Mass transport through solid helium

非技术文摘：液氦在低温下由正常液体变为超流体。它被称为超流体，因为它可以以零粘度流动而没有能量损失。超流体一旦在连续通道中流动，就永远不会停止。这一令人着迷的特性与液氦在低于2.176开尔文时进入量子态有关。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过固体氦样品。宾夕法尼亚州立大学的研究小组正在通过不同密度、厚度、晶体方向的固体氦样品测量氦原子的流速，以了解造成这种现象的确切机制。提出的实验为博士后、研究生和本科生提供了成为未来科学家的最严格训练的机会。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与工程中心（MRSEC）为K-12学生举办的全方位外展活动。这些有组织和协调的外联活动远比孤立的个人努力有效得多。技术摘要：固体氦样品以注入超流氦的多孔Vycor玻璃圆柱体的形式夹在超流导线之间。质量流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“三明治”进行的质量流实验正在进行中，以检验位错线模型的准确性。由于在高多孔二氧化硅气凝胶中生长的固体氦中没有位错线，因此在这种固体样品中的测量提供了对这种现象的位错线必要性的直接测试。正在研究不同厚度的固体样品，从8µm（其中位错线不形成连接的网络）到几mm，以了解位错网络连通性的影响。通过将石墨晶体的c轴指向或垂直于样品空间内的质量流动方向，研究了晶体取向的依赖性。通过固体氦的质量流是由施加在样品上的超流体喷泉压力驱动的。流速作为温度和喷泉压力的函数进行测量，以寻找分离无耗散区域和耗散区域的边界。

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.