Superfluid fraction
= Superfluid fraction
{title2=$\rho_s/\rho$}
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.