Search for Superfluid Helium-3 in Aerogel by Fourth Sound Technique

在本研究中，我们利用第四声技术研究了多孔硅气凝胶中的超流氦-3。我们使用了三种不同密度的气凝胶样品，一种是1%（对应孔隙度99%），一种是1.5%（对应孔隙度98.5%），另一种是2%（99%）。为了避免气凝胶本身的振动，三个气凝胶样品中的两个在烧结的银粉中生长，通常直径为100微米。这些气凝胶是由特拉华大学的Norbert Mulders教授制作的。我们使用第四声技术进行测量。通过测量在样品腔内共振的第四声声速，得到氦-3的超流密度。超流体密度对温度的依赖性给出了转变温度。我们在2.7巴到18巴之间的几种压力下测量了超流体密度。在所有压力下，超流体转变温度都被抑制，因为气凝胶骨架破坏了超流体氦-3的库珀对。气凝胶密度越大，转变温度越受抑制。转变温度对气凝胶密度的依赖性可以用Abrikosov和Gor'kov的含磁性杂质超导理论来解释。我们的结果在数量上与A-G理论一致，但与国外的研究结果不一致。关于超流体密度，我们的结果也可以用A-G理论来解释，但其他小组的结果不能。我们的结果与其他小组的结果之间的差异目前还不清楚。进一步的研究是必要的。

In this research, we studied Superfluid Helium-3 in porous silica aerogel by Fourth Sound technique. We used three aerogel samples which have different density, one is 1% (which is corresponding to the porosity 99%), another is 1.5% (corresponding to 98.5%), the other is 2% (99%). Two of the three aerogel samples were grown inside the sintered silver powder typically 100 micrometer diameter to avoid vibration of the aerogel itself. The aerogels were made by Professor Norbert Mulders in University of Delaware. We used Fourth Sound technique to measure. We get superfluid density of Helium-3 from a measured sound velocity of Fourth Sound, which resonating in the sample cavity. The temperature dependence of the superfluid density gives us transition temperature.We measured superfluid density at several pressures between 2.7 bar and 18 bar. At all pressures, the superfluid transition temperaure is suppressed from the bulk one, because aerogel skeleton breaks Cooper pairs of superfluid helium-3. The transition temperature is more suppressed in higher density aerogel. The aerogel-density dependence of the transition temperature can be explained by a theory of Abrikosov and Gor'kov for superconductivity including magnetic impurities. Our result agrees quantitatively with A-G theory, but this does not agree with foreign groups. Regarding superfluid density, our results also can be explained by A-G theory, but other groups not The difference between our results and other group's results is not understood now. Further study is necessary.