Crossover behavior in ³He 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 ³He 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 temperature T_c. Below T_c, we thus can make an accurate comparison with a recent calculation for the crossover of the coexistence curve. For the regime above T_c, an analysis of the compressibility demonstrates that the crossover regime in ³He is unexpectedly widened by a subtle interplay between quantum and critical fluctuations.