Phase transitions of helium in aerogel

The effect of dilute quenched impurities on the liquid-vapor critical point, superfluid and He³-He⁴ phase separation transitions were studied by introducing liquid helium into aerogel. Aerogel is a highly porous glass consisting of atomically thin silica strands interconnected at random sites. In spite of the random environment, the transitions were found to be remarkably sharp and wcll defined. Although the silica network constitutes as little as two percent of the total volume, the nature of the transitions is completely changed. When a He³-He⁴ mixture is placed inside aerogel of 98 percent porosity, the coexistence region is found to be detached from the supefluid transition line, giving rise to a new supefluid mixture that is rich in He³.