Transition from antiferromagnetism to superconductivity in the compensated metallic state of Ba_1-x K_x (Fe_1-y Co _y )₂ As₂

We have investigated the electronic properties of Ba_1-x K _x ( Fe_1-y Co_y)₂ As₂ (0≤x≤0.51,0≤y≤0.22) single crystals with a special focus on the carrier compensated regions (x=2y). With increasing x and y under the x=2y constraint, the ground state changes from an antiferromagnetic metal to a paramagnetic metal with an intervening superconducting phase. On the basis of the Hall coefficient measurements, we show that this transition occurs in a coherent alloy with a Fermi surface similar to BaFe₂ As₂, and that the antiferromagnetic fluctuations are slightly weakened with increasing degree of alloying. We discuss the role of the randomness introduced by the substitution of Co²⁺ on this transition.