Superfluid transition and solidification of 4He in aerogel

Superfluid transition and solidification of 4He in aerogel has been studied by longitudinal ultrasound. The superfluid transition temperature was determined up to the solidification pressure. The superfluid transition temperature in aerogel which was identified as a sharp absorption peak was suppressed from that in bulk liquid. The magnitude of suppression in aerogel was a few milli-Kelvin from the saturated vapor pressure to the solidification pressure and was much smaller than that in other porous media. The onset of solidification of 4He in aerogel was identified as an increase in sound absorption. It was shown that the overpressure required to initiate solidification was 0.2-0.3 MPa between 1.1 and 1.8 K. Small hysteresis was observed at the solidification and melting.