Studies on crystal growth of nuclear-ordered solid ^3He at ultra-low temperature

超低温下核有序固体^3He在超流体^3He中的晶体生长(和熔化)将探索晶体生长的详细微观机制，因为核有序固体的凝固潜热变得很小，过剩部分保证了晶体生长和熔化过程中良好的热量和质量传递，以及已经深入研究的^4He情况。由于固体和液体之间存在较大的磁化强度差，^3He的独特情况是自旋超流对晶体生长的主要贡献。(1)我们在1 mm的圆柱形样品池中研究了核有序的^3He晶体的生长和熔化，发现生长和熔化的速率有很大的差异。其生长机制应为螺位错辅助生长，台阶速度受超流B相对断裂速度的限制。另一方面，熔化的速度相当快，我们还无法测量任何定量数据，但t…更多的固体表面在熔化过程中似乎是一个粗糙的表面。利用核磁共振成像技术(MRI)进行了进一步的研究。(2)研究了固体在过冷液体中的形核问题。作为温度的函数。我们发现成核速率与温度无关，并认为成核是通过量子隧道机制进行的。我们将把这一研究推广到超低温下的^4He情形。(3)研究了^4He处于超态时，多孔介质中^4He的冻胀。提出了霜层生长的耗散机理，并与实验数据进行了比较。(4)发展了适用于超低温的磁共振成像技术。我们应用磁共振成像技术对^3He-^4He相分离边界进行了可视化。利用这项技术，我们获得了约10微米的二维空间分辨率。(5)结合上述核磁共振，研究了核有序固体~3He的自旋动力学，并在一定条件下观察到了核磁共振均一模的Suhl不稳定性。较少

Studies of crystal growth (and melt) of nuclear-ordered solid ^3He in superfluid ^3He at ultra-low temperatures will explore detailed and microscopic mechanisms of crystal growth since latent heat of solidification of nuclear-ordered solid becomes very small and superfluity guarantee good heat and mass transport during crystal growth and melt as well as the ^4He case which has been studied thoroughly. Unique feature of the ^3He case is the dominant contribution of spin supercurrent to the crystal growth since there exists a large magntization difference between solid and liquid.(1)We studied crystal growth and melt of nuclear-ordered ^3He in 1 mmphi cylindrical sample cell and found a large difference of the rates of growth and melt. The growth mechanism should be the screw-dislocation assisted one and the step velocity is limited by the pair-breaking velocity of superfluid B phase. On the other hand, the melt is rather very fast and we could not measure any quantitative data yet but t … More he surface of the solid on the melting process seems to be a rough surface. Further research is on progress by using the NMR-imaging technique (MRI) described below.(2)The nucleation problem of solid in a supercooled liquid ^4He has been investigated. as a function of temperature. We found that the nucleation rate becomes temperature-independent and concluded that nucleation occured by quantum tunneling mechanism. We will extend this studies to the ^3He case at ultra-low temperatures.(3)We studied frost heave of ^4He from porous media when ^4He is in the superfuild state. We proposed the dissipation mechanism of the growth of frost and compared it with our data.(4)We developed the MRI technique applicable at ultra-low temperatures. We applied the MRI to visualize ^3He-^4He phase separation boundary. We achieved 2-dimensional space resolution of about 10 mum by this technique.(5)With connection of the above MRI,spin dynamics of nuclear-ordered solid ^3He has been stueied and the Suhl instability of the uniform mode of NMR was observed under certain conditions. Less