Mass transport through solid helium

非技术摘要液态氦在低温下从正常液体转变为超流体。它被称为超流体，因为它可以以零粘度流动，而不会损失能量。一旦设置在连续的通道中流动，超流体将永远不会停止。这一令人着迷的性质与液氦在2.176开尔文以下进入量子态有关。值得注意的是，马萨诸塞大学和宾夕法尼亚州立大学最近的实验发现，氦原子可以像超流体一样流经固体氦样品。宾夕法尼亚州立大学的研究小组正在测量氦原子在不同密度、厚度、晶体取向的固体氦样品中的流动速度，以了解导致这一现象的确切机制。拟议的实验为博士后、研究生和本科生提供了机会，让他们接受最严格的培训，成为未来的科学家。首席研究员及其学生和博士后积极参与宾夕法尼亚州立大学材料研究、科学和工程中心(MRSEC)为K-12学生主办的全方位外联活动。这些有组织和协调的外展活动比单独的单独努力要有效得多。技术摘要固体氦样品夹在超流引线之间，超流引线的形式是注入了超流氦的多孔维科玻璃瓶。这种质量流动被认为与固体氦样品中边缘位错的运动和/或氦原子通过超流螺旋位错的输运有关。通过超流体/固体氦/超流体“夹心”的质量流动实验正在进行中，以检验位错线模型的准确性。由于在高度多孔的二氧化硅气凝胶中生长的固体氦中没有位错线，因此对这种固体样品的测量提供了对这种现象是否需要位错线的直接测试。对不同厚度的固体样品进行了研究，范围从8微米(位错线不形成连接的网络)到几毫米，以了解位错网络的连通性的影响。通过将石墨晶体的c轴(它是固体氦晶体的外延生长的核心)沿着或垂直于样品空间内的质量流动方向来研究晶体取向的依赖关系。固体氦的质量流动是由施加在样品上的超流体喷泉压力驱动的。通过测量流量随温度和喷泉压力的变化，寻找无耗散区域和耗散区域的分界线。

Non-technical abstractLiquid helium changes from a normal liquid to a superfluid at low temperatures. It is called a superfluid because it can flow with zero viscosity with no loss in energy. Once set flowing in a continuous channel, superfluid will never stop. This fascinating property is related to the fact that liquid helium enters a quantum state below 2.176 degrees Kelvin. Remarkably, recent experiments at the University of Massachusetts and Penn State found helium atoms can flow through solid helium samples like a superfluid. The Penn State team is measuring the flow rate of the helium atoms through solid helium samples of different densities, thicknesses, crystal orientations to understand the exact mechanism responsible for this phenomenon. The proposed experiments provide opportunities for postdocs, graduate and undergraduate students to receive the most rigorous trainings to be future scientists. The principal investigator and his students and post-docs are active participants in the full range of outreach activities sponsored by the Penn State Materials Research Science and Engineering Center (MRSEC) for K-12 students. These organized and coordinated outreach activities are far more effective than isolated individual efforts.Technical AbstractThe solid helium samples are sandwiched between superfluid leads in the form of porous Vycor glass cylinders infused with superfluid helium. The mass flow is thought to be related to the motion of edge dislocations and/or the transport of helium atoms through the superfluid screw dislocations in solid helium samples. Mass flow experiments through the superfluid/solid helium/superfluid "sandwiches' are in progress to test the veracity of the dislocation line model. Since dislocation lines are absent in solid helium grown inside highly porous silica aerogel, measurements in such solid samples provide a direct test on the necessity of dislocation lines for the phenomenon. Solid samples of different thicknesses ranging from 8 µm (where dislocation lines do not form a connected network) to a few mm are being studied to understand the effect of connectivity of the dislocation network. The dependence of crystal orientation is studied by affixing graphite crystals with their c-axis (which nucleates epitaxial growth of solid helium crystals) pointing either along or perpendicular to mass flow direction inside of the sample space. The mass flow through solid helium is driven by superfluid fountain pressure imposed across the samples. The flow rate as a function of temperature and the fountain pressure are measured to search for the boundary separating the dissipation-free and dissipative regions.