Acoustical Experiments on Superfluid 3He-4He Mixtures in Aerogel

This report discusses our results on the superfluidity of 3 He- 4 He mixtures in a 98% porosity silica aerogel. We have used low frequency sound to probe helium mixtures confined to aerogel, and have observed both the slow mode of superfluid 3 He in aerogel, which is manifested only below Tc, and an additional sound mode present only in the mixture. We attribute this novel sound mode to the slow-mode in the 4 He rich phase of the dilute 3 He- 4 He mixture. This mode exhibits positive frequency shifts below Tc in aerogel, while above Tc the mode is observed at a temperature independent frequency until close to T(lambda) where it shifts to zero frequency.