A fluid with directions

In the early 1970s Anthony Leggett developed a theory for the superfluid that is obtained when the rare gas isotope ³He is cooled down to very low temperatures. This fluid has magnetic properties, which makes it anisotropic; it has different properties in different directions. In addition, the fluid has several states with different properties, called phases, in which several types of ordering phenomena occur. Here the magnetic properties are linked to the atoms' movements.

Every pair of 3He atoms is described by its spin (fulldrawn green arrow) and its movement (broken red arrow). In Leggett's theory the phases A and B have different kinds of order. Which phase that appears depends on temperature, pressure and external magnetic field.