Study of Phase Transition of Superfluid Helium 3 in sub micrometer Space

通过对0.8 μm板状结构中^3He超流的实验研究，可以清楚地看到，表面准粒子的边界条件对超流相的稳定性有很大的影响。在真实的空间中，^3He超流态表现出各向异性，这种各向异性与样品池的各向异性相互竞争。因此，当我们把液态^3He放入一个尺寸接近相干长度的盒中时，我们有望观察到超流态之间的特殊相变（A-B相变）以及超流相变温度的抑制。相干长度随压力的变化而变化，压力越低，相干长度越长。从这个角度来看，上述期望在较低压力下变得更加明显。在我们的研究中，我们开发了用于NMR检测的高Q值调谐电路，并使用低温放大器来获得足够的信噪比。在所有压力下，我们观察到了比1.1 μm板状结构更低的A-B相变温度，同时也观察到了超流转变温度的大幅度抑制。如所预期的，这些在较低压力下增强，并且在低于约2巴时不发生A-B转变。临界厚度很好地解释了两种平板结构下的A-B相变现象。令人惊讶的是，当我们在界面上放置几个^4He层时，A-B相变温度变得更高。^4He薄层的超流性似乎改变了^3He准粒子的边界条件，提高了A-B转变温度。然而，迄今为止，所有的理论论证都显示了相反的结果。我们需要新的理论工作来讨论我们的结果。

It became clear that the boundary condition of ^3He quasi particles at the surface had a great effect on the stability of superfluid phases by our experimental study on superfluid ^3He in 0.8 μm slab geometry. The anisotropy of ^3He superfluid state appears in real space and this anisotropy competes with the anisotropy of sample cell. As a result it is expected that we can observe the particular phase transition between superfluid states (A-B phase transition) and the suppression of the superfluid transition temperature when we put liquid ^3He in a cell whose size is close to the coherence length. The coherence length varies depending on pressures, the lower pressure, the longer coherence length. From this point of view the above expectation becomes more obvious at lower pressures.In our study, we developed the high Q-value tuned circuit for NMR detection with the low temperature amplifier to get the enough S/N ratio. We observed the lower A-B phase transition temperatures than those in 1.1 μm slab geometry at all pressures and also observed the large suppression of superfluid transition temperature. These were enhanced at lower pressures as expected and no A-B transition occurred lower than about 2 bar. The critical thickness well explained A-B phase transition phenomena at both slab geometries. Surprisingly the A-B phase transition temperature became higher when we put a few ^4He layers on the boundary surface. The superfluidity of thin ^4He layer seems to change the boundary condition for ^3He quasi particle and to lift A-B transition temperature. All the theoretical arguments, however, showed the opposite result so far. We need new theoretical work to discuss about our results.