Collaborative Proposal: Synchrotron X-Ray Scattering Experiments on Solid Helium

*非技术摘要*这个合作奖项主要支持一个项目，该项目主要针对最近对超固态状态的实验观察所产生的问题。超固体是一种新的物质状态，其中固体中的部分原子相对于固体的其余部分或容纳它的容器表现出无摩擦的运动。这类似于超导和超流等现象。这个超级固体国家的确切性质目前是一个激烈辩论的问题。机械实验和X射线实验相结合的目的是将宏观层面上观察到的现象与固体的微观结构联系起来。这将为更全面地理解超固体提供关键信息。该项目是特拉华大学、西密歇根大学、北伊利诺伊大学和宾夕法尼亚州立大学研究小组之间密集和持续合作的工具，专注于阿贡国家实验室高级光子源的实验。这种设置让参与其中的本科生和研究生都比通常情况下更广泛地了解了物理事业，以及他们在其中的潜在角色。它还为学生提供了一种机制，让他们在早期建立同龄人之间的联系网络，并为科学生涯奠定良好的准备。*技术摘要*这个合作项目的中心目标是提供必要的关键信息，以解释最近对4He可能处于超固态状态的观察。一系列的X射线衍射、扭转振子(TO)和介电常数实验将被用来研究几个问题：1.X射线衍射中有没有直接指示超固态转变的特征？例如，仔细研究德拜-沃勒因子可能会对玻色凝聚分数提供限制。2.晶体质量的哪些方面对超固体状态的形成是重要的？X射线测量将用于确定晶体质量，并将与同步实验中的测量结果相结合。此外，通过在气凝胶等多孔介质中生长固体，将以受控的方式引入无序。3.在T=0时，4He是否形成相称的固体，或者零点空位浓度为0？高精度测量恒定体积下晶格常数与温度的关系，可以区分热空位和非热空位。研究生和本科生的培养是这一项目的重要组成部分。它的合作性质，以及部分工作是在APS/ANL完成的事实，使学生比通常情况下更广泛地接触到研究模式，并将为科学事业奠定良好的准备。

****NON-TECHNICAL ABSTRACT****This collaborative award supports a program primarily directed towards issues that stem from recent experimental observations of a supersolid state. A supersolid is a new state of matter in which part of the atoms in the solid show frictionless motion with respect to the rest of the solid, or the container that holds it. This resembles such phenomena as superconductivity and superfluidity. The precise nature of the supersolid state is currently a matter of lively debate. A combination of mechanical and x-ray experiments aims at correlating phenomena observed at the macroscopic level with the microscopic structure of the solid. This will provide critical information for a more general understanding of supersolidity. The project is a vehicle for intensive and sustained collaboration between research groups at the University of Delaware, Western Michigan University, Northern Illinois University, and Pennsylvania State University focused on experiments at the Advanced Photon Source at Argonne National Laboratory. This setting gives both the undergraduate and graduate students involved a much wider view of the physics enterprise, and their potential role in it, than is usually the case. It also provides a mechanism for the students to build up a network of contacts among peers early on and forms an excellent preparation for a scientific career.****TECHNICAL ABSTRACT****The central goal of this collaborative project is to provide critical information necessary to interpret recent observations of a possible supersolid state in 4He. A series of combined x-ray diffraction, torsional oscillator (TO) and dielectric constant experiments will be used to investigate several questions: 1. Are there features in the x-ray diffraction directly indicative of a supersolid transition? E.g., a careful study of the Debye-Waller factor may provide limits on the Bose condensate fraction. 2. What aspects of crystal quality are important for the formation of a supersolid state? X-ray measurements will be used to determine the crystal quality, and will be combined with TO measurements in a simultaneous experiment. In addition, disorder will be introduced in a controlled way by growing the solid in porous media such as aerogels. 3. Does 4He form a commensurate solid at T = 0, or is the zero-point vacancy concentration 0? High precision measurements of the temperature dependence of the lattice constants at constant volume, can distinguish between thermal and non-thermal vacancy populations. The training of graduate as well as undergraduate students forms an important part of this project. Its collaborative nature, as well as the fact that part of the work is done at the APS/ANL, gives students a much broader exposure to modes of research than is normally the case, and will form an excellent preparation for a scientific career.