Crossover behavior in He-3 and Xe near their liquid-vapor critical point

We present a detailed discussion of the crossover from mean-field to Ising critical behavior upon approach of the critical point, both for He-3 and Xe . By combining different sets of experimental data, we are able to cover an unusually large temperature range on either side of the critical temperatu re T-c. Below T-c, we thus can make an accurate comparison with a recent ca lculation for the crossover of the coexistence curve. For the regime above T-c, an analysis of the compressibility demonstrates that the crossover reg ime in He-3 is unexpectedly widened by a subtle interplay between quantum a nd critical fluctuations.