Model $F$ of Halperin, Hohenberg, and Siggia contains a phenomenological coupling ${\Gamma }_0^{\prime \prime }$ which has been treated previously as an adjustable nonuniversal parameter. For the application to ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ the a priori estimate ${\Gamma }_0^{\prime \prime }\simeq \frac{\hslash }{2m}$ is proposed, where $m$ is the helium mass. This is consistent with a nonlinear renormalization-group analysis of the critical thermal conductivity. It is pointed out that Pitaevskii's equations imply the universal identification ${\Gamma }_0^{\prime \prime }=\frac{\hslash }{2m}$.

• Received 19 September 1983

DOI:https://doi.org/10.1103/PhysRevB.29.1497