Depression of the superfluid transition temperature inHe4by a heat current

We report experimental results for the depression of the superfluid transition temperature ${\mathrm{T}}_{\lambda }$(Q) in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ by a heat current Q. The data were obtained by use of thermometry with a resolution of 10 nK, and cover the range 0.4≲Q≲10 μW/${\mathrm{cm}}^2$. They can be represented by 1–${\mathit{T}}_{\lambda }$(Q)/${\mathit{T}}_{\lambda }$(0)=(Q/${\mathit{Q}}_0$ ${)}_{\mathit{x}}$ with ${\mathit{Q}}_0$=568±200 W/${\mathrm{cm}}^2$ and x=0.813±0.012, and are in good agreement with theoretical predictions.