Raman Scattering Study of Fermi-degenerated Liquid ^3He

本项目的最终目标是通过拉曼散射澄清费米简并液体^3He中的基本激发行为。为了实现这个实验，我们需要大约100mK的低温，其中^3He作为费米简并液体。光散射稀释制冷机是该项目的重要设备。光窗是光散射所必需的，它能使入射激光和散射光通过，同时也增加了入射激光和热辐射在室温下引起的热泄漏。然后，我们必须提高^3He的循环速率，以获得高冷却功率，以补偿这些热输入。基于这些原因，我们考虑了预冷的方式，自行构建了稀释冰箱。为了检验循环速率与冷却功率之间的关系，我们采用了小型和大型两种泵送管路。得到的结果如下：小型的最低温度为150mK，大型的最低温度为100mK。在本例中，由于热辐射而产生的热量输入约为30w。此外，我们还设计了一种新的稀释制冷机，以获得更高的冷却功率。我们称之为蒸馏釜式，它在蒸馏釜中没有回流液体^3He热交换器。我们得到了70mK的最低温度和40w的冷却功率在100mK。40w对应于入射激光束4mW的热输入。因此，这种新型制冷机即使在相对较小的泵送系统中也能获得较高的冷却功率。本研究结果将为研制冷却功率为1mW、功率为100mK的稀释制冷机提供参考。通过本研究的继续，我们将用拉曼散射阐明费米退化液体^3He的基本激发行为。

The final goal of this project is to clarify behaviors of elementary excitations in Fermi-degenerated liquid ^3He by Raman scattering. To achieve this experiments, we need low temperature of about 100mK where ^3He plays as Fermi-degenerated liquid. Then a dilution refrigerator for light scattering is an important equipment for this project. Optical windows, which pass incident laser and scattered lights, are necessary for light scattering and increase the heat leaks due to the incident laser and thermal radiation a room temperature. Then we must increase circulation rates of ^3He in order to obtain high cooling powers which compensate with these heat inputs. From these reasons, we constructed the dilution refrigerator by ourselves, considering the way of precooling. In order to cheek the relationship between the circulation rate and the cooling power, we used two pumping lines, that is, small and large types. The obtained results are following. The lowest temperature was 150mK for the small type and 100mK for the large one, respectively. The heat input due to the thermal radiation was about 30 W for the present case. Furthermore we designed a new dilution refrigerator to obtain higher cooling power. We name the still-skip type, which does not have a heat exchanger of the returning liquid ^3He at the still. We obtained the lowest temperature of 70mK and the cooling power of 40 W at 100mK. 40 W corresponds to the heat input of the incident laser beam of 4mW. Thus this new type refrigerator is useful to obtain the high cooling power even for a relatively small pumping system. The present results will be the useful information to make the dilution refrigerator with the cooling power of 1mW at 100mK. By continuing this research, we will clarify the behavior of the elementary excitation in Fermi- degerierated liquid ^3He by Raman scattering.