Using the de Haas-van Alphen effect we have measured the evolution of the Fermi surface of BaFe2(As1-xPx)2 as a function of isoelectric substitution (As/P) for 0.41<x<1 (Tc up to 25 K). We find that the volumes 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 Tc. These results are not explained by simple band structure calculations, and it is likely that these trends originate from the same many-body interactions which give rise to superconductivity.
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