Quantum phase transition of liquid helium in aerogel

1997年，我们给出了98%气凝胶中氦-3超流相变的量子相变(QPT)证据。然而，QPT的细节到目前为止还不清楚。我们利用超声技术研究了氦3和氦4气凝胶中氦的QPT。本文报道了小孔多孔玻璃中氦4的气凝胶体系QPT。我们研究了氦-4在气凝胶中的超流转变，气凝胶的孔隙率在92-97%之间。这种转变被气凝胶抑制了。我们绘制了该体系的相图。然而，在气凝胶中没有观察到QPT。在这项研究中，我们观察到了许多有趣的现象。例如，在1.7K附近观察到97%的多孔气凝胶的异常声衰减，这一结果表明超流体中的第一、第二声与该复合体系中的气凝胶声模式之间存在模式转换的可能性。观察到了固-液界面的声传播。我们的实验表明，气凝胶中的二氧化硅对晶体生长有抑制作用。为本实验搭建了氦3和气凝胶系统的核消磁制冷机。声波小室具有气凝胶和散装液体的声腔，以供比较。用6 MHz的纵向超声观察了气凝胶的信号。由于冰箱的故障，只获得了初步的实验数据。实验仍在进行中。

We have presented evidence for quantum phase transition (QPT) on superfluid phase transition of helium 3 in 98% aerogel in 1997. However, the details of the QPT are not known so far. We studied the QPT of helium in aerogel for both helium 3 and 4 using ultrasound technique.1. helium 4 and aerogel systemQPT is reported for helium 4 in porous glass with small pore. We have studied superfluid transition of helium 4 in aerogel with porosity between 92 and 97 %. The transition was suppressed by aerogel. We have made the phase diagram for this system. However, QPT was not observed in aerogel. We have observed many interesting phenomena in this research. For example, anomalous sound attenuation was observed with 97 % porous aerogel around 1.7 K. This result suggested possibility of mode conversion between first, second sound in superfluid and aerogel sound mode in this composite system.Solidification of helium in aerogel was studied. Sound transmission trough the solid-liquid interface has been observed. Our experiment shows the crystal growth is suppressed by silica strands of aerogel.2. helium3 and aerogel systemA nuclear demagnetizing refrigerator has been constructed for this experiment. The sound cell has acoustic cavities with aerogel and bulk liquid for comparison. The signal with aerogel has been observed using longitudinal ultrasound at 6 MHz. Because of the trouble of refrigerator, only preliminary experimental data was obtained. The experiment is ongoing.