Second-sound velocity, scaling, and universality in He II under pressure near the superfluid transition

The velocity of second sound in He II was measured for $ϵ\equiv 1-\frac{T}{T_{\lambda }}>2\times {10}^{-5}$ between saturated vapor pressure (SVP) and the melting curve. For P>SVP, the corresponding superfluid fraction $\frac{{\rho }_s}{{\rho }^{{}^{\prime }}}$ cannot be described by a simple power law in ε. When reasonable general assumptions are made about higher-order singular contributions, the asymptotic behavior of $\frac{{\rho }_s}{\rho }$ may be deduced and agrees well with scaling and universality.