We improve the theoretical estimates of the critical exponents for the three-dimensional
XY universality class that apply to the superfluid transition in
4He along the $\ensuremath{\lambda}$ line of its phase diagram. We obtain the estimates $\ensuremath{\alpha}=\ensuremath{-}0.0151(3)$, $\ensuremath{\nu}=0.6717(1)$, $\ensuremath{\eta}=0.0381(2)$, $\ensuremath{\gamma}=1.3178(2)$, $\ensuremath{\beta}=0.3486(1)$, and $\ensuremath{\delta}=4.780(1)$. Our results are obtained by finite-size scaling analyses of high-statistics Monte Carlo simulations up to lattice size
L=128 and resummations of 22nd-order high-temperature expansions of two improved models with suppressed leading scaling corrections. We note that our result for the specific-heat exponent $\ensuremath{\alpha}$ disagrees with the most recent experimental estimate $\ensuremath{\alpha}=\ensuremath{-}0.0127(3)$ at the superfluid transition of
4He in a microgravity environment.
The critical behavior of semi-infinite systems in fixed dimensions
d<4 is investigated theoretically. The appropriate extension of Parisi's massive field theory approach is presented. Two-loop calculations and subsequent Pad\'e-Borel analyses of surface critical exponents of the special and ordinary phase transitions yield estimates in reasonable agreement with recent Monte Carlo results. This includes the crossover exponent $\ensuremath{\Phi}(d=3)$, for which we obtain the values $\ensuremath{\Phi}(n=1)\ensuremath{\simeq}0.54$ and $\ensuremath{\Phi}(n=0)\ensuremath{\simeq}0.52$, considerably lower than the previous $\ensuremath{\epsilon}$-expansion estimates.
In a recent letter, R.G. Brown and M. Ciftan (Phys. Rev. Lett. 76, 1352, 1996) reported high precision Monte Carlo (MC) estimates of the static critical exponents of the classical 3D Heisenberg model, which stand in sharp contrast to values obtained by four independent approaches, namely by other recent high statistics MC simulations, high-temperature series analyses, field theoretical methods, and experimental studies. In reply to the above cited work we submitted this paper as a comment to Phys. Rev. Lett.