Superfluid fraction ofHe4very close toTλ

Measurements of the superfluid fraction ${\mathrm{\rho }}_{\mathit{s}}$/ρ of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ are presented for the reduced-temperature range 3×${10}^{\mathrm{-}7}$≲t≡1-T/${\mathit{T}}_{\lambda }$≲${10}^{\mathrm{-}2}$. The data were obtained from the second-sound velocity ${\mathit{u}}_{20}$, which in turn was determined from nonlinear-pulse transit-time measurements. The exponent ζ, which describes the leading singularity of ${\mathrm{\rho }}_{\mathit{s}}$/ρ, is found to be 0.6705±0.0006. This result is in good agreement with existing theories and previous data much further away from ${\mathit{T}}_{\lambda }$.