| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1988: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1987: ¥1,300,000 (Direct Cost: ¥1,300,000)
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| Research Abstract |
Superfluid ^4He is the most typical Bose-condensed state. In the theoretical framework to clarify the properties of superfluid ^4He based on interatomic interactions, the dynamic structure factor derived by Feynman-Cohen is a very reliable mile-stone from a theoretical view point. However there is considerable difference between the theoretical expression and the experimentally observed dynamic structure factor. And to explain the difference is one of the most important problems of superfluid ^4He. The theoretical model by Ruvalds-Zawadowski has been regarded as an established theory to derive a theoretical dynamic structure factor, that can be quantitavely compared with experiments, from the Feynman-Cohen expression. We started this project with the purpose to investigate behavior of Fermi-particles in the Bose-condensed system. However, in the early research, we found an important fact that the two-roton Raman spectrum at elevated applied pressure does not fit to the Ruvalds-Zawadows
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ki model. The important finding made us concentrate our experiments on the precise detailed observation of two-roton Raman spectra of pure superfluid ^4He, to clarity the nature of interactions between elementary excitations. For that purpose, we developed a ^3He cryostat laser light scattering, with which we could make continuous observation over one week at 0.65 K with 400 mW laser imput. A 2-dimensional imaging photon detector was also introduced, with which we could reduce noises and increase accuracy considerably. With the experimental apparatus thus developed, we could observe very accurate two-roton Raman spectra at a temperature of 0.65 K with a resolution of 0.45 K. The pressure values were chosen at svp, 5, 10, 15, 20 kg/cm^2. The position of the two-roton peak was observed below at svp and above over 5 kg/cm^2 compared with twice the energy at roton minimum. The positive deviation was observed increase as the pressure increased. The spectral shape at svp can be explained by the conventional Ruvalds-Zawadoeski theory. However, the spectral shape observed over 5 kg/cm^2 connot be explained by the theory. We could thus claim necessity of development of new theories to interpret interacations between elementary excitations in Bose-condensed state. Less
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