Superfluid number density ($n_s$)

The particle-number density associated with the superfluid response. In two dimensions it is a number per unit area and gives the phase stiffness ${\rho }_s={\hslash }^2{n}_s/m$ for particles of mass $m$. It differs from total density and need not equal condensate density.

In the two-fluid model of superfluidity, the superfluid fraction is ${\rho }_s/\rho$, where ${\rho }_s$ is the superfluid mass density and $\rho$ the total mass density. For a one-component fluid of particles of the same mass this also equals $n_s/n$, using the superfluid number density $n_s$ and total number density $n$. It measures the superfluid response and is distinct from the condensate fraction, which measures occupation of a single-particle state. Interactions can make the two fractions very different.