  Using the de Haas-van Alphen effect we have measured the evolution of the
Fermi surface of BaFe_2(As_{1-x}P_x)_2 as function of isoelectric substitution
(As/P) for 0.41<x<1 (T_c up to 25 K). We find that the volume of electron and
hole Fermi surfaces shrink linearly with decreasing x. This shrinking is
accompanied by a strong increase in the quasiparticle effective mass as x is
tuned toward the maximum T_c. It is likely that these trends originate from the
many-body interaction which give rise to superconductivity, rather than the
underlying one-electron bandstructure.
