的性质和结构。如果能够确定这些气泡的电子波函数只有一小部分，这将导致物理学家对量子理论的看法发生根本性的变化。这项研究项目非常适合培养本科生和研究生，他们将学习广泛的实用技能，包括电子学、真空技术、低温物理、光学和计算机编程。技术摘要在这项研究中，我们的团队研究了在1K左右的超流氦中发现的奇异负电荷离子。在之前的实验中，我们的团队已经进行了迁移率测量，表明至少有18个不同的离子，每个离子具有不同的特征大小。值得注意的是，还有一些离子具有连续的尺寸分布。尽管不同的研究小组做了很多工作，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之前在氦气体中检测到的原子或分子氦离子联系起来。剩下的一种可能性是，这些离子是电子气泡，每个气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，并使存在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波函数的一小部分的气泡可以作为稳定实体存在的想法。在第一个项目中，对液体施加负压，并测量导致选定的外来离子爆炸所需的压力的大小。然后将负压的大小与理论预测进行比较。在第二个项目中，测量了离子的光吸收。这确定了可以诱导离子光激发的光子能量，并对该理论进行了第二次测试。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，为本科生和研究生提供了出色的培训。

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.