Superfluidity of liquid ^3He in aerogel

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Ultrasound measurement has been used to study super fluidity of liquid He in aerogel. We obtained the following results.1. Ultrasound spectrometer Ultrasound spectrometer using Phase Sensitive Detection has been made with a receiver purchased by this grant. This system enables to measure sound velocity sensitively. Main researcher, Matsumoto has moved to Kanazawa University so that we had to make a new system and has completed it.2. Complex sound mode in aerogel-He4 system For the test of the system, 10 MHz Ultrasound measurement has been done with liquid He4. We have observed complex sound mode in this system due to coupled motion between aerogel and liquid. An important result is that the sound velocity of this mode is extremely sensitive to the aerogel density. Theoretical analysis predicts the two complex sound modes and the observed mode has been shown to correspond to the fast mode. This is the first that this mode was observed in both normal and super fluid phase. Large absorption coefficient and no absorption peak due to Roton-Phonon interaction have been observed. These results have been published in Pysica B (as shown in the reference).3. Nuclear demagnetization refrigerator The main researcher, Matsumoto has move to Kanazawa University from Tokyo Institute of Technology in April 2000. We could not use nuclear demagnetizatio refrigerator in Tokyo Institute of Technology any more so that a new refrigerator had to be constructed at Kanazawa University. We have been trying to build nuclear refrigerator and main components have been completed. We continue improving it to achieve temperature below 1 mK which is necessary to study super fluidity of liquid He3.