Raman Scattering Study of Fermi-degenerated Liquid ^3He

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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.