RUI: Experimental Study of Dipolar Solid Helium

氦-4有两个质子和两个中子，是仅次于氢的最简单的原子之一。然而，尽管氦表面上看起来很简单，但由于量子力学效应，它在低温下的行为非常复杂。这些效应的一种类型是，氦在常压下不会冻结，而是一直保持液体状态，直到绝对零度(T=0K)。为了让氦冻结，它必须被挤压到超过大气压的25倍，基本上迫使原子变成固体。氦的量子性质的另一个结果是，这种奇怪的固体可能表现出超流体性质(即无摩擦流动)。这是一个想法，尽管非常奇怪，预计会在合适的条件下发生，但在检测到这样一个阶段的实验中，结果喜忧参半。这个项目研究了氦的电子性质，并利用这些性质开发了一种产生固体氦的新途径。这种生长固体氦晶体的新方法使研究人员能够对系统参数进行一定程度的控制，而这些参数通常是其他固体氦实验所无法获得的。这项研究还研究了用这种新方法生长的固体氦中的量子效应，目的是观察固体氦中的超流性。最后，该项目让加州州立大学(萨克拉门托州立大学)的本科生参与低温物理实验，该大学主要是一所本科院校，服务于庞大的、不同种族的学生群体。通过这项研究，代表不足的少数族裔学生发展了一系列高技术技能，有助于为STEM劳动力带来多样性。技术描述：到目前为止，已在液体和气体中确凿地观察到超流体特性。这就引出了一个问题：固体中是否也存在超流体性质？50多年来，这个问题一直困扰着低温物理学家。由于其量子性质，人们认为固体氦最有可能表现出超流体行为，尽管实验研究取得了不同程度的成功。本项目通过研究固体氦的电子性质，为固体氦的研究带来了一个新的视角，以开发一种新的产生固体氦的方法。具体地说，外加电场被用来控制偶极-偶极相互作用，这是中性氦原子之间原子间相互作用的基础。在适当的条件下，这些偶极相互作用会使原子形成晶格，产生“偶极固体氦”。利用这种方法，加州州立大学萨克拉门托分校的低温研究小组研究了经典固体和量子固体的行为差异，从而有可能观察到固体或超固体相中的超流体行为。该项目的具体目标是：1)观察偶极固体氦在不同相互作用强度下的形成和行为；2)测量常规和偶极固体氦的电子性质；3)建立一个有利于本科生研究的低温恒温器。这些实验为未来将研究偶极量子固体行为的大范围实验建立了框架。这些实验提供了对材料行为的洞察，随着技术变得越来越小，并接近量子效应变得重要的极限。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical Description:With its two protons and two neutrons, helium-4 is one of the simplest atoms next to hydrogen. However, although apparently simple, helium has very complex behavior at low temperatures due quantum mechanical effects. One type of these effects is that helium will not freeze under normal pressures, but rather will remain a liquid all the way down to absolute zero (T = 0 K). In order to get helium to freeze, it must be squeezed to over 25 times atmospheric pressure, essentially forcing the atoms into a solid. Another consequence of the quantum nature of helium is the possibility that this strange solid can display superfluid properties (i.e. flow without friction). This is an idea that, although very strange, is predicted to occur under the right conditions, but experiments have had mixed results in detecting such a phase. This project studies the electronic properties of helium and exploits these properties to develop a new route to creating solid helium. This new method of growing solid helium crystals gives researchers a certain amount of control over system parameters that have been typically inaccessible to other solid helium experiments. The research also studies quantum effects in solid helium grown using this new method, with the goal of observing superfluidity in solid helium. Finally, this project engages undergraduate students in low temperature physics experiments at California State University, Sacramento (Sacramento State), which is primarily an undergraduate institution serving a large, ethnically diverse student body. Through this research, under-represented minority students, develop a range of highly technical skills, helping to bring diversity to the STEM workforce.Technical Description:Superfluid properties have so far been conclusively observed in liquids and gasses. This begs the question: can superfluid properties exist in solids as well? This question has captivated low temperature physicists for over 50 years. Due to its quantum nature, it is believed that solid helium has the best chance of displaying superfluid behavior, although experimental searches have been met with varying success. This project brings a new perspective to solid helium research by studying electronic properties of solid helium in order to develop a new method of creating solid helium. Specifically, an external electric field is used to control the dipole-dipole interactions that are the basis of inter-atomic interactions between neutral helium atoms. Given the right conditions, these dipolar interactions cause the atoms to form a lattice, creating "dipolar solid helium". With this method, the low temperature research team at California State University, Sacramento, studies the differences in the behavior of a classically-behaved solid and a quantum solid leading to the possibility of observing superfluid behavior in the solid, or a, "supersolid" phase. Specific goals of this project are to: 1) observe the formation and behavior of dipolar solid helium under varying interaction strengths; 2) measure the electronic properties of both conventional and dipolar solid helium; and 3) build a low temperature cryostat that is conducive to undergraduate research. These experiments build a framework for a large range of future experiments that will study the behavior of a dipolar quantum solid. Such experiments offer insights into the behavior of materials that are important as technologies get smaller, and approach the limit where quantum effects become important.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.